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Stevenage Strategic Flood Risk Assessment Final Report
Stevenage Borough Council February 2009
Prepared by: ................................................ Approved by: ............................................. Barry Barton Philip Mcloughlin Project Manager Associate Director Stevenage Strategic Flood Risk Assessment
Rev No Comments Date
01 A Interim Draft Report Nov.2007
01 B Draft Report Apr.2008
01 C Revised Draft Report July 2008
01 D Revised Draft Report Aug 2008
01 E Revised Draft Report Oct 2008
01 F Final Report Feb 2009 1 The Forum, Minerva Business Park, Lynch Wood, Peterborough, PE2 6FT Telephone: 01733 391456 Fax: 01733 391139 Website: http://www.fabermaunsell.com Job No 55120 I PER Reference 01 F Date Created February 2009 This contains confidential and commercially sensitive information, which shall not be disclosed to third parties. f:\projects\55120i stevenage sfra\reports\stevenage\01f stevenage final rept.doc
Faber Maunsell Stevenage Strategic Flood Risk Assessment i
Contents ................................................................................................................................... i
Executive Summary .............................................................................................................. iii
1 Introduction ................................................................................................................ 1
2 Flood Risk ................................................................................................................... 5 2.1 Planning Policy Statement 25 .......................................................................... 5 2.2 Indicative Floodplain Maps .............................................................................. 6 2.3 Flood Zone Maps ............................................................................................. 7 2.4 Flood Maps ...................................................................................................... 7
3 Causes of Flooding .................................................................................................. 11 3.1. Overflowing of Watercourses ......................................................................... 11 3.2 Breaching of Embankments ........................................................................... 12 3.3 Mechanical, Structural or Operational Failure ............................................... 13 3.4 Localised Surface Water Flooding ................................................................. 13 3.5 Functional Floodplains and Washlands ......................................................... 15
4 Flooding in Stevenage ............................................................................................. 19 4.1 Hydrological Characteristics of Stevenage .................................................... 19 4.2 Hydrogeological Characteristics of Stevenage .............................................. 20 4.3 Urban Drainage Characteristics of Stevenage .............................................. 20 4.4 Classes of Flooding ....................................................................................... 21 4.5 Sources of Flooding ....................................................................................... 22 4.6 Records of Flooding ....................................................................................... 23 4.7 Operational and Emergency Planning ........................................................... 24
5 Flood Risk Alleviation Measures ............................................................................ 29 5.1 Flood Storage Reservoirs in Stevenage ........................................................ 29 5.2 Detailed Descriptions of Flood Storage Reservoirs ....................................... 30 5.3 Fairlands Valley Lakes ................................................................................... 35 5.4 Surface Water Sewerage System .................................................................. 36 5.5 Foul Sewerage System .................................................................................. 36
6 Strategic Assessment of Flood Risk ...................................................................... 41 6.1 General Methodology .................................................................................... 41 6.2 Topographic Divisions ................................................................................... 43 6.3 Anthropogenic Influences .............................................................................. 44 6.4 Hydraulic Modelling........................................................................................ 44 6.5 Strategic Flood Risk Mapping ........................................................................ 45 6.6 Climate Change ............................................................................................. 48 6.7 Rapid Inundation or Hazard Zones ................................................................ 48
7 Assessment of Flood Risk in Study Areas ............................................................ 53
8 Study Areas in Stevenage ....................................................................................... 59 8.1 WESTERN STUDY AREA ............................................................................. 61 8.2 NORTHERN STUDY AREA .......................................................................... 67
9 Planning Policy and Flood Risk.............................................................................. 73 9.1 Regional and Sub-Regional Strategies.......................................................... 73 9.2 Sustainable Urban Drainage Systems ........................................................... 75 9.3 Recommended Policy and Guidance Statements ......................................... 77
Continued over …..
Table of Contents
Faber Maunsell Stevenage Strategic Flood Risk Assessment ii
Continued …..
10 Conclusions .............................................................................................................. 83
11 Recommendations ................................................................................................... 89
References ............................................................................................................................ 93
Glossary ................................................................................................................................ 97 FIGURES PHOTOGRAPHS APPENDICES Appendix A Project Brief Appendix B Recommended Flood Risk Planning Policy & Guidance Statements LISTS of TABLES and FIGURES Tables
Table 2.1 PPS25 Flood Risk Zones Table 4.1 Local Flooding Problems in Stevenage Table 4.2 Locations of Properties Registered to Receive Flood Warnings Table 5.1 Flood Storage Reservoirs in Stevenage Table 6.1 Flood Zone Extents in Stevenage Table 6.2 Flood Hazard Zone Widths Table 7.1 Indicative Standards for Fluvial Flood Defence Table 7.2 High Probability Category – Flood Risk Sub-Divisions Table 9.1 Future Housing Development in the East of England Table 9.2 Future Housing Development in Hertfordshire Figures
Figure 1.1 General Location Plan Figure 1.2 Study Areas Location Plan Figure 2.1 Environment Agency Flood Map Figure 4.1 Rivers and Streams in the Stevenage Area Figure 4.2 Geology of the Stevenage Area Figure 4.3 Extent of Historic Flooding in Stevenage Figure 5.1 Flood Storage Reservoirs in Stevenage Figure 6.1 Strategic Flood Risk Maps of Stevenage (Three Sheets) Figure 8.1 Stevenage West Study Area Figure 8.2 Stevenage North Study Area
Faber Maunsell Stevenage Strategic Flood Risk Assessment iii
The Department for Communities and Local Government’s Planning Policy Statement 25
entitled "Development and Flood Risk" outlines how flood risk issues should be addressed in
regional planning guidance and Local Authorities' development plans.
Faber Maunsell Limited were commissioned by Stevenage Borough Council on 31st July
2007 to undertake a Strategic Flood Risk Assessment of Stevenage based on a Brief issued
by the Council. The Brief, prepared by the Council in partnership with the Environment
Agency, divided the study into two phases. Phase 1 involved the collation and evaluation of
data and information relating to the hydrological and hydraulic aspects of flooding in
Stevenage. Phase 2 of the study, based on the results and recommendations of Phase 1,
was to include the following main topics, including agreed work supplementary to the Brief:
• Identifying and mapping the areas of flood risk over the whole of the District within the
categories defined in Planning Policy Statement 25 (PPS25).
• Flood risk assessments of two large study areas on the western and northern edges
of the Borough.
• Preparation of guidance notes for developers, recommending procedures to be
followed by prospective developers for the assessment of flood risk and measures to
be adopted to minimise that risk and mitigate the effects of increased runoff from the
development on flood risk elsewhere.
The purpose of the Study was to provide a reference and policy document to inform the
Borough Council’s Local Development Framework and to ensure that the Council meets its
obligations under PPS25.
Because of delays in obtaining key data, Phases 1 and 2 of the study had to be run in
parallel in order to minimise the overall delays to the project. For that reason, and because
any deficiencies in the data and information obtained were not so serious as to prejudice the
successful progress to Phase 2 of the study, no separate Phase 1 Report was issued and
the findings and recommendations of Phase 1 were subsumed into the Final Report.
Executive Summary
Faber Maunsell Stevenage Strategic Flood Risk Assessment iv
In undertaking Phase 2 of the project, Faber Maunsell have carried out a general Strategic
Flood Risk Assessment for the whole of Stevenage Borough and more detailed Flood Risk
Assessments for the two specific study areas on the western and northern edges of the
Borough where future large scale urban development is considered to be most likely to
occur. The Strategic Flood Risk Assessment has been carried out in line with the
Environment Agency (Thames Region)'s guidance notes to local authorities.
The results of the Strategic Flood Risk Assessment are presented in this Report as a set of
three 1/10,000 scale maps covering the whole of the Borough and showing the flood risk at
any point in one of three categories of flood risk, taking into account the effect of existing
flood defences in reducing flood risk. Unfortunately the initial lack of any hydraulic modelling
results for Stevenage Brook or for the Borough’s surface water sewerage system meant that
it was not possible to prepare any rapid inundation (flood hazard) maps, although flood
hazard resulting from rapid inundation is not considered to present a serious risk in
Stevenage.
A separate Flood Risk Assessment was also presented for each of the two study areas
nominated by the Borough Council. For each study area the principal flood risk sources
were identified and the extent of each of the three flood risk categories within that area
described in detail. The salient flood risk and drainage features of the study areas are
illustrated in a set of 1/10,000 scale maps. These assessments showed that, apart from
marginal riparian land, neither of the study areas is at significant risk of flooding (i.e. PPS25
Flood Zones 2 and 3) but large scale urban development in each study area could have a
significant adverse impact on flood risk downstream.
It is very likely that urban runoff from the two study areas will be discharged to watercourses
which flow through built-up areas downstream of the study area and which are already
subject to a degree of flood risk. Whilst this certainly does not preclude development in the
study areas, particular attention should be paid to ensure that appropriate measures are
taken to attenuate surface water runoff from urban development in these areas to avoid
increasing flood risk to areas downstream. The possible effects of climate change on flood
risk have also been considered in this context.
This report includes recommendations for policy and guidance statements for prospective
developers suggested for inclusion in the Local Development Framework. These
statements have been drafted within a framework put forward by the Environment Agency
and taking into account the Agency's Flood Risk Standing Advice Matrix. The potential
impact of regional and sub-regional development proposals have also been considered in
this Report.
Faber Maunsell Stevenage Strategic Flood Risk Assessment v
After the completion of the draft Report in April 2008, the Practice Guide to PPS25 was
issued in June 2008, followed later in the same month by the Pitt Review. Faber Maunsell’s
draft report was therefore amended to take into consideration the contents of the Practice
Guide and the recommendations made in the Pitt Review.
The project was carried out in collaboration with the Environment Agency's Thames Region
and utilised detailed sewerage plans of the Borough provided by Thames Water and, to a
more limited extent, by Anglian Water. A draft of this Report was submitted to the Agency
for their comments and observations and, following discussion with the Agency, mutually
acceptable amendments have been incorporated into this Final Report, which has also been
the subject of a formal audit by the Agency.
Faber Maunsell Stevenage Strategic Flood Risk Assessment vi
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Introduction
Faber Maunsell Stevenage Strategic Flood Risk Assessment 1
1.1 The Borough of Stevenage is situated in Hertfordshire, 45km north of central London, on
elevated land at the eastern end of the Chiltern Hills on the watershed between the Thames
and Great Ouse river catchments. Stevenage Development Corporation was established in
1946 to promote the creation and growth of Stevenage New Town. Stevenage Borough
Council took on the roles previously assigned to the Development Corporation when the
Corporation was dissolved in 1980. The Borough, which covers a relatively small
geographical area (26 sq.km) is now largely urbanised. It is bounded by North Hertfordshire
District to the west and north and East Hertfordshire District to the south and east.
1.2 Faber Maunsell Limited were appointed by Stevenage Borough Council on 31st July 2007 to
undertake a Strategic Flood Risk Assessment of the Borough of Stevenage. The scope of
the study was described in a Brief issued jointly by the Borough Council and the
Environment Agency. A Location Plan showing Stevenage in relation to surrounding Local
Planning Authority districts is given in Figure 1.1.
1.3 Planning Policy Statement 25 - Development and Flood Risk ("PPS25") (Ref.1a) was
issued in December 2006 by the Department for Communities and Local Government and
superseded Planning Policy Guidance Note 25 (“PPG25”) (Ref.1b) which had been issued
by the Department for Transport, Local Government and the Regions in July 2001. PPS25
expects local planning authorities to apply a risk-based approach to the preparation of their
development plans in respect of possible flooding. PPS 25 (together with the Practice Guide
to PPS25 published in June 2008 which replaced the draft Practice Guide Companion
issued in February 2007) contains specific guidance for planning authorities and those
working on their behalf on the methodology to be used in undertaking strategic flood risk
assessments. PPS25 and its implications for this study are discussed in greater detail in the
next Section.
1.4 The Brief for the Strategic Flood Risk Assessment, which is reproduced in Appendix A,
requires the identification and mapping of the areas of flood risk over the whole of the
Borough within the categories defined in Planning Policy Statement 25 (PPS25). This
constitutes the principal objective of the study.
1.5 The East of England Plan (Ref.2) is a regional planning document that sets out a strategy to
guide the pattern of development in the East of England to the year 2021. It includes
specific policies for the management of the water cycle and places emphasis on the need for
flood risk assessments. The Plan was adopted in May 2008 and identifies Stevenage as a
Key Centre for Development and Change. The Plan states that a minimum of 16,000 new
homes should be provided in and around the town between 2001 and 2021. At least 6,400
of these homes are to be provided within Stevenage Borough and a further 9,600 in new
neighbourhoods outside the Borough boundary in North Hertfordshire, to the west and north
of the existing town.
1.6 This new housing provision has lead to the identification by the Borough Council of two
areas of study for more detailed flood risk assessment on the western and northern edges of
the Borough, straddling the Borough boundary. This is so that the risk of flooding to urban
development in those areas and the extent to which that development may increase the risk
of flooding elsewhere can both be assessed. Both study areas are located entirely within
Flood Zone 1 (Low Probability of Flooding) as shown on the Environment Agency’s current
Flood Map. The locations of these two study areas are shown in Figure 1.2.
1 Introduction
Faber Maunsell Stevenage Strategic Flood Risk Assessment 2
1.7 The Borough Council’s Local Development Framework (LDF) is a suite of documents
produced by the Council that will eventually replace the Stevenage Local Plan. Local
Development Plans must be specific to their local planning authority area, be capable of
being measured and should clearly set out where the responsibility for delivering objectives
and policies lies.
1.8 The Stevenage Strategic Flood Risk Assessment will provide an important evidence base for
the Stevenage LDF. It will determine how much growth the Borough can maintain,
recommend improvements to the existing pattern of development and propose future
patterns for development in the proposed growth areas. This is highlighted by the
requirement within the Brief for the preparation of guidance notes for developers,
recommending procedures to be followed by prospective developers and for the assessment
of flood risk and measures to be adopted to minimise that risk and mitigate the effects of
increased runoff from the development on flood risk elsewhere.
1.9 This study has taken into consideration the Department for the Environment, Food & Rural
Affairs (DEFRA)’s ongoing policy development programme entitled “Making Space for
Water” which commenced in Autumn 2005, and other related studies in the area which have
previously been published, including Catchment Flood Management Plans, the Stevenage
Water Cycle Strategy Study, currently under way and due for completion towards the end of
2008, and other SFRAs.
1.10 The Environment Agency's Flood Maps and Flood Zone Maps (see Section 2) have been
used as a starting point for this study, supplemented by other information supplied by the
Borough Council, the Environment Agency, Thames Water, Anglian Water Services and
other sources. Due consideration has also been given to the Environment Agency's
guidance notes on strategic flood risk assessment issued to Local Planning Authorities.
1.11 The methodologies used to carry out the strategic flood risk assessment and the individual
flood risk assessments for the study areas, together with the results obtained, are described
in detail in this study. The results are also presented in a series of three 1/10,000 scale
Strategic Flood Risk Maps of the Borough, together with site plans for each study area
showing the principal hydrological and drainage features affecting flood risk in that area.
1.12 This document has been prepared solely as a Strategic Flood Risk Assessment of the
Borough of Stevenage and as a Flood Risk Assessment of two study areas partly within
Stevenage and partly within an adjacent district for Stevenage Borough Council. Faber
Maunsell accept no responsibility or liability for any use which is made of this document
other than that by the Client for the purposes for which it was originally commissioned and
prepared.
Flood Risk
Faber Maunsell Stevenage Strategic Flood Risk Assessment 5
2.0.1 Flood risk can arise from both fluvial and tidal sources. Fluvial flooding occurs as a result of
the overflowing or breaching of river or stream banks when the flow in the watercourse
exceeds the capacity of the river channel to accommodate that flow. Tidal flooding occurs
when an exceptionally high tide, almost always accompanied by a storm tide surge,
overtops and/or breaches the tidal defences along a coastline or tidal estuary. There are no
areas within the Borough of Stevenage that are subject to tidal flooding so all flooding and
flood risk considered in this Report relates to fluvial flooding (including flooding from arterial
drainage, local drainage and sewerage systems), groundwater flooding and flooding from
man-made structures.
2.1 Planning Policy Statement 25
2.1.1 Planning Policy Statement 25 (PPS25) was published in December 2006 and superseded
Planning Policy Guidance Note 25 (PPG25) which had originally been introduced in July
2001. PPS25 was supplemented by a draft Practice Guide Companion issued for
consultation in February 2007 (Ref.3a). The definitive Practice Guide to PPS25 (Ref.3b) -
a considerably enlarged document - was eventually published in June 2007.
2.1.2 PPS25 defines three distinct zones of flood risk. These zones are based on the quantified
probability of flooding to which an area of land would be subject if no defences existed at the
time at which a land allocation decision is made or a planning application submitted. The
PPS25 flood zones and their associated fluvial flood risk designations and probability ranges
are summarised in Table 2.1 below.
Zone Designation Assigned Annual Flood Risk Probabilities
1 Low Probability Less than 0.1% (above 1 in 1000 years)
2 Medium Probability 0.1% to 1% (from 1in100 to 1in1000 years)
3 High Probability Greater than 1% (under 1 in 100 years)
Table 2.1 - PPS25 Flood Risk Zones
2.1.3 The PPS25 flood zones give a broad indication of flood risk. However, most areas which fall
within the High Probability zone (Zone 3) are on fluvial floodplains and many such areas
already enjoy some degree of protection from established flood defences. The actual
degree of flood risk to which these areas are subject may well be significantly less than that
implied by their PPS25 classification, provided of course that those defences are
maintained.
2.1.4 PPS25 requires Local Planning Authorities to adopt a risk-based approach to development
in areas at risk of flooding, and to apply a "sequential test" to such areas. This means that,
other factors being equal, the planning authority would favour development in areas with a
lower flood risk. It is obvious that different areas allocated for urban development within the
PPS25 "High Probability" zone may be at very different risks of flooding, although the
introduction of a quantitative definition of the Functional Floodplain in PPS25 goes some
way to addressing this problem.
2 Flood Risk
Faber Maunsell Stevenage Strategic Flood Risk Assessment 6
2.1.5 Nevertheless the basic problem still remains. For example, whereas the probability of
flooding in one area may be as much as 4% (1 in 25 years) the probability in a neighbouring
area may be as little as 1.25% (1 in 80 years), yet both are within PPS25 Zone 3. The
planning authority must therefore be able to rank study areas according to actual flood risk.
2.1.6 Table D1 in PPS25 sub-divides the "High Probability" Zone 3, as summarised below:
Zone 3a - Land not in the Functional Floodplain
Areas which may be suitable for water compatible or “less vulnerable” categories of
development. Essential infrastructure and development classified as “more vulnerable” is
only permitted if the Exception Test (see below) is passed. “Highly vulnerable” development
is not permitted in Zone 3.
Zone 3b - The Functional Floodplain
Areas only suitable for water-compatible uses (some recreation, sport, amenity or
conservation uses) or, if unavoidable, essential transport and utilities infrastructure.
2.1.7 Table D2 in PPS25 (Flood Risk Vulnerability Classification) defines the types of
development included in the Essential Infrastructure, Highly Vulnerable, More Vulnerable
and Less Vulnerable categories. The Exception Test, introduced in PPS25, enables LPAs
to approve, under restricted circumstances, certain categories of development within certain
flood zones (defined in PPS25 Table D3) where it would not normally be permitted.
2.1.8 PPS25 defines functional floodplains as "land where water has to flow or be stored in times
of flood” and “land which would flood with an annual probability of 1 in 20 (5%) or greater, or
is designed to flood in an extreme (0.1%) flood”. A functional floodplain can therefore be
either an area of floodplain which is known to flood frequently and where flooding is
tolerated, as it may prevent or ameliorate flooding elsewhere, or an area within a floodplain
that can be deliberately inundated during a flood event to provide temporary retention
storage for flood water.
2.2 Indicative Floodplain Maps
2.2.1 Under Section 105 of the Water Resources Act 1991 the Environment Agency produced a
series of maps covering the whole of England and Wales ("Circular 30/92 Maps") showing
areas of land considered to be at risk of fluvial and tidal flooding and the likely extent of that
flooding. These maps were then used as the basis for the Agency’s Indicative Floodplain
(IF) maps.
2.2.2 Indicative Floodplain (IF) maps, based on Ordnance Survey 1/10,000 scale base maps,
were first issued in 2000. The final version of the IF maps was issued by the Agency in
2002. They have since become obsolete and have been withdrawn and superseded by the
Agency’s Flood Zone Maps and Flood Maps described below.
2.2.3 "Indicative Floodplains" were defined by the Environment Agency as being those areas
which would naturally (i.e. without flood defences) be subject to flooding on average at least
once every hundred years. Where the flooding envelope of the highest recorded historical
flood is more extensive than that of the 1% (100-year) flood, the former was shown on the
Indicative Floodplain maps.
2.2.4 On the Indicative Floodplain maps, floodplains were shown to extend up river and stream
valleys only to the upstream limit of Main River or (in some Regions) where the catchment
area above that point falls below 10 sq.km. This arbitrary limit sometimes resulted in the
abrupt truncation of a floodplain, giving the potentially misleading impression that significant
flood risk ceased at the edge of the envelope shown on the map. Detailed local studies of
the floodplain sometimes revealed anomalies and inaccuracies in the position of the
floodplain envelope shown on the maps, but the Environment Agency readily admitted that
such anomalies would appear from time to time and indicated their willingness to modify the
IF maps in such cases.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 7
2.3 Flood Zone Maps
2.3.1 Following a comprehensive tidal and fluvial flood risk mapping exercise carried out across
the country, the Environment Agency issued a set of Flood Zone Maps to each Local
Planning Authority in England and Wales during Summer 2004 covering the whole of that
authority’s area in electronic format.
2.3.2 The Flood Zone (FZ) maps were prepared using nationally consistent methodologies for the
determination of flood risk zones for both tidal and fluvial flooding. Whereas the IF maps
showed only the indicative floodplain, which corresponded generally to PPG25 Flood Risk
Zone 3, the FZ maps show both PPG25 Flood Risk Zones 2 and 3. The FZ maps also
exclude the effect of existing flood defences, but go one step further than the IF maps in
removing the effects of de-facto defences such as road and railway embankments, major
artificial drainage channels etc.
2.3.3 The Flood Zone maps, like the earlier Indicative Floodplain maps, are based on OS
1/10,000 scale maps but, unlike the IF maps, the FZ maps are not limited to Main River
floodplains but include the floodplains of all watercourses with a catchment area of more
than 3 sq.km.
2.3.4 Flood Zone maps are not normally accessible to the general public as the existence of these
maps is not publicised by the Agency, but those wishing to undertake detailed flood risk
assessments can on request obtain excerpts from the FZ maps from the Environment
Agency.
2.4 Flood Maps
2.4.1 In October 2004 the Environment Agency issued a further set of flood risk maps covering all
of England and Wales. These maps, issued only on the standard 1/50,000 scale Ordnance
Survey base map (albeit expanded to 1/20,000 on the computer display), were intended for
use by the general public and are only available on the Agency’s website on the internet.
They are not intended, at this stage, to supersede the larger scale and more detailed Flood
Zone maps issued to Local Authorities but to be used in conjunction with them. The Flood
Zone envelopes displayed on the Flood Maps are the same as those shown on the Flood
Zone maps.
2.4.2 When viewed on the internet, the Flood Maps show two flood risk zones; a dark blue zone
described as “flooding from rivers or sea without defences” in which annual flood risk
probabilities are defined as greater than 1% for fluvial flooding (>0.5% for tidal), and a light
blue zone described as “extent of extreme flood” in which the annual flood risk probability is
greater than 0.1%. Like the IF and FZ maps, the dark and light blue areas show the
potential extent of flooding without defences but, unlike the IF maps, no distinction is made
on the Flood Maps between fluvial and tidal flood risk areas. Flood defences (and defended
areas) are shown where those defences are less than five years old and give a 1% (0.5%
tidal) standard of protection.
