appendix 12.1 flood risk assessment and drainage strategy

Appendix 12.1

Flood Risk Assessment and Drainage Strategy,

February 2015 (ICIS Design Limited)

icis design limited ref. 10007.gc

SPEN HILL DEVELOPMENTS LIMITED

FORMER SHREDDED WHEAT FACTORY WELWYN GARDEN CITY

Flood Risk Assessment & Drainage Strategy

Feb 2015

Rev D

Former Shredded Wheat Factory, Welwyn Garden City

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icis design limited 1

Contents

Summary ............................................................................................................................................... 2

1.0 Introduction .................................................................................................................................... 4

2.0 Flood Risk Assessment .................................................................................................................... 6

3.0 Drainage Strategy ......................................................................................................................... 13

4.0 Summary Conclusions and Recommendations ............................................................................. 19

5.0 Appendix A – Phase 1 Drainage Layout

6.0 Appendix B – Consultation Responses

7.0 Appendix C – MK Surveys

8.0 Appendix D – SFRA App 14

9.0 Appendix E – Extract from Arups Drainage Appraisal Report

10.0 Appendix F – WINDES Calculations

This report has been prepared for the private and confidential use of Spen Hill Developments Limited

and cannot be reproduced in whole or in part or relied upon by any third party for any use whatsoever

without the express written authorisation of Icis Design Limited. If any third party whatsoever comes

into possession of this report, they rely on it at their own risk and Icis Design Limited accepts no duty

or responsibility (including in negligence) to any such third party.

Authors: Gavin Clifford BEng CEng FICE [email protected]

Craig Taylor MSoPHE [email protected]

icis design limited Registered in England & Wales No. 8637234.

Barrington House, Heyes Lane, Alderley Edge, Cheshire SK9 7LA. tel 0161 883 1401

mailto:[email protected]



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Summary

This Flood Risk Assessment (FRA) and Drainage Strategy report is supplementary information to an

Outline Planning Application for a proposed mixed use development located between the A1000

Broadwater Road and the East Coast Main Line railway in Welwyn Garden City, Hertfordshire.

DESCRIPTION OF DEVELOPMENT

O/L Application

Erection of up to 850 Class C3 dwellings to potentially include up to 80 Class C2 care dwellings,

Class A1 retail, Class A3/A4 restaurants/cafés/bars/pubs, Class D1 community use and healthcare

and Class D2 gym/dance/exercise studio floorspace. Demolition of all buildings and structures

except the original 1920's silos, production hall, grain store and boiler house. Refurbishment and

change of use of the retained listed buildings to provide 2 Class C3 residential units, a Class C1

boutique/budget hotel, Class B1(a) offices, a Class A4 pub/bar, a Class D1 crèche and a Class D2

gym/dance/exercise studio. Provision of external space for leisure and recreation to include a linear

park, external games/play area, allotments and a skate park. Creation of internal estate roads,

paths, vehicle and cycle parking. Highway works comprising the widening of footways and the

provision of cycle ways to Broadwater Road and Bridge Road, works to Hyde Way, works to nearby

junctions and the erection of a new footbridge from Bridge Road leading into the scheme. Phase 1

to include the highway works - no matters are reserved. Phase 2 - layout, scale and means of access

are submitted for consideration.

LB Application

Demolition of all buildings and structures except the original 1920's silos, production hall, grain store

and boiler house. Refurbishment and change of use of the retained listed buildings to provide 2

Class C3 residential units, a Class C1 boutique/budget hotel, Class B1(a) offices, a Class A4 pub/bar,

a Class D1 crèche and a Class D2 gym/dance/exercise studio.

The total site area is approximately 8.7 hectares. Phase 1 of the development forms the northern

part and is approximately 5 hectares of the total.

The FRA considers the impact of fluvial, pluvial, infrastructure and groundwater derived flooding of

the subject site, and the effects of the proposed development on the local neighbourhood. A

drainage strategy has been developed to accommodate the design parameters derived from the

FRA.

The residual flood risk resulting from the proposed development of the site is considered to be LOW.

The findings of the flood risk assessment are summarised below:


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- The site lies within an area defined by the Environment Agency as Flood Zone 1 (<0.1% risk of

flooding in any one year). Although the site is in a Low Risk area, the size of the site at 8.7

hectares, triggers the requirement for a flood risk assessment in accordance with the National

Planning Policy Framework (NPPF).

- The site lies within the study area of the ‘Welwyn Hatfield Borough Council Strategic Flood

Risk Assessment’ (May 2009) undertaken by Scott Wilson Ltd for Welwyn Hatfield Borough

Council.

- The residual flood risks to the development site and the surrounding area as a result of the

proposed development are:

Fluvial Flooding from the River Mimram and River Lea - Low Risk

Pluvial Flooding – Low Risk on site. Improved off site.

From Sewers – Low Risk, although the impact of development on the sewer network

requires mitigation.

From Reservoirs, Canals and other artificial sources – Low Risk

Groundwater Flooding – Low Risk

Safe dry access and egress from the proposed development site to the public highway can

be achieved for a 1% (100 year) + Climate change and 0.1% (1000 year) + Climate Change

event.

