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Chelmsford Borough Council

Strategic Flood Risk Assessment Appendix B Chelmsford Supplementary Report

Report April 2008

Chelmsford Borough Council D115326 Strategic Flood Risk Assessment

Revision Schedule Mid Essex Strategic Flood Risk Assessment April 2008

Rev Date Details Prepared by Reviewed by Approved by

01 June 07 Draft Heather Rich Flood Risk Specialist

Peter Mansell Principal Engineer

Damon O’Brien Technical Director

02 July 07 Final Draft Heather Rich Flood Risk Specialist

Liz Williams Senior Consultant

Damon O’Brien Technical Director

03 October 07 Final Eleanor Cole Graduate Hydrologist


Jon Robinson Associate Director

04 April 08 Final Eleanor Cole Graduate Hydrologist


Jon Robinson Associate Director

Scott Wilson 6-8 Greencoat Place, London, SW1P 1PL Tel: 020 7798 5200 Fax: 020 7798 5001 www.scottwilson.com


PREFACE Purpose: The purpose of this report is to provide Strategic Flood Risk Assessment (SFRA) information specific to the Chelmsford Borough Council area. It outlines the main flood risks posed to the council area and how the long-term management fluvial flood sources could be addressed by the incorporation of Flood Storage Areas. The flood risks to potential sites have been identified on both greenfield and brownfield sites. Flood Hazard Mapping for South Woodham Ferrers along the River Crouch has been undertaken based on breach simulation modelling. Flood hazard and depth mapping has been provided for the climate change scenarios for the River Chelmer, using recent model information completed by Halcrow Ltd as part of this study. This information should be used in conjunction with guidance in the SFRA to apply the Sequential Test (Planning Policy Statement (PPS) 25: Development and Flood Risk, Communities and Local Government). It is presumed throughout this report that the reader is familiar with the contents of the SFRA main report (Mid Essex Strategic Flood Risk Assessment, Scott Wilson) and the methodologies presented therein. Objective The SFRA objective is to aid the partner Boroughs in their development process through the application of the Sequential Test as required by PPS25. It assesses the flood risks posed to the region and outlines the main hazard zones in order to further aid the development planning process. The objective of this report is to assess the flood risks posed to the Chelmsford Borough Council area by breaches in the tidal defences and fluvial flood sources and to assist the Council in applying the Sequential Test. Limitations The SFRA approach was outlined in the Inception Report & Stage 2- Brief & Specification (Scott Wilson, November 2006). All methodologies, including breach parameters, have been agreed with the Environment Agency in accordance with the current best practice at the time of completion. This report details the flood issues posed to the local authority of Chelmsford Borough Council, it does not include information for neighbouring council areas such as Uttlesford, which forms part of the River Chelmer catchment area.


Table of Contents

1 Non-Technical Summary.................................................................... 1 1.1 SFRA Background.......................................................................................................... 1 1.2 SFRA Planning Objectives ............................................................................................ 1 1.3 SFRA Report Layout ...................................................................................................... 2 1.4 Chelmsford Borough Council Considerations ............................................................ 2 1.5 Way Forward................................................................................................................... 5 1.6 A Living Document......................................................................................................... 5

2 Introduction and Background.............................................................. 6 2.2 Scope and Objectives .................................................................................................... 6 2.3 Report Structure............................................................................................................. 7

3 Level 1 Assessment ........................................................................... 8 3.1 Level 1 SFRA – Study Area, Flood Source Review and Data Review........................ 8 3.2 Chelmsford Borough Council Area .............................................................................. 9 3.3 Sources of Flooding..................................................................................................... 12 3.4 Climate Change ............................................................................................................ 13 3.5 Historic Flooding.......................................................................................................... 15

4 Level 2 SFRA ................................................................................... 17 4.2 What is the Exception Test? ....................................................................................... 18 4.3 What is Required to Pass the Exception Test? ......................................................... 18

5 Fluvial Flooding Sources .................................................................. 19 5.1 Sources ......................................................................................................................... 19 5.2 Fluvial Level 2 Study.................................................................................................... 24

6 Arterial Drainage Network Flooding Sources ................................... 26 6.1 Sources ......................................................................................................................... 26 6.2 Arterial Drainage Network Level 2 Study ................................................................... 27

7 Tidal Flooding Sources..................................................................... 29 7.1 Sources ......................................................................................................................... 29 7.2 Tidal Level 2 Study....................................................................................................... 31

8 Other Sources of Flooding................................................................ 36 8.1 Overland Flow............................................................................................................... 36 8.2 Surface Water ............................................................................................................... 37 8.3 Strategic Spatial Flood Source Summary.................................................................. 38


9 North Chelmsford Area Action Plan (NCAAP).................................. 41

10 Planning and Development Advice to Chelmsford Borough............. 42 10.2 Flood Risk ..................................................................................................................... 43 10.3 Sustainable Drainage Systems ................................................................................... 43 10.4 Flood Mitigation............................................................................................................ 49 10.5 Water Environment ...................................................................................................... 50

11 References ....................................................................................... 52

Annex A – Flood Alleviation Scheme Appraisal .......................................... 53 Background ................................................................................................................................ 53 Chelmsford Flood Alleviation Viability Study (2007) .............................................................. 53 Flood Storage Areas & Breach Scenarios ............................................................................... 54 Interim development period for Chelmsford ........................................................................... 56 FSA Appraisal............................................................................................................................. 57

Annex B – Halcrow Methodology for River Chelmer Modelling Updates ............................................................................................ 63

Introduction ................................................................................................................................ 63 Overview of Hydraulic Model .................................................................................................... 63 Methods and Assumptions ....................................................................................................... 64 Outputs ....................................................................................................................................... 65


1 Non-Technical Summary

1.1 SFRA Background 1.1.1 Scott Wilson Ltd was commissioned by the Mid Essex Area Liaison group (MEAL) to

undertake a ‘Stage 2’ Strategic Flood Risk Assessment (SFRA) of Mid Essex. An Inception Report, completed by Scott Wilson in November 2006, preceded this ‘Stage 2’ SFRA. The Inception Report located and identified available data and information that would be useful for completion of the SFRA. In addition the report outlined the extents of the study areas, the modelling approach and highlighted various specific flood risk issues within the Mid Essex area that should be covered within the main SFRA report.

1.1.2 MEAL incorporates the local councils of Chelmsford Borough Council, Colchester Borough Council, Braintree District Council and Maldon District Council. Uttlesford District Council does not form part of this study, although is part of the upper River Chelmer catchment area.

1.1.3 This project was carried out in collaboration with the Environment Agency’s Anglian Region, and a draft of the full report was submitted to the Agency for their comments and observations. Mutually acceptable amendments have been incorporated into the final SFRA report.

1.2 SFRA Planning Objectives 1.2.1 The primary objective of the study was to enable the four participating local authorities

to undertake Sequential Testing inline with Government’s flood risk and development policy document - Planning Policy Statement (PPS) 25: Development and Flood Risk - to inform the development of their emerging Local Development Framework (LDF) documents.

1.2.2 PPS25 requires local planning authorities to review flood risk across their districts, steering all development towards areas of lowest risk. Development is only permissible in areas at risk of flooding in exceptional circumstances where it can be demonstrated that there are no reasonably available sites in areas of lower risk, and the benefits of that development outweigh the risks from flooding. Such development is required to include mitigation/management measures to minimise risk to life and property should flooding occur.

1.2.3 The Strategic Flood Risk Assessment is the first step in this process, assisting in the development of the LDF’s by identifying flood risk areas and outlining the principles for sustainable development policies, informing strategic land allocations and integrating

Strategic FRA 1 April 2008


flood risk management into the spatial planning of the area. The SFRA thereby forms an essential reference tool providing the building blocks for future strategic planning.

1.3 SFRA Report Layout 1.3.1 The main background and methodology information, including guidance on using the

figures and potential measures for residual risk management are discussed in the main SFRA report. For each participating authority a suitable appendix has been compiled to include background information on that area with regards to flood risk. Additional information requested as part of the SFRA Brief in relation to each respective local authority and associated flood risk mapping for that area is also included.

1.3.2 There are four separate appendices for each authority. This report, Appendix B reflects the flood risk issues for Chelmsford Borough Council. The other Appendices reflect the flood risk issues in relation to each local authority.

1.4 Chelmsford Borough Council Considerations Background

1.4.1 The southern boundary of the Borough adjoins the Borough of Basildon and the northern bank of the River Crouch, which forms the only tidal boundary. The remaining landward boundaries of the Borough extend to Maldon in the east, Epping Forest and Brentwood in the west and Braintree and Uttlesford in the north. The main watercourses that influence flooding in the Chelmsford Borough are the River Can, River Wid and River Chelmer, which flow through the main town of Chelmsford. The other major settlements in the Borough are Writtle, Danbury and South Woodham Ferrers.

Objectives

1.4.2 The Chelmsford Borough Council Appendix within the Mid Essex SFRA has been undertaken to meet the following key objectives:

1. Provide information and guidance to enable the Local Planning Authority (LPA) to apply the Sequential Test within their district;

2. Present the detailed results of the flood hazard mapping and the breach analyses undertaken for the SFRA to provide the LPA with a more comprehensive planning tool specific to their district;

3. Provide a technical assessment of the River Chelmer Flood Risk Study to confirm whether proposed Flood Storage Areas within the Chelmsford region will allow future development within this area;



4. Consider the likely flow paths and ‘risk’ areas as a result of the proposed storage reservoir areas; and

5. Identify flood risks to proposed strategic locations and site specific land allocations relating to both greenfield and brownfield land.

The Sequential Test

1.4.3 The process of the Sequential Test outlined in PPS25 aims to steer vulnerable development to areas of lowest flood risk. The SFRA aims to facilitate this process by identifying the variation in flood risk across the Borough allowing an area-wide comparison of future development sites with respect to flood risk considerations.

1.4.4 The Borough of Chelmsford has been delineated into the flood zones outlined in PPS25 as Flood Zone 1, low probability, Flood Zone 2, medium probability and Flood Zone 3a, high probability. In addition, Flood Zone 3b, functional floodplain, has also been mapped. Table D.1 of PPS25 provides information on which developments might be considered to be appropriate in each flood zone, subject to the application of the Sequential Test and the Exception Test with a site-specific Flood Risk Assessment demonstrating safety.

1.4.5 In accordance with PPS25 Chelmsford Borough Council have completed a Sequential Test process for their spatial strategy and each of their proposed strategic locations. This identifies the flood risks and development vulnerability in order to assess the suitability of each development location, and where possible steers more vulnerable developments to areas of lower flood risk.

The Exception Test

1.4.6 Where it can be demonstrated by the Local Planning Authority that the Sequential Test is passed, it will also be necessary in some circumstances for the Council to demonstrate that all three elements of the Exception Test are satisfied.

Flood Sources

1.4.7 Chelmsford, Little Waltham and other rural villages within the Chelmer, Can and Wid catchments are susceptible to flooding. Surveys and modelling carried out in 2005 by the Environment Agency resulted in flood map changes reflecting a lower standard of protection than had previously informed planning policy and site specific flood risk assessments.

1.4.8 In the Chelmsford area, the flood defences upstream of the town centre are currently considered to provide protection to a 1 in 20 year standard. Downstream of the confluence of the River Can and Chelmer the standard of protection is considered as a 1 in 10 year standard on both banks. These low defence standards have repercussions on future development within the town centre, requiring flood



compensation storage and detailed design considerations for future development sites meeting the Exception Test.