2.4.3 The Flood Maps do not provide information on the depth or velocity of flood flow, nor do they
show flooding from other sources of flooding such as groundwater, overland flow, or flooding
from local drainage or sewerage systems. It should, however, be noted that the Flood Maps
do not show all areas benefiting from flood defences.
2.4.4 Users of the Flood Maps are invited to “click on” to any point on the map for which a specific
flood risk assessment is required. The user will then find the flood risk at that point
categorised and defined in accordance with the British Insurance Association’s “NaFRA”
classification as one of the following:
Faber Maunsell Stevenage Strategic Flood Risk Assessment 8
“Significant” annual probability >1.3% (once in less than 75 years)
“Moderate” annual probability between 1.3% and 0.5% (1 in 75 to 200 years)
“Low” annual probability less than 0.5% (1 in >200 years).
The 1.3% (1 in 75 years) annual probability level corresponds to the level currently adopted by the British Insurance Association and not that used in PPS25.
2.4.5 Users of Flood Maps who “click on” to a point in a dark blue zone on the map may find the
flood risk at that point classified as either “significant”, “moderate” or even “low” in
accordance with the BIA’s “NaFRA” classification given above. This classification will be
determined by the existence and standard of the flood defences at that point. Even where
no defences are shown specifically on the map, their presence may sometimes be inferred
from the flood risk categorisation given.
2.4.6 The Environment Agency updates the Flood Maps on a three-monthly basis in order to
ensure that the maps reflect the latest assessments of flood risk and to remove anomalies
as the Agency becomes aware of them. At some locations, for example, it is possible to
“click on” to a dark blue area on the map where no flood defences exist and where flooding
is known to occur and obtain a “low” flood risk classification.
2.4.7 The Environment Agency now publishes Historic Flood (HF) Maps and excerpts from the HF
Map for a specific area can be provided on request. These maps show the recorded flood
envelopes for specific flood events, usually commencing with the major widespread flooding
that occurred in 1947. It should, however, be emphasised that not all floods that have
occurred in every location have necessarily been recorded.
2.4.8 The Environment Agency’s recently published Thames Catchment Flood Management Plan
(CFMP) (Ref.4) provides a strategic overview on how the Agency suggest that flood risk
should be managed on a catchment-wide basis, considering both the upstream and
downstream impacts of flooding. Stevenage is located in the CFMP’s Upper Lee Policy Unit
which defines the priority actions necessary to implement the key recommendations for
managing flood risk in the Policy Unit. These are:-
• Appropriate land use planning
• Ensuring adequate stream channel conveyance in urban locations
• Implementing flood warning systems and promoting flood awareness procedures
• Appropriate emergency plans (including those of public utilities)
• Adding flood proofing and flood resilience features to existing buildings at risk of flooding.
The Thames CFMP was issued as a draft document for public consultation in January 2007
and publication of the definitive version of the CFMP is currently awaited.
Causes of Flooding
Faber Maunsell Stevenage Strategic Flood Risk Assessment 11
3.0.1. The great majority of the Borough of Stevenage lies within the catchment of the Stevenage
Brook, a tributary of the River Beane, with a small area in the northwest of the Borough
crossing the watershed into the adjacent Great Ouse catchment. The River Beane is one of
the principal catchments of the River Lee which drains a substantial area of Hertfordshire
and East London as well as the southern and western fringes of Bedfordshire and Essex
respectively.
3.0.2. The Borough of Stevenage is situated at the upstream end of the River Lee catchment
Stevenage is an area where the underlying geology is predominantly chalk. This has
produced the landforms typical of the Chiltern Hills with the often dry valleys of their gently
rolling uplands separated by broad chalk ridges. The rivers and streams of chalk areas are
fed by prolific chalk springs and therefore have a large baseflow component. The
hydrological characteristics of the Borough of Stevenage and the different sources of
flooding will be described in greater detail in Section 4.
3.0.3. Flood risks in chalk areas includes flooding from groundwater sources, although flooding
can also occur as a result of surface water runoff from glacial clay overlying the chalk or
from urban development. When flooding does occur it can manifest itself in two distinct
ways, as described in the following sub-sections.
3.1. Overflowing of Watercourses
3.1.1. When the flow in a river or stream exceeds the capacity of the channel to convey that flow,
either because of limited cross-sectional area, limited fall, or a restricted outfall, then the
water level in that channel will rise until the point is reached where the banks of the channel
are overtopped. Water will then spill over the channel banks and onto the adjoining land.
With an upland river the adjoining land is its natural flood plain, which will generally be of
limited extent and fairly well defined.
3.1.2. In the case of a major river, such as the Lee, the floodplain may be a kilometre or more in
width, though it may not be equally distributed on either side of the river channel. However,
due to local variations in geomorphology, the width of the floodplain may vary considerably
from point to point along the river valley. Floodplains are characterised by flat, riparian land
along the valley floor. In pre-industrial England, such land was regarded as liable to flooding
and was traditionally reserved for grazing and stock rearing and human settlements were
almost always established on higher land beyond the edge of the floodplain. In the industrial
age and more recent times with different priorities, pressures for development have resulted
in the widespread colonisation of floodplains, often with steps taken to mitigate the
associated risks of flooding.
3.1.3. When overtopping of an embanked watercourse occurs, the depth of water flowing over the
floodwall or embankment will probably be small, a few centimetres at most. The bank will
act like a weir and the rate of flow per unit length will be relatively modest and this,
combined with the limited duration of the overtopping, will limit the volume of water
cascading over the defences to cause flooding. If overtopping does occur and the protected
area is of considerable extent, any flooding that results will often be disruptive rather than
disastrous. The situation becomes far more critical if overtopping of an earth embankment
erodes its crest, leading to a breach in the embankment. This situation is considered in
paragraphs 3.2.1 to 3.2.6 below.
3 Causes of Flooding
Faber Maunsell Stevenage Strategic Flood Risk Assessment 12
3.2 Breaching of Embankments
3.2.1. An earth embankment may be breached as a direct result of overflowing. Overtopping of a
bank, especially when concentrated over a short length of bank, results in a rapid flow of
water down the rear slope of the bank. This can cause erosion, which starts at the rear of
the bank and works its way forward towards the channel. As the crest of the bank is washed
away the flow through the small initial gap increases and a small breach is created. This
becomes steadily bigger as water flows through it, eroding the sides and base of the breach,
and a rapid and progressive failure of the embankment follows. Complete collapse of the
bank may take only minutes. The contents of the embanked channel then pour through the
breach and across the surrounding land.
3.2.2. A tarmac road or dwarf floodwall along the crest of a floodbank may inhibit the rate of initial
erosion and postpone or even prevent the creation of a breach, depending upon the duration
of overtopping. Experience, fortunately limited, shows that when a fluvial floodbank
breaches, even if not by overtopping, it does so near the peak of the flood when the flow in
the river and hence flood levels are at or near their maxima. Experience also suggests that
breaches in river embankments usually extend from 20 to 30 metres in length and rarely
grow to more than 40 metres. Unlike tidal defence floodbanks, once a breach in a fluvial
floodbank has occurred there will be a reduction in flood levels in the river as water flows
through the breach. This reduces the stress on neighbouring floodbanks along the same
reach of river, thus considerably reducing the risk of further breaches in the same area.
3.2.3. The design of a floodbank (or floodwall) incorporates a certain level of freeboard to allow for
uncertainties, bank settlement, wave action, etc. but the height of any floodbank is
determined primarily by the peak height of the design flood. Because of freeboard, the
return period of the flood which gives rise to overtopping must be greater than that of the
design flood. The return period of flooding from a breach caused by overtopping will be
essentially the same as for the far less severe flooding resulting from that overtopping alone,
but it must be borne in mind that breaches in earth embankments can occur from
causes other than overtopping and may thus have return periods significantly less
than the that for which the embanked channel was designed.
3.2.4. Apart from overtopping, breaches in floodbanks can occur where weak spots in the bank
have been created over a long period by gradual leakage through the bank at old, forgotten
structures buried in the bank such as culverts or sluices, or where the activities of burrowing
animals such as rabbits or coypu have impaired the integrity of a floodbank. These inherent
weaknesses may not be readily apparent under normal conditions but when an exceptional
hydraulic gradient through the bank arises during flood conditions, a failure may occur,
quickly giving rise to a breach. This may well happen in a flood of considerably lesser
magnitude and return period than the design flood.
3.2.5. Furthermore, since the inherent weakness tends to increase slowly with age, the fact that a
bank did not fail in an earlier flood does not guarantee that it will not fail in a comparable (or
even a lesser) flood at some time in the future. If, however, a floodbank is of recent
construction it may be assumed that it has been properly engineered and, provided that
there is an adequate inspection and maintenance regime, the risk of breaching as a result of
the factors outlined above is negligible.
3.2.6. Although there are no known areas in Stevenage where rivers or streams in flood are
contained within raised embankments, there are a dozen or so flood storage reservoirs and
amenity lakes in the Borough most of which are impounded behind earth embankment
dams. The comments made in the preceding paragraphs about the breaching of earth
floodbanks apply equally to these reservoir embankments.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 13
3.3 Mechanical, Structural or Operational Failure
3.3.1 Although less common than overtopping or breaching of defences, flooding can also be
caused by the mechanical or structural failure of engineering installations such as land
drainage pumps (or their power supplies), sluice gates (or the mechanism for raising or
lowering them), lock gates, outfall flap valves etc. Such failures are, by their nature, more
random and thus unpredictable than the failures described in the previous sub-Sections, and
may occur as a result of any number of reasons. These include poor design, faulty
manufacture, inadequate maintenance, improper operation, unforeseen accident, vandalism
or sabotage.
3.3.2 Structural failure, in this context, is also taken to include the failure of "hard" defences in
urban areas such as concrete floodwalls. "Hard" defences are most unlikely to fail by the
overtopping / erosion / breaching sequence experienced by earth embankments. Their
failure tends to be associated with the slow deterioration of structural components, such as
rusting of steel sheet piling and concrete reinforcement, or the failure of ground anchors.
Such deterioration is often difficult to detect and failure, when it occurs, may well be sudden
and unforeseen.
3.3.3 Structural failure of "hard" defences is most likely to happen at times of maximum stress,
when water levels are at their highest during a flood. Failure of hydraulic structures and
"hard" defences can, under certain circumstances, be precipitated by the scouring of
material from beneath their foundations by local high velocity flows or turbulence, especially
under flood conditions.
3.3.4 Flooding can also be caused or exacerbated by the untimely or inappropriate manual
operation of sluices, or by the failure of the person or organisation responsible to open or
close a sluice at a critical time. Responsibility for the operation of sluices rests with various
public bodies as well as riparian landowners. Operational failures of this nature generally
occur during a flood event and their results are to exacerbate rather than to cause flooding,
and their impact is therefore normally limited in extent.
3.3.5 Flooding, especially that caused by overflowing of watercourses, can be exacerbated by
other operational failures. These failures can also include neglected or inadequate
maintenance of watercourses resulting in a reduction of their hydraulic capacity. Flooding
can also be caused or exacerbated by bridge or culvert blockages, although these are not
necessarily due to maintenance failures and may be caused by debris, natural or man-
made, swept along by flood flows.
3.3.6 The risks associated with this category of failures are almost impossible to quantify,
especially as experience has shown that there is a joint probability relationship between this
class of failure and flooding resulting directly from extreme meteorological events. It can of
course be argued that if a risk of this type was quantifiable and found to be finite then action
should already have been taken to alleviate the risk. Even an assessment of relative risk for
failures of this type must depend on a current and detailed knowledge of the age and
condition of plant, its state of maintenance, operating regime etc at a significant number of
disparate installations. These types of risk are generally regarded as ‘residual’ risks.
3.4 Localised Surface Water Flooding
3.4.1 Almost all localised flooding of a serious nature occurs as a result of a severe convective
storm, limited in extent and duration and generally occurring during the summer. This
flooding can, however, be exacerbated by two factors, blockages in the local surface water
drainage system or by "floodlocking". Each of these factors is considered separately below.
In some instances, in what would otherwise have been a relatively moderate rainstorm,
these factors can themselves be the cause of flooding.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 14
3.4.2 Intense storm rainfall, particularly in urban areas, can create runoff conditions which
temporarily overwhelm the capacity of the local sewer and drainage system to cope with the
sudden deluge. Localised “flash” flooding then occurs. In upland areas with small, relatively
steep, impermeable catchments, this may result in quite severe flooding over a limited area,
often with a considerable depth and velocity of flood water. The duration of such flooding is
usually relatively short but this does not mitigate its impact for those affected, especially
when the flooding may have developed suddenly and unexpectedly.
3.4.3 Localised flooding can also occur in urban areas where a stream or watercourse has been
extensively culverted. In its natural state, if the channel capacity of a stream is exceeded
the channel will overflow along a considerable length and the resultant flooding is distributed
over a wide area. If, however, the stream runs through a long culvert and the hydraulic
capacity of that culvert is exceeded under flood conditions the culvert becomes surcharged
at its upstream end. Water levels will then rise rapidly and localised flooding upstream of the
culvert, often quite serious, can occur. The flood water, in attempting to follow the natural
line of the culverted watercourse, may also flow through the built-up area above the line of
the culvert. This applies equally to many larger surface water sewerage systems in urban
areas which are, in effect, culverted watercourses.
3.4.4 Local flooding is often exacerbated by deficiencies in the local surface water drainage
system, but these can usually be remedied by relatively minor works once they have been
exposed by a flooding event. Local flooding can also be caused by temporary blockages or
obstructions in a drainage system, especially one that has been extensively culverted. Such
flooding can therefore be virtually random in its occurrence, although the prevalence of
blockages at a particular location would suggest a systematic problem, justifying action to
modify the drainage system at that location in order to resolve it.
3.4.5 In recent years some urban watercourses considered to be particularly at risk from such
blockages were designated "Critical Ordinary Watercourses" (COWs) although this
designation did not have any statutory status. COWs were designated in their respective
areas by Local Authorities (and, where applicable, by Internal Drainage Boards) as well as
by the Environment Agency. The Environment Agency has recently completed the process
of designating all COWs as Main River. Where a COW was separated from the Main River
system by a length of non-Main River the intervening watercourse has also been en-mained.
3.4.6 Although COWs have ceased to exist as such, the designation of an ordinary watercourse
as “critical” indicates that it has at one time been regarded as a potential flood risk in an
urban area. Watercourses designated as COWs are therefore of significance in any
strategic flood risk assessment. However, only one COW was designated in Stevenage.
This was at the upstream end of Stevenage Brook from Six Hills Way (TL 2402 2380) to the
then head of Main River near Langley Junction (TL 2420 2266).
3.4.7 In inland areas, all local surface water drainage systems discharge to a major stream or
river. Except where pumps have been installed, this discharge is by gravity. If the receiving
stream or river is in flood, especially where that watercourse is contained within raised
floodwalls or banks, the flow in the local drainage system can no longer drain to the river
and is impounded behind the defence line for the duration of the flood. This is known as
‘floodlocking’. This can result in secondary flooding within the defended area, even though
the defences may not have been breached or overtopped. Fortunately, this secondary
flooding is almost always much less severe or widespread than primary flooding from the
main river would have been.
3.4.8 In Stevenage the conditions that give rise to floodlocking, as described above, do not
generally occur because there are no significant areas of land behind raised flood defences.
There are a few isolated locations in Stevenage where surface water runoff is pumped to a
surface water sewer at a higher level (generally associated with underpasses etc) but
provided that the pumps operate satisfactorily ‘floodlocking’ should not arise.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 15
3.4.9 The occurrence of secondary flooding depends on the coincidence of heavy rain over the
local drainage catchment with ‘floodlocking’ of its outfall. Often the rainfall event that caused
the flood conditions in the main stream or river will also have caused high flows in the local
drainage system, but because of the much slower hydrological response of the larger
watercourse, the rapid runoff from the local catchment will have discharged to that
watercourse before the flood peak in the receiving watercourse arrives at the local drainage
outfall. In Stevenage, however, if floodlocking did occur the similarity in size between the
catchment of the ‘floodlocked’ drainage system and that of the receiving watercourse will be
relatively small and a simultaneous occurrence of the two flood peaks is not unlikely.
3.4.10 Because secondary flooding, where blockages or ‘floodlocking’ is involved, depends upon
what are either random events or a complex coincidence of events, its probability of
occurrence is difficult to quantify and it thus falls within the category of ‘residual risk’.
3.5 Functional Floodplains and Washlands
3.5.1 Functional floodplains form the basis for Flood Zone 3b. Although described in PPG25, they
are now defined quantitatively in PPS25 as those parts of Zone 3 where flooding may be
expected to occur with an annual probability of at least 5% or at least once in 20 years.
3.5.2 Washlands are areas within functional floodplains where flooding, whether controlled or
uncontrolled, can be expected to occur with an annual probability of at least 10% (once in
ten years). The sacrificial flooding of washlands will reduce the flood risk to land further
downstream. Flooding of washlands can be active or passive, depending on whether
human intervention (e.g. opening or closing a valve or sluice) is required to initiate the
flooding. The land within the basins formed by the Stevenage flood storage reservoirs
should obviously be regarded as washland, although the flooding to which they are all
subject is passive.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 16
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Flooding in Stevenage
Faber Maunsell Stevenage Strategic Flood Risk Assessment 19
4.1 Hydrological Characteristics of Stevenage
4.1.1 The Borough of Stevenage in Hertfordshire exists as the local government comprises
Stevenage New Town which was established shortly after World War II and has since
expanded to occupy most of the land area within the borough boundary. The Borough is
fairly small in geographical extent (a modest 2,606 hectares) but with a population of 78,000
and very little ‘greenfield’ land remains within the Borough, nearly all of which is now
urbanised.
4.1.2 Stevenage is situated at the northern, upstream end of the catchment of the River Lee,
which drains a large part of Hertfordshire and south west Essex. The Lee is a major
tributary of the River Thames into which it flows in East London. Although most of the
Borough lies within the Lee catchment, the northern end of the Borough straddles the
watershed between the Rivers Thames and Great Ouse and hence the boundary between
the Environment Agency’s Thames and Anglian Regions.
4.1.3 The catchment area of the River Beane lies between Stevenage, Buntingford and Hertford.
Stevenage Brook occupies the western side of the catchment. The main channel of the
River Beane rises near the village of Rushden, 7km northeast of Stevenage, from where it
flows in a southerly direction through Walkern, parallel with but beyond the eastern edge of
Stevenage. Together with the Rivers Mimram from the west (143 sq.km) and Rib from the
east (153 sq.km), the River Beane (174 sq.km) meets the River Lee (172 sq.km) at Hertford.
4.1.4 Most of Stevenage Borough lies within the catchment of Stevenage Brook, a major tributary
of the River Beane which it joins at Frogmore Hall, 1.5km downstream of the borough
boundary. At their confluence the river and brook have catchment areas of 61.6 sq.km and
39.5 sq.km respectively. Within Stevenage the main channel of the Stevenage Brook
(catchment area 11.3 sq.km upstream of Fairlands Valley) drains the western side of the
Borough and the town centre.
4.1.5 The Brook has two principal tributaries; the Fairlands Valley Stream (3.16 sq.km) which
drains the central part of Stevenage, and the Aston End Brook (6.51 sq.km) which drains the
eastern side of the Borough. All three streams flow from north to south. The catchments of
the first two streams are almost entirely urbanised, that of the Aston End Brook slightly less
so. The pattern of rivers and streams in the Stevenage area is shown in Figure 4.1.
4.1.6 Ground levels within the Borough fall from 142mOD on the northern boundary east of
Chesfield Park to 66mOD at the Borough’s southern extremity near Hook’s Cross. North of
Stevenage ground levels rise to nearly 150mOD on the Thames / Anglian watershed at
Weston and Hickman’s Hill. Within the north eastern part of the Borough the land rises
locally to above 140mOD in Hampson Park and Wellfield Wood.
4.1.7 To the north and west of Stevenage lies the catchment of the River Hiz, a major tributary of
the Anglian Region’s River Great Ouse. West of the A1(M) motorway, the western edge of
the borough falls within the Ippollitts Brook catchment. The northwestern corner of the
borough drains to the Ash Brook catchment. These two relatively small streams (catchment
areas 22.6 sq.km and 15.5 sq.km respectively) combine on the eastern side of Hitchin to
form the River Purwell which then meets the River Hiz in Walsworth, a northern suburb of
Hitchin.
4 Flooding in Stevenage
Faber Maunsell Stevenage Strategic Flood Risk Assessment 20
4.2 Hydrogeological Characteristics of Stevenage
4.2.1 The chalk outcrop which forms the Chiltern Hills to the west of Hertfordshire continues
eastwards and then northwards into East Anglia. Stevenage lies just south of the crest of
the ridge which forms the Thames / Anglian watershed and which separates the scarp slope
of the chalk to the north from its dip slope to the south. Chalk is a highly permeable stratum
and has a dominant influence on the hydrological characteristics of the rivers and streams
which drain it.
4.2.2 Natural chalk streams are largely spring-fed and characterised by dry valleys (bournes)
above the springs. Below the springline, flows in chalk streams are notable for their large
baseflows and long, attenuated flood hydrographs. In extreme events, however, when the
chalk becomes saturated after prolonged periods of heavy rainfall, surface flow may appear
in the bournes and flooding from groundwater can occur.
4.2.3 In many places the underlying chalk has been covered with a blanket of Boulder Clay and
other glacial deposits (sands, gravels and clays) during the ice ages. This blanket may be
many metres thick in places. Clay forms a hydrologically impermeable layer and this
produces the characteristic hydrological response of natural streams on clay strata - a low
baseflow and a rapid response to rainfall, with short, sharply peaked flood hydrographs.
4.2.4 Where the glacial clay layer (which for the purposes of this Report includes Boulder Clay) is
discontinuous and the underlying chalk outcrop is exposed in places, as in the Stevenage
area, the hydrological response of streams may often be complex, reflecting the very
different characteristics of chalk and clay. As the Boulder Clay cover tends to be
concentrated on higher ground, it is not uncommon to find flow in clay streams disappearing
underground when the stream crosses the boundary between clay and the downstream
chalk outcrop, emerging further downstream as springflow and leaving a dry bourne
between the chalk/clay boundary and the spring.
4.2.5 Throughout Stevenage most of the chalk is covered by a capping of Boulder glacial clay,
with the exception of a strip of exposed chalk stretching south from Chesfield Park to the
Fairlands Valley at Bedwell. There is also a small exposure of chalk in the northeast corner
of the Borough at Box Wood.
4.2.6 The most notable geological feature of the Stevenage area is the pair of buried glacial
valleys which run beneath the present day Stevenage Brook valley and, west of Stevenage,
the Langley Valley. These buried valleys were formed during the ice ages by melt water
flowing south from glaciers north of Stevenage incising deep valleys in the chalk, but
subsequently became filled with glacial sediments to form buried valleys. The main buried
valley enters the northeast corner of the Borough at Whitney Wood. For most of its length it
is between 1000m and 1,500m wide but diminishes to about 500m in width at Bragbury End.
The smaller buried valley under Langley Valley joins the main buried valley beneath
Stevenage Brook under the A1(M) at Junction 7. The geology of the Stevenage area is
illustrated in Figure 4.2.
4.3 Urban Drainage Characteristics of Stevenage
4.3.1 Because the Borough straddles the watershed between the Environment Agency’s Thames
and Anglian Regions it also straddles the surface water drainage areas administered by
Thames Water and Anglian Water respectively, although it should be noted that the
boundary between the these two water companies does not correspond exactly with the
topographic watershed. It should also be noted that Thames Water only provides sewerage
services in Stevenage; public water supply and distribution is in the hands of the Three
Valleys Water Company.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 21
4.3.2 Although Stevenage is a predominantly urban area, the controlled evolution and
development of the new town has resulted in an urban area which incorporates large public
open spaces such as grassland (17% of the land area of the Borough), woodland (5%) and
scrubland (2%). This means that a smaller proportion of storm rainfall falling on the town will
give rise to surface water runoff than in older or more traditional urban areas. Nevertheless,
a total of twelve flood storage reservoirs (locally known as “water meadows”) have been
established within or adjacent to the town to attenuate storm runoff from impermeable areas.