The FRA and Drainage Strategy for the site have been developed taking in to account the Welwyn

Hatfield Borough Council SFRA (Strategic Flood Risk Assessment) and site specific FRA, in accordance

with the NPPF (National Planning Policy Framework).

The existing site drainage (both surface water and foul) discharges at an unrestricted rate in to

separate surface and foul water public sewers, via connections to the north and east of the site.

To maximize the benefit of redevelopment and reduce the impact of the site on the surrounding area,

the proposed surface water drainage design reduces discharge to ‘greenfield’ run-off rates, based on

rainfall of a 1 in 100 year event plus 30% for potential climate change. The reduction is to be achieved

by the wide-scale use of SuDS techniques to reduce and delay surface water flows. Methods used

include large areas of soft landscape at ground level together with brown and green roofs, providing

the facility for evapo-transpiration and infiltration. The podium levels are enhanced further by the

use of a geo-cell layer beneath planting for surface water detention, with remaining capacity

requirements catered for by deeper geo-cell systems under hard landscaping and attenuation tanks.

The ground conditions at the site are not suitable for soakaway, as the underlying chalk has been

found to suffer from at least two dissolution features (‘sink holes’). However, the systems

incorporated in to the design represent a sustainable approach, significantly better than the existing

condition.

Foul water drainage also incorporates attenuation via storage, in order to mitigate against restricted

capacity of the downstream sewer network. The drainage strategy allows for 24 hour storage

capacity, with controlled (timed and flow rate) discharge.


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1.0 Introduction The following report describes the assessment of flood risk and a drainage strategy for a proposed

development by Spen Hill Developments Limited. The assessment is based on the review of historic

and contemporary published information, supplemented by topographical and on site drainage

survey.

The flood risk has been assessed in order to inform and influence the development of an Outline

Planning Application and Reserved Matters Application designed by EPR Architects.

The assessment has been carried out in accordance with the National Planning Policy Framework

(NPPF), Planning Practice Guidance (PPG), the Welwyn Hatfield Borough Council Strategic Flood Risk

Assessment and in consultation with relevant regulatory bodies including the Local Planning Authority

and the Environment Agency.

The site is centred at approximate OS Grid Reference TL24121 12870, ie 482970E, 213855N and lies

immediately west of the A1000 Broadwater Road, south of Bridge Road to the east of Welwyn Garden

City railway station.

Fig. 1 Site Location – Extract from EPR Architects drawing 2455-T-00-0002-ZXX


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The site is broadly rectangular in plan, measuring approximately 8.7 hectares in total. In accordance

with the NPPF (Footnote 20) an assessment of flood risk is therefore required due to the size of the

site, irrespective of the Flood Zone in which the site lies.

The Outline Planning Application is for the full site and this FRA considers the full area. However,

Phase 1 for which a Reserved Matters application is being made in parallel, encompasses only the

northern end of the site, to an area of approximately 5 Hectares. Whilst the principles of the drainage

strategy will therefore apply to the whole site, the detail for only Phase 1 is presented in this report

and associated project drawings.

In this report where reference is made to the ‘project drawings’ or ‘development proposals’, the

relevant drawings, which should be read in conjunction with this report, are:

D-GA-XX-601, 602, 603, 604, 605, 606, 607 & 608. Copies are provided in Appendix A.

Full Site Boundary

Area of Phase 1

Listed Buildings to be Retained

Fig. 2 Site Phasing Diagram


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2.0 Flood Risk Assessment

Background Information

The Welwyn Hatfield Borough Council Strategic Flood Risk Assessment (May 2009, Scott Wilson Ltd)

(‘SFRA’) identifies current and future flood risk in Welwyn Garden City and the wider area. The SFRA

outlines the identified risks within the borough and describes the preferred surface water

management strategy. In the context of national policy, it provides local policy recommendations and

guidance for the implementation of sustainable drainage.

Flood Risk maps published by the Environment Agency are referenced in the SFRA, indicating Flood

Zones defining probability of flooding. In accordance with the NPPF (Table 1), the identified Flood

Zone is one of the main triggers for the consideration of flood risk and is intended to provide a starting

point for the determination of Flood Risk Areas by the Local Planning Authority. The Flood Zone for

this site has been identified via the Environment Agency flood mapping as Flood Zone 1 – Low Risk.

Policy recommendations of the SFRA (Paragrpah 10.3.1) require surface water run off rates to be

restricted to the Greenfield run off rate and to attenuate all storm events up to 1 in 100 years plus a

30% allowance for climate change.

Consultation

Pre-application consultation has been undertaken with the Local Planning Authority, Environment

Agency (‘EA’) and Thames Water. Feedback from the Hertfordshire Flood Risk Management Team and

the Environment Agency Sustainable Places Planning Advisor has been consistent with national and

local policy, confirming the status of the site as a Zone 1 Flood Risk.

The EA states that their aspiration for the site would be to achieve a surface water run off rate

equivalent to Greenfield rates.