1.4.9 It is important for the town of Chelmsford that flood defence protection is maintained and improved to at least the 1 in 100 year fluvial standard of protection inclusive of climate change as required in Planning Policy Statement 25 (PPS25). A study is currently being carried out to assess the viability of managing flood risk through the provision of a flood alleviation scheme. The effects of this potential future management option are assessed in this report.

1.4.10 At the time of writing this report the viability study was approaching completion. The preferred option of a Flood Storage Area on the River Wid combined with some minor flood defence work, would provide an increased standard of protection of 1 in 200 years (equivalent to 1 in 100 years and climate change) for existing developments and at least 9 proposed future developments in the Chelmsford town centre. Further work is ongoing to identify the additional works required to increase the standard of protection to the remaining proposed sites in the town centre in collaboration with Chelmsford Borough Council. The scheme may currently not be eligible for grant aid and therefore other sources of funding, such as developers contributions, must be sought.

1.4.11 Prior to the construction of a Flood Alleviation Scheme, proposed development situated within Flood Zones 2 or 3 will require flood risk mitigation measures to ensure that development is ‘safe’. These measures will be determined on a site-by-site basis, as part of a site-specific flood risk assessment.

1.4.12 Areas of ‘functional floodplain’ (i.e. Flood Zone 3b) have been identified. These areas are generally open space areas that flood relatively frequently, and are not defended.

1.4.13 In areas at risk from tidal flooding in South Woodham Ferrers, residual risk of flooding (when defences are overtopped or breached) is identified as an issue in relation to both existing and future development. For this reason an overtopping and breach analysis of the tidal defences was conducted as part of the SFRA to provide information on the potential consequences of flooding in such circumstances and assist in future emergency planning for the area should such an event occur.

1.4.14 Surface water and groundwater flooding data for the study area was limited, although some postcode locations were identified from Anglian Water records. These reports were reviewed and incorporated into the report.

1.4.15 There are no land use or development restrictions within Flood Zone 1, Low Probability (i.e. all remaining areas of the Borough). These include the large greenfield areas proposed as broad locations for growth in the Core Strategy. However, it is important to note that surface run-off from development within these areas, if not carefully designed and managed, may exacerbate existing flooding and/or drainage problems



further downstream. Therefore in accordance with PPS25 all developments should seek to incorporate Sustainable Drainage Systems (SuDS).

1.5 Way Forward 1.5.1 The risk of flooding posed to properties within the Borough arises from a number of

different sources including river flooding, tidal flooding and surface water flooding.

1.5.2 The main settlement in the Borough is situated in proximity to a main river and associated floodplain. Much of Chelmsford town centre is within Flood Zones 2 and 3. Therefore a considerable proportion of proposed site-specific allocations in the town centre are at risk of flooding.

1.5.3 A spatial planning solution to flood risk management should be sought wherever possible. It is necessary for the local authority to consider through the PPS25 Sequential Test how to steer vulnerable development away from areas affected by flooding. This should also take into consideration other relevant strategies and studies in the area seeking to reduce flooding to those already at risk within the Borough. Specific planning recommendations have been provided at the end of this report.

1.5.4 Where other planning considerations must guide the allocation of sites and the Sequential Test has been satisfied, further studies have been carried out to assist the local authority and developers to meet the Exception Test.

1.5.5 Engagement with the Emergency Planning Team and ‘Blue Light Services’ is imperative to minimise the risk to life posed by flooding within the Borough. It is recommended that the Council review their adopted flood risk response plan in light of the findings and recommendations of the SFRA.

1.6 A Living Document 1.6.1 The Mid Essex SFRA has been completed in accordance with PPS25 and the current

guidance outlined in the draft Development and Flood Risk: A Practice Guide Companion to PPS25 ‘Living Draft’ (Feb 2007).

1.6.2 The SFRA has been developed by building heavily upon existing knowledge with respect to flood risk within the Borough. Ongoing modelling in respect of proposals for flood alleviation measures may significantly improve current knowledge of flood risk within the Borough over time, and may alter predicted flood extents within the Borough through improved defence (or alleviation measures). This may therefore influence future development control decisions within these areas.

1.6.3 In summary, it is imperative that the SFRA is adopted as a ‘living’ document and is reviewed regularly in light of emerging policy directives and an improving understanding of flood risk within the Borough.



2 Introduction and Background 2.1.1 Scott Wilson was commissioned by the Mid Essex Area Liaison Group (MEAL) to

undertake a Strategic Flood Risk Assessment (SFRA) on behalf of the local authorities of Braintree District Council, Chelmsford Borough Council, Colchester Borough Council and Maldon District Council.

2.1.2 The SFRA identifies flood risk issues relevant to both existing and proposed developments within the area of Mid Essex and the individual planning authorities, allowing a direct input into the strategic planning of the Mid Essex region through local development frameworks. The SFRA process also aids local authorities to meet the requirements of Planning Policy Statement 25 (PPS25) ‘Development and Flood Risk’ which was published in December 2006.

2.1.3 In addition to the main report, Scott Wilson was commissioned to produce four reports to address the flood risk concerns specific to each local authority. This is the report for Chelmsford Borough Council. The scope and objectives for the Chelmsford area are addressed in the following section.

2.2 Scope and Objectives 2.2.1 This report has been undertaken for the local authority of Chelmsford Borough

Council.

2.2.2 The purpose of this report is to:

• Provide information and guidance to enable the Local Planning Authority (LPA) to apply the Sequential Test within their district;

• Present the detailed results of the flood hazard mapping and the breach analysis undertaken for the SFRA to provide the LPA with a more comprehensive planning tool specific to their district;

• Provide a technical assessment of the Environment Agency flood study to confirm whether proposed Flood Storage Areas within the town will allow future development within this area;

• Consider the likely flow paths and ‘risk’ areas as a result of the proposed storage reservoir areas; and

• Identify flood risks to proposed allocation areas in both greenfield and brownfield sites.



2.3 Report Structure 2.3.1 The report deals in turn with fluvial, overland, drainage and tidal flood risk; shows the

results of a breach analysis and provides guidance for the Sequential Test and Flood Risk Assessments (FRA’s).

2.3.2 This report is comprised of one volume, which forms an appendix of the main SFRA report: Appendix B – Chelmsford Borough Council. The Main Report (Mid Essex Strategic Flood Risk Assessment, Scott Wilson) should however be referred to for Flood Mapping and Application and Methodology information.



3 Level 1 Assessment

3.1 Level 1 SFRA – Study Area, Flood Source Review and Data Review

3.1.1 The objective of the Level 1 SFRA is to collate and review available information on flood risk for the study area. The Inception Report, which is in effect a Level 1 report without mapping any growth areas with respect to flood risk considerations, was completed prior to the release of PPS25 Draft Practice Guide. Therefore to ensure this document is consistent with future policy, the Inception Report should be used in conjunction with the Level 1 Assessment tables presented in this section.

3.1.2 The Level 1 SFRA addresses Objective 1 (Section 1.4) and forms part of the evidence base (Objective 5, Section 1.4) for the study area.

3.1.3 A Level 1 SFRA is designed to be sufficiently detailed to allow the application of the Sequential Test on the basis of Table D1 of PPS25 and to also identify whether application of the Exception Test is expected to be necessary. Information from this stage can also be used to assess how any environmental objectives relating to flooding, as defined in the sustainability appraisal, may be affected by any additional proposed developments.

3.1.4 This stage in the SFRA is primarily a desk-based study, which should use existing information for a number of sources, outlined below.

• Environment Agency Flood Map

• Regional Flood Risk Appraisal (RFRA) where available, (including all the sources of data referred to in the guidance provided on their preparation)

• National Flood and Coastal Defence Database (NFCDD) and National Flood Risk Assessment (NaFRA)

• Any available expert advice from the Environment Agency. This may be in the form of reports containing the results of detailed modelling and flood mapping studies, including critical drainage areas and on historic flood events.

• Consultation with other flood risk professionals including: Internal Drainage Boards (IDB’s), water companies, highways authorities, local authorities (in their role as statutory drainage (operating authority)), navigation authorities and informed local sources

• Maps of geology and soil. These allow the potential for the implementation of source control and infiltration techniques, groundwater and surface water flood risk to be investigated and assessed.



3.1.5 This information as a whole should be sufficient to allow application of the Sequential Test and subsequently inform the Sustainability Appraisal and any succeeding plan policies.

3.1.6 In the event of the Level 1 SFRA demonstrating the potential need of the application the Exception Test, either due to current levels of flood risk or due to an increase in flood risk resulting from climate change, further data collection and/or analysis will need to be carried out, this should be investigated and incorporated into the Level 2 SFRA.

3.1.7 Figures B6-B23 are split into two series of maps, Figure BX-1 and Figures BX-2. The BX-1 series show the current flood zones in accordance with PPS25 for 2007.

3.1.8 PPS25 states an appropriate allowance should be included for climate change over the lifetime of the development, considered for residential development to be 100 years. Therefore the Figure BX-2 series show the flood zones with 100 years of climate change for 2107.

3.1.9 Table D.1 of PPS25 provides information on which developments might be considered to be appropriate in each flood zone, subject to the application of the Sequential Test and either the Exception Test or a site-specific Flood Risk Assessment demonstrating safety.

3.2 Chelmsford Borough Council Area Human Geography/Demographics

3.2.1 The Chelmsford Borough covers over 33,800ha with a population of 157,748 (see Figure B1). The main town is Chelmsford, with fringe towns of Great Baddow and Writtle. The other major settlement in the Borough is South Woodham Ferrers.

Development Plan

3.2.2 The Borough Council submitted its core strategy and development control policies and Chelmsford town centre area action plan documents to the Secretary of State for formal examination in November 2006 with hearings commencing on 11 September 2007. These set out how the Borough Council will meet its future development needs until 2021. This includes identifying land for the provision of 14,000 new dwellings. The bulk of the new housing requirement will be provided on previously developed land within the urban areas of Chelmsford, South Woodham Ferrers and key villages.

3.2.3 In accordance with PPS25 Chelmsford Borough Council have completed a Sequential Test process for their spatial strategy and each of their proposed strategic locations. This identifies the flood risks and development vulnerability in order to assess the suitability of each development location, and where possible steers more vulnerable developments to areas of lower flood risk.



3.2.4 The core strategy will be developed through area action plans. These will be used to provide the planning framework for areas where significant change and conservation is proposed, focusing on the planned growth (growth options) and conservation areas (the green wedges).

3.2.5 The Chelmsford Town Centre Area Action Plan reinforces Chelmsford’s position as a regional focus for business, shopping and community functions. Chelmsford town centre has a role to play in assisting to deliver the Borough’s housing requirements and encourage the re-use of previously developed land (brownfield sites), identified in the plan as opportunity sites that in most cases will provide a mix of uses including residential.

3.2.6 Other sites with potential for residential development will be allocated in the forthcoming site allocation DPD’s based on the Council’s urban capacity study. The principle of concentrating development, where possible, within existing urban areas follows central government’s advice and guidance.

3.2.7 There are currently 38 potential development sites within Chelmsford town centre; all are brownfield land and 20 of these lie within flood zone 2 and 3.