These reservoirs will be described and discussed in greater detail in Section 6.
4.4 Classes of Flooding
4.4.1. Because of the absence of any significant areas of pump-drained land within the Borough,
potential flooding in Stevenage can be limited to two of the three general classes determined
by the predominant landforms in the area. These three classes are outlined below.
Lowland Plains
4.4.2. This type of flooding results from the overflowing of relatively large, slow moving rivers
(and tributaries with which they are in hydraulic continuity) onto a wide, extensive
flood plain. Flooding of this will vary in depth but arises from continuous, frontal
rainfall, usually on an already saturated catchment, and it develops over a period of
hours or even days. The duration of flooding will, however, be correspondingly
prolonged. Away from the river itself the flood flow velocities are low. The land
adjacent to the main river may, in certain places, be protected by floodbanks but
these will be of modest height and, even if breached, the consequences will be very
much less severe than in fenland areas. Although flooding of this class will be
encountered lower down the Lee catchment it will not arise on the narrower
floodplains of the River Beane and the Stevenage Brook. Its description is, however,
included here for completeness.
Upland River Valleys - Type A
4.4.3. Closer to their headwaters, rivers in these areas will be smaller but faster flowing.
Flooding will occur by overtopping of the banks but as the valley, and hence its
floodplain, is relatively narrow the flooded area will be of limited extent. Nevertheless,
the depth of flooding may be considerable, especially where the river flow is impeded
by obstructions. The velocity of the water over the floodplain may be considerable,
although in the gentler topography of Stevenage it is unlikely to reach life endangering
velocities encountered, for example, in steep, narrow Pennine valleys. This type of
flooding is typically more "flashy" than that experienced in the Lowland Plains class
and, especially with smaller watercourses, arises from exceptionally heavy, but
shorter duration and more intense rainstorms.
Upland River Valleys - Type B
4.4.4. This has the same origin as Upland River Valleys Type A flooding but occurs in urban
areas where the watercourse has been extensively culverted, as described in
paragraph 3.4.4.
Localised Flooding - Type A
4.4.5. In theory this results from a local urban drainage system being unable to cope with
the rate of runoff from a particularly heavy, intense storm larger than that for which it
was designed. In practice, the problem is usually exacerbated by an obstruction or
blockage of the drainage system, either by a long term accumulation of silt or debris,
or by larger debris carried along with the storm runoff. In such cases the flooding can
occur almost at random, and on a relatively moderate storm event and hence comes
under the heading of "residual risk" flooding in PPS25. Unless the problem is chronic,
due (e.g.) to under-design, once the obstruction has been removed the flood risk can
be drastically reduced.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 22
This type of flooding is normally associated with the local surface water or combined
(surface and foul) sewerage systems and its effects are generally limited to part of a
street or small cluster of properties
Localised Flooding - Type B
4.4.6. This occurs at the downstream end of small local drainage systems, either urban or
rural, as the result of "floodlocking" of the local drainage outfall by flood levels in the
river or watercourse into which the local system discharges. This has been described
in some detail in paragraph 3.4.7.
4.4.7. Within Stevenage, the Upland River Valley Type A and, to some extent, Type B flooding
would have been the predominant types of flooding before Stevenage New Town was
established. Since then the spread of urbanisation throughout the Borough means that
Localised Type A flooding is probably now the dominant type and could occur almost
anywhere within the Borough but is most likely along the routes of large surface water
sewers. For reasons given in para.3.4.8, there are very few areas in Stevenage where
Localised Type B flooding is considered to be a significant or widespread risk.
4.5 Sources of Flooding
4.5.1. From the above it will be obvious that the principal source of flooding in Stevenage will be
Stevenage Brook and its major tributary, the Aston End Brook. Although the Borough is
situated on chalk strata and chalk is associated with groundwater flooding, Stevenage lies at
the upstream end of the Chiltern chalk outcrop, close to the watershed and well upstream of
the point where groundwater flooding would be expected to appear in typical chalk bourne or
valley, even under extreme conditions. Groundwater flooding can therefore be disregarded
as a flood risk source in this study.
4.5.2. The downstream portions of Stevenage Brook and Aston End Brook within the Borough flow
in open watercourses. Through most of the heavily urbanised area, Stevenage Brook flows
in a concrete lined or canalised channel but downstream of Ashdown Road the stream
channel takes on a much more natural appearance. Aston End Brook runs down the
eastern edge of the built up area and, although canalised in places, it has retained relatively
natural over much of its length downstream of Aston End. These open watercourses are all
potential flood risk sources.
4.5.3. Upstream of Six Hills Way Stevenage Brook and its tributaries have all become principal
components of the town’s surface water drainage system and the original (pre-1946)
watercourses are now surface water sewers. It should however be noted that upstream of
the sewered area, the rural headwaters of Stevenage Brook upstream of Rectory Lane and
the western fringe of the catchment west of the A1(M) are still in their natural condition.
4.5.4. Although its main channel is still an open watercourse, the urbanised western tributaries of
Aston End Brook are all now surface water sewers. The urban area in the northeast corner
of Stevenage is outside the Stevenage Brook / Aston End Brook catchment and surface
water sewers in this area discharge direct to the River Beane at Walkern via Boxbury Flood
Storage Reservoir and the pipeline from the reservoir to the river.
4.5.5. These surface water sewer systems have all been designed to accommodate flows
expected to occur in a rainfall event of a specific return period and duration. In a more
severe event the system becomes surcharged and overflows, resulting in overland flow
along the line of the sewer. The sewered tributaries should therefore also be considered as
sources of flood risk.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 23
4.5.6. Although as a modern town, Stevenage has almost entirely separate foul and surface water
sewerage systems, some surface water runoff will inevitably find its way into foul sewers
during heavy rainfall. The volume of this runoff will probably be small but the very large
Stevenage Trunk Sewer, which conveys the whole of the town’s foul drainage, should also
be regarded as a possible, albeit residual risk of flooding along the downstream portion of its
route through the southern end of the town.
4.5.7. Most of Stevenage’s flood storage reservoirs, as well as the Fairlands Valley Lakes,
impound water behind earth embankments. These embankments, if they breach, are
therefore potential flood risk sources, but since the risk they present is a residual risk they do
not have any influence on the flood risk envelopes associated with specific probabilities of
occurrence.
4.5.8. There are no canals, aqueducts, bulk water transfer pipelines or other artificially elevated
watercourses in Stevenage which could present a source of flood risk.
4.6 Records of Flooding
4.6.1. Stevenage Borough Council (including ex-employees of the Council), the Environment
Agency and Thames Water were all contacted to obtain information on flooding records and
drainage problems in the Borough. (There are no Internal Drainage Boards in the Thames
Region of the Environment Agency.) The responses received are summarised below.
STEVENAGE BOROUGH COUNCIL
4.6.2. The Borough Council and ex-employees of the Council have provided details of localised
drainage and flooding problems in Stevenage of which they are aware. These details,
supplemented by information obtained from other sources of information, are summarised in
Table 4.1 below.
Location Details Date
93 & 94 Minheadway n/a n/k
25 Bragbury Close n/a n/k
92 – 104 Kymswell Close n/a n/k
7 Colts Corner, Shephall Flooding from Peartree Park n/k
Peartree Spring Infants School ditto n/k
1 Medalls Link, Shephall ditto n/k
Roebuck Gate (see below) Flooding from Stevenage Bk n/k
Table 4.1 - Local Flooding Problems in Stevenage
4.6.3. Stevenage Borough Council have appointed Jacobs Babtie Ltd to carry out a hydrological
study of the Peartree Park area with a view to alleviating flooding in that area.
4.6.4. It is understood that minor flooding from foul sewers has occurred at a number of locations
in Stevenage in the past but that these problems have been resolved by local sewer
improvement schemes. There has also been a localised flooding problem at Roebuck Gate
in the vicinity of Stevenage Brook but works have been undertaken to solve this problem
also. A flooding problem at Warwick Road was remedied by the construction of the Camps
Hill Park Flood Storage Reservoir - see Section 5.2.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 24
4.6.5. A major storm event occurred in Stevenage in 1963 when a number of properties were
flooded. Unfortunately no further details of this event have been discovered.
ENVIRONMENT AGENCY
4.6.6. The Environment Agency has provided ‘shapefiles’ in electronic format showing the extent of
the flooding events which occurred in Stevenage in 1947, 1978, 1992 and 1993. These are
understood to be the most significant flood events to have occurred in the Borough since
World War II. The Agency has pointed out that in the Stevenage area the Summer 2007
flood event was far less severe than that of 1993. This information has been used to
compile Figure 4.3 which gives a plan showing the flood envelopes of these four events.
4.6.7. The Environment Agency also provided a list of 367 groundwater flooding incidents and their
locations recorded in the Thames Region from 2000 to 2007. None of these incidents
occurred at locations within or in the vicinity of Stevenage.
WATER COMPANIES
4.6.8 Thames Water record flood incidents by postcode and undertook a data search for the two
postcode areas (SG1 and SG2) which the Borough Council state cover the whole of the
Borough. No records of sewer flooding of properties in either postcode area were found.
There are no records of flooding from the Stevenage Trunk Sewer.
4.6.9 Anglian Water have no records of sewer flooding in the small area of north west Stevenage
for which they are responsible. Problems have been encountered with the 150/175mm foul
sewer that runs from Graveley towards the Lister Hospital and the sewage pumping station
at Coreys Mill (Hitchin Rd) but sections of this sewer have subsequently been re-laid.
4.6.10 The Borough Council had intended to undertake a Stevenage Water Cycle Strategy
following the completion of this Strategic Flood Risk Assessment, but to minimise delays to
the preparation of their Local Development Framework the Council is proceeding with the
Water Cycle Strategy study in parallel with this SFRA.
4.7 Operational and Emergency Planning
4.7.1 Elsewhere in the Thames and Anglian Regions, the Environment Agency has issued flood
defence and land drainage emergency operational plans for high flood risk areas in
conjunction with the local authority. These documents are intended to clarify areas of
responsibility for the operation and maintenance of flood defence structures within that local
authority's area and summarise the agreed joint emergency response by each of the public
bodies involved. These documents are not considered necessary for most areas of the
country, including Stevenage.
4.7.2 It is possible, albeit very unlikely, that serious flooding in Stevenage could trigger the
declaration of a major incident. Should a major incident be declared, Hertfordshire County
Council (HCC)’s Emergency Plan and Stevenage Borough Council (SBC)’s Emergency Plan
would both be activated. Local authorities are classed as Category 1 Responders under the
Civil Contingencies Act 2004.
4.7.3 The HCC Emergency Plan may be found on the County Council’s website,
www.hertfordshire.gov.uk . Advice on action to be taken in the event of flooding may be
found on this website and on the Borough Council’s website, www.stevenage.gov.uk .
4.7.4 Specific responsibilities and actions by the Environment Agency (EA) and Stevenage
Borough Council (SBC) are detailed in the following paragraphs.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 25
STEVENAGE BOROUGH COUNCIL
4.7.5 Stevenage Borough Council’s Emergency Plan (Version 1.0)’ was issued in April 2008 and
gives details of current emergency plans for a variety of possible emergencies, including
flooding.
4.7.6 The Borough Council’s Contract / Direct Services Department is responsible for providing,
where applicable, plant, equipment, transport and labour as part of the overall response to
emergency incidents. The Council’s Operational Unit holds sufficient materials at their
Cavendish Road depot to make up approximately 500 sandbags. These are intended for
distribution to residents but not to commercial properties.
ENVIRONMENT AGENCY
4.7.7 The Environment Agency have not issued any flood emergency plans and procedures that
are of specific relevance to Stevenage.
4.7.8 The Environment Agency issues flood warnings via their Automatic Voice Messaging (AVM)
system to residents and property owners in areas at risk of flooding. There are a total of 270
properties in Stevenage that fall within the Stevenage Brook Flood Warning Area but only 57
of these have registered to receive flood warnings. The roads in which those properties that
have registered to receive flood warnings are located are listed in the table below.
Location Postcode
Ashburnham Walk SG2 8DZ
Badminton Close SG2 8SR
Belgrave Mews SG2 8QE
Berkeley Close SG2 8SG
Bittern Close SG2 9PE
Brookhill SG2 8RR
Brook Drive SG2 8TP
Burghley Close SG2 8SX
Falcon Close SG2 9PG
Hertford Road SG2 8SA
Kingfisher Rise SG2 9PF
London Road SG2 8DT
Nursery Close SG2 8SD
Ranworth Avenue SG2 8SL
Roebuck Gate SG2 8DP
The Glynde SG2 8SY
Thornbury Close SG2 8SQ
Woburn Close SG2 8SW
Table 4.2 - Locations of Properties Registered to Receive Flood Warnings
Faber Maunsell Stevenage Strategic Flood Risk Assessment 26
4.7.9 The Environment Agency recommends that everyone in flood risk areas signs up to the
Agency’s flood warning system, Floodline Warnings Direct (FWD), which is a multi-media
system that is used to issue flood warnings by telephone, mobile phone, fax or pager. This
is a free service and people are encouraged to receive flood warning messages if they are
at risk of flooding.
4.7.10 The aim of this service is to increase awareness of the risk of flooding and provide advice on
how people can limit the damage that flooding can cause. The Environment Agency’s
monitoring of flood risk continues throughout the year and further information on the
Agency’s ‘Floodline’ service can be had by telephoning 0845 988 1188.
Flood Risk Alleviation Measures
Faber Maunsell Stevenage Strategic Flood Risk Assessment 29
5.1 Flood Storage Reservoirs in Stevenage
5.1.1 Because of Stevenage’s location close to a watershed at the head of a river catchment,
there are no major flood risk sources in the Borough and, as far as we have been able to
ascertain, there are no raised flood defences of any significance along any of the
watercourses in Stevenage.
5.1.2 However, from the start of the development of Stevenage New Town, the Development
Corporation was aware that the extensive urbanisation of what had hitherto been ‘greenfield’
land would generate substantial volumes of additional surface water runoff. In order to
minimise the impact of this runoff on the receiving watercourses the new town was planned
to incorporate numerous small flood storage reservoirs (FSRs), to be known in Stevenage
as “water meadows” to reflect their value as public open space in the urban environment.
As Stevenage has expanded over the past fifty years, additional reservoirs have been added
and some of the original flood storage reservoirs have been reconfigured and enlarged.
5.1.3 The Development Corporation’s surface water drainage policy was to maintain the existing
channel of the Stevenage Brook as the new town’s principal watercourse but, because of
the Brook’s limited hydraulic capacity, storm runoff from urban development was to be
routed through a number of balancing ponds or flood storage reservoirs (“water meadows”).
These were all sited in valley bottoms as public open spaces. Each FSR was designed to
cope with a 20-year return period storm over its catchment and it is believed that the
Development Corporation was consented to discharge a total of up to 3.0 cu.m/sec urban
runoff into Stevenage Brook.
5.1.4 The original proposals were for five FSRs, one on the Stevenage Brook itself and four in
tributary valleys. Construction of two of these FSRs began before 1952 and all five had
been completed by 1960. By this time a revised growth forecast for the new town indicated
that additional FSRs would be needed to cope with the additional storm runoff that the
expanded urban area would generate. Two more FSRs were therefore added in 1964
followed by an eighth in 1966. Two further FSRs were subsequently constructed to bring
the total to ten in 1972. An eleventh FSR was added in 1975/6 and a twelfth has since been
added to deal with a specific localised flooding problem.
5.1.5 As water was abstracted from the River Lee downstream of Stevenage for public water
supply, it was imperative to prevent pollution of Stevenage Brook from urban runoff from the
new town. For this reason oil interceptors were incorporated throughout the town’s surface
water drainage system and the FSRs were designed with this in mind.
5.1.6 The winding up of the New Town Development Corporation in 1980 and successive local
government reorganisations have resulted in the transfer of ownership and responsibility for
some FSRs to other organisations, notably, via its predecessors, to the Environment
Agency. These processes have unfortunately resulted in discontinuities and the dispersal of
records relating to the Stevenage FSRs, as a result of which some technical information is
no longer readily available. It is, however, believed that all of the known Stevenage “water
meadows” have been identified and they are described individually in Sub-section 5.2 below.
A plan showing the location of all twelve flood storage reservoirs is given in Figure 5.1.
5.1.7 A recent (2006/7) study for the Environment Agency (Ref.5) has shown that the flood current
defence standards provided by their three FSRs range from between 2 and 5 years
(Wychdell) to between 100 and 200 years (Ridlins Wood). No reason is given for this
surprisingly large discrepancy.
5 Flood Risk Alleviation Measures
Faber Maunsell Stevenage Strategic Flood Risk Assessment 30
5.2 Detailed Descriptions of Flood Storage Reservoirs
5.2.1 Table 5.1 lists Stevenage’s twelve flood storage reservoirs and gives their salient details. All
but one of these FSRs are situated within the Borough and all but one are located within the
Environment Agency’s Thames Region. A detailed description of each FSR is given in the
following paragraphs.
Name of FSR Catchment OS Grid Ref. Date
Sainsbury’s River Hiz TL 2250 2670 pre-1960
Meadway Stevenage Bk TL 2265 2475 pre-1960
Burymead Stevenage Bk TL 2350 2600 1964
Elder Way Stevenage Bk TL 2395 2340 pre-1960
Old Knebworth Ln Stevenage Bk TL 2430 2195 pre-1960
Broad Oak Stevenage Bk TL 2445 2260 1964
Wychdell Stevenage Bk TL 2645 2155 pre-1960
Camps Hill Park Aston End Bk TL 2595 2465 post-1980
Ridlins Wood Aston End Bk TL 2650 2235 pre-1972
Aston Valley Aston End Bk TL 2655 2175 1966
Bragbury End Stevenage Bk TL 2690 2095 1975/6
Boxbury River Beane TL 2725 2665 pre-1972
Table 5.1 - Flood Storage Reservoirs in Stevenage
Note: The dates given in Table 5.1 are the date when that FSR was originally established.
5.2.2 The larger FSRs have fairly well defined natural catchments and estimates of their
catchment areas have been obtained from the Flood Estimation Handbook (FEH) (Ref.6)
and are given in the text. The areas draining to the smaller FSRs are determined as much
by the configuration of the surface water sewerage system as by their ‘natural’ (i.e.
topographic) catchment boundaries which are therefore not necessarily applicable.
Sainsbury’s (Corey’s Mill) FSR
5.2.3 This flood storage reservoir is situated in the angle between Hitchin Road (A602) and the
A1(M) motorway. It has a compact shape, approximately 80m by 60m, and is bounded by a
Borough Council depot on the north, a supermarket to the south and the motorway on the
west. Its top water level is about 85mOD.
5.2.4 This FSR was one of the “water meadows” constructed by the Development Corporation and
was originally known as Corey’s Mill FSR. It was enlarged to about 16,000 cu.m circa 1992,
the work being funded by the supermarket developer. It is unusual in that it lies to the north
of the Thames / Anglian watershed and drains into the head of Ash Brook, a tributary of the
River Hiz. Unlike all the older FSRs, it retains a substantial depth of water at all times and
acts as a de facto nature reserve.
5.2.5 The reservoir has been constructed on-line but with a piped inflow. The outfall from the
reservoir is to a culvert beneath the motorway which discharges to an open watercourse
beyond. The embankment at the west end of the reservoir has a sheet piled retaining wall
because of its close proximity to the motorway embankment. The natural catchment
draining to this FSR, which includes the village of Graveley, is about 2.4 sq.km.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 31
Meadway FSR
5.2.6 A very small (approx. 1,500 cu.m) on-line flood storage reservoir (also known as Symonds
Green FSR) situated on the west side of Gunnels Wood Road (A1072) and north of
Meadway. The watercourse that flows into the reservoir from the north and into which it
discharges is culverted both upstream and downstream of the reservoir. This watercourse is
one of the headwater tributaries of Stevenage Brook.
5.2.7 Much of the small earth embankment at the south end of the reservoir is taken up with the
concrete outfall structure. There is a Thames Water surface water sewer pumping station
immediately to the east of the outfall structure which is understood to supplement the low
level drainage of the upstream catchment.
5.2.8 The relatively narrow reservoir basin extends for about 80m upstream of the reservoir
embankment and is heavily overgrown with scrub. In September 2007 the reservoir basin
was dry although there was a small flow in the watercourse through the reservoir. When full
the reservoir’s top water level is at about 95mOD.
Burymead FSR
5.2.9 Another small flood storage reservoir, it is situated just east of Burymead and north of
Martins Way (A1072) in the valley of the Chesfield Park stream, a major tributary of
Stevenage Brook. Upstream of the FSR its catchment area is 1.9 sq.km although only
about 13 hectares at the catchment’s downstream end is at present urbanised. The stream
is culverted beneath the northern outskirts of Stevenage and the inflow to and outflow from
the reservoir is piped. The reservoir basin was dry in September 2007.
5.2.10 There is an orifice plate control structure embedded in the embankment at the southern end
of the reservoir. The public footpath along the western edge of the reservoir is embanked
and appears to form part of the reservoir embankment. The land to the east of the FSR is a
public recreation area and the reservoir basin is maintained as open grassland.
5.2.11 It is possible that this FSR may operate (like Camps Hill Park FSR) as an off-line reservoir
with an in-line throttle in the piped section of stream beneath the reservoir which diverts
excess flows into the reservoir (top water level about 100mOD) when the capacity of the
pipe is exceeded.
Elder Way FSR
5.2.12 This substantial off-line flood storage reservoir is situated alongside Stevenage Brook a
short distance downstream of Stevenage town centre. Stevenage Brook is an open
watercourse both upstream and downstream of the FSR. The Main River section of the
Brook extends as far as the St Georges Way / Six Hills Way roundabout, 300m upstream of
the reservoir. The reservoir is bounded by London Road to the west and Monkswood Way
(A602) to the east, with Elder Way on the south.
5.2.13 There was originally a much smaller FSR just upstream of the existing reservoir but this
disappeared and was replaced by the existing FSR when the Stevenage College site was
redeveloped. The existing reservoir was originally operated as an on-line FSR but was
modified to off-line operation when it was enlarged from about 7,000 cu.m to its present
capacity of about 15,000 cu.m. The catchment area upstream of the reservoir is
approximately 10.1 sq.km.
5.2.14 The Environment Agency state that flows from the Brook to the reservoir are controlled by
the operation of “a bank of penstock valves” in a concrete structure across the Brook near
the downstream end of the reservoir. When water levels in the Brook exceed predetermined
levels, excess water overflows side-spillway weirs in the left bank of the stream and
cascades into the normally dry reservoir. Top water level, when full, is approximately
85mOD.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 32
5.2.15 Elder Way FSR is divided into two separate but hydraulically linked compartments by a line
of trees along the central axis of the reservoir basin, although each compartment has its own
outflow structure for the return of flood water to the stream after a flood event. The pre-
existing trees were left in situ for amenity purpose when the reservoir basin was excavated
on either side of them. The reservoir basin is maintained as rough grassland.
Old Knebworth Lane FSR
5.2.16 Old Knebworth Lane FSR operates as an off-line reservoir situated in the angle between Old
Knebworth Lane and the East Coast Main Line railway, south of the lane and west of the
railway at the north end of Knebworth golf course. At this location Old Knebworth Lane is
the borough boundary and the reservoir lies just outside the borough. This FSR superseded
an earlier, smaller FSR located north of the lane when the site of the earlier FSR was
required for industrial development.
5.2.17 Excess flood flows in Stevenage Brook are piped to Old Knebworth Lane FSR, a distance of
about 100m. Flows are diverted to the reservoir by a control structure on the Brook which
has a 15.1 sq.km catchment upstream of the diversion. The same pipe serves both inflows
to and outflows from the reservoir. The capacity of the reservoir is about 24,000 cu.m,
slightly less than would make it subject to the provisions of the Reservoirs Act. Its top water
level is about 80mOD.
5.2.18 Because of the proximity to the railway embankment, the reservoir itself is formed by
concrete capped steel sheet pile walls along its northern and eastern sides. The reservoir’s
inlet / outlet structure is located at the angle of the two sheet pile walls.