Feedback from Thames Water is that there has been no significant upgrade of sewer infrastructure

within the relevant network area since the publication of the SFRA.

Copies of consultation responses, where received in writing, are provided in Appendix B.

Topographical Setting

The site is located at OS Grid Reference TL24121 12870, to the east of Welwyn Garden City centre,

between the East Coast Main Line railway and Broadwater Road. A topographic and drainage survey

by MK Surveys, drawing L6080-2D, shows the site and immediate boundary conditions. It is included

as Appendix C.

The site itself is generally flat, having previously been developed with a range of industrial buildings

and warehouses. The external site area lies at an elevation of approximately 85.0m Above Ordnance

Datum (‘AOD’).


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The northern boundary of the site is formed by an embankment forming the approach ramp to a rail

over-bridge (Bridge Road), rising to approximately 92.0mAOD. Other than Bridge Road, existing

buildings and highway kerbs, the site and its boundaries do not form any abrupt changes in level or

other topographical features that would channel water flow.

The surrounding geography continues to be near flat or gently undulating for considerable distance.

Urban commercial and residential estates exist to all sides of the site, with the railway separating these

to the west.

Approximately half of the site area has already been cleared by demolition to ground level. Derelict

buildings occupying the rest of the site are to be cleared or refurbished as part of the redevelopment.

Other than those disturbed by demolition, there are currently no significant areas of soft landscape or

permeable surface on the site.

Fluvial Flooding

Fluvial flooding is flooding from existing water courses. This occurs when water levels in a stream or

river channel rise beyond their capacity, causing floodwater to spill over the banks of the watercourse

on to adjacent land. The main reasons for water levels rising are:

Intense or prolonged rainfall within the catchment area of the watercourse and its tributaries.

This can be exacerbated by previously wet conditions and where there are significant

contributions from groundwater;

Constrictions within the channel causing flood water to back up;

Blockage of structures within the channel (bridges, culverts etc.) causing flood water to back

up and over-top the banks.

All watercourses in England and Wales are classified as either ‘Main Rivers’ or ‘Ordinary

Watercourses’. The difference between them is based largely on the perceived importance of the

watercourse and in particular the potential to cause significant and widespread flooding.

The two rivers closest to the site are the River Lee and River Mimram, although both are at least 2km

from the site and at a lower level. The flood risk maps published by the Environment Agency show

their maximum flood extents to be relatively narrow and of no significance to the site.

The site is identified by the EA as lying wholly within Flood Zone 1, with an annual probability of flood

risk of <0.1%. The site is therefore considered to be at Low Risk of flooding from these sources and

the EA have confirmed this in writing (see appendix B).


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Pluvial Flooding

Pluvial Flooding or Surface Water Flooding is the term used to describe flooding that occurs due to

intense, often short duration rainfall, when it is unable to soak in to the ground or enter drainage

systems and therefore runs over the land surface before entering drainage or water courses. It is

most likely to occur when soils are already saturated so that they cannot accept further infiltration, or

in urban areas where buildings and hard landscaping prevent water soaking in to the ground. The

excess water can pond in low points and result in the development of flow pathways along roads or

through built up areas and open spaces.

The potential volume of surface run-off is directly related to the size and shape of the catchment to

that point and is a function of geology, slope, rainfall, saturation, soil type, urbanisation (hard

landscape) and vegetation.

The SFRA states that there are no records of pluvial flooding affecting the borough, although this may

be due to a lack of recording. The site owner is not aware of any records of significant flooding

affecting the site.

Inspection of the EA Surface Water Flood Risk Map shows that the majority of the site is unaffected

by ponding of surface water, although some small areas are shown. (It has not been possible to

reproduce the EA map in this report, but it can be viewed using this link: http://watermaps.environment-

agency.gov.uk/wiyby/wiyby.aspx?topic=ufmfsw&scale=8&ep=map&layerGroups=default&lang=_e&y=214140&x=526362#x=523971&y=212676&scale=11 ).

When comparing the EA map with the topographical survey, it can be seen that the ponding occurs in

localised, low lying areas of the site, where there is no effective drainage. As the site is generally flat,

any ponding water will flow at slow rates and is not a significant hazard. Design proposals should

reduce this effect further by providing positive drainage and designed falls to landscaping.

Site

Fig. 3 Extract From Environment Agency Flood Map for Planning (Rivers and Seas)

http://watermaps.environment-agency.gov.uk/wiyby/wiyby.aspx?topic=ufmfsw&scale=8&ep=map&layerGroups=default&lang=_e&y=214140&x=526362#x=523971&y=212676&scale=11

http://watermaps.environment-agency.gov.uk/wiyby/wiyby.aspx?topic=ufmfsw&scale=8&ep=map&layerGroups=default&lang=_e&y=214140&x=526362#x=523971&y=212676&scale=11


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We therefore consider the risk associated with pre-development pluvial flooding to be low and

reduced further by development proposals that include landscape design to the full site area.

Proposals to boundaries do not include level or step changes that would significantly affect the flood

risk.