3.2.8 The North Chelmsford Area Action Plan will be put in place to assist in achieving the Borough’s housing requirements as not all the housing requirements can be achieved through brownfield sites.

3.2.9 The Council’s greenfield housing allocation will be achieved through the creation of two new neighbourhoods to the north of urban Chelmsford, in the vicinity of Broomfield Hospital and to the north of Springfield. These will deliver approximately 4,100 new homes and will be accompanied by a range of infrastructure improvements including a Northeast Chelmsford bypass, a link road access the Chelmer Valley to Broomfield Hospital and a new rail station at Springfield.

Physical Geography

3.2.10 The southern boundary of the Borough follows the northern bank of the River Crouch with the other major watercourse in the Borough being the River Chelmer, which flows through the town of Chelmsford.

3.2.11 The River Chelmer runs from its source in Thaxted in a southeasterly direction to the tidal discharge point in the Blackwater Estuary at Beeleigh Falls in the neighbouring district of Maldon. It has two major tributaries; the River Can and the River Wid. The total catchment area of the River Chelmer to Beeleigh Falls is 650km2, of which the River Can catchment is 228km2 and the River Wid is 137km2. The River Chelmer catchment is generally low lying and topography is gently undulating.

3.2.12 The upstream main river limit of the River Chelmer is situated at Thaxted. The river passes through Great Dunmow and the villages of Great Waltham and Little Waltham



before flowing through the town of Chelmsford. Downstream of Chelmsford the Chelmer is canalised forming part of the Chelmer and Blackwater navigation systems. The main watercourses that influence flooding in the Chelmsford Borough are the River Chelmer, River Can (the upstream extents are situated north west of Chelmsford before it converges with the River Chelmer in the town centre) and the River Wid (this is located south west of Chelmsford and converges with the River Can just west of Chelmsford). The River Can is the major tributary of the Chelmer and in turn the Wid a tributary of the River Can. Apart from the Can and Wid, most of the tributaries within the Chelmer catchment do not pose any significant risk of flooding to developed areas.

3.2.13 The other watercourses within the Borough of Chelmsford are:

• Roxwell Brook

• Boreham Brook

• Newlands Brook

• One Bridge Brook Chignall

• Baddow Meads Ditch

• Fen Brook

• Rettendon Ditch

• Runwell Brook

• Margaretting Brook

• Sandon Brook

• Sandon Brook East Arm

• Eyotts Farm Ditch

3.2.14 The location of these watercourses can be seen in Figure B2. Detailed flood risk mapping has been obtained from the Environment Agency for all the watercourses listed above apart from the Runwell Brook and the Eyots Farm Ditch, where the information is not available.

3.2.15 The River Crouch extends through Maldon and Rochford before reaching the Borough of Chelmsford with the upper tidal limit of the River Crouch at Battlesbridge within the Borough of Chelmsford. Downstream of Battlesbridge is South Woodham Ferrers, a new town that has developed from a small settlement with a population of 16,500 and lies on the north bank of the River Crouch. As part of this study, flood hazard mapping of South Woodham Ferrers has been undertaken.



Geology

3.2.16 The solid geology of the Chelmsford Borough can be separated into two areas; Northern areas of the district are underlain by the London Clay Formation (composed of clay, or silty clays with small calcareous nodules and selenite crystals), southern areas are characterized by outcrops of the Claygate Beds (silts and silty clays with inter-bedded fine grained sands) overlying the London Clay and are generally found associated with higher ground. Occasionally the Bagshot Beds (fine grained sands) are found overlying the Claygate Beds. Near Tye Green the Bagshot Beds are overlain by the Bagshot Pebble Bed (approximately 4m of rounded black flint pebbles)

3.2.17 Drift deposits overlying the solid geology consist mainly of the Lowestoft Formation in the northern area of the district, which comprises Glaciofluvial Deposits, Till and Glaciolacustrine Deposits except in the areas around large river channels where Head Deposits are prevalent. In the southern area of the district the predominant superficial deposit are the Head Deposits.

3.2.18 River Terrace Deposits and alluvium tend to be located around river channels.

3.3 Sources of Flooding 3.3.1 The Main SFRA report covers the whole of the Mid Essex area. This appendix of the

report provides a summary of the flooding sources for the Borough of Chelmsford. Information regarding the different mechanisms of flooding is contained in the main SFRA.

3.3.2 Chelmsford Borough contains both fluvial (river) and estuary (tidal) watercourses. This document assesses the flooding sources that are considered to have the largest potential consequences.

3.3.3 Covered by the Level 1 SFRA:

• Only major rivers (River Chelmer, including the River Can and River Wid tributaries, and the River Crouch) have been assessed as fluvial sources of flood risk due to the limited information available on the smaller river systems.

• Likewise structural failures, such as breaches in sea defences or the failure of barriers, have been assessed only at specific points identified in the Inception Report (Scott Wilson November 2006).

3.3.4 Excluded from the Level 1 SFRA:

• The mechanical or structural failure of localised defence barriers or mechanisms such as demountable flood boards, pumps designed to drain individual properties and the like has not been considered in this report as it is not appropriate for a strategic level study.



• Flood risk associated with smaller localised streams and failure of property specific flood defence systems will be covered under site specific flood risk assessments when planning consent is sought.

3.3.5 The River Crouch is the main pathway of tidal flooding in the Chelmsford Borough. Tidal flooding can result from a storm surge, high spring tides or both events combined over undefended land. Figure B3 shows the areas at risk from fluvial and tidal flooding which have been classified as Flood Zones 2 and 3 dependent on the assessed risk.

3.4 Climate Change 3.4.1 In the UK the effects of climate change over the next few decades is estimated to

result in milder wetter winters and hotter drier summers. An increased frequency of heavy, intense precipitation and storms will lead to different rainfall patterns resulting in changes in peak river flows. The rise in sea levels will increase the duration and magnitude of tide locking affecting all tidal areas. Although the combined effect of climate change and sea level rise at the river catchment scale is uncertain, these factors are expected to have a major influence on the potential for future flooding.

3.4.2 Consequently PPS25 requires flood risk studies to consider the potential impacts of climate change on flood risk for the lifetime of proposed developments.

3.4.3 Climate change has been addressed in both the fluvial and tidal flood zone mapping. As discussed in the Methodology (Chapter 6 in the Main Report), PPS25 includes allowances for climate change on a yearly increment scale (table 6-1 and 6-2). One of the main uses of the SFRA is to assist in the Sequential Test and development of local planning policy including the core strategy. As a result residential development tends to be the prime concern, and should consider a minimum of 100 year for climate change, with commercial and industrial development considering either the design lifetime of the development or 60 years of climate change (In accordance with PPS25 and Environment Agency guidance the lifetime of residential developments should be considered as 100 years). The 100-year climate change mapping represents a conservative estimate that would be required for future commercial development.

3.4.4 When considering flooding from the sea, allowances for regional rates of sea level rise should also be taken into account, in combination with the sensitivity ranges for wave height and wind speed. Climate change allowances and sensitivity ranges for peak rainfall intensities and peak river flows, offshore wind speeds and wave heights under PPS25 are outlined below, Table B3-1.



TABLE B3-1 RECOMMENDED CONTINGENCY ALLOWANCES FOR NET SEA LEVEL RISE (FROM TABLE B.1, PPS25).

New Sea Level Rise (mm/yr)

Relative to 1990

Administrative Region

1990 to 2025

2025 to 2055

2055 to 2085

2085 to 2115

East of England, East Midlands, London, SE England (south of Flamborough Head)

4.0 8.5 12.0 15.0

South West 3.5 8.0 11.5 14.5

NW England, NE England (north of Flamborough Head)

2.5 7.0 10.0 13.0

TABLE B3-2 RECOMMENDED NATIONAL PRECAUTIONARY SENSITIVITY RANGES FOR PEAK RAINFALL INTENSITIES, PEAK RIVER FLOWS, OFFSHORE WIND SPEEDS AND WAVE HEIGHTS (FROM TABLE B.2, PPS25).

Parameter 1990 to 2025

2025 to 2055

2055 to 2085

2085 to 2115

Peak rainfall intensity +5% +10% +20% +30%

Peak river flow +10% +20%

Offshore wind speed +5% +10%

Extreme wave height +5% +10%



3.5 Historic Flooding TABLE B3-3: RECORDED FLUVIAL FLOODING ON THE RIVER CHELMER DURING THE 20TH CENTURY Report Reference

Location Major Flood Event (date)

Reported Flooding

Essex River Board Engineer’s Report, 1960

Chelmsford 13th and 14th

March 1947 Flooding occurred due to a winter type of flood. Heavy rain and snow fell between January and March, freezing on the ground due to prolonged cold weather. When temperatures rose quickly in mid-March the runoff from the snowmelt caused extensive damage in Chelmsford.

Essex River Board Engineer’s Report, 1960

River Can Catchment

6th September 1958

This was a summer type of flood arising from intense rainfall of short duration on a saturated catchment over the River Can catchment.

Flooding Report, 1968

River Chelmer

17th September 1968

Flooding recorded downstream of Paper Mills Bridge, on the road but negotiable by car. Flooding at Felsted Mill and Church End. No properties were affected in Chelmsford.

Media Reporting and River Chelmer Flood Risk Study

River Chelmer Catchment

October 2000 The flooding was the result of what proved to be the wettest Autumn since records began in the 1700s; river catchments were saturated and did not hold water which therefore ran straight into rivers. The worst affected town was Little Waltham where 8 properties were flooded. The peak flow on the River Can was larger in October 2000 then 2001.

Media Reporting and River Chelmer Flood Risk Study

River Chelmer Catchment

October 2001 The catchment was inundated with a large quantity of rain which resulted in flooding. Peak flow was larger on the River Chelmer in 2001 than 2000. Many properties were flooded including 10 properties in Great Dunmow, 14 in Little Waltham, 2 in Broomfield, 2 in Brook End and the Rivermead Industrial Estate in Chelmsford

* Taken from the River Chelmer Flood Risk Study (Black and Veatch 2006)

3.5.1 The most significant flooding events in this area in recent times were the fluvial flooding events in October 2000 and October 2001 (River Chelmer Flood Risk Study 2006). These events were initially assessed by the Environment Agency as having a return period of 1 in 200 years within the Chelmer catchment (i.e. a 0.5% probability of occurring in any year). This assessment was based on results of flood studies carried



out in an earlier report (‘Flood Defence Standards of Protection in Chelmsford: River Can and Chelmer at Chelmsford’ (Halcrow Water 2000)).

3.5.2 Black & Veatch later re-assessed this estimate in the River Chelmer Flood Risk Study (2006) suggesting that the peak flows recorded during these events had a return period in the range 1 in 20 to 50 years, (discussed in the ‘River Chelmer Hydraulic Modelling Report’ (Black and Veatch 2006)).

3.5.3 Following these flooding events the Environment Agency initiated a series of flood risk studies to identify the current standards of protection and potential works to improve these standards. One of these studies was the River Chelmer Flood Risk Study. This study was completed in August 2006 and was a strategic study for the Chelmer Catchment. To carry this work forward and identify if there is a viable flood alleviation scheme for Chelmsford, a viability study was commissioned.



4 Level 2 SFRA 4.1.1 The objective of the Level 2 SFRA is to reduce the level of uncertainty regarding flood

sources for any development sites that cannot be located in Flood Zone 1 after application of the Sequential Test. The principal purpose of a Level 2 SFRA is to facilitate application of the Exception Test. This is achieved through a more in depth study, with particular attention paid to the details and nature of flooding taking into account the presence of any flood risk management strategies.