5.2.19 This FSR is situated at the downstream end of the Knebworth Park valley which has a
catchment area of 3.7 sq.km. Although the downstream end of this valley appears to be a
dry bourne under normal conditions, there is a stream-fed lake in the park. The Ordnance
Survey map also shows this lake discharging into an open watercourse which disappears
700m downstream of the lake and 900m upstream of the FSR. It must be assumed that in
an extreme event flood flows continue down the valley along the normally dry bourne. It is
not known whether the design of the FSR allows for any such surface water inflow from
Knebworth Park.
Broad Oak FSR
5.2.20 Broad Oak FSR is situated near the downstream end of the Fairlands Valley Stream, about
200m upstream of its confluence with Stevenage Brook. The reservoir is located in open
woodland south east of Stevenage Town Football Club’s stadium, between Broadhall Way
(A602) and London Road (B197). This reservoir is also known as Fairlands Valley FSR but
will be referred to in this Report as Broad Oak FSR to avoid confusion with the Fairlands
Valley Lakes which are 2 km upstream.
5.2.21 Broad Oak is an on-line reservoir with open watercourse inflows and outflow and a top water
level of between 80m and 85mOD. There is an orifice plate control structure set in the
embankment at the southern end of the reservoir. It is understood that this FSR has not
been enlarged and its capacity is not known. The catchment area upstream of the reservoir
is about 3.1 sq.km but this includes the Fairlands Valley Lakes.
Wychdell FSR
5.2.22 The largest of Stevenage’s FSRs when built, with a capacity of about 44,500 cu.m
(Environment Agency data) and therefore subject to the provisions of the Reservoirs Act. It
is situated just upstream of the confluence of Stevenage Brook and Aston End Brook,
between Ashdown Road and Broadhall Way (A602). The earth embankment is located at
the east end of the elongated reservoir basin, much of which is open scrubland. This is an
on-line FSR and Stevenage Brook, in which there was a substantial flow in September 2007,
runs from end to end through the reservoir basin.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 33
5.2.23 There is a flow control structure consisting of a penstock and an overspill weir in a concrete
chamber set in the reservoir embankment. No formal spillway could be seen on the crest of
the embankment (approx. 75mOD) and flood water has been observed overflowing the
embankment during a flood event. Wychdell FSR is operated and maintained by the
Environment Agency who state that the whole length of the embankment constitutes the
emergency spillway “that is protected to receive the probable maximum flood”. The Agency
has produced evidence that this protection is provided by a buried layer of scour protection
matting (Enkamat) over the whole downstream face of the embankment. This protection
was installed in 2004 as a consequence of recent housing development adjacent to
Stevenage Brook at Bragbury End, less than 1km downstream, which has resulted in the
reservoir being reclassified for safety purposes as ‘Category A’.
5.2.24 Upstream of the reservoir Stevenage Brook has a catchment area of 22.3 sq.km. Recent
hydrological and hydraulic modelling (Ref.5) has found that the storage capacity of Wychdell
FSR would currently be exceeded in a flood event with a return period of between only 2
and 5 years.
Camps Hill Park FSR
5.2.25 A small off-line reservoir situated in a children’s play area south west of Chells Way, in open
parkland between Harvey Road and Warwick Road. The most recent of Stevenage’s FSRs,
Camps Hill Park FSR was built to alleviate recurring localised flooding of residential
properties in the Warwick Road area from a trunk surface water sewer. The valley in which
the reservoir lies drains south eastwards to the head of Aston End Brook.
5.2.26 Inflow and outflow to the reservoir are both piped from the nearby trunk sewer. Flows from
the sewer to the reservoir are controlled by an orifice plate in the section of sewer between
the junctions with the FSR inflow and outflow pipes. The reservoir capacity is not known. Its
top water level when full is about 105mOD.
Ridlins Wood FSR
5.2.27 Ridlins Wood FSR is situated on Aston End Brook, west of Gresley Way and just north of
Broadwater Lane. It is an on-line reservoir with the Brook in open channel both upstream
and downstream of the reservoir. The reservoir, which is now operated and maintained by
the Environment Agency, was enlarged by the then Thames Water Authority in about 1982
and now has a capacity of 51,800 cu.m (Environment Agency data) and a top water level of
about 75mOD.
5.2.28 Aston End Brook has a catchment area of 6.1 sq.km upstream of the reservoir. Recent
hydrological and hydraulic modelling (Ref.5) has found that currently the storage capacity of
Ridlins Wood FSR would only be exceeded in a flood event with a return period of between
100 and 200 years. Aston End Brook is a Main River as far as Tatlers Lane at Aston End,
1.5km upstream of the reservoir.
5.2.29 Broadwater Lane runs along a berm on the downstream side of the reservoir embankment
with the reservoir outflow piped beneath the road. The road originally ran along the crest of
the embankment and the reservoir was enlarged by raising the embankment on the
upstream (north) side of the road and extending the embankment for a short distance along
both sides of the reservoir. There is a control structure consisting of a penstock and internal
overspill weir in a concrete chamber embedded in the raised section of the embankment.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 34
5.2.30 Despite the size of the reservoir, there appears to be no formal spillway over the reservoir
embankment, although the Environment Agency state that it has a spillway, occupying about
80% of the embankment’s length and some 400mm lower than the remainder of the
embankment “that is protected to receive the probable maximum flood”. The 1982
enlargement involved the placing of a gabion mattress under a thin layer of soil along the
downstream face of the embankment. It is not known whether the downstream face of the
embankment is also lined with a layer of scour protection matting (Enkamat). The presence
of the road along a berm on the downstream side of the embankment will also provide some
degree of scour protection.
Aston Valley FSR
5.2.31 Aston Valley FSR is located at the downstream end of Aston Valley, just upstream of the
confluence between Aston End Brook and Stevenage Brook. The substantial on-line
reservoir lies east of Broadhall Way (A602) on the western edge of Stevenage golf course.
5.2.32 This reservoir is operated and maintained by the Environment Agency and although it might
therefore be expected to have a capacity greater than 25,000 cu.m, the Reservoirs Act
minimum, the Agency state that the capacity of this reservoir is “somewhere around 14,000
cu.m.” The reservoir is believed to have been enlarged, though further enlargement is
limited by the proximity of recent housing development at Goddard End, close to the
northern (upstream) end of the reservoir.
5.2.33 Outflow from the reservoir is controlled by an orifice plate structure and an internal overspill
weir in a concrete chamber set in the reservoir embankment. The embankment has a crest
level of between 70m and 75mOD. As with Wychdell and Ridlins Wood FSRs, there does
not appear to be any formal spillway on the crest of the embankment and the Environment
Agency state that the full width of the embankment constitutes the spillway. Since this
reservoir does not come within the provisions of the 1975 Reservoirs Act it is not known
whether scour protection matting was installed along the downstream face of the
embankment as at Wychdell and possibly Ridlins Wood.
5.2.34 The catchment of the Aston Valley upstream of the reservoir is about 6.5 sq.km. Recent
hydrological and hydraulic modelling (Ref.5) has found that the storage capacity of Aston
Valley FSR would only be exceeded once in between 10 and 20 years.
Bragbury End FSR
5.2.35 This small on-line flood storage reservoir is located behind the residential development
along the east side of Bragbury Lane. It was constructed in 1975/6 to deal with urban runoff
from new residential development in the southeastern corner of the borough at Bragbury
End, between the Stevenage - Hertford railway line and the A602 road.
5.2.36 There is a piped inflow to and piped outflow from the reservoir. Discharge into the piped
outflow is controlled by an orifice plate structure in the embankment at the north end of the
reservoir. The top water level in the reservoir is between 70m and 75mOD.
5.2.37 The reservoir has a natural catchment area of 4.6 sq.km extending as far as Knebworth and
Datchworth. The chalk catchment is mainly rural but includes much of the large village of
Knebworth. There are no surface watercourses shown within the catchment on the OS map
- the valley between Knebworth and the FSR is a dry bourne - and the presence of the
railway embankment across the valley less than 100m upstream of the reservoir means that
there is no open channel inflow to the reservoir.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 35
Boxbury FSR
5.2.38 Boxbury FSR is the only one of the Stevenage “water meadows” that lies well outside the
borough boundary. It is situated 150m northwest of Boxbury Farm, 1.5km west of Walkern
village, in a tributary valley of the River Beane. This valley is a dry bourne as far
downstream as Walkern. Above Walkern the valley has a catchment area of 5.1 sq.km.
This diminishes to 3.7 sq.km upstream of the reservoir although only about half of this area
is at present urbanised. Top water level in the reservoir is approximately 100mOD.
5.2.39 This small on-line FSR has two separate piped surface water sewer inflows from urban
development in the northeast corner of the Borough. There is an orifice plate control
structure set in the reservoir embankment that discharges directly into a piped outflow. As
there is no stream channel down the Boxbury Valley this pipe extends down the valley as far
as Walkern where it discharges, via an oil interceptor south of Stevenage Road, into the
River Beane.
5.2.40 The capacity of this FSR is less than 25,000 cu.m and it is understood that it has not been
enlarged. The capacity of the 2 km pipeline down the Boxbury Valley to the River Beane
could prove to be a constraint to the enlargement of this reservoir.
5.3 Fairlands Valley Lakes
5.3.1 Fairlands Valley is a 3km ribbon of undeveloped land running from north to south through
the centre of the Borough from Pin Green to London Road (B197). Most of Fairlands Valley
is managed by the Borough Council as the Fairlands Valley Park although the downstream
end of the valley south of Broadhall Way (A602) is scrubland and rough woodland.
Broad Oak FSR (see above) is located at this end of the valley.
5.3.2 The three Fairlands Valley Lakes, completed in 1973, are situated towards the northern
(upstream) end of Fairlands Valley Park. Although there are actually four lakes in series, the
first lake is little more than an enlarged stream channel and the second and third lakes are
relatively small. The fourth and most downstream lake is, however, of considerable size
(90,910 cu.m capacity) and subject to the provisions of the Reservoirs Act. It has a
catchment area of 2.0 sq.km and its top water level is 101.7mOD.
5.3.3 Although the lakes were established as amenity lakes, it is understood that the three smaller
lakes were intended to have a secondary function as oil interceptors. The three largest
lakes were formed by constructing earth embankments across the Fairlands Valley Stream.
The largest lake, also known as the Sailing Lake, is used for dinghy sailing (Stevenage
Sailing Club is situated at the upstream end) and coarse fishing.
5.3.4 It should be emphasised that the Fairlands Valley Lakes were created as amenity lakes and
were never intended or designed as “water meadows” or flood storage reservoirs. They are
operated by the Borough Council as amenity lakes and maintained full for that purpose.
Nevertheless, it is inevitable that the largest lake will have an attenuation effect on surface
water runoff entering and leaving the lake and the Environment Agency state that the
Boating Lake “is considered to be an FSR and is registered as such”.
5.3.5 All surface water inflows to the Fairlands Valley Lakes are piped. The Boating Lake has a
concrete overflow structure embedded in the reservoir embankment which discharges into a
pair of large diameter pipes. This lake has a siphon spillway in the centre of the
embankment but no conventional overflow spillway on the crest of the embankment. There
is a 450mm diameter valve-controlled outlet pipe through the base of the embankment.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 36
5.3.6 In flood conditions these pipelines become surcharged and surplus flood water is designed
to emerge from a concrete overflow chamber located immediately downstream of Six Hills
Way. From here the surplus water flows overland through Fairlands Valley Park into an
emergency flood storage area in the valley bottom, upstream of Broadhall Way. This flood
storage area is designed to accommodate the peak of the 1000-year return period flood
event which will fill the storage area in five minutes.
5.4 Surface Water Sewerage System
5.4.1 Although much of the discharge from the New Town’s surface water sewerage system goes
straight into the town’s “water meadows” they are not considered here as part of the town’s
surface water sewerage system and have been dealt with separately in Sub-sections 5.1
and 5.2 above. This sub-section therefore deals solely with other flood alleviation works
incorporated into the town’s surface water sewerage system.
5.4.2 A single large scale (1/1,250) sewerage plan of the whole of Stevenage was provided by the
Borough Council in electronic format at the end of January 2008. The plan, based on
Thames Water sewerage plans, showed both foul and surface water sewerage
infrastructure, including the Stevenage Trunk Sewer, and most but not all of the town’s flood
storage reservoirs were shown. Unfortunately the plans were not accompanied by manhole
reference tables. For analytical convenience the single plan was divided into twenty
separate but overlapping plans. Sewerage plans for those areas of Stevenage served by
Anglian Water were obtained direct from Anglian Water.
5.4.3 Stevenage’s smaller ‘water meadows’ constitute its surface sewerage system’s flood
attenuation facilities. In a small number of locations on-line flow attenuation devices
(‘hydrobrakes’) have been installed in connection with recent commercial or industrial
development and are now incorporated into the public sewerage system.
5.4.4 The town’s surface water sewerage system includes a few small pumping stations but these
are all associated with local topographic anomalies such as underpasses or road cuttings.
In general the surface water drainage system relies essentially on gravity discharge.
5.5 Foul Sewerage System
5.5.1 Before the New Town was established, the original town of Stevenage was served by a
small sewage treatment works located at Roaring Meg, off London Road. The treated
effluent from this works was discharged to Stevenage Brook. As early as the 1940s it was
foreseen that the creation of New Towns at Stevenage and Harlow and other widespread
urban development in the Lee catchment would justify the provision of a single large sewage
treatment works for the whole of the upper part of the catchment.
5.5.2 To meet this projected demand a large modern sewage works was established in the Lee
Valley at Rye Meads, 3km southeast of Hertford. The new sewage works was completed by
1954 and foul sewage from the whole of Stevenage New Town was conveyed to Rye Meads
by a 26.5 km trunk sewer - the North Western Outfall Sewer - which continues to the
present day to serve the whole of Stevenage.
5.5.3 Stevenage New Town was from the start provided with separate foul and surface water
sewerage systems and there are no significant areas of the town, even in the core of the old
town, that are served by combined (i.e. foul and surface water) sewers such as are
frequently found in older urban areas. Hence there appear to be very few storm overflows
from foul sewers to surface water sewers or open watercourses in the Borough.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 37
5.5.4 Although Knebworth lies outside the Borough its sewerage system is connected to the Rye
Meads trunk sewer and the main sewers from Knebworth run through the southern edge of
Stevenage. There are underground storm tanks at Braemar Close, south of Hertford Road,
at the downstream end of a main sewer serving the northern end of Knebworth and these, if
surcharged in a major storm event, could pose a minor localised flood risk within the
Borough.
5.5.5 As would be expected, no specific flood attenuation facilities (storm tanks etc) associated
with the foul sewerage system were found, except where the main foul sewers from
Knebworth entered the Borough at Braemar Close.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 38
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Strategic Assessment of Flood Risk
Faber Maunsell Stevenage Strategic Flood Risk Assessment 41
6.1 General Methodology
6.1.1 For the purposes of this study, the quantitative limits of the flood risk categories used will
correspond exactly with the three zones defined in PPS25 (Table 2.1). Since the
Environment Agency's Flood Zone (FZ) maps represent an important attempt to define the
limits of the zones of inherent flood risk as expressed in PPS25 Flood Zones 2 and 3 (and
hence the boundary between PPS25 Zones 1 and 2) they will be used as a starting point for
the detailed strategic (i.e. Borough-wide) assessment of actual flood risk within Stevenage.
6.1.2 A previous study (Ref.7) has shown that although they were generally accurate and reliable,
close inspection of the Agency's original Indicative Floodplain (IF) maps revealed various
anomalies in the plotting of the flood envelope. These anomalies were grouped into six
types, as follows.
• Where the flood level on one side of a floodplain was significantly different from that
on the other.
• Where the flood envelope did not follow a closely adjacent contour line where
"ponded" flooding was known or could be assumed to occur.
• Where the edge of the flood envelope indicated that the flood level at a point
downstream was higher than the level a significant distance upstream.
• Where the presence of an "island" in the floodplain had been overlooked.
• Where the water level gradient implied by the flood envelope boundary was clearly
at variance with the general land level gradient along the valley floor (thalweg) in
that area, except where due to an obvious obstruction to flow.
• Where the presence of an obvious obstruction to overbank or channel flows
(artificial embankment, restricted waterway at bridge, etc) had been overlooked.
6.1.3 The Agency’s FZ maps were compiled using a different and more systematic methodology
than their predecessor IF maps and many of the anomalies identified in the IF maps have
largely been eliminated, although a detailed investigation, outside the scope of this study,
would be needed to confirm this. It should, however, be noted that the sixth anomaly listed
above is no longer relevant in the context of FZ maps in which the impact of all such artificial
obstructions has been removed. These anomalies are, nevertheless, presented here as
they give a useful indication of potential errors applicable to all forms of flood risk mapping.
6.1.4 A number of sources of data and information are available which can be used to check and,
where necessary, refine the Environment Agency's FZ maps and thus the outer limits of the
High Probability and Medium Probability zones (Flood Zones 3 & 2). These are :-
LiDAR data
6.1.5 The Environment Agency has established a national database of topographical spot-
level data derived from an airborne laser imaging process. Contoured plots of
LiDAR data are found to be of greatest use in open country as the presence of
buildings is found to give rise to clearly anomalous results in built-up areas.
6.1.6 At present the LiDAR data coverage does not extend over the whole country,
although in Stage 1 of this Study a small scale plan made available by the Agency
indicated that there was a comprehensive but not total LiDAR coverage of
Stevenage and the area around the Borough. The LiDAR data for this area was
supplied in electronic format by the Agency in Stage 2 of this Study.
6 Strategic Assessment of Flood Risk
Faber Maunsell Stevenage Strategic Flood Risk Assessment 42
6.1.7 LiDAR data is supplied in two types, filtered and unfiltered. The unfiltered data does
not differentiate between ground levels and roof levels or even, in some cases, the
top of dense vegetation canopies. The filtered data has been processed
automatically to ‘remove’ all buildings and vegetation and give only inferred ‘bare
earth’ levels. The filtering process is not wholly reliable in heavily urbanised areas.
Ordnance Survey Maps
6.1.8 1/25,000 scale OS maps are contoured at 5m intervals which is adequate to give a
general indication of the shape of the floodplain at any location. The contours are
supplemented by spot heights to the nearest 1m on roads. It should, however, be
noted that road levels can, particularly in floodplains, be significantly higher than
adjacent land levels.
6.1.9 A complete 1/2,500 scale OS map coverage of Stevenage has been provided by the
Borough Council on CD which can be accessed using "Mapinfo" software. These
maps are not contoured but include spot heights on roads to the nearest 0.1m
(though some of these metric spot heights are conversions from earlier imperial
units and are therefore only accurate to the nearest 1ft / 0.3m). The Borough
Council also supplied OS mapping of the District at 1/50,000 scale.
Ordnance Survey "Profile" and SAR Data
6.1.10 Ordnance Survey "Profile" data, large scale contoured plans, is available to all Local
Authorities although many, including Stevenage, have not availed themselves of the
opportunity to acquire this data. Faber Maunsell have, however, been given to
understand that "Profile" data is derived from the spot heights and contour lines on
existing large scale OS maps and provides no more accurate information than can
be inferred or deduced from the OS maps. In any case, the widespread availability
of LIDAR data in the Stevenage area removed the need to obtain “Profile” data.
6.1.11 SAR data, because it is inherently less accurate than LiDAR survey data, should
only be used to supplement LiDAR data where gaps exist in the LiDAR data
coverage or where urban area interference has rendered LiDAR data unintelligible
or unreliable.
Flood Zone Maps and Flood Maps
6.1.12 The Environment Agency’s Flood Zone Maps are based on OS 1/10,000 scale
maps. They show both Flood Risk Zones 2 and 3 and, by inference, Zone 1. These
maps are not contoured but show spot heights to the nearest 1m. In many cases
these are at the same locations as the spot heights on the 1/25,000 scale maps but
additional spot heights are shown on the 1/10,000 maps. Copies of the FZ maps for
the whole of Stevenage were supplied to Faber Maunsell on CD.
6.1.13 The Agency’s Flood Maps are available from the Agency’s website and are based
on an OS 1/50,000 scale map base. The representation of some recent flood
defences (and, by inference, the presence of others) is useful but the small scale of
these maps is a considerable drawback to their use, even at a strategic level.
Isolated instances have already been found where the textual flood risk information
obtained from Flood Maps for a fixed point by the “click-on” procedure is at variance
with that shown graphically on the map for the same point.
Flooding Records
6.1.14 Records of past flooding events, where available, (see Section 4) can be used to
verify or amend the floodplain envelope.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 43
6.1.15 In order to define the extent of all three actual flood risk categories (corresponding
numerically to the PPS25 flood risk zones) it is necessary only to delineate the boundaries
between Categories 2 and 3 and between Categories 1 and 2 - i.e. the 1% (1in100 year)
and 0.1% (1in1000 year) flood envelopes. The delineation of a more accurate 1% line by
the identification and removal of local anomalies in the FZ maps has been outlined above,
but the delineation of the 0.1% line presents more of a problem.
6.1.16 Research has demonstrated that the channel capacity of a natural river or stream is
approximately equivalent to the mean annual flood flow which can be shown statistically to
have a return period of 21/3 years (Ref.8). In other words, the natural river channel will start
to spill over onto its floodplain nearly every other year. It is therefore reasonable to assume
that there will also be a natural return period at which the floodplain is completely covered.
6.1.17 If a 1in1000 year flood were to occur there is no reliable means of determining whether or
not it was the 1in1000 year flood, rather than (say) a 1in500 year or a 1in10,000 year flood.
In an area of sedimentary rocks, such as Hertfordshire, which was glaciated during the ice
ages, the landforms are relatively recent. Since the last ice age there will, on the balance of
probabilities, only have been a single 10,000 year flood and about a dozen floods in the
1000 to 5000 year return period range. It is therefore reasonable to assume that the fluvial
floodplains have, since the ice ages, been shaped by a relatively small number of floods with
return periods not greatly in excess of 1000 years. This suggests that the 1000-year flood
would cover the entire flood plain but not to such a depth that the flood plain would be
radically reshaped. Conversely, the outer limit of the flood plain roughly defines the 1000
year (0.1%) flood envelope.
6.1.18 Since there is often a marked discontinuity in ground slope between the floodplain and the
land on either side, it is quite likely that in many places the 1000-year flood envelope is not
significantly wider than the 100-year flood envelope. The respective depths of flooding over
the flood plain could, of course, be significantly different.
6.1.19 The Environment Agency's Indicative Floodplain and Flood Zone maps of Stevenage have
been studied in detail for any anomalies in the six types listed in para.6.1.2 above, in
conjunction with the data sources described in paras.6.1.5 to 14. Any obvious anomalies
identified have been corrected and the 1in100 year flood envelope amended accordingly.
The methodology outlined in paras.6.1.15 to 18 has then been used to obtain an estimate of
the 1in1000 year flood envelope for comparison with that shown on the FZ maps. This
process has resulted in the production of the set of three 1/10,000 scale strategic flood risk
maps of Stevenage included as Figure 6.1 in this report.
6.2 Topographic Divisions
LOWLAND AREAS
6.2.1 These are extensive areas of relatively flat land which, unlike fenland, have sufficient slope
to drain by gravity but are characterised by broad floodplains and wide, slow flowing, often
embanked rivers.
6.2.2 The lower Lee Valley downstream of Hertford is clearly a lowland area, and the valleys of
the Lee’s major tributaries, such as the River Beane, can also be regarded as lowland
areas. Although the relatively broad valley of the River Beane below Watton at Stone can
still be regarded as lowland, the Beane’s headwaters and tributaries are more upland in
character.
UPLAND AREAS
6.2.3 The upland areas within Hertfordshire are mainly associated with the numerous small
headwater valleys of the Lee and its principal tributaries such as the Stort, Rib, Beane and
Mimram. Many of these valleys are chalk bournes. The fact that some of these headwater
valleys may now be heavily urbanised does not affect their classification as upland areas.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 44
6.2.4 It can therefore be deduced that Stevenage, situated within the catchments of two of the
River Beane’s headwater tributaries, the Stevenage and Aston End Brooks, and close to the
Lee / Great Ouse watershed, may be regarded as an upland area within the definition used
in this study.