Flooding from Sewers

Flooding from sewers can occur due to lack of capacity within the system, or localized damage or

blockage.

The SFRA indicates that the site and surrounding area is not affected by sewer flooding due to

surcharge. Appendix 14 of the SFRA is reproduced as Appendix D, showing that reported problem

areas exist only to the south of Welwyn Garden City. Consultation with regulatory authorities for this

site specific FRA has not revealed any change from this status.

A drainage survey of the site has been undertaken by MK Surveys (reproduced as Appendix C), which

confirms connectivity of the site for separate surface and foul drainage. Existing discharge is via gravity

at unrestricted rates.

Surface Water attenuation and SuDS applied to the site during redevelopment would therefore

significantly reduce the impact on the local Surface Water sewer network. We therefore consider the

risk of surface water sewer flooding to be low and reduced further by the proposed development.

Previous investigation and consultation with Thames Water on behalf of the client by ARUPS has

indicated that at times of peak flow, the Foul Sewer network downstream of the site may be at full

capacity. (An extract from the report is reproduced as Appendix E). It was reported by ARUPS that

the modelling of the sewer network at the time may be incorrect, or that improvements to the

network may change Thames Water’s view.

We have contacted Thames Water by telephone and in writing, to enquire whether this situation has

been resolved since the ARUPS investigation was undertaken, by further investigation or

improvements to the network. We have received verbal response that infrastructure improvement

has not been undertaken, but that further network analysis may have proved that this was not

necessary. We have been advised by Thames Water that they may require further network analysis

to be undertaken before they can confirm allowable discharge rates for the site, but that they are

willing for this to be subject to Planning Condition.

Development of the site will result in an increased foul water discharge and therefore the drainage

strategy should allow for appropriate control, so that the risk of sewer flooding is mitigated. This is

discussed further in the Drainage Strategy section of this report.


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Flooding from Reservoirs, Canals and other Infrastructure

Structural failure of water retaining structures and infrastructure is a potential source of flooding. The

NPPF (Paragraph 14) and PPG require that site specific FRA considers the risk of such failure.

There are no reservoirs, canals or artificial water sources of any significance within at least 2 miles of

the site. The EA Flood Risk Map does not indicate risk of flooding from reservoirs within WGC and the

SFRA (Fig 21) concurs.

We therefore consider the risk of flooding to the site from reservoirs, canals and other built

infrastructure to be low.

Groundwater Flooding

Groundwater flooding is caused by the emergence of water originating from sub-surface permeable

strata, ie where the groundwater level rises such that the water table intersects with the ground

surface.

The groundwater regime in Welwyn Garden City is described in the SFRA (Chapter 5.6) as low risk. The

SFRA (Fig21) indicates that the site is not in an area of rising groundwater.

Ground Investigation undertaken by Delta Simons Environmental (report ‘Factual and Interpretive

Geotechnical Report 2342.18 Nov 2014’, submitted with planning application) indicates that standing

groundwater level varies between 20m and 26m below existing ground level. At this level the ground

water is of negligible influence on the site and we consider the risk of flooding to the site from

groundwater to be low.

Safe Site Access and Egress

The NPPF (Paragraph 39) and PPG require that Flood Risk Assessment include an assessment of

whether safe access is available to and from a site during a flood event.

The NPPF (Paragraph 40) requires only that ‘judgement be used’ but a joint DEFRA and Environment

Agency document “Flood Risks to People Phase 2, Guidance Document, FD2321/TR2 (March 2006)”

brings together information on flood depth and speed (velocity) of floodwater to create a rating of

hazard to people within each area that experiences flooding.

The hazard rating categorises flood risk in terms of Caution, Danger for Some, Danger for Most and

Danger for All, with the hazard becoming dangerous to more people as depths and velocities increase.


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The DEFRA guidance defines the categories thus:

Velocity m/s

Depth (m)

0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Description of categories:

Degree of Flood Hazard

Colour Codes Description

Low Caution Flood zone with shallow flowing water or deep standing water

Moderate Dangerous for some (i.e. children) Danger: Flood zone with deep or fast flowing water

Significant Dangerous for most people Danger: Flood zone with deep fast flowing water

Extreme Dangerous for all Extreme danger: Flood Zone with deep fast flowing water

The guidance states that the maximum permissible hazard for safe access should correspond to the

depth-velocity combinations shown in blue (low hazard).

The safety of wading is affected by underwater hazards such as unevenness of the ground and

presence of depressions or unsecured manhole covers.

With respect to requirements for safe access and exit from new developments in flood risk areas, the

following should be provided (in decreasing order of preference):

- Safe dry route for people and vehicles;

- Safe dry route for people;

- A route where flood hazard is low and should not cause a risk to people; and

- A route for vehicles where the flood hazard is low to permit access for emergency vehicles.

Fig. 4 Flood Hazard Rating


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Previous sections of the report have shown that the flood risk (from all forms of flooding) to the site

in the event of a 1 in 100 year storm is low. Even in an extreme event above the 1 in 100 year

threshold, the topography of the site and surrounding area is such that flood depths and velocities

would be below the threshold values for Low Risk.