4.1.2 The Level 2 SFRA will then facilitate the adoption of the sequential approach within a flood zone when allocating sites. In addition, this increased level of detail will allow the policies and practices required to ensure developments within a flood zone satisfy the requirements of the Exception Test.

4.1.3 The additional modelling and hazard mapping presented in Figures B24-B37 presents the Level 2 SFRA information sufficient for the application of the Exception Test to those sites that cannot be located in lower flood risk zones. This modelling presents details on the nature of the flood hazard within a flood zone and includes flood depths, flood velocities, and flood hazard.

4.1.4 This modelling allows the consideration of the variation in flood risk within a flood zone. In order to achieve this the following outputs are contained within this Level 2 SFRA:

• An appraisal of current condition of flood defence infrastructure and of likely future policy with regard to its maintenance and upgrade

• An appraisal of the probability and consequences of overtopping or failure of flood risk management infrastructure, including an appropriate allowance for climate change

• Maps showing the distribution of flood risk across flood zones

• Guidance on the Exception Test for sites within flood zones

• Guidance on the content of Flood Risk Assessments for sites with varying flood risk

4.1.5 Guidance on flood risk management options for sites situated within Flood Zones 2 and 3 protected from flooding by defences will be included within this Level 2 SFRA.

4.1.6 The assessment has been made at a strategic level and is intended only to inform how the locations may be at risk from each source. This SFRA should therefore form a ‘stepping-stone’ for site-specific FRA’s, considering the recommendations discussed throughout in this report and the main SFRA report.



4.2 What is the Exception Test? 4.2.1 The Exception Test is only appropriate for use when there are large areas of

development in Flood Zones 2 and 3, where the Sequential Test alone cannot deliver acceptable sites, but where continuing development is necessary for wider sustainable development reasons. There must be evidence to prove that the Sequential Test has been applied to a particular area to support the outcome.

4.3 What is Required to Pass the Exception Test? 4.3.1 For the Exception Test to be passed:

• It must be demonstrated that the development provides wider sustainability benefits to the community that outweigh flood risk, informed by a SFRA;

• The development should be on developable previously developed land or, if not, it must be demonstrated there is no such alternative land available; and

• A FRA must demonstrate that the development will be safe, without increasing flood risk elsewhere, and, where possible, reducing flood risk overall.

4.3.2 All three parts of this test must be satisfied in order for the development to be considered appropriate in terms of flood risk. There must be robust evidence in support of every part of the test.

4.3.3 A Level 1 SFRA should identify sites that, through application of the Sequential Test, require the Exception Test. Details of how to undertake the Exception Test are addressed in a Level 2 SFRA.

4.3.4 The Level 2 SFRA corresponds to a more in-depth study of flood risk required to facilitate the application of the Exception Test, and to allow a sequential approach to site allocation within a Flood Zone i.e. preferentially developing those sites situated in an area of lower hazard within a Flood Zone.



5 Fluvial Flooding Sources

5.1 Sources 5.1.1 The main fluvial flooding source in the Borough of Chelmsford is the River Chelmer.

The River Can is a major tributary of the River Chelmer, which drains in a southeasterly direction. The River Wid is a tributary of the River Can. Apart from the Rivers Can and Wid, most of the other tributaries within the catchment pose little or no flood risk issues.

River Chelmer

5.1.2 The source of the River Chelmer can be found 2.5km upstream of Thaxted. The river is approximately 75km in length and flows in a southeasterly direction. The outfall of this river is at the Blackwater Estuary. The upstream tidal limit of the River Chelmer is at Beeleigh Falls. There are many tributaries of the River Chelmer, of which the Rivers Can and Wid are the most major.

5.1.3 The catchment is largely low-lying in nature with gently undulating topography. The total area of the river catchment is 650km2. The upper reaches of the Chelmer flows through a number of towns, including Great Dunmow, Great Waltham and Little Waltham, and farmland before entering Chelmsford from the north. The river then flows through Little Badow, Utling and Langford.

5.1.4 The catchment is characterised by agriculture including cereals production and livestock. The solid geology of the catchment is characterised by Chalk in the upper catchment and London Clay in the lower reaches. Both the Chalk and the London Clay are overlain by Boulder Clay (River Chelmer Flood Risk Study 2006).

5.1.5 The River Can and Wid tributaries originate to the west of Chelmsford and join downstream of Writtle and upstream of Chelmsford.

5.1.6 The river channel is generally natural but downstream of Chelmsford the river has been canalised and makes part of the Chelmer and Blackwater Navigation. The Chelmer and Blackwater navigation was constructed in the 1790’s to enable the transport of goods up the River Chelmer, (previously goods were transported by horse from Maldon). This route runs from Springfield Basin in Chelmsford to Heybridge near Maldon. Springfield Basin was constructed to allow the transport of goods, e.g. coal, timber, into this rapidly developing industrial area and was fed by the River Chelmer. As railways began to transport the majority of goods, the Chelmer and Blackwater Navigation and Springfield Basin became redundant and fell into decline. The Springfield Basin was however restored in the mid 1990’s.



River Can

5.1.7 The River Can is a major tributary of the River Chelmer and runs in a south westerly direction into the River Chelmer to the south of the town centre. The catchment is 228km2 in total. Again, the terrain is generally low-lying and gently undulating.

River Wid

5.1.8 The River Wid is a tributary of the River Can. The total catchment is 137km2 in area. This river drains to the north and into the River Can to the north of Writtle.

5.1.9 Chelmsford and several other rural villages adjacent to the River Chelmer are susceptible to infrequent flooding. This has been noted by the historical flood events detailed in Table B1.

Pathways

5.1.10 The main pathway of fluvial flooding is as a result of high rainfall events in the catchments of the Rivers Can, Wid and Chelmer. Water then travels via a number of routes into the river channels and results in high flows, or possibly out of bank flows, depending on the rainfall intensity and the antecedent conditions of the catchments. As the River Wid flows into the River Can and the River Can flows into the River Chelmer, the River Chelmer receives a large volume of floodwater. The peaking of the Rivers Can and Chelmer may occur concurrently, or with a lag, depending on the circumstances of the storm and the conditions of the catchment. Halcrow assume a coincidence in the peak flows on both rivers, which may be conservative. This is however fully explained below (paragraph 5.2.4 and onwards).

5.1.11 High water levels along any of these rivers resulting in out of bank flow may cause flooding to Chelmsford as these rivers run through the settlement (the Can and Wid converge upspream of Chelmsford but high flows on one tributary may lead to flooding downstream).

Receptors

5.1.12 The Environment Agency’s detailed flood maps produced as part of the River Chelmer Flood Risk Study shows the predicted extent of fluvial flooding in the River Chelmer catchment during the estimated 1 in 100 year (1%) and the 1 in 1000-year (0.1%) flood event. In the upper reaches the flooding is confined to strips of land adjacent to the watercourses and only villages on the watercourses such as Little Waltham are at high risk. In the lower reaches villages such as Broomfield, Chelmsford and Brook End have large floodplain extents associated with them.



Defences

5.1.13 The River Chelmer Flood Risk Study outlines significant river improvement works that were carried out in Chelmsford in the late 1960s following the very severe floods of March 1947 and September 1958. These included:

• Installation of Chelmsford Sluice Gates, located downstream of the confluence of the Rivers Chelmer and Can and downstream of the main urban area. There is one tilting gate and two radial gates, which are all fully automatic and dependent on river levels. These are used to retain an artificially high water level of 22.7 mAOD through Chelmsford and were designed to pass flood flows of up to the magnitude of the severe March 1947 flood event;

• River realignment comprising river widening, straightening and deepening, from upstream of Springfield Gauging Station at Victoria Road Bridge on the River Chelmer to the new sluice gates;

• River realignment comprising of river widening, straightening and deepening, upstream of Beach’s Mill Gauging Station on the River Can to the confluence with the River Chelmer; and

• Raising of flood embankments on the River Can (approximately 601m in length) and within the main shopping centre on the River Chelmer (930m in length).

5.1.14 The scheme was designed to reduce to a minimum any loss of flood plain, therefore areas such as the recreation ground, cricket ground and tennis courts beside the River Can were not defended.

5.1.15 The River Chelmer upstream of the tidal limit (located at Beeleigh Weir to the north west of Maldon) is currently classed as undefended (i.e. current defences do not offer the statutory protection to the 1 in 100 year standard). Defences are present along the Chelmer, such as the Chelmsford Sluice Gates located downstream of the confluence of the River Chelmer and River Can and downstream of the main urban area, further detail on each of the defences can be found in the River Chelmer Flood Risk Study (Black and Veatch & The Environment Agency 2006).

5.1.16 The Chelmsford Sluice Gates include one tilting gate and two radial gates. The gates are fully automatic and are dependent on river levels in their operation. The presence of these gates retains an artificially high water level throughout Chelmsford for aesthetic reasons but were also designed to control flood flows up to the magnitude of those seen in the severs March 1947 flood event.

Structures

5.1.17 There are 12 mills and 12 locks in the study area. The Agency’s mill rights for Felsted Mill and Little Waltham Weir require them to maintain the brickwork of the structures. The mill owner is responsible for maintenance of the gates at Felsted Mill. However, as



with all the other mill structures on the River Chelmer the current assessment is that if the owners fail to carry out maintenance work to the gates, there is believed to be only a local effect on water levels.

5.1.18 The River Chelmer Flood Risk Study identified through hydraulic modeling that if the mills and control structures were either not opened during flood events, or alternatively, were not maintained and allowed to collapse thereby blocking the channel, there would be an increase in water levels immediately upstream.

5.1.19 Significant river improvement works were carried out in Chelmsford in the late 1960s following the very severe floods of March 1947 and September 1958. These included the installation of the Chelmsford Sluice gates, river alignment works (widening, straightening and deepening of the channel) and the raising of flood embankments in selected areas.

5.1.20 The only area where there is a significant backing up is at the Chelmsford Sluice Gates, where leaving the gates closed would cause extensive flooding in Chelmsford town centre. During flood conditions the sluice gates downstream of the confluence of the Chelmer and Can are operated by the Environment Agency and may at certain times be opened to alleviate high flows.

5.1.21 Two areas on the River Can where defences have been constructed are located on Prykes Drive (north bank of the river) and Beach’s Drive (north bank of the river).

5.1.22 The residual life of the defences through Chelmsford are likely to be in excess of 20 years.

Flood Defence Options

5.1.23 The River Chelmer Flood Risk Study has been undertaken by the Environment Agency to assess flood risk within the River Chelmer catchment. The River Chelmer Flood Risk Study includes an appraisal of various flood defence options on the River Chelmer, Wid and Can and the likely benefits of these works of varying scale from ‘Do Nothing’ to ‘Flood Storage Options’.

5.1.24 The Environment Agency has undertaken hydraulic modelling of the River Chelmer as part of a larger hydrological study of this river. Detailed flood outlines for the return periods 1 in 20, 1 in 100 and 1 in 1000 year for the rivers Chelmer, Can and Wid have been mapped as part of the study.

Functional Floodplain

5.1.25 PPS25 stipulates that functional floodplain (Flood Zone 3b) is defined as any land that:

• would flood with an annual probability of 1 in 20 (5 per cent) or greater in any year, or at another probability to be agreed between the LPA and the Environment Agency (EA), or:



• is designed to flood in an extreme (0.1 per cent) flood, or at another probability to be agreed between the LPA and the EA.