6.2.5 The floodplains in these upland valleys follow the course of the streams but are also narrow,
rarely extending for more than much more than 50m on either side of the watercourse. At
the scale of 1/25,000 generally used for Strategic Flood Risk maps in rural areas,
differences between the floodplain defined by the Agency and that defined in this study will
generally be of a second order of magnitude.
6.2.6 The 1in1000 year flood envelope in upland areas has been assumed to extend across the
whole of the floodplain, but for reasons given in para.6.1.17, the difference between the
1in100 year and 1in1000 year flood envelopes in these areas will be fairly small. This is
readily apparent on the Agency’s FZ maps.
6.3 Anthropogenic Influences
6.3.1 Anthropogenic influences in this context are any man-made topographic features that have
an impact on the flow of flood water in a natural channel or along a floodplain. These
features can be considered in two separate categories – those established specifically to
modify or impede natural flow patterns and those established for other purposes but have a
coincidental but significant effect on the flow of flood water.
6.3.2 In Stevenage the first category obviously includes the town’s many flood storage reservoirs
but will also include measures taken to increase the natural capacity of a watercourse to
convey flood flows. This category can also include enlarging the watercourse or lining the
bed and banks of the channel with (e.g.) brickwork or concrete to decrease their roughness
factor. Although not prominent in Stevenage, floodbanks and floodwalls come into this
category.
6.3.3 The second category, ‘passive’ influences, are structures such as roads or railways which
are usually raised or embanked to a greater or lesser extent above the natural ground level.
Even where there are openings through an embankment it may still create a significant
obstruction to overland flows on a floodplain. Within the watercourse itself, bridge piers and
abutments, or the soffits of bridge decks and arches can also obstruct the flow in the
channel and create a considerable afflux upstream. Weirs and sluices, unless erected for
flood control purposes, also fall into this category. In lowland areas, large arterial drainage
channels may intercept and divert flood flows.
6.3.4 In heavily urbanised areas, buildings in a floodplain or in the vicinity of a watercourse can
have a major influence locally on flood flow paths and hence on flood depths and velocities.
This influence can be particularly marked where the buildings are contiguous as, for
example, with rows of terraced houses.
6.4 Hydraulic Modelling
6.4.1 Computer-based hydraulic modelling of a watercourse, if done for that purpose, enables the
modeller to obtain estimates of water levels and depths along the watercourse and its
adjoining floodplain for a range of flood flows corresponding to flood events of
predetermined annual probabilities or return periods. Standard hydrological techniques
applicable to whole of Great Britain are available to estimate the flood flow rates in that
watercourse (Ref.6).
Faber Maunsell Stevenage Strategic Flood Risk Assessment 45
6.4.2 The results of hydraulic modelling give flood levels at all points along the whole length of the
watercourse modelled, corresponding to specified return periods, and allow for the effects of
anthropogenic influences. Combined with a detailed topographical survey of the floodplain
(e.g. LiDAR) the modelling results can be used to plot the extent of the flood envelopes for
the specified return periods, normally the 20, 100 and 1000-year return period events to
correspond with the Flood Zone 2, 3a and 3b boundaries. The hydraulic model can then be
re-run to derive equivalent estimates of flood levels up to the year 2115, using the predicted
effects of climate change on fluvial flood flows given in Table B2 of PPS25.
6.4.3 In any Strategic Flood Risk Assessment, unless the Brief specifically includes hydraulic
modelling of watercourses in the study area, the assessment utilises the results obtained
from pre-existing hydraulic modelling exercises in the study area. In most cases these
models will have been commissioned by the Environment Agency and carried out on their
Main Rivers.
6.4.4 For watercourses where no hydraulic modelling has been carried out, the Agency’s Flood
Zone maps will give first-order estimates of the Zone 2 and 3 flood envelopes. Under these
conditions, the FZ maps can be used as a starting point for strategic flood risk mapping,
especially on smaller rural watercourses. It should, however, be borne in mind that the FZ
maps were initially derived using a national-scale methodology and not by modelling specific
to that watercourse.
6.4.5 At the outset of this study it was found that no hydraulic modelling of open watercourses or
the surface water sewerage system had been done or, with one exception, neither the
Environment Agency nor the Borough Council had any knowledge of such modelling.
Although the Borough Council believed that the promoter of an industrial development had
undertaken hydraulic modelling of a short length of the Stevenage Brook in the Bragbury
area, all attempts to obtain information on this modelling from the developer concerned were
abortive.
6.4.6 As far as we have been able to ascertain, no modelling of the Stevenage surface water
sewerage network has been carried out in recent years and there are no estimates of the
hydraulic capacity of the system when surcharging and overflow occurs.
6.4.7 An Environment Agency project involving the hydraulic modelling and flood mapping of the
River Beane, including much of Stevenage Brook and most of Aston End Brook, was
completed in May 2008. The model results did not become available for this study until
September 2008 but they included not only estimates of flood levels along the modelled
watercourses for a range of return periods (5, 10, 20, 50, 100 and 1000years and 100 years
with climate change) but also those flood levels transposed on to a LiDAR-based plan of the
stream corridor.
6.5 Strategic Flood Risk Mapping
6.5.1 Two separate methodologies must be used for the strategic flood risk mapping of the
Borough of Stevenage - one for the open watercourse sections of Stevenage Brook and
Aston End Brook and the other for the culverted and sewered sections of those
watercourses and their tributaries. It is evident that the impact of flooding from the culverted
and sewered watercourses, as regards the number of properties potentially at risk, is at least
equal if not greater than the impact of flooding from the open channels as in most areas of
the town urban development has been kept at a reasonable distance from the stream
channels.
6.5.2 The phenomenon of ‘floodlocking’ has already been described and discussed in Section 3,
but the absence of any significant lengths of raised flood defences along open watercourses
in Stevenage means that ‘floodlocking’ can be disregarded in this strategic flood risk
assessment.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 46
Open Watercourses
6.5.3 The strategic flood risk mapping of those areas of Stevenage at risk of flooding from open
watercourses and the preparation of the relevant parts of the set of three 1/10,000 scale
flood risk maps (Figure 6.1) was originally based on the Zone 2 and 3 flood envelopes
shown on the latest (2007) version of the Environment Agency’s Flood Zone maps. The
lengths of watercourses for which this information is available on the Flood Zone maps are
listed in Table 6.1 below. It should be noted that the Flood Zones only extend along the
larger open watercourses in the Borough.
Watercourse Downstream Limit Upstream limit
Stevenage Brook Hooks Cross (Boro bdy) Six Hills Way (A1070)
Bragbury End Stream Stevenage Bk confl’nce Bragbury End FSR
Aston End Brook Stevenage Bk confl’nce Edmonds Drive
Fairlands Valley Stream Stevenage Bk confl’nce Broadhall Way (A602)
Knebworth Stream Stevenage Bk confl’nce 100m u/s Old Kn Ln FSR
Wymondley Brook Chantry Lane (Boro bdy) Graveley Rd (Boro bdy)
Table 6.1 - Flood Zone Extents in Stevenage
6.5.4 The LiDAR data supplied by the Environment Agency was plotted, initially at 1.0m contour
intervals, on an overlapping series of six 1/2,500 scale working plans, arranged to give a
complete coverage of the Borough. The data could then be re-plotted to a larger scale and
at smaller contour intervals to determine local topography. Along the principal open
watercourse floodplains these contoured plots were compared with the flood envelopes on
the Flood Zone maps and used to modify those envelopes, where appropriate, to take into
account the local topography as well as the local impact of anthropogenic influences such as
bridges and culverts.
6.5.5 Implicit in the flood risk assessment of open watercourses is the assumption that the full
width of the floodplain will be affected by all floods in excess of the 100-year return period.
In reality, the difference in width between the 100-year and 1000-year flood envelopes on
minor watercourses with narrow floodplains is usually too small to be represented on
strategic-scale maps.
6.5.6 When the detailed results of the Agency’s River Beane modelling became available in
September 2008 (see para.6.4.7) these results automatically superseded the previous less
precise assessments and have enabled the strategic flood risk maps to incorporate the
results of the latest available hydraulic modelling of most of the open watercourse sections
of Stevenage Brook (to Broadhall Way) and Aston End Brook (to Woodcock Road).
Culverted Watercourses and Sewered Areas
6.5.7 The sewer plans of Stevenage show the effective pipe diameter of all lengths of foul and
surface water sewer in the town. Knowing the pipe diameter, roughness factor and pipe
gradient, standard hydraulic pipe-flow equations can be used to determine the pressure
head and maximum flow capacity in any length of sewer before surcharging and subsequent
overflow occurs. Unfortunately the sewer plans originally provided by the Borough Council
were not accompanied by schedules of sewer type, sewer invert and manhole cover levels.
This meant that local ground levels had to be used to determine pipe gradients and
generalised pipe roughness factors assumed. Although Thames Water sewer plans
became available in July 2008, by this time the deficiencies in the available LiDAR data (see
para.6.5.11) had become a determining factor in the analysis.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 47
6.5.8 The Flood Estimation Handbook (FEH) software and database was used to derive storm
rainfall estimates for the 100-year and 1000-year events corresponding to the critical storm
duration for the sewered sub-catchment under consideration. The corresponding 100 and
1000-year storm runoffs from the sub-catchment were then compared with the sewer
capacity to determine the degree of surcharge to which that length of sewer was subject and
hence the volume of excess storm runoff that would be forced to take an overland flow
route.
6.5.9 It was then intended to use the LiDAR data to identify the likely width of the path of the
excess flow, based on an assumption as to the average flow velocity along that path. The
width of the flow path thus obtained would define the flood envelope. Ideally a two-
dimensional (2D) hydraulic modelling procedure would be used to model the depth, velocity
and lateral extent of the excess flow on its overland path but to analyse the whole of the
sewered area by this method is beyond the scope of this study.
6.5.10 Both filtered and unfiltered LiDAR data was supplied for this study (see para.6.1.7). Since
the pattern of overland flow of flood water through an urban area will be heavily influenced
by the presence of buildings etc in the flow path unfiltered LiDAR data should be used as
filtered (‘bare earth’) LiDAR data will remove these obstructions and give unrealistically high
velocities but correspondingly low depths of water and narrow flood envelopes. On the
other hand, 2D modelling experience has shown that in dense urban areas the LiDAR grid is
too coarse to identify passageways and gaps between buildings through which flood water
would flow, leading to over-estimates of flood depth and extent and under-estimates of
velocity.
6.5.11 Unfortunately the routes of the town’s large surface water sewers were through heavily built-
up areas with dense residential or commercial development over or close to the line of the
sewer. This was in contrast to the open channel lengths of Stevenage and Aston End
Brooks modelled which ran through public open spaces where the LiDAR data could
reasonably be relied on. Where sewer flooding occurs, the flooding mechanism -
surcharging of buried pipes followed by overland surface flow - is very different from that of
flooding from open channels. This, and the problems with the reliability of LiDAR data in
dense urban areas made it impossible to produce flood envelopes for sewer flooding with
the data available. It was therefore found that areas at risk of flooding from surcharged
sewers could only be identified in general terms on the Strategic Flood Risk maps.
6.5.12 One notable feature of Stevenage’s foul sewerage network was the relatively small size of
many of the pipes, including those serving substantial numbers of properties. As the New
Town’s sewerage systems had clearly been designed from the start on the assumption of
entirely separate systems, with little or no surface water being allowed to enter the foul
sewers, it had been possible to achieve an economical design of the foul sewer system.
In this study a de minimis surface water pipe diameter of 450mm was taken, and it was
assumed that any flooding from a smaller sewer would have only a local impact and would
not be of any strategic significance.
6.5.13 Recent research (Ref.9) has, however, revealed that illegal mis-connections to sewerage
systems increase steadily over time. These mis-connections include both foul sewage into
surface water sewers and surface water into foul sewers. Whereas the consequences of the
former will be pollution of surface watercourses, the consequences of the latter could be the
surcharging of foul sewers and localised sewer flooding during heavy rainfall. Sewer mis-
connections of this type, if they become prevalent in the town, could create problems of
localised flooding from foul sewers in the future.
6.5.14 In the light of the problems encountered in producing flood risk envelopes for sewer flooding
outlined in paragraph 6.5.11, it is recommended that a comprehensive programme of
detailed hydraulic modelling of Stevenage’s principal surface water sewers is undertaken to
determine and define more precisely those locations in the town at significant risk of flooding
at the 1% and 0.1% annual probability levels.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 48
6.5.15 It is further recommended that the hydraulic modelling of Stevenage’s principal surface
water sewers should be used as the basis for a Surface Water Management Plan for the
Borough or those areas considered to be at greatest risk of flooding from surface water
sewers, as suggested in the recently published Pitt Review (Ref.10), in order to achieve a
greater degree of coordination between the Borough Council, the Environment Agency and
Thames Water.
6.6 Climate Change
6.6.1 In Strategic Flood Risk Assessments the impact on flood risk of the predicted (PPS25)
effects of climate change on fluvial flood flows is normally evaluated by re-running existing
hydraulic models with the hydrological inputs to the model increased by the amount
indicated in Table B3 of PPS25. As many river models pre-date PPS25, even if the model
was originally run to investigate the impact of climate change this would have been done
under PPG25 and the results would now be obsolete.
6.6.2 The potential effect of climate change up to the year 2115 would be modelled by increasing
the flood flow hydrograph in each sub-catchment of the distributed model by 20%. This
adjustment would be applied to the current 100 and 1000-year flood flows and hence gave
the "with climate change" sets of flood levels and corresponding flood envelopes.
6.6.3 Unfortunately in this study the absence of any existing hydraulic models of the Stevenage
Brook means that there is no reliable means of assessing the impact of climate change on
the present day flood envelopes, which are themselves less precise than they would be had
they been obtained from modelling results. For this reason it was considered inappropriate
and potentially misleading to produce strategic “with climate change” flood risk maps.
6.7 Rapid Inundation or Hazard Zones
6.7.1 In a major flood event where a river is confined within flood defences, there may be an
appreciable difference between the water level on one side of the flood defence and the
ground level in the defended area behind that defence. If that defence were then to fail,
whether through the collapse of a floodwall or the breaching of an embankment, there would
be a sudden inrush of flood water into the defended area. The velocity and depth of water
cascading through the breach could, initially at least, be sufficiently great to sweep a person
off their feet resulting in their death by injury or drowning. The premature failure of a flood
defence structure is by its nature a residual risk, but its potentially fatal consequences
dictate that it be given serious consideration in flood risk assessment.
6.7.2 As flood water pours through a breach it will fan out, and its velocity and depth will decrease
with distance from the breach. At some distance from the breach the velocity and depth of
water will have diminished to a point where an adult is capable of standing upright in the
flow. This is deemed to be the outer edge of the rapid inundation hazard zone. The
distance of this point from the defence line, and hence the width of the hazard zone, will be
determined by the flood level / ground level difference (head of water) and the width of the
breach.
6.7.3 A simple methodology for determining hazard zone width utilising a two-parameter matrix
(head of water and breach width) was developed for the Environment Agency in connection
with their Humber Estuary Shoreline Management Plan and recent investigation of flood risk
along the Humber estuary. This methodology assumes that the critical point occurs when
the product of depth (m) and velocity (m/s) reaches a value of 1.0 m2/s (e.g. 1m deep water
flowing at 1m/s, or 0.5m deep water flowing at 2m/s). The resultant matrix for determining
the width of the hazard zone behind any flood defence is given in Table 6.2 below.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 49
Height of water level above base of breach
Distance (metres) to the outer edge of the Hazard Zone for a breach width of ---
10 m 20 m 30 m 40 m
1.0 m 9 17 25 32
2.0 m 19 38 57 76
3.0 m 39 79 118 157
4.0 m 63 126 189 252
Table 6.2 - Flood Hazard Zone Widths
Note: The hazard zone widths for the 1m breach depth have been obtained by extrapolation of the plotted data.
6.7.4 Apart from the difficulty of plotting the narrow hazard zone widths in Table 6.2 with any
reasonable degree of accuracy on strategic-scale flood risk maps, the large local variations
in hazard zone width renders any generalised representation of them very misleading.
Table 6.2 is therefore presented simply to give a broad indication of potential hazard zones
widths, and they have not been shown on the strategic flood risk maps for the reasons given
above.
6.7.5 Where two-dimensional hydraulic modelling of the passage of a flood wave has been used,
the results will normally give the variation with time of flood depths and velocities at any
point in the Hazard Zone. These results can then be used in conjunction with matrix given in
Table 13.1 of the Flood Risk Assessment Guidance for New Development (R&D Technical
Report FD2320/TR2) published in 2005 (Ref.11) which gives an assessment of the physical
danger to people for different combinations of flood depth and velocity.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 50
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Assessment of Flood Risk in Study Areas
Faber Maunsell Stevenage Strategic Flood Risk Assessment 53
7.1 Any useful assessment of flood risk within the two study areas identified by the Borough
Council as being potential locations for future large scale development requires an
evaluation of actual flood risk over the whole of those study areas. This will enable the
Council to apply the sequential test required by PPS25, both as regards the variation of
flood risk within a study area and also for the purposes of ranking the two study areas and
others outside the Borough in accordance with their respective overall degree of flood risk.
7.2 As the PPS25 Flood Zones are based on the situation that would obtain in the absence of
any flood defences, it follows that within any PPS25 flood risk zone the actual level of flood
risk will depend on the existence of flood defences and the standard of protection provided
by those defences. Assuming that its envelope has been correctly drawn, within any
PPS25 Zone 3 (High Probability) there may be areas which are protected to a higher
standard than 1% (1in100 years). In these cases their actual flood risk classification would
be Zone 2 (Medium Probability). In practice, this will only apply to Zone 3 since flood
protection to a higher standard than 0.1% (1in1000 years) is almost unheard of.
7.3 In order to be able to apply the sequential test within the PPS25 Flood Zone 3, it will be
useful to consider sub-divisions of actual flood risk within Zone 3. This has been done using
the indicative fluvial flood defence standards laid down in the Department for the
Environment, Food, and Rural Affairs' Flood & Coastal Defence Project Appraisal Guidance
(Economic Appraisal) ("FCDPAG 3") (Ref.12). These are summarised in Table 7.1 below.
Land Use
Band
Return Period (Years)
Annual Probability of Failure
Typical Development
A 50 to 200 0.5% to 2% Intensively developed urban areas.
B 25 to 100 1% to 4% Less intensive urban areas with some high-grade agricultural land
C 5 to 50 2% to 20% Large areas of high-grade agricultural land with some properties at risk.
D 11/4 to 10 10% to 80%
Mixed agricultural land with occasional properties at risk.
E < 21/2 > 40%
Low-grade agricultural land with isolated agricultural properties at risk.
Table 7.1 - Indicative Standards for Fluvial Flood Defence
7.4 Based on the indicative standards shown in Table 7.1 the following flood risk sub-divisions
within PPS25 Flood Zone 3 have been derived. As these sub-divisions refer to actual (as
opposed to unprotected) flood risk they will be referred to as "categories" in this report to
avoid confusion with the PPS25 Flood Zones. For the same reason, roman numerals will be
used for the Category 3 sub-divisions to distinguish them from the lower case letters used in
Table D1 of PPS25 (Zones 3a and 3b) to suggest and define, in support of Table D2 in
PPS25, the appropriate planning response for land in PPS25 Flood Zone 3a.
7 Assessment of Flood Risk in
Study Areas
Faber Maunsell Stevenage Strategic Flood Risk Assessment 54
Sub-Division Range
Type of Development Retn. Period (Yrs) Ann. Prob.
Category 3(i) less than 20 > 5% Mainly agricultural.
Category 3(ii) 20 to 50 2% to 5% Less intensive urban.
Category 3(iii) 50 to 100 1% to 2% More intensive urban
Note: Category 3(i), even with defences, corresponds to the Functional Floodplain (Zone 3b).
Table 7.2 - High Probability Category – Flood Risk Sub-Divisions
7.5 The sub-divisions of Flood Risk Category 3 used in this study are given in Table 7.2 above. Food Risk Categories 1 (Little or No Risk) and 2 (Low to Medium Risk) are for all practical purposes equivalent (as well as being numerically equivalent) to the PPG25 Flood Risk Zones 1 and 2. These sub-divisions should enable an appropriate planning response to be made in individual cases, matching the nature of the proposed development with the relevant flood risk within Category 3.
7.6 The degree of actual flood risk throughout each of the study areas, expressed in terms of
actual Flood Risk Categories, has been assessed from a combination of factors, sources of
information and engineering judgment. Flood risk is assessed as current flood risk - no
allowance can be made for enhanced flood risk within the study area which could arise as a
result of inappropriate future development. If the predicted effects of climate change over
the next hundred years are considered to be sufficient to transfer a study area to a higher
Flood Risk Category this will be stated in the assessment.
7.7 Flood risk sources considered in the assessment include all open watercourses (rivers,
streams, canals, arterial drains and riparian drains) and, where applicable, principal surface
water and combined (foul + surface water) sewers. Possible flooding from foul sewers is not
included in the assessments as this can occur from a variety of causes, often with no direct
or quantifiable relationship to extreme rainfall events.
7.8 The risk of flooding of a development site is not the only consideration. The potential
increased flood risk posed by the urbanisation of a ‘greenfield’ development site to other
areas downstream of the development site also has to be evaluated. This risk can arise not
only from the additional runoff volumes and higher peak runoff rates generated by newly
impermeable areas but also from the reduction in natural floodplain storage capacity if the
development takes place in a floodplain.
7.9 Both of the Stevenage study areas are situated at the upper ends of relatively small
catchments with extensive built-up areas downstream. Development in such areas could
have a significantly adverse impact on flood risk for existing urban developments further
downstream. It is, however, assumed that no development would be permitted that would
so restrict the capacity of any watercourse that flooding would be caused or exacerbated
upstream of the development.
7.10 The two individual flood risk assessments in Section 8 will be presented in a common
format, under the following headings:-
a) General description of the study area
b) Hydrology of the study area (including hydraulic structures etc)
c) Flood risks within the study area
d) Flood risks to downstream areas
Faber Maunsell Stevenage Strategic Flood Risk Assessment 55
7.11 Assessment of flood risk in study areas must also consider the particular case of the
potential hazard to life and limb from fast flowing flood water that could occur in close
proximity to a sudden breach in a flood defence. This could occur within a defended area in
the event of the collapse of a floodwall or embankment. As the depth and/or velocity of an
inrush of flood water increases, there comes a point where an adult is no longer capable of
standing upright in the flow and could be swept away and drowned.
7.12 The area immediately behind the flood defence line within which a serious risk to life and
limb could occur is known as the Hazard (or Rapid Inundation) Zone. This has already been
discussed in some detail in Section 6 and a matrix for determining the potential width of the
Hazard Zone at any location presented in Table 6.2. Although no critical rapid inundation
zones were found in Stevenage or either of the two study areas assessed in Section 8 of
this Report, the general principles outlined above could still apply in any future assessment
of flood risk in areas for potential development in Stevenage and the two study areas.
7.13 In a built-up area, the flow of water into the defended area behind a breach is very unlikely
to conform to a uniform pattern. The actual pattern of flow through a breach, and hence the
width of the hazard zone at that point, will be distorted by the presence of buildings, walls,
street furniture, parked vehicles, etc. Whilst this prevents the meaningful representation of
hazard zone widths on strategic flood risk maps, the likely extent of the hazard zone, where
applicable, should still be determined in any site-specific flood risk assessment.
7.14 The flood risk assessments of the study areas made in Section 8 for strategic planning
purposes do not preclude the necessity for site-specific flood risk assessments of individual
development sites within the wider study areas. The flood risk assessments of the study
areas should nevertheless be used as a general framework within which site-specific flood
risk assessments are undertaken.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 56
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Study Areas in Stevenage
Faber Maunsell Stevenage Strategic Flood Risk Assessment 59
8.0.1 The locations of the two study areas for which more detailed flood risk assessments are
required are shown in Figure 1.2. Both of these large study areas are at present ‘greenfield’
land and are located on the western and northern outskirts of Stevenage respectively. Each
study area is situated partly within Stevenage Borough and partly within the neighbouring
North Hertfordshire District.