Access routes on to the site are proposed from Broadwater Road and Bridge Road as well as via the

existing pedestrian access across the railway, providing multiple means of egress. We therefore

consider that the NPPF and PPG requirement for safe access and egress is satisfied by the proposed

development.


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3.0 Drainage Strategy

The NPPF Core Planning Principles (Paragraph 17) encourage sustainable development, mitigation of

flood risk and adaptation for potential climate change. As outlined in Section 2 of this report, the

policy recommendations of the SFRA (Paragraph 10.3.1) and aspiration of the Environment Agency

are that surface water discharge from the site be limited to Greenfield runoff rates.

In order to meet these aspirations and recommendations, the proposed drainage strategy provides

for attenuation of discharge using a variety of SuDS.

Existing Drainage

An Existing Drainage survey has been undertaken by MK Surveys which confirms the connectivity of

the site. Their drawing No. L6080-2D is included in Appendix C.

Private on-site drainage has been identified, consisting of separate foul and surface water systems,

serving both the north and south of the site. No storage tanks or flow control devices were found,

indicating that the existing drainage discharges at unrestricted rates. Where buildings have been

demolished these drains are now redundant but elsewhere the drains remain live.

There are numerous existing lateral connections in to the Thames Water sewers, all of which connect

to the sewer network in Broadwater Road and Bridge Road. There do not appear to be any existing

lateral connections running to the west or south of the site.

Thames Water sewers pass across the north and east of site, located (in part) inside the site boundary.

For the purposes of the development proposals, 6m wide easements have been respected along the

full length of the Thames Water sewers and are indicated on the project drawings thus:

Fig 5. Extract from Drainage Plan D-GA-XX-603


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Proposed Development Drainage Strategy

The foul and surface water drainage strategy for the whole site will be based on common principles.

However, for the purposes of providing detail for the Reserved Matters application for Phase 1 the

design has been developed based on separating the Phase 1 area in to four dedicated drainage cells.

The main reasons for this are:

- To provide resilience to the various buildings in terms of continuity of connection to the public

sewer network in case of phased occupation, maintenance or accident.

- To utilise existing lateral connections onto the foul and surface water public sewer network.

- To limit the depth of the drainage runs, thereby minimising associated excavations and

allowing discharge under gravity where possible.

Foul Water

Each of the four drainage cells will require a dedicated foul drainage system, as generally indicated on

the development proposal drawings listed in Section 1.0. The foul drainage will discharge to the

Thames Water sewer network via proposed and existing lateral connections.

The foul drainage system will be designed to convey foul water from all buildings, collecting discharge

from the following areas;

- Toilets

- Kitchens

- Bathrooms

- Plant areas

The foul water drainage system will collect the above ground drainage at both ground and basement

levels, discharging to the sewer infrastructure via a system of below ground drainage access chambers.

The location of the access chambers will be coordinated with the architectural layouts, foundations

and the above ground drainage proposals to ensure that they are located in positions that provide

sufficient access for maintenance and in locations which are aesthetically acceptable.

As reported in the FRA section of this report, it is understood that the existing Thames Water foul

sewer network may have insufficient capacity to accept unrestricted discharge. Therefore, for the

purposes of this drainage strategy report, it has been assumed that foul water will be discharged from

each cell at a restricted discharge rate. To allow for the necessary temporary storage, each of the four

drainage cell designs has been designed to incorporate a packaged foul water pumping station with a

24 hour storage capacity. 24 hour storage is in line with industry good practice but also allows for the

possibility of timed discharge, to further control the impact on the sewer network.


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Based on EPR Architects’ area schedule, Table 3.1 summarises the pumping station requirements.

Discharge rates have not yet been agreed with Thames Water, however for the purposes of this report,

it is proposed that 5 l/s will be an acceptable rate from each drainage cell. This is a worst case scenario

and storage tank volumes could be reduced following agreement of discharge rates with Thames

Water.

Table 3.1 – Foul Water Pumping Station Schedule

Ref Cell Buildings Volume

(m³)

Discharge

(l/s)

Outfall Location

FWPS01 01 02 & 03 50 5 Via re-use of existing and/or

formation of new lateral

connections to 525 dia TW sewer

flowing east at the north of the

site adjacent to Bridge Road.

Exact locations tba with Thames

Water.

FWPS02 02 & 03 01, 04 & 05 35 5

FWPS03 04 06 36 5

FWPS04 04 07 30 5

Notes:

1. The storage volumes are approximate and subject to further design development during the next design stage.

2. The foul water discharge rates are subject to agreement with Thames Water.

Each pumped main will discharge in to a pressure break chamber and each foul water drainage cell

will discharge to the Thames Water sewer network via a disconnecting chamber on the boundary of

each cell.

Further consideration should be given to the location of storage tanks, ideally to place them at a level

where discharge can be achieved without pumping. However, this will need to be considered in

conjunction with options for timed discharge and discharge rates, so is dependent on input from

Thames Water at detailed design stage.