5.1.26 According to PPS25 this definition includes water conveyance routes. Flood storage areas play an essential role in the storage of flood water they are classified as Functional Floodplain.

5.1.27 This definition in PPS25 does not differentiate between developed and undeveloped areas as developed areas can still contribute to the storage and conveyance of flood water. For example, certain developed areas can be designed to flood periodically to preserve flood storage volumes, i.e. car parks located close to a watercourse. The functionality of these areas should be considered when defining Flood Zones 3a and 3b and strategic flood risk management policies should be taken into consideration.

5.1.28 The delineation of Flood Zone 3b has been undertaken using revised 1 in 20 year outlines from the River Chelmer modelling as part of this study provided by Halcrow Group Ltd (Halcrow’s methodology for the additional modelling is included in Annex B of this document). An outline has been provided for the River Chelmer and the lower River Can and Wid, these are mapped as Flood Zone 3b in the Figure BX-1 series depicting the current PPS25 flood zones for 2007.

5.1.29 To make an allowance for climate change for Flood Zone 3b, the 1 in 20 year event plus a 20% increase in flows was mapped. The River Chelmer model was re-run to determine these outlines which are mapped in the Figure BX-2 series which depict the PPS25 flood zones inclusive of climate change (in accordance with PPS25, 100 years of climate change have been mapped appropriate for residential development considerations).

5.1.30 Some areas of functional floodplain have been identified in Chelmsford town centre. Flood Risk Assessments for sites identified as 3b would need to provide a robust case regarding flood zone classification, identifying site specific flood pathways, topographic levels, and informal defences. If the FRA demonstrates to the satisfaction of the Environment Agency the site is located in 3a, more vulnerable and less vulnerable uses would be permitted provided they passed the Sequential Test and where necessary the Exception Test. If the FRA resolved that the site was located in 3b, then only water compatible or essential infrastructure uses would be permitted.

5.1.31 In areas where Flood Zone 3 has not been differentiated into Flood Zone 3a and 3b, all areas should be considered as Flood Zone 3 until an appropriate Flood Risk Assessment has distinguished this zone into the constituent parts. Only then can an area be considered as located within Flood Zone 3a.



5.2 Fluvial Level 2 Study 5.2.1 The Borough’s development allocations and urban capacity sites have been identified

in the Figures B6-B23, some of which are located within the current Environment Agency floodplain. The extent of the floodplain within each potential site varies significantly.

5.2.2 There may be some smaller watercourses in the Borough that are not mapped by the Environment Agency at present, but have localised flooding associated with them. At this time there is insufficient information available to be able to map these accurately as part of the SFRA. These may need to be addressed in site-specific FRA’s and the Borough should, where possible, keep records of flooding associated with these watercourses.

5.2.3 The redefinition of the Environment Agency floodplain in this report, based upon detailed site topography and adjacent design flood levels will improve the accuracy of the Environment Agency floodplain extent and should be undertaken as part of a site-specific FRA.

Modelling

5.2.4 Halcrow have produced the following text on hydrograph phasing and joint probability flows in January 2008. This text explains the occurrence of ‘dual peaking’ and the methodology behind the modelling of this phenomenon as part of the Environment Agency Flood Compensation Study (2007 and ongoing). The updated flood outlines mapped in this SFRA and produced by Halcrow were created using this methodology.

Hydrograph Phasing

For the purpose of modelling the design events, up to and including the 200 year flood event, it has been assumed that a uniform and concurrent storm occurs over the Chelmer, Can and Wid catchments. This results in the Can catchment (i.e. the catchment area upstream of the Beach’s Mill River gauging station, which includes the Wid catchment) peaking approximately 6 hours before the Chelmer catchment. This accurately reflects the rate of runoff from the more impermeable soil characteristics, which dominate the catchment. This has been observed many times in the past, most recently during the October 2000 and 2001 flood events, although a review of the peaks over threshold data shows 40 occurrences from 177 when the Can and Chelmer peak on different days. The assumption of coincidence of peaks is therefore potentially conservative.

As part of the modelling exercise, a sensitivity analysis was undertaken on hydrograph phasing to test the robustness of this assumption. This involved separating the hydrograph peaks by increasing the time lag by 6 hours and also reducing the time lag by 6 hours to achieve a concurrent peak. The results indicated that hydrograph phasing is relatively insensitive, in terms of its effect



on peak water levels through the town centre. As such the impact on flood related damages is also found to be insensitive. For design purposes the assumption is therefore considered robust and would not merit further assessment.

Joint Probability

During the hydraulic modelling it has been assumed that the 1 in 100 year design flood event (or 1% Annual Exceedence Probability) within central Chelmsford will be produced by a 1 in 100 year flood event from both the Chelmer and Can catchments. The same assumption was made for all other return periods. This is considered a reasonable assumption given that the catchment centres are only 40km apart from one another and therefore low probability (extreme) frontally-driven storm events are likely to affect both catchments. For the purpose of delivering a robust and future-proof flood defence design, and in the absence of sufficient data to support any other approach, the assumption of equal probabilities is considered appropriate.

Halcrow January 2008

Hazard Mapping

5.2.5 To provide a greater level of detail on the fluvial flood risks, an assessment has been made on the hazard associated with the River Chelmer (Can and Wid). The hazard mapping was based on the outputs from the River Chelmer modelling (by Halcrow Group Ltd) and associated assumptions (see above paragraph 5.2.4 and onwards) for the 1 in 100 and 1 in 1000 year plus climate change outlines. The modelling is 1-dimensional and does not have an associated velocity output; therefore the hazard has been classified as a function of depth, assuming zero velocity.

5.2.6 The Hazard categories have been mapped using the “FD2320/TR2 – Flood Risk Assessment Guidance for New Development” depths and associated hazard with an assumed zero velocity as shown in Table 13.1 of that document. This is explained in more detail in Section 6 of the main SFRA.

5.2.7 An estimation of velocity to refine the hazard classification for a site could be made on a site-specific basis in relation to distance from the river, local topography, flow paths etc.



6 Arterial Drainage Network Flooding Sources

6.1 Sources 6.1.1 Localised flooding can occur as a result of severe storms, which are localised in extent

and duration. The intensity of the rainfall in urban areas can create runoff volumes that temporally exceed urbanised sewer and drainage capacities, creating ‘flash’ flooding.

6.1.2 Flooding may occur when the rainfall intensity exceeds the capacity of the storm water sewer system. This is likely to become a more common occurrence in the future, due to climate change and an increase in the number and intensity of convective storms. It is now widely accepted that one of the main effects of climate change in the southeast of England will be a higher intensity rainfall and more frequent winter storms, which will increase the risk of flooding from all sources of flooding.

6.1.3 As part of this study a request was made to Anglian Water for information regarding existing storm water flood risk in the area. A series of post codes were provided for general areas in which flooding from this source had been recorded in the past.

6.1.4 Comparison with the recorded events and allocation sites will help to identify the risk local sewers may have upon particular sites. However, it should be noted that all sewers represent a degree of flood risk through restricted capacities for transporting large volumes of water. Therefore irrespective of their flooding history, an assessment from this source should be made as part of a site specific FRA.

6.1.5 Anglian Water does not hold information with regard to flood risk from drainage networks. In general there is little information available regarding previous flooding incidents from this source and therefore no means of assessing on a strategic scale where flooding from the drainage network may occur in future.

Pathways

6.1.6 If the capacity of sewers is inadequate or a blockage were to occur, manholes are likely to become surcharged, forcing flood water to spill out of the manholes and flood the surrounding area.

6.1.7 The extent of the flood will be defined by the surrounding topography and volume of floodwater involved. For example, if a manhole surcharges in a low lying area/depression, surrounded by higher ground, flooding may be relatively deep but affect an isolated and relatively small area. If flooding were to occur near the top of a hill, floodwater may route down gradient into areas of lower ground. This could potentially exacerbate flooding elsewhere rather than cause flooding near the responsible sewer.



Receptors

6.1.8 Anglian Water has provided a list of post codes where incidents of flooding have been reported and recorded for general households and businesses to date. These are discussed in Section 7.3.

6.2 Arterial Drainage Network Level 2 Study 6.2.1 This section discusses the results of the assessment made with regard to the locations

and the information that has been provided by Anglian Water.

6.2.2 There were found to be 4 historic sewer flood incidents within the Borough that were within 500m of a particular site allocation. These are summarised in Table B6-1 below.

TABLE B6-1 SUMMARY OF POSTCODE AREAS AFFECTED BY FLOODING FROM SEWERS IN THE PAST

Postcode

CM1 1TS

CM1 2BS

CM1 2EJ

CM2 8BS

6.2.3 The information identified above summarises the closest site only. The particular pathway from the sewer to a specific site is a crucial aspect of a site-specific assessment of flood risk. A more distant site may be located down gradient of the sewer and become flooded, whilst the site identified may be raised above the particular sewer and therefore may be considered unlikely to flood. Furthermore, as discussed previously this is not exhaustive, additional sewers are likely to undergo flooding in the future, especially considering the anticipated affects of climate change.

6.2.4 Two of the postcode areas are within close proximity, which suggests that the local sewers flood relatively frequently. A more thorough investigation should be undertaken under these circumstances.

6.2.5 If a particular area is subject to relatively frequent flooding from sewers, this should not prevent the site from undergoing development. Certain mitigation measures can be designed into the development to manage flood risk of this kind.

6.2.6 In those worst affected areas Anglian Water may upgrade the sewer networks in an attempt to alleviate flooding. However, this is a relatively lengthy process considering



the large region that Anglian Water serve. Anglian Water were unable to provide details where future alleviation schemes may be undertaken.

6.2.7 It should be ensured that trunk sewers located within close proximity to any new developments have sufficient capacity for the increase in use associated with new developments. This should avoid surcharging. New developments could incorporate surface water drainage design to direct water away from properties and could also incorporate raised entrances by 300mm to mitigate potential ‘burst sewer’ occurrences, which could result in flooding.



7 Tidal Flooding Sources

7.1 Sources 7.1.1 The River Crouch extends through the districts of Maldon on the north bank and

Rochford on the left bank before narrowing as it extends into the Borough of Chelmsford. The River Crouch is the source of tidal flooding in the Chelmsford Borough. Tidal flooding can result from a storm surge, high spring tides or both events combined over undefended land. In the case of land protected from flooding by sea defences, tidal flooding can occur through either a breach in the sea defences, failure of a mechanical barrier or overtopping of defences.

7.1.2 The upper tidal limit of the Crouch is at Battlebridge, in the Borough of Chelmsford. Downstream on the north bank lies South Woodham Ferrers, which is a relatively new town. The town has grown to become one of the largest settlement areas on the Crouch and Roach Estuary.

7.1.3 The North Essex Catchment Flood Management Plan, due to be published by the Environment Agency in summer/autumn 2007, will assess how flood risks might change and be managed over the next 50 to 100 years. The Scoping Report published in April 2006 outlined the current understanding of flood risk in the North Essex CFMP area. It provided a broad picture of flooding processes (i.e. flooding through overtopping or breaching of defences) and how these may change in the future in relation to the existing defences.