8.0.2 The Western study area (approximately 281 hectares) is currently being promoted by the
West Stevenage Consortium, a consortium of developers comprising Messrs Taylor
Woodrow, Persimmon Homes and the Garden Village Partnership (Leach Homes, Redrow
Homes and Taylor Woodrow). The Northern study area (approximately 119 hectares) is
currently being promoted by a consortium of developers, comprising Messrs Croudace
Strategic, Miller Strategic, Bellway Homes and Wheatley Homes.
8.0.3 Neither of the study areas contains any land within a flood risk zone (i.e. Zones 2 and 3) as
shown on the Environment Agency’s Flood Zone Maps. Apart from a single very small
stream in each, there are no open watercourses of any significance in either study area.
Neither stream has a floodplain, and any flood risk associated with these streams is of
purely local concern.
8.0.4 However, the relatively elevated location of each study area on a watershed or in the
headwaters of a river or large stream means that the impact of urban runoff from extensive
development within that study area on flood risk downstream of the study area could be
significant, particularly if that runoff has to be conveyed through existing urban areas.
Assessment of the two study areas will therefore concentrate on the potential effects of
urban runoff from the study areas on flood risk outside the study area, and on the hydrology
and hydrogeology of the receiving watercourses.
8.0.5 Foul sewage from substantial parts of both study areas will inevitably, for topographical
reasons, drain into the Stevenage sewerage system. All foul sewage from Stevenage is
discharged to the Stevenage Trunk Sewer which follows the line of Stevenage Brook (as
shown on the Strategic Flood Risk maps) and then continues for 20km down the valley of
the Rivers Beane and Lea, conveying all foul sewage from Stevenage and other settlements
along the valley to the large regional sewage treatment works at Rye Meads, near
Hoddesdon. The capacities of the Stevenage Trunk Sewer and the Rye Meads treatment
works are both limited (Ref.13) and this should be taken into account in the planning of
development in both study areas, especially as regards the possible surcharging of flows in
the Stevenage Trunk Sewer.
8.0.6 Details of the current development proposals for the Stevenage Western Study Area can be
found on the West Stevenage Consortium’s website (www.weststevenage.co.uk).
Documents on this website include an Environmental Statement (Ref.14) which sets out a
variety of options for the disposal of surface water runoff from the development. The
preferred options were for local or area infiltration to groundwater to minimise the effect of
the development on the hydrogeology of the underlying chalk aquifer, although the feasibility
of these options was not tested. The Development Principles & Design Guide document
(Ref.15) envisages that surface water runoff from the old Norton Green landfill site will be
discharged to the Stevenage surface water sewerage system.
8.0.7 A general appraisal of flood risk, drainage and public utility services within the Stevenage
Northern Study Area is given in a report issued for the development consortium in July 2007
(Ref.16).
8 Study Areas in Stevenage
Faber Maunsell Stevenage Strategic Flood Risk Assessment 60
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Faber Maunsell Stevenage Strategic Flood Risk Assessment 61
8.1 WESTERN STUDY AREA
General Description of the Study Area
8.1.1 Stevenage Western study area is situated partly along the western edge of Stevenage
Borough and partly within the neighbouring District of North Hertfordshire. The 281 hectare
study area is situated entirely to the west of the A1(M) motorway which forms its long,
curving eastern boundary. The study area has an irregular but roughly rectangular shape,
extending for nearly 3km in a southeast to northwest direction and averaging around 800m
in width. A plan showing the principal features of the study area is given in Figure 8.1.
8.1.2 The most prominent physical feature of the study area is the ridge which runs up the centre
of the area from Norton Green to Almshoe Bury, rising gently from 110mOD near Norton
Green to 122mOD in the centre of the site and then falling to 112mOD at Almshoe Bury.
A subsidiary spur of high land runs north from the ridge towards Todds Green. Along the
western side of the ridge the land slopes down to the valley of Langley Brook which falls
from 94mOD at the southern corner of the study area to below 80mOD at Chapelfoot, west
of Almshoe Bury. The north western side of the area includes the upper slopes of the small
Almshoe and Titmore valleys.
8.1.3 Along the east side of the ridge the land falls more gently towards the motorway. The lowest
points along the study area’s eastern boundary are 95mOD at Todds Green about 100mOD
at Bessemer Drive.
8.1.4 The study area extends north from just outside Norton Green to the southern edge of Todds
Green but, apart from a pig and poultry farm and farm bungalow at the southern end of
Todds Green, includes neither of these small settlements. The study area also extends as
far west as, but does not include, the hamlet of Almshoe Bury. Apart from a travellers’ site
and gas pipeline valve compound (see below), a small handful of properties at its northern
extremity and an isolated house near Almshoe Bury, the study area consists entirely of open
country.
8.1.5 Most of the study area is arable land, although there are a number of small meadows along
its eastern edge towards Todds Green. There is a small area of scrubby woodland, Lucas’
Wood, in the northern corner and another small isolated area of woodland, High Broomin
Wood, on its south western boundary.
8.1.6 Towards the southern end of the study area’s eastern boundary, close to Norton Green,
there is what was once a substantial landfill site (the Norton Green landfill site) which was
operated from 1971 to 1989 but all activities there have now ceased. The 13.4 hectare site
has since been capped with soil, but no further restoration has taken place and the site is at
present immature scrubland. On the northern edge of the old landfill site there is a small
travellers’ site provided by the Borough Council.
8.1.7 At present the only public highways giving access to the site from Stevenage are Bessemer
Drive and Clovelly Way. Bessemer Drive is a minor road which runs southwest from the
A1072 to serve the light industrial area between the A1072 and the motorway, but continues
on beneath the motorway. West of the motorway Bessemer Drive branches; one branch
(Chadwell Road) leading south to Norton Green and the other north to the travellers’ site
and gas pipeline valve compound.
8.1.8 There are three other public accesses to the study area under the A1(M) along the area’s
eastern boundary. One of these is a private vehicular access at the western end of
Meadway. The other two are pedestrian subways at Norton Green and Symonds Green.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 62
8.1.9 Clovelly Way, the road from Stevenage to Todds Green, crosses over the motorway at
Fishers Green, just beyond the northern tip of the study area, and forms the northern
boundary of the study area for about 200m, just west of the motorway. At the northwest
corner of the study area the public road through Lower Titmore Green terminates at the
boundary of the area where it becomes a green lane, farm track and bridleway.
8.1.10 There are also three other access routes into the study area from the west. These are a pair
of farm tracks heading east from Almshoe Bury and, in the south, a farm road extending
southeast from the end of Dyes Lane. Dyes Lane is a public road which leaves the B656
road at Rush Green and heads southeast along the line of Langley Brook.
8.1.11 Within the study area there a number of farm tracks and green lanes with public byway or
bridleway status. Kitching Lane runs north from a junction with Dyes Lane on the borough
boundary to meet the west end of Meadway at the motorway. Another green lane /
bridleway runs south from Lower Titmore Green towards the centre of the study area and is
linked to Kitching Lane and Almshoe Bury by a network of bridleways.
8.1.12 The proposed development in the Stevenage Western study area will be phased but is
eventually intended to include about 5000 dwellings grouped into three linked ‘villages’ as
well as three dedicated employment areas, a large mixed-use centre and two other local
centres, and four schools. The majority of the urban development will take place on the
eastern (Thames) side of the Thames / Anglian watershed but there will still be significant
development on the upper slopes along the eastern side of the Langley Valley, in the head
of the Almshoe Valley and on the hillsides above the Titmore Valley.
Hydrology of the Study Area
8.1.13 The hydrology of the Stevenage Western study area is heavily influenced by its geology.
The dominant chalk ridge from Norton Green to Almshoe Bury is almost wholly capped with
glacial deposits; glacial clay in the south but with an extensive cover of sands and gravels
over the highest part of the area. The glacial clay cap appears again at the northern end of
the subsidiary spur towards Todds Green. However, at Almshoe Bury and on the north side
of the small valley northeast of Almshoe Bury (the Almshoe Valley) the underlying chalk is
exposed, as it is along a line on the northeastern side of the Langley Valley, between the
southwest boundary of the study area and Langley Brook. Figure 8.1 (together with Fig.4.2)
shows the salient physical and hydrological features of the study area.
8.1.14 As discussed in Section 4, the relatively impermeable glacial clay across much of the study
area will tend to produce surface runoff in ditches and watercourses. However, when this
runoff reaches the exposed chalk strata further down the hillsides and valleys it will be
intercepted and percolate into the highly permeable chalk, emerging as springflow a
considerable distance downstream. Langley Brook and Ippollitts Brook clearly illustrate this
phenomenon. The permeable sand and gravel capping over the highest point of the ridge
southeast of Almshoe Bury will, however, permit rainfall to percolate through the glacial
cover and into the chalk beneath.
8.1.15 Langley Brook rises in the woodland north of Knebworth Park at the head of Langley Valley.
It flows northwest down the valley for about 3km until it reaches a point 250m upstream of
White Lane, south of Almshoe Bury, where it disappears into a swallowhole into the
underlying chalk aquifer which is here close to the surface. From White Lane to Little
Almshoe, a distance of 1.5km, the valley is a dry valley with no surface watercourse.
Downstream of Little Almshoe the stream (now Ippollitts Brook) reappears, but with an
ephemeral or seasonal flow, until it reaches the southern outskirts of Hitchin where it
receives a substantial perennial flow from chalk springs.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 63
8.1.16 About 250m downstream of Little Almshoe the Little Almshoe Stream enters Ippollitts Brook.
The Little Almshoe Stream drains a catchment stretching from Almshoe Bury to Titmore
Green and Todds Green, including the Almshoe Valley and the north western part of the
study area. This stream is hydrologically very similar to the Langley Brook / Ippollitts Brook
system.
8.1.17 There are very few open watercourses within the study area, apart from a few small ditches
in the vicinity of Lucas’ Wood and west of Symonds Green, and two dewponds near the
highest point in the area. There is a short, isolated length of stream which rises in a wood
near the head of the Almshoe Valley northwest of Almshoe Bury, but this stream disappears
after 500m into a swallowhole on the western boundary of the study area and into the
underlying chalk. The stream does not reappear until it emerges further down the valley at
Little Almshoe Cottage as flow in the Little Almshoe Stream, of which it is a tributary.
8.1.18 The ridge and spur of higher land (para.8.1.2) forms the watershed between the
Environment Agency’s Anglian and Thames Regions. The eastern slopes of the ridge will
drain towards Stevenage Brook, but the motorway along the eastern boundary of the study
area creates a significant obstruction to the natural west to east flowpaths along its route.
However, there are bridges beneath the motorway at the three topographical low spots
along this section of the route, at Symonds Green, Meadway and Bessemer Drive, and
these gaps in the motorway embankment will provide an outlet for the eastward drainage of
water from the study area.
8.1.19 As far as we area aware, there are no Anglian Water sewers within the study area. Although
the farm bungalow at the northern extremity of the area is connected to the head of the
public foul sewer in Stevenage Road, Todd’s Green. Thames Water’s 1/1,250 scale sewer
plans show the head of a 375mm surface water sewer just west of the A1(M) at the west end
of Bessemer Drive. Just south of the study area Anglian Water surface water (225mm) and
foul (150mm) sewers serve the hamlet of Norton Green, discharging into the heads of
Thames Water sewers at the west end of Six Hills Road.
Flood Risk within the Study Area
8.1.20 With one small exception, nowhere in the study area falls within Zones 2 or 3 on the
Environment Agency’s Flood Zone map. The Flood Zone map shows a narrow strip of Flood
Zone 3 land along the Langley Valley as far upstream as Pigeonswick Wood, southwest of
Norton Green. For the most part the Zone 3 land is well down the valley side from the
western boundary of the study area, but 250m southeast of Dyes Farm the southern corner
of the study area meets Langley Stream at the junction of Dyes Lane and Kitching Lane. At
this point a 1,500 sq.m triangle of land just inside the study area comes into Zone 3.
8.1.21 The Flood Zone map shows the Zone 3 land terminating at the swallowhole 250m upstream
of White Lane, although the Agency’s Flood Map shows the Zone 3 continuing on down the
valley to where the Ippollitts Brook appears. On the Flood Zone map the Zone 3 land is
fringed throughout with Zone 2.
8.1.22 The undulating topography of the study area and the absence of any significant hollows or
basins within the study area indicate that no part of the study area is at present subject to
any significant risk of flooding or waterlogging. The whole of the study area (apart from
about 0.15ha in the southern corner of the area) may therefore be considered as falling
within Flood Risk Category 1.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 64
8.1.23 This satisfactory situation should still continue after development, provided that the surface
water drainage systems are designed to an appropriate standard (e.g. a 30-year return
period storm event, with allowance for climate change over the life of the development). The
layout of the development should be designed so that in an even more extreme storm (up to
a 100-year return period, with an allowance for climate change) surplus runoff in excess of
that for which the drainage system has been designed will be retained in temporary storage
in public open spaces and on roadways or paved areas below kerb level so that residential
properties in the study area are unaffected.
8.1.24 The design of the surface water drainage systems on the eastern side of the study area may
present a more complex problem due to the disruption of natural drainage paths created by
the A1(M) motorway and the possible constrictions to flow presented by the existing piped
discharge paths beneath the motorway at Symonds Green, Meadway and Bessemer Drive.
Drainage of the restored landfill site north of Norton Green could also pose problems locally
as the natural subsoil characteristics and drainage patterns of that area have been
destroyed.
Flood Risk to Downstream Areas
8.1.25 It is assumed that the western side of the study area will drain either to Langley Brook or to
the Little Almshoe Stream, both of which are tributaries of the Ippollitts Brook and thence the
River Hiz. The smaller, eastern side of the study area will drain into the right bank
headwaters of Stevenage Brook.
8.1.26 The discharge of substantial volumes of runoff from the extensive impermeable areas
created by the large scale urban development of the Stevenage Western study area into the
Langley Brook / Ippollitts Brook system or to the Stevenage Brook system has the potential
for causing significant increases in flood risk downstream. Both these systems exhibit
specific hydrological characteristics that could exacerbate the potential problems and
therefore require careful consideration. The particular problems associated with each
system will be discussed in the following paragraphs.
8.1.27 Because Langley Brook already receives a significant volume of surface runoff from the
glacial clay which blankets a considerable part of its catchment, it has a well defined stream
channel capable of conveying an appreciable flood flow. The absence of any stream
channel in the dry valley downstream of White Lane suggests that the swallowhole
(para.8.1.15) is capable of accepting almost all flood flows in Langley Brook. Only in the
most extreme flood events will the swallowhole become surcharged and flood flows be
forced to continue on down the valley as overland flow. Information from Stevenage
Borough Council states that although overland flow occurred in the extreme conditions
experienced in June/July 2007 this was the first such occurrence for at least fifteen years.
8.1.28 The unattenuated discharge of large volumes of urban runoff into Langley Brook would
upset the existing natural balance, with the result that the frequency of overland flows down
the dry valley between White Lane and Little Almshoe could increase significantly. This
could have potentially serious consequences for property in Little Almshoe itself.
8.1.29 Although under flood conditions the presence of the swallowhole at the downstream end of
Langley Brook will certainly provide a significant degree of flood risk alleviation downstream
of the swallowhole, the disappearance of large volumes of storm runoff down the
swallowhole and into the chalk aquifer below could also have serious implications for water
quality, as the groundwater in the chalk aquifer in the Hitchin area is heavily utilised for
public water supply abstractions.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 65
8.1.30 Discharge of urban runoff into the Little Almshoe Stream could result in a very similar
situation, though perhaps not as acute, as that in Langley Brook. Even in Ippollits Brook the
existing watercourse, which only appears as an open channel downstream of Little Almshoe,
will have a totally inadequate capacity to accept any significantly increased storm runoff
without overtopping as far downstream as the springs at Gosmore. Although only a handful
of properties might be put at increased risk of flooding, the attractive character of the stream
through St Ippollitts could be compromised.
8.1.31 Downstream of the A602 road, Ippollitts Brook (and the River Purwell into which it flows)
runs alongside and then through the residential suburbs of Hitchin for 3.5km before reaching
the River Hiz. Any substantial increase in flood flows in the stream at Hitchin could therefore
significantly increase the flood risk to a large number of properties in the town. Even below
the confluence with the River Hiz the impact could still be felt.
8.1.32 The eastern side of the study area lies within the Stevenage Brook catchment. Although the
A1(M) motorway, which runs along the eastern boundary of the study area, forms a barrier
to shallow flowpaths, the permeability of the underlying chalk strata and the absence of any
surface watercourses on the eastern side of the watershed (apart from the ditches west of
Symonds Green) indicate that the natural drainage is by infiltration through the relatively
permeable chalky till and percolation to the chalk. The hydrogeological watershed in this
area is believed to lie to east of the topographical watershed and the majority of the
percolation will appear as streamflow in Langley Brook.
8.1.33 Following urban development of the study area, the amount of surface runoff generated on
the eastern side of the watershed, and although it will be possible to attenuate this runoff by
means of Sustainable Urban Drainage Systems (SUDS) a considerable volume of urban
runoff will have to be discharged to Stevenage’s surface water drainage system. This must
be done without increasing flood risk from the sewerage system or from Stevenage Brook
itself. The design criteria for the surface water drainage systems associated with the study
area should conform to the general pattern described in para.8.1.23.
8.1.34 At present the only existing surface water sewers into which the study area’s surface water
drainage could be connected are the 375mm sewer at the west end of Bessemer Drive or
possibly the 225mm sewer at Norton Green. However the natural topography of the study
area produces three distinct sub-catchments. From north to south these sub-catchments
drain to the underpasses beneath the A1(M) at Symonds Green, the western end of
Meadway, and Bessemer Drive respectively. There is a drainage pipe conveying a small but
apparently perennial flow through the Symonds Green pedestrian underpass and thence to
the ponds on Symonds Green, east of the motorway. There is, however, no obvious sign of
a drainage pipe beneath the motorway at Meadway.
8.1.35 It is clear that the restriction presented by the limited surface water drainage paths through
the motorway and the limited capacity of the surface water sewerage system east of the
motorway together make it essential for urban runoff from the eastern side of the proposed
development to be rigorously attenuated before it leaves the study area. As well as the
extensive use of SUDS, it will almost certainly be necessary for small flood storage
reservoirs to be constructed just west of the motorway at the downstream end of each of the
three small sub-catchments described above.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 66
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Faber Maunsell Stevenage Strategic Flood Risk Assessment 67
8.2 NORTHERN STUDY AREA
General Description of the Study Area
8.2.1 The Stevenage Northern study area is situated in open countryside on the north side of
Stevenage, between the village of Graveley and the present edge of the Stevenage urban
area. The study area, which is shown in Figure 8.2, covers an area of about 119 hectares.
8.2.2 The study area has an irregular but compact shape. Its southwestern boundary is the limit
of the existing residential development off Granby Road and Chancellors Road and the old
Great North Road (B197) forms the area’s western boundary. To the east the study area is
bounded by Weston Road, although Park Plantation and existing scattered development
along the west side of the road (Rooks Nest Farm etc) is excluded from the study area. The
area’s north eastern boundary is the southern edge of Chesfield Park, which coincides with
the Borough boundary. The northern part of the study area, almost half of its total area,
extends beyond the Borough boundary to the west of Chesfield Park. The undemarcated
northern boundary of the study area runs eastwards across open fields on the hillside south
of Graveley, roughly on a line with the village cricket field.
8.2.3 The dominant physical feature of the study area is the ridge which runs from northeast to
southwest across the area. North of the study area the ridge is quite pronounced but it
diminishes in elevation as it crosses the area. Chesfield Park lies along the eastern side of
the ridge. Almost the highest point in the study area, 135mOD, occurs where the crest of
the ridge crosses the area’s northern boundary at the southwestern corner of Chesfield Park
but by the time the ridge has reached the area’s southern boundary its crest level has fallen
to about 115mOD. (On the study area’s eastern boundary at Park Plantation the land rises
to about 138mOD.)
8.2.4 East of the ridge the land falls to a small but well defined valley. West of the ridge the study
area’s topography is more broken and undulating, with no distinct valleys. The lowest points
in the study area are 107mOD in the valley bottom on the southern boundary and about
98mOD in two separate depressions on the eastern side of the B197 road.
8.2.5 The entire study area consists at present of arable land, divided into extensive fields by
hedgerows. The zigzag line of the Borough boundary which crosses the study area from
west to east is, however, marked by a belt of young trees. Two small areas of woodland,
Ten Acre Plantation and Park Plantation, are situated adjacent to the area’s north eastern
boundary. St Nicholas’ Church, its graveyard and the municipal cemetery are just outside
the southern corner of the study area at The Bury.
8.2.6 At present, direct vehicular access to the study area is limited to agricultural field entrances
off Weston Road and the old Great North Road, and a field track down the western edge of
Chesfield Park. It appears that provision has been made for a future access road to the
study area across Weston Road from Great Ashby Way opposite Canterbury Park where a
gap has been left through recent housing development. Other than field-edge tracks, there
are no hard surfaced roads anywhere in the study area.
8.2.7 There are at present no buildings of any sort within the study area. However two parallel
lines of high voltage electricity transmission pylons cross the centre of the study area from
west to east.
Hydrology of the Study Area
8.2.8 The study area is bisected by the watershed between the Environment Agency’s Thames
and Anglian regions - the ridge that forms the western edge of Chesfield Park and which
runs southwest from Chesfield Park towards the northern outskirts of Stevenage. The
eastern half of the study area drains southwards into the top end of Stevenage Brook. The
western half of the area drains westwards into the headwaters of Ash Brook.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 68
8.2.9 The geology of the Stevenage Northern study area is predominantly exposed chalk. There
is a very narrow strip of glacial clay along the southern boundary, isolated patches of clay on
the area’s eastern edge, and a small cover of glacial deposits at one point on the crest of the
ridge, but none of these will have any significant effect on the hydrological characteristics of
the study area. The western edge of the study area overlaps the edge of the buried glacial
valley at Graveley and there is a more substantial area of glacial sand in the northwest
corner of the area but this, too, will not affect the area’s general hydrological character.
8.2.10 A small stream, little larger than a ditch, runs south down the valley which lies on the east
side of the ridge. On reaching the southern boundary of the study area the stream is
culverted beneath a farm track, runs alongside the track for a short distance, and then
appears as a grassy depression in the small pasture field between the study area and
Rectory Lane. The depression ends about 70m into the field and there is no indication as to
where any flow in the stream would go from that point. There was no flow in the stream
when the site was inspected in August 2007. The stream appears to rise about 300m
upstream of the southern boundary of the study area. There is no sign of any corresponding
watercourse within Chesfield Park although the parkland occupies the head of the stream
valley.
8.2.11 The Ordnance Survey maps of the area show the line of a “drain” along the hillside parallel
with and west of the stream. There is no sign of any open watercourse on the line of the
drain indicated on the map. It is now merely a hedgerow and any field ditch previously on
this line has been infilled or piped.
8.2.12 There are no open watercourses of any sort on the western side of the ridge, that part of the
study area which drains into the Ash Brook catchment and towards Graveley. The natural
topography of this part of the study area suggests three separate flow paths. The main one
is westwards towards the old Great North Road near the southwest corner of the study area.
Of the two smaller flow paths, one runs west to the North Road southwest of Graveley
cricket ground and the other follows a shallow depression northwestwards towards the east
end of Graveley village.
8.2.13 Thames Water’s 1/1,250 scale sewer plans show a 600mm surface water sewer flowing
south under Rectory Lane, discharging into the Burymead Flood Storage Reservoir from a
1050mm outfall pipe. The 600mm sewer originates as a 450mm sewer in Chancellors
Road, its size suggesting that it serves a larger catchment, but there is no indication on the
sewer plans of any connection between the Chesfield Park stream and the
450/600/1050mm public surface water sewer.