Grease Removal

Generally for retail units and buildings where it is proposed to incorporate a commercial kitchen, a

suitable method for removal of fats and grease must be considered during the next design stage.

Consultation with the kitchen specialist will be required to determine a suitable system which is sized

to allow for anticipated flows from the kitchen and food preparation areas and in accordance with BS

EN 1824 – Grease Separators.


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Surface Water

The SFRA (Clause 10.3.1) and Environment Agency aspiration (see consultation letter in Appendix B)

require that surface water discharge from the site be reduced to greenfield runoff rates. There are

several alternative methods available for calculating greenfield run off rates, however we have based

the drainage design on a rate of 5.0 l/s per hectare, as a lower bound figure.

In accordance with the NPPF Core Planning Principles (Paragraph 17), Environment Agency guidance

(See appendix B) and SFRA policy recommendations (Chapter 10.3.1), attenuation should be provided

using Sustainable Drainage Systems (SuDS) where possible.

SuDS are based on drainage techniques that seek to mimic run off from the site in a natural state. The

aim of a SuDS based system is to manage rainwater on the surface, close to its source, with the

consequence that water is stored and released slowly, thus reducing flood risk and improving water

quality. Examples of SuDS tchniques include permeable paving, soakaways, green roofs, detention

basins and ponds. The accepted hierarchy of preferred methods is shown in Fig 6.

Fig. 6 Extract from EA Publication: SUDS – A Practical Guide


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Good urban design is only possible where the design and management of water is integrated as an

important part of the whole concept. Therefore engagement with the multi-disciplinary team is

important, so that SuDS can be considered early to ensure that the water management features are

not disconnected with the character and aesthetics of the development.

Making SuDS attractive and integrating their design with other urban features (ie roads and public

open space) is vital to their acceptance, performance and longevity. Communities and stakeholders

are more likely to approve, operate and maintain SuDS when they are delivered using good urban

design principles.

Taking these considerations in to account, the design team have incorporated SuDS techniques in to

Phase 1 of the development by following the preferred hierarchy to include extensive areas of green

and brown roofs. These are illustrated on BMD Landscape Architects drawings and translate to the

following as a minimum:

Living (Green) roofs 11% of Phase 1 site area

Brown roofs 8% ‘’

Permeable soft landscaping / infiltration trenches/strips 13% ‘’

Permeable paving on podium and roofs linked to local geo-cell 14% ‘’

Stormwater attenuation (tanks)

Soakaways

The ground investigation undertaken by Delta Simons Environmental (‘Factual and Interpretive

Geotechnical Report 2342.18 Nov 2014’, submitted with planning application) has identified two

significant dissolution features (sink holes) within the chalk bedrock. These features typically form

over very long periods but are at risk of collapse where ground above them is subject to increased

water flow or inundation, such as that created by the use of soakaways. The geology of the site has

been found to consist of impermeable clays over gravels and chalk; a typical situation for these

circumstances to exist.

In the event of collapse, the sink holes are a danger to buildings and population and therefore

appropriate mitigation will need to be undertaken. In terms of flood risk and drainage strategy, the

site is therefore not suitable for the use of soakaways and surface water that is not disposed of via

evapo-transpiration and soft landscape will require discharge to the public surface water sewer.

Drainage Design

In a similar manner to the foul drainage, the proposed drainage design for the Phase 1 area is split in

to four drainage ‘cells’. Each of the four drainage cells will incorporate dedicated storm water

attenuation systems, so that each area is independent and self-sufficient. The stormwater attenuation

tanks will have a restricted discharge and be sized to accommodate 100 year storm events plus 30%

for Climate Change.


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As a conservative approach for preliminary allocation of maximum storage volume, a calculation of

attenuation storage requirements has been undertaken using WINDES Quick Storage (see Appendix

F).

Based on a 1 in 100 year storm event plus 30% for climate change, the maximum storage requirement

for Phase 1 is calculated as 3,339m3. Table 3.2 summarises how this volume would be divided

between the four drainage cells, based on the site area of each. The drainage design proposals, as

shown on the project drawings, provide these volumes and total 3,370m3.

Table 3.2 – Stormwater Attenuation Tank Schedule

Cell Buildings Volume

(m³)

Discharge

(l/s)

Method of

discharge

Outfall Location

01 02 & 03 990 7.5 Hydro-

brake

Via existing or proposed new lateral onto TW 600mm dia

surface water sewer flowing

east at the north of site

adjacent to Bridge Road, tba

with Thames Water.

02 04 640 5.0 Hydro-

brake

03 05 720 5.0 Hydro-

brake

Via proposed 225mm dia.

lateral onto TW MH2802 in

Broadwater Road

04 06 & 07 1,020 7.5 Pumped Via existing 225mm dia. lateral

onto TW MH2802 in

Broadwater Road

Detailed design, incorporating the full beneficial effect of the SuDS techniques that have been

employed, will allow reduction of the above stated storage volumes. The opportunity should then be

taken to locate smaller tanks in areas and at levels where, ideally, all discharge can be achieved under

gravity. Ideally, pumped systems are to be avoided so that maintenance, energy use and running costs

are minimized.