7.1.4 The Essex Estuarine Strategies has similar generic flood management policies. These are Hold the Line, Advance the Line, Managed Realignment and No Active Intervention (Table B7-1, Environment Agency et al. 2006). Of these methods, the one with the greatest potential to affect development proposals within Mid Essex is Managed Realignment. This essentially involves placement of a new defence structure on the landward side of the existing structure. In addition, it can include either partial or complete removal of the existing structure, thus increasing the flood risk to the newly created hinterland.



TABLE B7-1 ESSEX ESTUARINE STRATEGIES FLOOD MANAGEMENT POLICIES

Flood Management Policies Details

Hold the Line Maintaining the existing flood defences and control structures in their present positions and increase the standard of protection against flooding in some areas.

Advance the Line The construction of a new flood management scheme in front of existing flood defences

Managed Realignment The policy of Managed Realignment involves the placement of a new Managed Realignment flood defence landward of the existing flood defences or realignment to higher ground. This policy would be achieved through the partial or complete removal of the existing flood defences or through regulated tidal exchange. This policy would be gradually implemented and regularly monitored in order to study any potential effects on the overall estuary shape.

No Active Intervention There would be no further active intervention by the Environment Agency. Without intervention the defences would eventually fail and areas currently protected from flooding would no longer be protected. This would happen gradually over a long period of time. However, land owners may be entitled to pay for the continued maintenance of the flood defences or undertake maintenance themselves following the preparation of an Exit Strategy.

7.1.5 The Roach and Crouch Flood Management Strategy (R&CFMS) is one of the management plans operating under the generic Essex Estuarine Strategies. It was developed in several stages. Firstly the flood risk strategy objectives and flood management options were defined through a series of consultation exercises. Following this a short list of flood management options was appraised. Such management options included holding the line, managed realignment, advancing the line and no intervention. For the area from these, a preferred flood management strategy was produced. The outcomes of the flood management strategy are not definitive and are only intended to provide an indication of potential locations and measures.

7.1.6 For the first 5 years the R&CFMS indicates little change would be required to the existing defences, recommending some minor sections of realignment that could be possible. The R&CFMS suggests that ‘holding the line’ will be sufficient for South Woodham Ferrers in the short term, as demonstrated in Figure B4.



7.1.7 After 50 years, the R&CFMS foresees the vast majority of the area as requiring potential realignment following detailed studies. The strategy identifies a small area of managed realignment along the Chelmsford coastal boundary (see Figure B5). Chelmsford Borough Council should consider the R&CFMS in their long-term strategy with respect to the flood defences that currently protect their coastal boundary.

Pathways

7.1.8 The River Crouch provides the main pathway for tidal flow in Chelmsford Borough. Defences along the River Crouch protect the areas from tidal flooding and prevent tidal inundation. In the event of a breach in the River Crouch defences, the low-lying areas and drainage channels in and around South Woodham Ferrers provide pathways for floodwater.

Receptors

7.1.9 Based on the methodology presented in Section 6 of the SFRA, the defended areas for the Chelmsford Borough have been separated into flood cell embayments, defined by topographic features, flooding characteristics and potential flood pathways. Figure B24, shows the Chelmsford Borough flood cell in relation to the breach modelling.

Standard and Condition of Sea Defences

7.1.10 Information pertaining to the standard of the Coastal Flood defences has been taken from the National Flooding and Coastal Defence Database (NFCDD) in this area. Flood defences include both man-made raised defences and maintained channels. A man-made tidal wall (concrete revetment) protects the area of South Woodham Ferrers and extends from Clements Green to Marsh farm (TQ 82052 96943) - North Bank Crouch Estuary (TQ 81962 96936). NFCDD classifies the flood defence as ‘good (2)’ condition. The Environment Agency is responsible for maintaining the flood defences in this area.

7.2 Tidal Level 2 Study 7.2.1 One breach location was specified and tested for the Borough of Chelmsford at South

Woodham Ferrers. It was modelled to occur just east of the intersection of Broughton & Creekview Roads. The invert level within the breach was then assumed to be that of the land surrounding the tidal defences that fail, in this case 2.8 m AOD.

7.2.2 Breaches were simulated under four different tidal event scenarios. The 1 in 200 year and 1 in 1000 year events were tested using a model set up that assumed the current levels of flood defences in the area were in place (that is, according to the recently surveyed levels). Hence, some overtopping of defences occurred in addition to the breach itself.



7.2.3 In addition, the model was run to simulate the effects of 100 years of climate change on the 1 in 200 year and 1 in 1000-year events at the existing defence height. Further details of the methodology are outlined in Section 6 of the main SFRA.

7.2.4 The breach model provides a general indication of likely flood extents and depths in the event of a breach in the defences at this location. However, this is a strategic assessment and the model has not included all possible details. Limitations of the model include:

• Only one breach location has been considered - South Woodham Ferrers - therefore the results may vary spatially;

• The effects of major linear features: e.g. Roads and rail embankments have been considered in the modelling, small unofficial defences have not been included in a model;

• Effects of buildings: buildings have not been considered within the modelling and would be expected to reduce flood speeds in the event of a breach.

7.2.5 Considerable parts of the study area are either urbanised or associated with man-made features (e.g. roads, embankments, walls, bridges) that may affect the free flood flow on the floodplain.

7.2.6 Embankments, flood defences, significant water courses and other linear features that may have been misrepresented due to the resolution of the hydraulic model used, have been incorporated into the hydraulic model by adding break lines (i.e. the mesh orientation is forced to follow the alignment of the features and the localised elevations are amended within the ground model). A detailed modelling methodology is presented in Section 6 of the main SFRA Report.

7.2.7 Smaller linear features and individual buildings have not been included in this model however more specific detailed models could be produced as part of a site specific flood risk assessment to refine the results in specific areas of interest.

Hazard Mapping

7.2.8 Table B7- presents the results of the flood risk analysis at South Woodham Ferrers. These results may be used to determine the potential consequences from different scenarios. Hazard zones of high, medium and low hazard have been identified for each breach scenario. The table includes the details of the breach, the maximum floodwater depths resulting from the breach and a summary of the hazard zone analysis.

7.2.9 The 2D breach modelling produces variables for both depth and velocity during the tidal inundation as a result of a breach. The hazard methodology closely follows the Flood Hazard guidance provided in DEFRA/EA R& D publication FD2320/TR2 table




13. However in this instance does not include a debris factor, as this cannot accurately be calculated on a strategic scale.

7.2.10 Breaches in other locations along the defences may result in different hazard zoning. This should be taken into account when determining whether a particular property is within a certain hazard zone. In addition to this, the hazard zone results for this study were produced at a strategic scale not appropriate for determining hazards zone for individual properties on the edge of the zones within flood inundation areas. A precautionary approach is recommended when using the study results at the fringes of hazard zones.



TABLE B7-2: SOUTH WOODHAM FERRERS BREACH RESULTS

GENERAL INFORMATION BACKGROUND BREACH DETAILS

Flood Risk Source: CHELMS – South Woodham Ferrers OS Location: 582000, 197000 Location: Breach runs running from 581970, 196750 to 581970, 196700

Flood Risk Type: TIDAL

Modelled Scenario: Current (no land raising or improvement works)

DTM Generation: From LIDAR data Breach Width: 50 metres wide

Flood Cell(s): South Woodham Ferrers Source of Flood Level Information:

EA, Anglian Region, Eastern & Central Areas, Draft Report on Extreme Tide Levels, Schedule 2.

Base Level of Breach: 2.80m AOD

Figures: Figure B26 Hazard Map of CH01 (1 in 200 year - 2107), Figure B28 Hazard Map of CH01 (1 in 1000 year - 2107)

Defence Type: Clay seawall - Essex blockwork revetment Repair Time (hrs): 18 hours

RESULTS FLOOD DEPTH RESULTS

Flood Cell Description Figure B24 shows the flood cell extent.

Location of Main Flood Depth (1 in 200 Year event)

The location of deepest flood water occurs on the open area within Saltcoast Park and Compass Gardens, with depths of almost 5m AOD. The maximum extent of inundation occurs at approximately 15.5 hours after the breach. At this point there is approximately 4.4m AOD of inundation at the sports ground to the east of South Woodham Ferrers. The east of the town itself experiences inundation depths of between 0.25 and 2.1m, these water depths reduce towards the town centre. The location of the maximum flood depth differs in the 1 in 200 year climate change scenario from that in the 1 in 200 and 1 in 1000 year scenarios. There are marginally less areas of High Hazard witnessed during the climate change, most notably the western edge of Saltcoats Park towards Ferrers Road is now only Medium Risk. This difference arises because the 200 and 1000 year scenarios include the effects of overtopping. The 200 year scenario including the effect of climate change assumes an increase in defence height so the water is focused through a breach and therefore concentrated in a different area. The depth mapping for the 1 in 200 year climate change scenario can be seen in Figure B27.

Return Period (flood probability):

1 in 200 Yr 1 in 200 Yr + CC 1 in 1,000 Yr 1 in 1,000 Yr + CC

Max Depth of Floodwater: 4.4 5.0 5.0 5.9

Location of Max Depth: 582170, 197086 (element 20717) 582170, 197086 (element 20717) 582170, 197086 (element 20717) 582170, 197086 (element 20717)

Time Max Depth first occurs:

15.50 15.00 16.50 15:50

Maximum Flood Velocity: 2.60 3.10 2.88 3.22

Rate of Onset: of Flooding: 20 minutes 20 minutes 20 minutes 20 minutes



HAZARD ZONE RESULTS

1 in 200 Year event The main area of high hazard is situated to the east of the breach location within South Woodham Ferrers. The high hazard zone covers Saltcoate Park and Compass Gardens to the east of South Woodham Ferrers including a school and commercial property area. The medium hazard zone extends into the residential area near Clements Green lane and Overmead Drive. The low hazard area extends further into more of the residential areas and also the commercial area including the police station and fire station. 1 in 1000 Year event There are three main areas of high hazard covering Saltcoate Park and Compass Gardens to the east of South Woodham Ferrers and Marsh Country Park to the south. The flood depth and hazard situation for the 1 in 1000 year case is similar to that described above, but more High Risk zones are seen in the land between Inchbonnie Road and Ferrers Road. Almost all of the Compass Gardens and Saltcoats Park area is now classified as High Risk, as are the school & grounds and much of the Tutors Way and Overmead Drive area. The Low and Medium Risk areas are now more widespread to the west. A band of medium and low flood Hazard runs along the periphery of the breach envelope to the east of South Woodham Ferrers.

Relevance To Development Main Hazard Zones The modelling results indicate that there are three main areas of high hazard in the event of a breach scenario at this location: 1. Saltcoate Park and Compass Gardens to the east of South Woodham Ferrers; 2. Marsh Country Park to the south of the town; 3. To the west of South Woodham Ferrers the sewage treatment works and Woodham Fen nature Reserve. Considerations for South Woodham Ferrers CH01 causes inundation to the east of South Woodham Ferrers and small areas to the periphery of the town to the south and south west. Flood depths near to the school in South Woodham Ferrers are approximately 2m. Specific Development Locations There are four sites allocated in South Woodham Ferrers (urban capacity sites), located within the 1 in 1000 year extent in a medium hazard areas. The land allocated for residential development or redevelopment is located within either the high or medium flood hazard zones. As this land is generally surrounded by existing residential development there may be strong non flood related pressures to redevelop these areas. If the increased flood risk from redevelopment is considered to be outweighed by the overriding pressure for new housing, developers should aim to minimise the increase in flood risk as a result of development. It may be possible to reduce the tidal flood risk by localised land raising however this must be offset against considerations of how land raising would fit in with surrounding land levels. If land were to be raised for this residential development an appropriate flood warning and evacuation procedure would still be required to reduce the risk to life from inundation. The areas of land proposed for residential development would be strongly discouraged by the Environment Agency. In any case the finished floor levels of residential development could be raised above existing levels and flood compatible building materials could be used to reduce the effects of flooding on property.