8.2.14 Anglian Water’s 1/1,250 scale sewer plans of the area show no sewers of any sort within
their part of the study area. The residential development in the Granby Road area, south of
the study area, is served by a local surface water sewerage system which drains to the Ash
Brook via Sainsbury’s (Corey’s Mill) FSR. The main foul sewer from Graveley village runs
down the valley of Ash Brook, parallel with the stream, to Corey’s Mill sewage pumping
station from where it is pumped into Thames Water’s foul sewerage system in Stevenage.
Flood Risk within the Study Area
8.2.15 No part of the study area falls within Zones 2 or 3 on the Environment Agency’s Flood Zone
map. East of the A1(M) motorway the map shows a narrow strip of Flood Zone 3 land which
extends up the valley of the Ash Brook and through the centre of Graveley village as far
upstream as a point 200m north of Graveley parish church. At the southern end of the
village the Zone 3 land comes within 100m of the edge of the northwestern tip of the study
area but is at least 5m below it.
8.2.16 Apart from the small stream in the valley on the eastern side of the study area, there are no
watercourses of any significance in the study area, either open or culverted, which could
present a flood risk to the area. The whole of the study area may therefore be regarded as
being within Flood Risk Category 1.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 69
8.2.17 There is no reason why this satisfactory situation should not continue after development of
the study area has taken place, so long as the surface water drainage systems are designed
to an appropriate standard (e.g. a 30-year return period storm event, with allowance for
climate change over the life of the development). The layout of the development should be
designed so that in an even more extreme storm (up to a 100-year return period with an
allowance for climate change) surplus runoff in excess of that for which the drainage system
has been designed will be retained in temporary storage in public open spaces and on
roadways or paved areas below kerb level so that residential properties in the study area are
unaffected.
Flood Risk to Downstream Areas
8.2.18 It is assumed that large scale urban development of the study area will not have a significant
effect on the general drainage pattern of the area and that the north – south ridge down the
centre of the area will continue to form the de-facto watershed between the Anglian and
Thames regions. The eastern side of the study area will drain to the Stevenage Brook and
the western side will drain to the Ash Brook. The implications of extensive urban
development will be slightly different in each case and the impacts on flood risk downstream
will be considered separately in the following paragraphs.
8.2.19 In the eastern half of the study area the local topography is more pronounced, with a well
defined valley running down from Chesfield Park to the northern edge of the present built-up
area. As might be expected from the local geology, this headwater valley is a chalk bourne,
with the small stream only appearing as a surface watercourse well down the valley. The
exposed chalk strata means that infiltration to the aquifer is significant, originally reappearing
as springflow a considerable distance downstream but now almost certainly contributing to
public water supply groundwater abstractions in the area.
8.2.20 The urbanisation of this valley will, unless appropriate measures are taken, have two closely
linked results. Without such measures the widespread replacement of naturally permeable
soil surfaces with impermeable surfaces (roads, roofs, paved areas etc) will reduce the
volume of infiltration to the chalk aquifer whilst at the same time a corresponding volume of
runoff will enter the surface water drainage system. Since there appears to be no
continuous surface watercourse downstream of the study area (see para.8.2.10) it must be
assumed that the area’s future surface water drainage system will discharge directly or
indirectly into the head of the Stevenage sewerage system in the Rectory Lane area.
8.2.21 The Martins Way / North Road area of Stevenage is served by a 750mm surface water trunk
sewer that follows the line of the tributary valley from Chesfield Park along Old Stevenage
High Street and eventually joins the culverted section of Stevenage Brook at the Six Hills
Way / St George’s Way roundabout. This trunk sewer was probably created by the
culverting of the downstream reaches of the Chesfield Park stream. Flood flows in this
sewer are attenuated by the small off-line flood storage reservoir at Burymead (Burymead
FSR). This is one of the original Stevenage “water meadows” and although it has not been
possible to discover its design parameters, it must be assumed that it was never designed
with sufficient capacity to attenuate urban storm runoff from the Stevenage Northern study
area.
8.2.22 Urban storm runoff from the eastern half of the study area will have to pass through the
heavily culverted headwaters of Stevenage Brook, through the urban area upstream of
Stevenage town centre and the Elder Way FSR. Downstream of the town centre, although
the Brook is largely in open channel, it flows for a further 3km within the built-up area before
reaching Wychdell FSR, including an area of the town (Roebuck Gate) where there has in
the past been a flooding problem. It is abundantly clear that any additional storm runoff from
urban development in the Northern study area could give rise to an appreciable increase in
flood risk along the Stevenage Brook valley unless the hydraulic capacity of Stevenage
Brook through the town is significantly increased. This is likely to be prohibitively expensive.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 70
8.2.23 There are two other ways by which this unacceptable increase in downstream flood risk
could be minimised, although the design criteria for the surface water drainage systems
associated with the development of the study area should of course conform to the general
pattern described in para.8.2.17.
8.2.24 The first of these options is by the incorporation of Sustainable Urban Drainage Systems
(SUDS) into the design of the development’s surface water drainage system on a large
scale, ensuring that as much runoff as possible is held in swales and thus allowed to
percolate into the underlying chalk. The use of the chalk aquifer for public water supply
purposes will, however, necessitate the use of oil interceptors and/or the separation of roof
etc drainage from the runoff from road surfaces and vehicle parking areas. Although the
permeable local geology will make the use of SUDS highly effective in the study area, even
the widespread use of SUDS may not obviate the need for the provision of a substantial
volume of additional runoff storage in the form of an additional flood storage reservoir or
reservoirs.
8.2.25 This is the second option, and there are two ways by which it might be achieved. The more
obvious is the construction of a flood storage reservoir at the point where the Chesfield Park
stream crosses the southern boundary of the study area. It may, however, be possible to
enlarge the capacity of Burymead FSR (it is situated in a large public open space) although
the capability of the trunk sewer upstream of Burymead to accept increased flows would
have a major bearing on the feasibility of this option. In practice it may prove necessary to
minimise additional runoff to the Stevenage Brook by a combination of both options outlined
in paras.8.2.24 and 8.2.25 above.
Planning Policy and Flood Risk
Faber Maunsell Stevenage Strategic Flood Risk Assessment 73
9.1 Regional and Sub-Regional Strategies
9.1.1 The East of England Region, one of the largest of the English regions, has a population of
5.5 million and stretches from the northern fringe of London to the North Norfolk coast.
It includes the counties of Essex, Suffolk, Norfolk, Cambridgeshire, Bedfordshire and
Hertfordshire. The East of England Regional Assembly (EERA) has the overall
responsibility for spatial planning at the regional level.
9.1.2 EERA has produced the East of England Plan. When this study began the latest draft
revision to the Regional Spatial Strategy (RSS) had been issued in December 2004 (Ref.2a)
but the definitive East of England Plan incorporating the Revised RSS was not published
until May 2008 (Ref.2b). The Revised RSS includes a section on the future provision of
housing in the Region to meet predicted population changes and growth forecasts. Core
spatial strategy Policy H1 indicates that 25,400 additional dwellings will be required annually
in the Region between 2001 and 2021, a total of 508,000 dwellings.
9.1.3 Housing development forms the principal component of urban development, and regional
forecasts of housing allocations between sub-regions and local planning authorities gives a
broad indication of the areas where flood risk may be a constraint to growth and also where
large scale urban development could have a significantly adverse impact on flood risk
elsewhere. It is therefore necessary to consider proposals for future urban growth on a
regional scale, not only within local authority areas but within neighbouring areas.
9.1.4 Within the region (including those parts of Bedfordshire within the Milton Keynes South
Midlands sub-region) these additional dwellings will be distributed as follows :-
Sub-Region Totals (2001 – 2021)
Draft RSS (2004) RSS (2008)
Essex (incl.Thurrock & Southend) 123,400 127,000
Suffolk 58,600 61,700
Norfolk 72,600 78,700
Cambridgeshire (incl.P’boro) 89,300 98,300
Bedfordshire (incl.Luton) 54,500 59,100
Hertfordshire 79,600 83,200
Totals 478,000 508,000
Table 9.1 - Future Housing Development in the East of England
9.1.5 Within Hertfordshire the allocation of housing growth between the various Local Planning
Authorities (LPAs) within the county, including Stevenage, is shown in Table 9.2 on the next
page. The relatively small allocations to many LPAs is notable, and the largest allocations
are to two of Stevenage’s neighbours, East Hertfordshire and North Hertfordshire.
9 Planning Policy and Flood Risk
Faber Maunsell Stevenage Strategic Flood Risk Assessment 74
Local Authority Totals (2001 – 2021)
Draft RSS (2004) RSS (2008)
Broxbourne 5,100 5,600
Dacorum 6,300 12,000
East Hertfordshire 20,800 12,000
Hertsmere 4,200 5,000
North Hertfordshire 15,800 6,200
St Albans 7,000 7,200
Stevenage 6,400 16,000
Three Rivers 3,600 4,000
Watford 4,600 5,200
Welwyn Hatfield 5,800 10,000
Totals 79,600 83,200
Table 9.2 - Future Housing Development in Hertfordshire
9.1.6 The large increase from Stevenage’s relatively modest allocation of new housing in 2004 for
the period from 2001 to 2021 to that in the Revised RSS published in 2008 can be explained
by the fact that the 2008 figure now includes provision for 9,600 new houses outside the
Borough boundary in North Hertfordshire. This is because parts of both the Stevenage
West and Stevenage North Study Areas fall within North Hertfordshire. It will be noted that
there is a corresponding reduction in the North Hertfordshire housing allocation between
2004 and 2008. Stevenage now has the largest allocation of new housing of any Local
Authority in Hertfordshire.
9.1.7 In the Revised RSS Stevenage is identified as a Key Centre for Development and Change
(Policy SV1). One of the main elements of the strategies in Policy SV1 is “the overall growth
of 16,000 dwellings within and on the edge of the built-up area by 2021”. It is also
envisaged that “sustainable urban extensions will also be required to the west and north
including at least 5,000 dwellings west of Stevenage.”
9.1.8 Because of its location in the headwaters of the River Beane catchment, Stevenage is not
vulnerable to the potential increase in flood risk resulting from large scale urban
development outside its boundaries (with the partial exception of development in the
Stevenage North Study Area but this can be regarded as development over which the
Borough Council has a direct influence). On the other hand, large scale development in
Stevenage and its two study areas could have a significant impact on flood risk in North
Hertfordshire (from the River Hiz) and East Hertfordshire (River Beane).
9.1.9 In 2007, within the context of their Regional Spatial Strategy, EERA published the East of
England Housing Investment Plan for the period from 2008 to 2011 (Ref.17). This plan
identifies specific strategic (i.e. major development) sites within the Region where short to
medium term development is expected to take place. One of the 25 strategic sites across
the East of England identified in this document is Stevenage West.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 75
9.1.10 In the EERA Housing Investment Plan the Stevenage West site will initially have 3,600
dwellings, rising to a total of 5,000 by 2015. The ultimate capacity of the site is given as
10,000 dwellings, but no timescale is stated. Construction is due to begin in 2008/09 with
completion forecast for 2015/16. The document also states that outline consent for this site
has been achieved. The Stevenage North site is not included in the list of strategic sites
and, despite the large housing allocation for East Hertfordshire in the RSS, there are no
strategic sites in that District.
9.1.11 Section 10 of the Revised East of England RSS also sets out four policies relating
specifically to water and the water cycle. These four policies deal respectively with Water
Efficiency (WAT1), Water Infrastructure (WAT2), Integrated Water Management (WAT3) and
Flood Risk Management (WAT4). Although there is a passing reference to “the effects of
floods and droughts” in one of the explanatory paragraphs to policy WAT3, Strategic Flood
Risk Assessments and their relationship to Local Development Documents are dealt with in
some detail in policy WAT4. The purpose of SFRAs is also set out in paragraph 10.14 of the
Revised RSS.
9.2 Sustainable Urban Drainage Systems
9.2.1 Policy WAT4 (Flood Risk Management) in the Revised East of England RSS states that
Local Development Documents should require that SUDS are incorporated in all appropriate
developments. SUDS are also considered in the explanatory paragraphs to WAT4
(paras.10.14 and 10.17) in the Revised RSS.
9.2.2 It is therefore probable that Local Planning Authorities throughout the Region will, as a
matter of course, ensure that these major urban developments incorporate Sustainable
Urban Drainage Systems (SUDS) (Ref.18a) and that appropriate physical features, such as
adequate runoff retention storage and flow retarders, are included in the design of these
systems. These features are necessary in order to limit the surface water runoff from the
newly impermeable areas created by these urban developments to the rates and volumes of
runoff which would have been generated by those ‘greenfield’ areas prior to urbanisation,
taking into account predicted (PPS25) increases in runoff over the lifetime of the
development as a result of climate change. The Environment Agency (Thames Region) has
also issued guidance for developers on the selection and design of SUDS (Ref 18b)
9.2.3 The Brief requires the Study to consider responsibilities for the long term maintenance of
Sustainable Urban Drainage Systems (SUDS). SUDS are frequently installed where there is
a need to attenuate the additional surface water runoff generated by the impermeable
surfaces of a newly developed urban area on what was previously ‘greenfield’ land. These
impermeable surfaces include roofs, paved areas and roads or any surface through which
the natural infiltration into the subsoil has been artificially impeded. SUDS may include
retention ponds, lagoons, buried tanks or oversize sewer pipework, swales and hydraulic or
mechanical flow retarders.
9.2.4 In Stevenage the generally permeable nature of the soil, subsoil and underlying strata
makes the disposal of runoff to groundwater by means of SUDS incorporating soil infiltration
processes a desirable and potentially feasible option. There should therefore be an initial
presumption within Stevenage in favour of using these types of SUDS in preference to those
that merely attenuate peak discharges to sewers or watercourses by the retention of runoff
in temporary storage facilities. Since chalk is the dominant stratum in the Stevenage area,
developers should be made aware of the presence of a number of groundwater source
protection zones in the area and it is essential that the chemical and bacteriological quality
of the runoff disposed of by infiltration is fully taken into account.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 76
9.2.5 Although SUDS are now routinely included in the design of new urban drainage systems in
Britain they have not yet been in use for long enough or widely enough to ascertain how
effective they are at the catchment scale in controlling runoff from urban areas. Even
though SUDS are likely to become a mandatory feature of all urban development, they are
designed to attenuate runoff from storm events of a specific return period, or less. The
attenuation of runoff from storms greater than that for which the drainage system was
designed will only be partial. It is therefore inevitable that, even with the universal use of
SUDS, future urban development will in extreme events result in greater runoff volumes and
higher flood peaks than was hitherto the case. This caveat should not, however, discourage
the use of SUDS as their widespread introduction will have a major beneficial effect in
reducing the impact of urban runoff in all but the most extreme events.
9.2.6 The diminished effectiveness of SUDS in extreme events, combined with the potential
increase in storm runoff and flood flows resulting from climate change, could have an
adverse effect on flood risk in river catchments subject to large scale urbanisation,
particularly where that urbanisation takes place in the headwaters of the catchment. This
situation is likely to arise as a result of major urban development in the upper reaches of the
Rivers Beane and Hiz, as a consequence of which there is likely to be an increase of flood
risk downstream in extreme events.
9.2.7 Until recently, SUDS have often been provided by the developer in compliance with a
planning consent condition imposed by the local planning authority (usually in consultation
with the Environment Agency and/or the appropriate Water Company) where they are
considered necessary to attenuate the additional runoff from a development before it is
discharged to the receiving watercourse or sewer. This situation should change with the
growing public understanding and acceptance of SUDS and it is anticipated that
consideration of the incorporation of SUDS into the surface water drainage systems of all
urban development proposals will automatically be included in their flood risk assessments.
9.2.8 In many cases, especially that of an estate-scale development, the developer intends to sell
the development to multiple prospective purchasers of the houses, industrial units etc and
has no wish to have any interest or involvement in the development thereafter. However, for
SUDS to provide consistent and effective long-term attenuation of runoff from the
development they have to be maintained in an efficient condition for the life of the
development. This may involve the control of weed growth in ponds and lagoons, the
frequent removal of debris, both natural and man-made, from watercourses and
weedscreens, the clearance of blockages, sometimes at short notice, from pipes and
culverts, and the repair of malicious damage and vandalism. A routine inspection regime is,
of course, essential to ensure that any such problems are identified and dealt with in a timely
manner.
9.2.9 This raises the question of the responsibility for the maintenance of SUDS, which may be of
particular relevance where the development (e.g. a housing estate) ultimately becomes the
property of numerous private individuals. Even where the outflow from a lagoon or retention
pond discharges to a public sewer, it is usually found that the water company owning that
sewer will be unwilling to accept responsibility for the lagoon. Similarly, even where the
receiving watercourse is a Main River, the Environment Agency is unlikely to accept
responsibility for any SUDS discharging to that watercourse.
9.2.10 It is therefore important that the future responsibility for the maintenance of any SUDS
constructed in connection with a major development should, if at all possible, be agreed and
resolved before planning consent is granted. The type and configuration of SUDS adopted
in any site-specific situation should be determined by the need to obtain the optimum
solution in each individual case and justified in the site-specific flood risk assessment.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 77
9.2.11 Many of the issues raised above have for some time been under discussion at national level
by the parties most immediately involved. The widespread flooding events of Summer 2007,
which have resulted in the publication of the Pitt Review (Ref.10), have given a greater
urgency to the need to resolve these issues and for a policy decision at national level on the
overall responsibility for surface water drainage at all levels between the various statutory
bodies with their currently diverse responsibilities.
9.2.12 Further detailed information on SUDS can be found in Annexe F of PPS25, the Interim Code
of Practice for Sustainable Drainage Systems (Ref.19), the Construction Industry Research
& Information Association (CIRIA)’s Sustainable Urban Drainage Systems Manual (Ref.18a)
and their website www.ciria.org.uk
9.3 Recommended Policy and Guidance Statements
Recommended Flood Risk Planning Policy
9.3.1 In recognition of the need to incorporate robust flood risk management policies in Stevenage
Borough Council's Local Development Documents, these guidance notes have been
expanded to become the basis of recommended policy and guidance statements for
prospective developers. In preparing these Recommended Policy and Guidance
Statements care was taken to ensure that they are in full accordance with the Environment
Agency's draft flood risk policies issued in October 2003. The full text of the proposed
Recommended Flood Risk Planning Policy and Guidance Statement will be found in
Appendix B at the end of this Report.
9.3.2 The policy recommendations made in this Report and those implicit in the guidance for
developers are intended to assist the Council in applying the Sequential Test in PPS25 at
the strategic level and, since it is often possible for the test to be applied at a site-specific
level in large developments, to enable the Council to ensure that developers have applied
the Sequential Test to their proposals. The recommendations made in this Report should
also assist the Council in deciding whether or not the Exception Test can be applied in
individual cases.
9.3.3 Although the Sequential and Exception Tests are described in PPS25, the Council should
also take into consideration the more detailed guidance on the application of these tests
contained in the Practice Guide Companion to PPS25.
9.3.4 To assist LPAs, the Environment Agency has produced standing advice on their
requirements regarding the consultation process on flood risk issues to be followed when
dealing with planning applications. Central to this standing advice is a Flood Risk Matrix to
enable Local Planning Authorities to carry out an initial evaluation of development
applications and, according to the location, size and scale of the proposed development,
decide on the procedure to be adopted by the planning authority in determining that
application. The procedures suggested in the matrix focus on the degree of consultation
between the Planning Authority and the Environment Agency appropriate to the application.
9.3.5 The current version of the Environment Agency's Flood Risk Matrix, together with the
accompanying explanatory notes, will be found on the Agency’s website at
www.environment-agency.gov.uk/aboutus/512398/908812/908815/ . On the introductory
page ‘clicking on’ to >> Flood Risk Standing Advice produces a series of documents.
These are titled respectively -
Welcome An introduction to the Agency’s Flood Risk Standing Advice for England.
User Flowchart A flowchart with parallel routes for LPAs and developers, guiding
each to those parts of the Standing Advice that will be most relevant to them.
Consultation Matrix The fundamental Flood Risk Matrix referred to above, which
sets out when and at which stage LPAs should consult the Environment Agency on
proposed developments.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 78
Sequential Test Results Table Presents a framework to guide LPAs through the
evidence that the Agency will require to demonstrate that the Sequential Test has been
appropriately carried out.
Applicant & Agent Advice Advice and guidance for developers and their agents on
flood risk assessment procedures and useful links to flood risk information and the Agency’s
consent procedures.
Householder & Other Minor Extensions Similar to the above but with specific guidance
to cater for minor developments such as domestic extensions etc.
General Surface Water Drainage Information Advice and guidance on sustainable
surface water management and dealing with storm runoff from developments to avoid
exacerbating flood risk elsewhere.
Planning Policy Statement 25 A summary and explanation of the purpose and
requirements of PPS25.
Flood Risk Assessment Note 1 Guidance on the control of storm runoff from
developments greater than one hectare in extent in Flood Zone 1.
Flood Risk Assessment Note 2 Guidance on minor extensions but where the
cumulative effect of development needs to be addressed.
Flood Risk Assessment Note 3 Guidance on development (other then minor
extensions) in Flood Zones 2 and 3.
9.3.6 In considering applications for the development of ‘windfall’ sites, the same fundamental
criteria for the consideration of flood risk to the site should be applied as they would for sites
in areas considered for larger scale development. These criteria should, however, be
tempered to take into account the site’s previous development and flooding history and the
need to avoid ‘planning blight’ in an area of established and sustainable development.
9.3.7 Implicit in these policies is the need to reduce surface water runoff from development,
combined with improving public awareness of flood risk and the need to foster multi-agency
working and collaboration.
Guidance for Developers
9.3.8 Included in the Brief for this study was the requirement to prepare Guidance Notes for
Developers. These notes would provide prospective developers of land with guidance on
the Borough Council's policies on flood risk and the development of land deemed to be at
risk of flooding. These notes would also give guidance to developers on the design of
surface water drainage systems to minimise the generation of storm runoff from paved and
impermeable areas in accordance with best practice as exemplified by Sustainable Urban
Drainage Systems (SUDS).
9.3.9 Developers should be encouraged to consult the Environment Agency when considering any
proposed development to obtain all relevant information relating to flood risk at the
development site and to seek the Agency’s guidance on the requirements for a site-specific
Flood Risk Assessment (FRA), if applicable, and the Agency’s opinion as to the acceptable
level of flood risk to the proposed development. General guidance for developers can be
found on the Agency’s website at www.environment-agency.gov.uk/developers . A link to
more detailed advice for large and complex developments can be found in para.9.3.5 above.
9.3.10 The Environment Agency is an essential source of information for preparing site-specific
flood risk assessments. For developments in Stevenage the Agency’s External Relations
Team at their Hatfield office should be contacted as early as possible to obtain information
relating, for example, to historic flooding records, the availability of hydraulic modelling
results, and detailed topographical survey (LiDAR) data. Although the information contained
within the SFRA is the best available at the time of its publication, the developer should be
aware that more up-to-date information may subsequently become available from the
Agency.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 79
9.3.11 Developers should be aware that other sources of flood risk, in addition to the strategic flood
risk sources identified in this Report, may be encountered. These other flood risk sources
include flooding from surface water; combined flow or foul sewers; groundwater or springs;
overland flow from paved areas; inadequate, obstructed or collapsed culverts; or flooding
that can occur when natural soil or subsoil drainage paths have been impeded or obstructed
by human activities, notably at the base of a hillslope. These local flood risk sources should
all be considered in a site-specific flood risk assesment.
9.3.12 Developers should be made aware of the possibility of large underground structures,
including deep foundations, impeding or interfering with the flow of groundwater in
superficial geological deposits (e.g. gravel beds) raising the water table upstream of the
obstruction and resulting in waterlogging or even the emergence of groundwater above
ground level. Geophysical investigations may be necessary in some cases to ensure that
such problems do not arise.
9.3.13 Although a proposed development may itself be safe from flooding, the site-specific flood
risk assessment should also demonstrate that there is safe access to and egress from the
development, especially residential areas, in flood events of a magnitude likely to occur
during the life of that development. In this context the definition of what is considered ‘safe’
for different combinations of flood flow depths and velocities will be found in the Safe Access
and Exit Lookup Table (Table 13.1) in Research Report 2320 (Ref.11).