Currently it is proposed that large diameter pipework or geo-cellular tanks wrapped in an

impermeable membrane will act as the attenuation tanks. The tanks will be located below the ground

floor car parks or externally below paving, as shown on the project drawings. Discharge rates will be

controlled by incorporating a flow control chamber downstream of the attenuation systems within

each cell.


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4.0 Summary Conclusions and Recommendations

Flood risk of all types is considered to be LOW.

Proposed development includes surface water attenuation in accordance with the NPPF, SFRA and

Environment Agency guidance, to achieve discharge equivalent to greenfield run-off rates. The impact

of the site on the surrounding area is thereby reduced as a result of the proposed development.

This reduction is to be achieved by the implementation of SUDS techniques, prioritizing low

maintenance methods. Within Phase 1 this includes:

- Green and brown roofs to approximately 19% of the total Phase 1 site area.

- Soft landscaping at Ground Level to a minimum of 13% of the total Phase 1 site area.

- Permeable paving at podium and roof level linked to a geo-cell attenuation layer of an area

equivalent to 14% of the total Phase 1 site area.

Drainage is to be designed to discharge under gravity where possible, although invert levels are such

that some pumped systems are probable. Detailed design should seek to remove or reduce this where

possible.


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5.0 Appendix A – Phase 1 Drainage Layout

Drawings:

D-GA-XX-601

D-GA-XX-602

D-GA-XX-603

D-GA-XX-604

D-GA-XX-605

D-GA-XX-606

D-GA-XX-607

D-GA-XX-608

Project

Client

ApprovedCheckedDrawn Date Scale @ A1 Drawing No. RevisionProject No.

Drawing Status

Drawing Title

icis design limitedCivil & Structural Consulting Engineers

Rev. Description Date

www.icis-design.co.ukt: +44 (0)161 883 1401Barrington House, Heyes Lane, Alderley Edge, Cheshire SK9 7LA

17.11.2014Stage C IssueP1P1D-GA-XX-60110007

Stage C

Below Ground Drainage, Basement

1:200Nov 14GCRSCT

Welwyn Garden City

Spen Hill Developments Limited

Project

Client


Drawing Status

Drawing Title




17.11.2014Stage C IssueP1P1X-GA-60210007

Stage C

Below Ground Drainage, Basement

1:200Nov 14GCRSCT

Welwyn Garden City


Project

Client


Drawing Status

Drawing Title





Stage C

Below Ground Drainage Layout

1:200Nov 14GCRSCT

Welwyn Garden City


CL 84.96

CL 85.20

CL 85.05

CL 85.14

CL 83.88

CL 83.75

CL 84.96

CL 84.81

CL 84.75

CL 84.35

Project

Client


Drawing Status

Drawing Title





Stage C

Below Ground Drainage

1:200Nov 14GCRSCT

Welwyn Garden City


Project

Client


Drawing Status

Drawing Title





Stage C


1:200Nov 14GCRSCT

Welwyn Garden City


CL 85.56

CL 84.96

CL 85.20

CL 85.69

CL 85.60

CL 85.52

CL 85.05

CL 85.14

TLC

Project

Client


Drawing Status

Drawing Title





Stage C


1:200Nov 14GCRSCT

Welwyn Garden City


Project

Client


Drawing Status

Drawing Title





Stage C


1:200Nov 14GCRSCT

Welwyn Garden City


CL 85.73

CL 85.52

CL 85.46

CL 85.46

CL 85.47

CL 85.82

TLC

TLC

Project

Client


Drawing Status

Drawing Title





Stage C


1:200Nov 14GCRSCT

Welwyn Garden City



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6.0 Appendix B – Consultation Responses

Consultation Responses:

09 October 2014 Herts CC Flood Risk Management Team

31 December 2014 Environment Agency Sustainable Places Planning Officer

7 January 2015 Thames Water

HCC Flood Risk Management Team

-----Original Message----- From: Sophie Williamson [mailto:[email protected]] Sent: 09 October 2014 16:07 To: [email protected] Cc: Richard Aston Subject: FW: Consult Message: Former Shredded Wheat Factory, Broadwater Road, Welwyn Garden City {-13428-207215-}

Response to below consultation:

Thank you for consulting HCC Flood Management Team on this pre-application.

Apologies but there was diffculty adding comments to the Data Space consultation page. Please therefore find our comments attached.

If you have any questions relating to this response please contact me direct.

Should there be any further pre-application meetings and consultations HCC are able to provide further comment on an advisory basis.