General Considerations Future development in South Woodham Ferrers should be steered away from the high and medium hazard zones into areas of reduced flood risk. Flood hazard mapping shows that evacuation and access routes to the town are cut during a breach event. Emergency access routes to and from the town should be improved over time to improve response times for breach closure and the ability of emergency services to access flooded areas. In accordance with PPS25, residential development in these areas would be discouraged. Although if through the Sequential and Exception Tests it can be demonstrated that there are no alternative sites, mitigation measures such as raising finished floor levels, flood proofing construction and secondary defences could be implemented to ensure the development was ‘safe’ from flooding.

Functional Floodplain The functional floodplain outlines were derived for the 1 in 20 year and 1 in 20 year + 100 years climate change cases using the maximum tidal levels for such events (3.98 and 5.00 mAOD, respectively) and the LiDAR digital terrain data. The outline was taken as the points where the tidal level intersected the land levels, taking into account any local flood defences.


8 Other Sources of Flooding

8.1 Overland Flow 8.1.1 In the Borough of Chelmsford there are rural areas as well as the major urban areas

and smaller settlements. The surrounding areas of the Borough are rural in nature, comprising of agricultural land and some wooded areas. Infiltration rates, (which reflects the capacity of the soil to allow rainfall to soak into the ground), vary with the nature of the ground surface. Within the agricultural and rural areas, infiltration rates coupled with the volume or intensity of the rainfall, will define the volume and rate of overland flow (runoff) generated from any particular area. Any generated overland flow would naturally route towards the nearest watercourse, stream or ditch, down gradient defined by the local topography.

8.1.2 Following urbanisation, drainage routes are likely to have been disturbed significantly. As a result, many urban areas receive overland flow from surrounding rural hill slopes.

8.1.3 Overland flow often forms one of the first hydrological responses to rainfall within a particular catchment. As such, overland flow from surrounding hill slopes can be observed before the river itself has responded to the rainfall and can potentially result in an early phase of flooding.

8.1.4 The hydrological response of particular catchments throughout the country varies significantly. The underlying geological conditions of the area are a key factor in determining the hydrological response of the river. The underlying geology of the Chelmsford area can be split into two areas, the northern area is underlain by London Clay and the southern area is characterised by outcrops of the Claygate Beds overlying London Clay. These two geological conditions will result in slightly different hydrological responses to rainfall, as both geologies are silty and clayey in nature. Catchments characterised by London Clay deposits will permit little infiltration and result in larger quantities of overland flow (Table 8-1).

TABLE B8-1 TYPICAL INFILTRATION CO-EFFICIENTS FOR DIFFERENT SOIL TYPES

Soil Type Typical Infiltration Co-efficient (m/hr)

Chalk 0.001 - 100

Sandy clay loam 0.001 – 0.1

Clay <0.0001

*Taken from Table 4.7 of the CIRIA C697 SuDS Manual



Pathways

8.1.5 Pathways of overland flow are largely defined by the local topography. Features, both natural and man made may influence the route that floodwaters will take. In urban areas, features such as roads will influence the routing of water travelling overland.

Receptors

8.1.6 Flooding from overland flow (or indeed any source) is not discriminatory on any specific receptors. However, flooding from overland flow is more likely to occur where water is routed down steep slopes or down gradient on hard standing areas. The catchment for the River Chelmer has an undulating topography and is generally low-lying, therefore on a strategic scale overland flow is considered a low risk. However on a site specific scale the risks from this flood source should be identified within a flood risk assessment.

8.2 Surface Water Sources

8.2.1 Localised flooding can occur as a result of severe storms, which are localised in extent and duration. The intensity of the rainfall in urban areas can create runoff volumes that temporarily exceed the urbanised sewer and natural drainage capacities, creating ‘flash’ flooding, referred to in this document as surface water flooding.

8.2.2 Flooding may occur because of overflow from the surface water systems when the rainfall intensity exceeds the capacity for the drainage systems. This is likely to become a more common occurrence in the future due to climate change and an increase in the number and intensity of rainstorms. It is now widely accepted that one of the main effects of climate change in the South East England will be higher intensity rainfall and more frequent winter storms, which will increase the risk of flooding from surface water.

Pathways

8.2.3 Chelmsford Borough Council has been unable to provide details pertaining to specific drainage networks such as highways drains, drainage ditches or non-main river watercourse that may pose flood risk within the Borough.

Mitigation

8.2.4 Chelmsford Borough Council have provided a list of incidents of flooding that have been reported by householders; it can be seen in Annex 1. The flood history database includes records of blocked culverts, surcharged manholes and river flooding.



8.2.5 The goals of SuDS, (see Chapter 9 in the Main Report and Chapter 8 in this report), can be achieved by utilising a management plan incorporating a chain of techniques, (as outlined in Interim Code of Practice for Sustainable Drainage Systems 2004), where each component adds to the performance of the whole system. These are prevention measures, source control, site control and regional control strategies.

8.3 Strategic Spatial Flood Source Summary 8.3.1 Table B8-2 identifies potential impacts to example sites from a combination of the

three key flood sources within the district (i.e. fluvial and tidal, residual flood risk and the arterial drainage network, Table B8-1). This is intended as a tool to summarise the potential consequence of flooding from a site. A weighting factor has been applied to each source. This is intended to represent the different scale or magnitude of the associated flood risk, with the presence of fluvial and tidal flood risk forming the most heavily weighted factor. Weighting factors associated with each flooding source are outlined below, (Table B8-1).

8.3.2 Information pertaining to fluvial and tidal flood risk is also deemed to be the most reliable source of information and forms the key consideration of the Sequential Test, and hence determines the appropriate location of development. It should be noted that some potential sites are large, with only a small portion lying within Flood Zones 2 and 3. In such cases Flood Zone 1 could be preferentially utilised for the proposed development.

8.3.3 The process of the Sequential Test outlined in PPS25 aims to steer vulnerable development to areas of lowest flood risk. The SFRA aims to facilitate this process by identifying the variation in flood risk across the Borough allowing an area-wide comparison of future development sites with respect to flood risk considerations.

8.3.4 Where there is a stream/ditch network present within a potential site (a floodplain value of 1), a site specific FRA should fully investigate the impact of this source of flooding to the potential site area.

8.3.5 There is a lack of overland flow information. It is however considered unlikely that flood risk from this source will render a site un-developable. Arterial drainage networks should be considered and investigated as part of any site specific FRA, which should identify what mitigation measures may be necessary to protect against flooding from each source.

8.3.6 The total maximum value for any individual potential site equates to a value of 6, which would suggest that the development allocation is potentially at significant flood risk.



TABLE B8-1 FLOOD SOURCE WEIGHTINGS

STRATEGIC SPATIAL FLOOD SOURCE SCORING SYSTEM

Value Classification Notes

Fluvial Flood Sources

5

Potential site is located within the Environment Agency floodplain zone 2 or 3.

Presence of the floodplain within the settlement buffer zone has been attributed the heaviest weighting due to the implications on breach locations. The breach locations used will affect what settlements are associated with a residual risk (or breach hazard) score. Particular breach locations were specified, however a breach could occur elsewhere and affect a settlement distant to those identified with a score of 3 in table 10, thus is subjective. In this report, the score relating the presence of the floodplain is anticipated to be the most crucial in assessment of flood risk.

3

A small stream (or equivalent) flows through the site/adjacent to the site boundary, which is not defined by an Environment Agency floodplain

1 Sites characterised by an isolated ditch network or a series of ponds No water features exist

0 No water features exist

Arterial Flood Sources

1 Historical evidence suggests that site may be at risk from arterial flooding

This information is based on the above arterial drainage network information (Table B5)

0 No flood arterial flood incidents identified

Flood Sources Resulting from a Breach

The breach analysis did not identify any of the potential employment or housing allocations to be at risk. This residual risk has not been considered as part of this summary. A breach could occur anywhere, as such all areas within the floodplain are potentially associated with a residual risk. This source should be addressed in a site specific FRA.

Overland Flow Flood Sources

Overland flow was not considered to be a significant problem in the Borough. This source has not been considered as part of this summary but should be considered in a site specific FRA.



8.3.7 Table B8-2 is an example of how Table B8-1 could be used to identify what sources of flooding may be anticipated to affect a particular site. It is intended that this would then inform a site-specific FRA. The summary is by no means conclusive; it could however be used to provide a useful guide of flood risk associated within individual sites. The methodology undertaken is considered to be very subjective, but is however based upon the key factors identified and used within this report to define flood risk from each individual source.

TABLE B8-2 EXAMPLE OF USE OF STRATEGIC SPATIAL FLOOD RISK SCORING SYSTEM TO DETERMINE RISK AT POTENTIAL DEVELOPMENT ALLOCATIONS

Site Floodplain Drainage Total

e.g. 1 5 0 5

e.g. 2 3 0 3

e.g. 3 5 1 6



TABLE B8-1 FLOOD SOURCE WEIGHTINGS

STRATEGIC SPATIAL FLOOD SOURCE SCORING SYSTEM

Value Classification Notes

Fluvial Flood Sources

5

Potential site is located within the Environment Agency floodplain zone 2 or 3.

Presence of the floodplain within the settlement buffer zone has been attributed the heaviest weighting due to the implications on breach locations. The breach locations used will affect what settlements are associated with a residual risk (or breach hazard) score. Particular breach locations were specified, however a breach could occur elsewhere and affect a settlement distant to those identified with a score of 3 in table 10, thus is subjective. In this report, the score relating the presence of the floodplain is anticipated to be the most crucial in assessment of flood risk.

3

A small stream (or equivalent) flows through the site/adjacent to the site boundary, which is not defined by an Environment Agency floodplain

1 Sites characterised by an isolated ditch network or a series of ponds No water features exist

0 No water features exist

Arterial Flood Sources

1 Historical evidence suggests that site may be at risk from arterial flooding

This information is based on the above arterial drainage network information (Table B5)

0 No flood arterial flood incidents identified

Flood Sources Resulting from a Breach

The breach analysis did not identify any of the potential employment or housing allocations to be at risk. This residual risk has not been considered as part of this summary. A breach could occur anywhere, as such all areas within the floodplain are potentially associated with a residual risk. This source should be addressed in a site specific FRA.

Overland Flow Flood Sources

Overland flow was not considered to be a significant problem in the Borough. This source has not been considered as part of this summary but should be considered in a site specific FRA.



8.3.7 Table B8-2 is an example of how Table B8-1 could be used to identify what sources of flooding may be anticipated to affect a particular site. It is intended that this would then inform a site-specific FRA. The summary is by no means conclusive; it could however be used to provide a useful guide of flood risk associated within individual sites. The methodology undertaken is considered to be very subjective, but is however based upon the key factors identified and used within this report to define flood risk from each individual source.