9.3.14 A flowchart to assist developers in deciding whether a site-specific flood risk assessment is
required and at what stages they should seek advice and information from the Borough
Council and the Environment Agency is given overleaf. The full text of the proposed
Guidance Statement for Developers (see para.9.3.8 above) will be found incorporated in
Appendix B at the end of this Report.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 80
YES NO NO YES YES NO
YES NO NO YES
Guidance for Developers on FRAs for Site-Specific Planning Applications
Is the Development Site allocated for the proposed Land Use in the Local Development
Document (LDD)?
Submit Planning Application
without an FRA
Identify the Vulnerability Category of the Proposed Development
(PPS25 Table D2)
Contact Stevenage BC to find out if a Site Specific FRA
is required.
Is the Development likely (with modification if necessary)
to pass the Sequential Test and/or Exception Test ?
Consult Environment Agency & Sewerage Undertaker
for advice and information. Are there any Flood Risk Related
Constraints that make the Development undesirable ?
Consider an Alternative
Development or Site.
Consult Stevenage BC to agree the scope of s/s FRA.
Undertake the FRA
Do the FRA’s conclusions and recommendations enable Stevenage BC to meet the requirements of the
Sequential Test ?
Submit the s/s FRA with the
Planning Application
Review and revise the s/s FRA after consultation
with SBC and EA.
Conclusions
Faber Maunsell Stevenage Strategic Flood Risk Assessment 83
10.1 Local Planning Authorities (LPAs) require estimates of actual flood risk throughout their
district for spatial planning and development control purposes. However, the Environment
Agency only provides Flood Maps and Flood Zone Maps which show inherent flood risk,
ignoring the reduction in flood risk due to the presence of existing flood defences and the
effect of any artificial obstructions on the floodplain such as road or railway embankments
etc. LPAs therefore need a document detailing the ‘actual’ flood risk within their District and
hence the need for a Strategic Flood Risk Assessment.
10.2 Where the standard of protection provided by flood defences is less than the 1in100 year
fluvial flood event this does not necessarily invalidate the flood risk classification shown on
the Flood Zone maps, although it gives no indication of relative flood risk within Zone 3.
However, in urban areas that were subject to a significant fluvial flood risk but are now
defended to a standard of protection in excess of the 100-year event, the Agency's Flood
Zone maps are of limited practical help to LPAs in determining the actual or even the relative
degree of flood risk at different locations within defended areas.
10.3 For public reference, the Environment Agency made available its Flood Zone maps on the
internet as Flood Maps in Autumn 2003. These Flood Maps are based on the Agency’s
Flood Zone maps but at the much smaller base-mapping scale of 1/50,000. However, as
well as PPS25 Flood Zones 2 and 3 (and, by inference, Zone 1) the Agency’s Flood Maps
also enable the user to “click on” to any point on the map to obtain an estimate of actual
flood risk at that point. These estimates take some recent flood defences into account and
use three categories of risk - “significant” (more than once in 75 years), “moderate”
(between once in 75 and once in 200 years) and “low” (less than once in 200 years). These
categories do not correspond to those defined in PPS25.
10.4 The studies and investigations undertaken in connection with this Report have identified the
various potential sources of flood risk that could cause flooding in the Borough of
Stevenage. The principal sources of flood risk for almost all of Stevenage are the
Environment Agency’s Main Rivers (the Stevenage and Aston End Brooks) and Thames
Water’s principal surface water sewers. Taking natural and man-made topographical
features into consideration, the areas of influence of the principal flood risk sources have
been identified and defined. The three Flood Risk Categories adopted for this study are, for
consistency, numerically equivalent to the three Flood Risk Zones defined in PPS25.
10.5 Utilising other available information on the existence, nature and condition of existing fluvial
flood defences, each of these areas of influence were sub-divided into areas considered to
fall within the three categories of actual flood risk (i.e. with existing flood defences taken into
account) as defined in this Report. This has enabled a set of three 1/10,000 scale maps
covering the whole of Stevenage (Figure 6.1) to be prepared, showing an estimate of actual
flood risk at any point within the Borough. These maps should enable the Borough Council
to determine both the actual and relative degree of flood risk to which different areas of the
Borough may be subject, thereby enabling informed planning decisions, both strategic and
site-specific, to be made and justified with confidence.
10.6 It had at the start of the study been hoped that the Main Rivers in the Borough (Stevenage
and Aston End Brooks) would previously have been subject to hydraulic modelling and that
sequences of flood levels along those watercourses for events of specific return periods
would be available. It was similarly hoped that the surface water sewerage networks in the
Borough would also have been hydraulically modelled to determine their conveyance
capacities.
10 Conclusions
Faber Maunsell Stevenage Strategic Flood Risk Assessment 84
10.7 Unfortunately no hydraulic modelling results of any sort were available until the results of the
Agency’s River Beane hydraulic model (which included the Stevenage and Aston End
Brooks) became available in September 2008, over a year after the start of this study. This,
and particularly the absence of any surface water sewer modelling results, proved to be a
major handicap in the overall flood risk assessment.
10.8 The absence of any sewer modelling results in Stevenage made it impossible to produce
strategic flood risk maps of the Borough showing the extent of the Flood Risk Category 2
and 3 envelopes beyond the upstream limits of the River Beane model on the open
watercourse sections of the Stevenage and Aston End Brooks. In those parts of the
Borough drained by culverted watercourses or surface water sewers it was only possible to
give a geographical indication of locations where there was considered to be a risk of
flooding, albeit an unquantified one.
10.9 Since the River Beane modelling results did not include estimates of the 1000-year ‘with
climate change’ scenario and since no sewer modelling results whatsoever were available, it
would not have been possible to produce meaningful Strategic Flood Risk maps of the
Borough for the ‘with climate change’ situation in year 2115.
10.10 In addition to the strategic flood risk maps described above, this Report also contains
detailed Flood Risk Assessments of two large study areas identified by the Borough Council
as areas of potential major urban development on the western and northern edges of the
Borough respectively. For each study area the assessment includes a description of the
extent of those parts of the study area within each of the three flood risk categories defined
in this Report. Each assessment is accompanied by a 1/10,000 scale plan of the study area
showing the main flood risk sources, and the salient topographical and drainage features
likely to influence flood risk within that area.
10.11 Detailed consideration of the two study areas has revealed that although future large scale
urban development in each of the study areas would not significantly increase the flood risk
in the study area itself, it could have significant implications for increased flood risk to land
and property downstream of the study area unless substantial runoff attenuation measures
are adopted. This is of particular concern in the Stevenage North study area, where one of
the small streams draining the area is culverted beneath the northern suburbs of Stevenage
immediately downstream of the study area.
10.12 Although there are virtually no significant areas within Stevenage that are protected by
raised flood defences, a considerable degree of flood alleviation is provided by the network
of flood storage reservoirs, both on-line and off-line, that exist throughout the Borough. Most
of these flood storage reservoirs (known locally as “water meadows”) were constructed
some forty years ago by the then New Town Corporation and unfortunately no records of
their design parameters could be obtained. Three of the largest of these reservoirs are now
maintained and operated by the Environment Agency.
10.13 Even though there are no known areas in Stevenage where rivers or streams in flood are
contained within raised embankments, almost all of the dozen or so flood storage reservoirs
and amenity lakes in the Borough are impounded behind earth embankment dams, the
failure of which could release a significant volume of water and thus create a hazard to life
and limb in the immediate vicinity. It should, however, be emphasised that only very small
areas of land in Stevenage are subject to this risk and the problem, where it exists at all, is
extremely localised. The existence of this hazard should nevertheless be borne in mind in
assessing flood risk at specific sites immediately downstream of these embankments.
10.14 Groundwater flooding is, in general, not considered to pose a significant problem in
Stevenage but the potential effect of developments, especially those with deep foundations,
on shallow groundwater flows in fluvial sands and gravels in valley bottoms could impact on
flood risk locally if not taken into account in the site-specific flood risk assessment.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 85
10.15 This Report recognises that flood risk can be reduced by appropriate spatial planning at
strategic level and makes suggestions for flood risk alleviation policies to be adopted by the
Borough Council in dealing with applications for planning consent from prospective
developers. In this context, the Report also puts forward policy and guidance statements for
developers to ensure that their proposals incorporate measures to reduce not only any flood
risk to their own development but also, by attenuating storm runoff from impermeable areas
created by their development, the flood risk to property downstream of the development.
10.16 This Report identifies the importance of the continuing long term inspection and
maintenance of SUDS installed in connection with urban development for runoff attenuation
purposes. Since it is inevitable that in many cases the de-facto responsibility for this will
devolve upon the Borough Council, adequate provision for the funding of this responsibility
should be considered and secured at the planning stage.
10.17 It is hoped that this Report will form a sound and reliable basis for Stevenage Borough
Council to make informed and confident decisions on planning issues, both at the strategic
and site-specific levels, thereby reducing the time taken to reach decisions and the
resources employed in reaching those decisions.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 86
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Recommendations
Faber Maunsell Stevenage Strategic Flood Risk Assessment 89
11.1 In accordance with the findings of this study and the conclusions reached in Section 10 of
this Report the following recommendations are made:-
• Stevenage Borough Council utilise the set of three Strategic Flood Risk Maps
prepared for the Borough (Figure 6.1 in this Report) in devising strategies and policies
for incorporation in their forthcoming Local Development Documents.
• The flood risk assessments made for the two study areas identified in this Report -
Stevenage West and Stevenage North - are used to apply the sequential test
advocated in Planning Policy Statement 25 (PPS25) in deciding the most appropriate
areas for development and the sequence in which those areas should be developed
within those study areas.
• The Borough Council should consider updating this SFRA before 2012, by which time
it is hoped that the results of a comprehensive programme of hydraulic modelling of
Stevenage’s surface water sewerage system will be available. The Borough Council
should therefore attempt to persuade Thames Water to undertake the comprehensive
hydraulic modelling of Stevenage’s surface water sewerage system at an early date,
as recommended above.
• The Borough Council should also encourage the Environment Agency to extend the
existing River Beane hydraulic model to the head of Main River on Stevenage Brook
and to re-run the extended model to include (inter alia) the ‘1000-year with climate
change’ scenario to enable Strategic Flood Risk maps for the ‘with climate change’
situation to be produced.
• The Borough Council should be fully aware of the potential impact on flood risk in
North and East Hertfordshire resulting from the large scale urban development in the
Stevenage West and North study areas and ensure that the development proposals
take this potential impact fully into account and include all necessary physical
measures to alleviate this risk.
• As a consequence of the above recommendation, the Borough Council in
collaboration with the Environment Agency, give initial consideration to the design
standard of protection provided by the flood alleviation measures to maintain or
reduce the current degree of flood risk to land and property downstream of the two
study areas.
• In those areas of the Borough where there appear to be significant surface water
flooding issues, the Council, in conjunction with the Environment Agency and Thames
Water, should consider the implementation of a Surface Water Management Plan for
that area.
• Because of the importance of ensuring that any SUDS installed in connection with a
large scale urban development are properly inspected and maintained over the life of
the development, the Borough Council may, de-facto, find itself faced with this
responsibility. This possibility should be anticipated in each case at the planning
stage and, if applicable in that case, provision made at that stage for the funding of the
future inspection and maintenance of the installation by the Council.
• The Borough Council should adopt the flood risk alleviation policies suggested in this
Report for dealing with applications for planning consent from prospective developers,
together with the proposed policy and guidance statements for developers to assist
developers in putting forward proposals in line with the Council's flood risk alleviation
policies.
11 Recommendations
Faber Maunsell Stevenage Strategic Flood Risk Assessment 90
• In implementing these policies, the Council should take every opportunity to enhance
and restore river corridors, and to safeguard Functional Floodplains and land required
for possible future flood alleviation schemes.
• The Council should, in collaboration with the Environment Agency, ensure that their
emergency planning procedures include measures to increase flood risk awareness
by residents and landowners in the Borough.
• The Council should, where they are applicable to Stevenage Borough, pay due regard
to the conclusions and recommendations contained in the Environment Agency’s
recently published Thames Region Catchment Flood Management Plan.
11.2 The recommendations made and policies advocated in this Report should be used to inform
the Borough Council’s preparation of its various Local Development Framework documents
and reduce the risk of flooding within the Borough through the spatial planning process.
References
Faber Maunsell Stevenage Strategic Flood Risk Assessment 93
1a. Planning Policy Statement 25 - Development and Flood Risk (PPS25)
Department for Communities and Local Government December 2006
1b. Planning Policy Guidance Note 25 - Development and Flood Risk (PPG25)
Department for Transport, Local Government and the Regions July 2001
2a. East of England Plan (Draft Revision to the Regional Spatial Strategy)
East of England Regional Assembly December 2004 2b. East of England Plan (Revision to the Regional Spatial Strategy for the East of England)
Government Office for the East of England May 2008
3a. Development and Flood Risk - a Practice Guide Companion to PPS25
(Consultation draft) Department for Communities & Local Government February2007 3b. Planning Policy Statement 25 - Development and Flood Risk - Practice Guide
Department for Communities & Local Government June 2008 4. Thames Region - Catchment Flood Management Plan (Technical Document)
Environment Agency (Thames Region) July 2008 5. Stevenage Flood Storage Areas Performance Study - Final Report
Halcrow Group Ltd (for Environment Agency) March 2007 6. Flood Estimation Handbook (‘Refreshed’ Version)
Centre for Ecology & Hydrology, Wallingford 2006 7. North Kesteven Strategic Flood Risk Assessment.
Bullen Consultants, Peterborough (for North Kesteven D C) September 2002 8. A Study of the Bankfull Discharges of Rivers in England and Wales
M. Nixon Proc. I.C.E. Vol.12 February 1959 9. Investigation into the Impacts of Wrong Connections – Implications for the
Environment Agency (Final report, Vol.1) CES (for Environment Agency) 1999 10. Learning Lessons from the 2007 Floods - An Independent Review by Sir Michael Pitt
(“The Pitt Review”) The Cabinet Office June 2008 11. Flood Risk Assessment Guidance for New Development (Phase2)
(R&D Tech.Rept.FD2320/TR2) HR Wallingford (for EA & Defra) October 2005 12. Flood and Coastal Defence Appraisal Guidance - Economic Appraisal (Vol 3)
Ministry of Agriculture Fisheries & Food (now DEFRA) December 1999 13. Rye Meads Water Cycle Strategy - Scoping Study
Halcrow Group Ltd (for Environment Agency) August 2007 14. West of the A1(M) at Stevenage – Environmental Statement
Barton Willmore Master Planning (for the West Stevenage Consortium) July 2001 Continued over ….
References
Faber Maunsell Stevenage Strategic Flood Risk Assessment 94
Continued …. 15. West of the A1(M) at Stevenage – Development Principles & Design Guide
Barton Willmore Master Planning (for the West Stevenage Consortium) July 2001 16. Stevenage & North Herts Joint Area Action Plan - Land at North Stevenage
PFA Consulting, Swindon (for the Development Consortium) July 2007 17. East of England Housing Investment Plan, 2008 – 2011
East of England Regional Assembly June 2007 18a. Sustainable Urban Drainage Systems Manual
Construction Industry Research & Information Assn. Report C697 February 2007 18b. SUDS - A Practical Guide
Environment Agency (Thames Region, Development Control) October 2007 19. Interim Code of Practice for Sustainable Urban Drainage Sytems
National SUDS Working Group 2004
Glossary
Faber Maunsell Stevenage Strategic Flood Risk Assessment 97
Baseflow
The flow in a river or stream during a dry period, when the flow has not been temporarily increased by
runoff generated by a specific rainfall event.
Berm
A strip of unobstructed level land along the bank of a river or stream, usually between the river bank
and the adjacent floodbank.
Brownfield
Brownfield land is land previously developed for another purpose which is or has been occupied by a
permanent structure or structures and associated fixed surface infrastructure. HM Government states
that development of brownfield land shall take precedence over development of ‘greenfield’ (q.v.)
land. A full definition of what constitutes brownfield land can be found in Annexe C to Planning Policy
Guidance Note 3.
Catchment
The area of land enclosed within a watershed (q.v.) upstream of a specific point on a river or stream
location and which contributes to the flow in the river or stream at that point.
Critical Ordinary Watercourse (COW)
In the 1990s, those small streams or watercourses, generally in an urban area, which were
considered to present a particular flood risk to adjacent land or property were designated as "Critical
Ordinary Watercourses" (COWs) by statutory authorities, although this designation did not have any
statutory status. Most COWs have subsequently been adopted as Main Rivers (q.v.) by the
Environment Agency.
District Plan (SDP2R) See Local Plan.
East of England Plan (EoEP) See Regional Spatial Strategy.
East of England Regional Assembly (EERA)
Established in 2001, the EERA is the government sponsored voluntary regional chamber charged as
the Regional Planning Body responsible for producing the Regional Spatial Strategy (qv). EERA
comprises representatives of all 54 county, unitary and district/borough councils in the region
(comprising the counties of Hertfordshire, Bedfordshire, Cambridgeshire, Essex, Norfolk and Suffolk)
together with a wide range of other public and private sector organisations. For further information
about EERA’s composition and constitution visit www.eera.gov.uk.
Flood Estimation Handbook (FEH)
A standard method of calculating the flood flow in any river or stream in Great Britain and Ireland
originally developed and published by the Institute of Hydrology (see References).
Floodlock
The siuation that occurs when the drainage of an area of land by gravity to an adjacent watercourse is
inibited or prevented if the receiving watercourse is in flood, especially where the drained area is
defended from flooding from that watercourse by raised floodwalls or floodbanks.
Glossary
Faber Maunsell Stevenage Strategic Flood Risk Assessment 98
Floodplain The flat land along the bottom of a river or stream valley that under natural
conditions is subject to occasional inundation from the river or stream during a flood event.
Flood Storage Reservoir (FSR)
A small impounding reservoir created specifically for the temporary retention of flood water to reduce
flood risk downstream of the reservoir. Also known as Runoff Retention Ponds, Runoff Storage
Lagoons, and (in Stevenage) as “water meadows”.
Fluvial To do with rivers and streams.
Freeboard
The difference in height between the level of water in a watercourse at any point and the top of the
bank of the channel at that point.
Gabion
A flexible wire basket filled with rocks and stones placed in the bed or banks of a watercourse to
stabilise the channel of that watercourse and protect against erosion.
Greenfield
Greenfield land is land upon which there has never been any previous development.
Hydraulic Modelling
The use of a computer-based mathematical model of the flow of water in a network of pipes or open
watercourses to determine the depth and velocity of that flow at any point in the network.
Hydrograph The graphical representation of the variation with time of flow in a watercourse.
LiDAR
Light Detection and Ranging - a modern high-definition aerial surveying technique using a laser
beam to determine the elevation of a grid of points on the ground surface.
Local Development Document (LDD)
These are the individual components of the Local Development Framework (see below) and can be
Development Plan Documents (DPDs) or Supplementary Planning Documents (SPDs), both of which
are used by the Local Planning Authority to guide development and in the determination of planning
applications.
Local Development Framework (LDF)
This statutory document, prepared by LPAs (qv), comprises a number of different types of document -
the Local Development Scheme, Annual Monitoring Report, Statement of Community Involvement,
Local Development Documents, Development Plan Documents and Supplementary Planning
Documents.
Local Development Scheme (LDS)
This sets out which documents are part of the Local Development Framework and the timetable for
their review and the preparation of new documents. It is essentially a programme management
document (qv). This is a statutory document, although not a Local Development Document (LDD).
Faber Maunsell Stevenage Strategic Flood Risk Assessment 99
Local Plan
A document produced under the old planning system that set out all the council's policies on the
development and change of use of land and buildings. The existing Local Plan – called the
Stevenage District Plan (Second Review) (SDP2R) - will be automatically ‘saved’ for three years
from its adoption in December 2004. This means that its policies and provisions continue to have
statutory force during that three years period. If the council wishes to save its local plan policies
beyond this period it must seek the permission of the government.
Local Planning Authority (LPA)
A local authority charged by central government with a statutory duty to prepare development plan
documents and undertake other duties under the Town and Country Planning Acts (and other,
ancillary legislation). District Councils, sometimes styled as Borough Councils, have planning powers
for all development in their administrative areas with the exceptions of minerals and waste. County
Councils have planning powers for minerals and waste within their administrative areas. Unitary
authorities (i.e. those not within the jurisdiction of a County Council) have all the planning powers of
both county and district councils within their own administrative areas. There are no unitary
authorities in Hertfordshire.
New Neighbourhoods
A new neighbourhood (also known as an urban extension) is a sustainable option for providing
additional housing after building on appropriate sites within urban areas. These neighbourhoods will
be particularly appropriate where it is possible to utilise existing physical and social infrastructure and
there is good access to public transport, employment, schools, shopping and leisure facilities. Some
planned new neighbourhoods can be so large that they make their own provision for the facilities
listed above. Stevenage West would be a new neighbourhood.
Orifice Plate
A thin plate inserted in a pipe (or other aperture) in which an orifice has been cut or drilled in order to
throttle the flow of water in that pipe or aperture.
Penstock
A small mecahnically operated and vertically opening sluice gate or valve inserted in an open
watercourse or pipe to control the flow of water in that watercourse or pipe.
Pitt Review
A report, published in June 2008, by Sir Michael Pitt who was appointed by the Government to
investigate the serious and widespread flooding that took place in England in June and July 2007.
Planning Blight
The adverse effect of a proposed development or policy which could result in a reduction in property
values in the affected area.
Planning Policy Guidance Notes (PPGs)
Statements of government planning policy and best practice produced prior to the 2004 Planning and
Compulsory Purchase Act. PPGs are now being superseded by PPSs (qv).
Planning Policy Statements (PPSs)
Statements of government planning policy produced since the 2004 Planning and Compulsory
Purchase Act. PPSs are slowly superseding PPGs (qv).
Regional Spatial Strategy (RSS)
This is the successor to both the non-statutory Regional Planning Guidance and to the statutory
Structure Plan (qv). It will set the strategic context for development across the region, including
setting the level of new housing to be accommodated by each LPA(qv). The RSS for the East of
England Region is the East of England Plan which was prepared by the East of England Regional
Assembly (qv) and adopted in May 2008.
Faber Maunsell Stevenage Strategic Flood Risk Assessment 100
Reservoirs Act
An Act of Parliament passed in 1975 (but not implemented until 1983) regulating the design,
construction, supervision and monitoring of all large raised reservoirs in Great Britain, superseding the
Reservoirs (Safety Provisions) Act of 1930.
Return Period
The average time, normally expressed in years, which can be expected to elapse between
occurrences of an extreme event of a given magnitude.
Riparian Land
Land along the banks of an open watercourse immediately adjacent to the channel of that
watercourse.
SAR
Synthetic Aperture Radar - a modern relatively high-definition aerial surveying technique using a
radar beam to determine the elevation of a grid of points on the ground surface, but less precise than
LiDAR (q.v.)
Shapefiles
Computer files containing a separate layer of graphical information which combines with other such
layers to form a single, complex plan or diagram.
Structure Plan
A document produced by Hertfordshire County Council under the old (pre-2004) planning system
which considers strategic issues. Now effectively superseded by the Regional Spatial Strategy(qv).
Swallowhole
A hole or fissure in the bed or banks of a watercourse through which all or part of the flow in that watercourse disappears into the groundwater aquifer beneath the watercourse to emerge elsewhere as springflow.
Thalweg
The physical profile of a river or stream, shown graphically as elevation of the channel plotted against
distance along the watercourse from the watershed (q.v.) to the lowest point in the catchment (q.v.).
Watershed
The line along the crest of a ridge of high land separating two adjacent catchments (q.v.) and from
which runoff flows in either direction.
Windfall Site
A site that comes forward for development that has not been allocated specifically for housing.
Windfall sites are typically ‘brownfield’ (q.v.) sites or underused urban sites. Common examples are
offices that are no longer needed for their current use, houses with large gardens, and houses for
conversion to flats.
Figures
Photographs
Appendix A - Project Brief
Appendix B Recommended Flood Risk Planning Policy
& Guidance Statements