Kind Regards

Sophie Williamson

Sophie Williamson Senior Flood Risk and SuDS Officer

Flood Management Team Environment and Resource Planning

Hertfordshire County Council County Hall Pegs Lane Hertford SG13 8DE

Tel: 01992 556492 Ext: 26492E-mail: [email protected]

HCC Flood Risk Management Team response

Pre-development application: Former Shredded Wheat Factory, Broadwater Road, Welwyn Garden





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HCC Flood Risk Management Team response

Pre-development application: Former Shredded Wheat Factory, Broadwater Road, Welwyn Garden

City 13428-207215

Due to the limited information available we are not in a position to provide detailed comments at

this stage however we are able to provide the following principal advice and guidance:

Surface Water Flood Risk

Based on the EA surface water flood maps for areas susceptible to surface water which were

published and available on their website from December 2013, these maps show there are areas

within the site susceptible to surface water flood risk.

We would therefore recommend that any FRA carried out to support any future planning

application should assess this in more detail to determine the risk to the property and any impact

the development may have on this risk to the surrounding area i.e. if building takes place within an

existing surface water flow this could intercept and divert these flows and increase flood risk

elsewhere.

Management of surface water run-off from new development

Based on the location of this site and current use, this should be seen as an opportunity to improve

surface water run-off and water quality by incorporating a sustainable drainage scheme that

follows the SuDS philosophy of maintaining surface water flows on the surface utilising open green

spaces, innovative ways of using multi space use such as car parking, verges, play areas etc.

The location of this site is directly outside of the railway station and as Welwyn Garden City this

development could provide a green and blue space as commuters and the local community leave

the station.

HCC have recently published its interim guidance for the implementation of SuDS in Hertfordshire

http://www.hertsdirect.org/services/envplan/water/floods/suds/. This guidance document pulls

together references to all existing best practice technical guidance for SuDS and provides a vision of

how SuDS can be implemented across Hertfordshire. For examples of developments that have

incorporated SuDs schemes the applicant is advised to refer to the Susdrain website

http://www.susdrain.org/ . This website provides case studies, information on different SuDS

techniques and provides information on how the implementation of SuDS can be cheaper than

conventional drainage systems and provide other benefits to the development.

Water sensitive urban design

Water sensitive urban design promotes an integrated way of incorporating the water cycle within

new developments, towns and cities. More information on how this can be achieved can be found

on the CIRIA website;

‘Water Sensitive Urban Design is an opportunity to create beautiful, successful and resilient places.

It is undeniable that the relationship between water and our urban areas needs to be given a higher

priority to provide integrated solutions to flood risk management, sustainable water use and supply

and the improvement of water quality in our treasured watercourses. This priority needs to be

applied in an integrated way by the people and partners that plan and design the built

environment. In doing so, we can bring together the skills and creativity of practitioners who plan

and design the places we live in to bring much wider benefits to communities’

http://www.hertsdirect.org/services/envplan/water/floods/suds/

http://www.susdrain.org/


10007 – FRA


priority to provide integrated solutions to flood risk management, sustainable water use and supply

and the improvement of water quality in our treasured watercourses. This priority needs to be

applied in an integrated way by the people and partners that plan and design the built

environment. In doing so, we can bring together the skills and creativity of practitioners who plan

and design the places we live in to bring much wider benefits to communities’

http://www.ciria.org/Resources/Free_publications/Water_Sensitive_Urban_Design.aspx

Role of Lead Local Flood Authority

Whilst HCC are not a statutory consultee and not the SuDS Approval Body we have an invested

interest as having an overall view of flood risk within the county as the Lead Local Flood Authority

and are responsible for managing flood risk from all local sources. We therefore need to ensure

that any new development does not increase flood risk from existing sources and from any

increases in surface water run-off rates and volumes working with our Local Planning Authorities.

We also have to have due regard to the EU Water Framework Directive to ensure there is no

detriment to water quality within local watercourse and its ecology. SuDS are the best way to

manage and prevent pollution to watercourses when a treatment train approach is implemented.

This allows surface water run-off to pass through stages of drainage:

7. Source Control- catches first ‘flush’ of surface water which provides initial treatment

catching silt and diffuse pollution. This can be through permeable paving, infiltration

measures etc

8. Conveyance- this conveys surface water on the surface to the next stage of the drainage

system which further filters the surface water run-off. This can be through swales,

infiltration trenches etc

9. Regional storage- this is usually the last stage where water is stored for more extreme

rainfall events providing attenuation and the final treatment before discharging into the

nearest watercourse or local sewer network.

To learn more about our roles and responsibilities please refer to the Hertfordshire Local Flood Risk

Management Strategy

http://www.hertsdirect.org/services/envplan/water/floods/floodrisk/lfrmsherts/

http://www.ciria.org/Resources/Free_publications/Water_Sensitive_Urban_Design.aspx

http://www.hertsdirect.org/services/envplan/water/floods/floodrisk/lfrmsherts/


10007 – FRA


Environment Agency Sustainable Places Planning Officer


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Thames Water


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7.0 Appendix C – MK Surveys Drg 19040


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8.0 Appendix D – SFRA App 14


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9.0 Appendix E – Arups Report

Extract from Arups Drainage Appraisal Report – Welwyn Garden City ref REP/207043/R001, Feb

2009


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10.0 Appendix F – WINDES

appendix 12.1 flood risk assessment and drainage strategy

Documents