TABLE B8-2 EXAMPLE OF USE OF STRATEGIC SPATIAL FLOOD RISK SCORING SYSTEM TO DETERMINE RISK AT POTENTIAL DEVELOPMENT ALLOCATIONS

Site Floodplain Drainage Total

e.g. 1 5 0 5

e.g. 2 3 0 3

e.g. 3 5 1 6



9 North Chelmsford Area Action Plan (NCAAP) 9.1.1 The North Chelmsford Area Action Plan is being developed to assist in achieving the

Borough’s housing requirements as not all the housing requirements can be achieved through brownfield sites. There are two new neighbourhoods proposed to the north of Chelmsford; one to the northeast of Springfield and one to the west/northwest of the Broomfield area.

9.1.2 In accordance with PPS25 the local authority and Environment Agency would require that any development on a greenfield site should retain the existing greenfield runoff rates post-development and should allow for climate change. Sustainable Drainage Systems should be included in new developments where possible to manage surface water.

9.1.3 PPS25 requires the use of SuDS as an opportunity for managing flood risk, improving water quality and increasing amenity and biodiversity. Sustainable Drainage Systems should be located in accordance with the restrictions set out in Policy and Practice for the Protection of Groundwater (1998).

9.1.4 The ground conditions of the areas allocated as part of the NCAAP have not been assessed as part of this SFRA and should be subject to ground investigations at the site scale when determining their suitability for SuDS.

9.1.5 Information regarding surface water drainage from section 2.1.18 should be considered and incorporated in the NCAAP developments.

9.1.6 Developing greenfield areas for housing will result in increased impermeable areas. Retaining greenfield run off rates and attenuation of increased surface water volumes post-development is required to ensure that there is no net increase flood risk downstream. A surface water management strategy should be put in place as part of the development process to ensure there is no increase in flood risk as a result of the development.



10 Planning and Development Advice to Chelmsford Borough

10.1.1 PPS25 was issued in December 2006 and further reinforces the guidance issued in PPG25 on managing development and flood risk. PPS25 strives to reinforce the importance of a flood risk assessment and mitigation of flood risk, to a development. The flood risk should be considered as part of development proposal, and the acceptability of the risk is dependant on the scale and nature of the development.

10.1.2 PPS25 puts a greater emphasis on the importance of managing surface water and its safe disposal in order to ensure flood risk is not increased elsewhere as a result of a development. This should be carefully considered when developing areas such as developments on greenfield sites i.e. as part of North Chelmsford Area Action Plan (NCCAP).

10.1.3 In the context of PPS25, if the Flood Storage Schemes were to become operational and defend areas in Chelmsford up to the 1 in 200 year standard this would not change the PPS25 position in how the Sequential Test is undertaken. PPS25 ignores the presence of defences and therefore the floodplain will remain as defined without defences for Flood Zones 1, 2 and 3. The Sequential Test should still be undertaken on the risk based approach and thereafter steering development into Flood Zone 1, where no other sites reasonably available development should be steered to Flood Zone 2 and then Flood Zone 3a.

10.1.4 Funding for the Flood Storage Areas, as it stands, is likely to require developer contributions, as although the scheme would be highly beneficial, the priority score of 13 is not at the national requirement of 31 to obtain government funding.

10.1.5 National and local policies have been reviewed against the local flood risk issues and objectives identified by the Environment Agency in the CFMP. From these policies the following catchment wide and specific area strategies have been developed under the headings Flood Risk, SuDS, Flood Mitigation and the Water Environment. Integration of these suggested policy considerations into LDF / LDD should ensure that the objectives and aspirations of the Environment Agency and national policy are met whilst strengthening the position of the Local Planning Authority with regard to Flood Risk.



10.2 Flood Risk Catchment Wide Strategies (based on PPS25)

10.2.1 Ensure the Sequential Test is undertaken for all allocations to reduce the flood risk to the allocation and ensure that the vulnerability classification of the proposed development is appropriate to the flood zone classification;

10.2.2 Flood Risk Assessments should be undertaken for all developments within Flood Zones 2 and 3 to assess the risk of flooding to the development and identify options to mitigate the flood risk to the development, site users and surrounding area, and where flood plain storage is removed, the development should provide compensatory storage on a level for level basis to ensure that there is no loss in flood storage capacity.

10.2.3 Flood Risk Assessments are required for all developments in Flood Zone 1 that are greater than 1.0ha. However, if a critical drainage problem has been identified on a development in Flood Zone 1 Flood Risk Assessments will then required for developments that are greater than 0.5ha or over 10 dwellings.

10.2.4 Flood Risk to development should be assessed for all forms of flooding;

10.2.5 If a critical drainage area is identified, then all developments that may have an impact on the local drainage should produce a FRA/drainage strategy, not just major developments.

Area Specific Strategies

10.2.6 Consideration of flooding from overland flow should be given for developments occurring throughout the Borough, but with particular regard to areas such as Chelmsford town centre, affected by flooding from the Chelmer. This is reinforced by Policy CP10 of the Chelmsford Borough Council Core Strategy DPD.

10.2.7 Through integration of these suggestions, the emerging LDF is anticipated to comply with PPS25 and the aspirations and policies contained within the Chelmsford Borough Council Core Strategy DPD, more specifically policy DC56, CP4 and CP10.

10.3 Sustainable Drainage Systems Sustainable Drainage Policies should address the following issues:

Catchment Wide Strategies

1. Sustainable Drainage Systems must be included in new developments as a way to manage surface water.

2. PPS25 requires the use of SuDS as an opportunity for managing flood risk,

improving water quality and increasing amenity and biodiversity.



3. Flood risk assessments should be undertaken for developments in Flood Zone 1

that are greater than 1ha in size, to ensure that flood risk is not increased to other properties due to increased site runoff;

4. Runoff rates from new developments should not increase following

redevelopment, including an allowance for climate change; 5. Runoff rates should be restricted to greenfield runoff rates in areas known to have

a history of sewer flooding;


1. Runoff rates should be restricted for both greenfield and brownfield developments in Chelmsford, South Woodham Ferrers and Boreham in particular. This is also likely to be appropriate within other settlements to ease surface water flooding and drainage capacity exceedence;

2. Infiltration techniques are unlikely to be appropriate where sites are underlain by London Clay which is present throughout the Borough. Their use should be determined by a site-specific ground investigation. Attenuation techniques should be imposed in these circumstances.

10.3.1 Through integration of these suggestions, the emerging LDF is anticipated to comply with PPS25 and the aspirations and policies contained within the Chelmsford Core Strategy DPD, more specifically policy DC27- Water efficiency and Sustainable Drainage systems.

10.3.2 As defined within the 2002 Amendment of Building Regulations Part H (3) surface water should discharge into one of the following, in order of preference:

• An adequate soakaway or some other adequate infiltration system; or where that is not reasonably practicable;

• A watercourse; or where that is not reasonably practicable a sewer.

10.3.3 If surface water is discharged into a watercourse and/or sewer, a practical restriction upon the rate of discharge should be imposed and assessed on a site-by-site basis.

Sustainable Drainage Systems Specific to Chelmsford

Geology

10.3.4 The Solid geology of the Chelmsford Borough can be separated into two areas; the northern areas of the Borough underlain by the London Clay Formation, and the southern areas of the Borough characterised by outcrops of the Claygate Beds.

10.3.5 The following tables (Table B10-1&Table B10-2) highlight the main solid and drift geology deposits with an indication of potential corresponding appropriate SuDS




techniques. Specific geological conditions at individual sites should be ascertained from geological maps and where relevant, during more detailed stages of design, verified as part of a site investigation.

10.3.6 It is important to note that various geological formations have variable permeability, which can mean that the particular formation may consist of differing geological deposits. For example, the alluvial deposits of a particular river may consist of widespread sands and gravels, characterised by a high permeability. More isolated pockets may exist where the alluvial deposits consist of clay and silts, with a low permeability.

10.3.7 Furthermore, the Upper Chalk Formation located throughout the northern section of the Borough could potentially form an ideal receptor of storm water generated on an overlying site. However, in many circumstances the solid geology is not exposed at or near ground level and is overlain by drift deposits, which can be in excess of 200 metres thick. Under these circumstances the presence of the drift deposits could negate the practicality of infiltration techniques.



TABLE B10-1: SPECIFIC SOLID GEOLOGY TO THE CHELMSFORD DISTRICT

Solid Geology Permeability General Characteristics Locations Appropriate SuDS Techniques

Bagshot Beds (Bagshot Pebble Bed)

Permeable fine grained sands the pebble bed consists of rounded black flint pebbles

Localised deposits around Woodham Ferres, Galleywood, east of Hanningfield, south of Writtle and north of Ramsdon Heath

Infiltration and combined infiltration/attenuation systems and attenuation systems e.g. permeable surfaces, sub surface infiltration, basins and ponds, swales and filter strips i.e. a combined system

Claygate Beds Variably Permeable

Silts and silty clays with interbedded fine grained sands)

South of Writtle, around Galleywood, Hanningfield, Woodham Ferrers and South Hannginfield


London Clay Formation Impermeable

Clay, Orange brown becoming blue grey with depth, variably silty with thin sand and rare pebble beds. Some siltstone nodules and bands and Selonite Crystals, occasional shell fragments.

The dominant solid lithology across the district.

Attenuation systems e.g. basins and ponds, green roofs, tanks, rainwater harvesting etc

sford Borough Council rategic Flood Risk Assessment


TABLE B10-2: SPECIFIC DRIFT DEPOSITS TO THE CHELMSFORD DISTRICT

Drift Deposit Permeability General Characteristics Locations Appropriate SuDS Techniques

Alluvium Variably Permeable Generally clay with some gravel sand and silt

Found along the line of the rivers and tributaries and brooks


River Terrace Deposits

Variably Permeable

Sandy gravel, clayey in places local veneer of clayey silt

Found along the line of the rivers and tributaries and brooks


Lowestoft Formation

Variably Permeable

The Lowestoft Formation consists of Glacial deposits comprising Till, Glaciofluvial Deposits and Glaciolacustrine Deposits. Till deposit: sandy clay with chalk fragments (formerly known as Boulder Clay) Glaciolacustrine Deposits: calcareous silt which is found finely interbedded with the Glaciofluvial Deposits which occur below within and above the Till deposits. The Glaciofluvial Deposits consist of gravelly clayey sand with interbeds of clayey sand and silt.

Mainly located in the northern area of the Borough (to the north of Chemsford and Writtle.

Attenuation systems e.g. basins and ponds, green roofs, tanks, rainwater harvesting etc are likely to be the most suitable techniques. Infiltration systems may be appropriate within more the deposits characterised by sands and gravels.

Head Deposits Impermeable Silty or sandy clay

Found within the southern area of the Borough, overlying the London Clay Formation, generally found in low lying areas.

Attenuation systems e.g. basins and ponds, green roofs, tanks, rainwater harvesting etc

ChelmD115326 St


SuDS Recommendations

10.3.8 PPS25 indicates that Regional Planning Bodies and Local Authorities should promote the use of SuDS for the management of storm water runoff generated by development. Runoff rates from new developments should not increase following redevelopment, including an allowance for climate change.

10.3.9 PPS25 recognises that flood risk and other environmental damage caused by traditional drainage systems can be managed through the use of SuDS. Flood risk can be reduced by systems that minimise the changes in the volume and rate of surface runoff from development sites. This is complementary to the control of development within the floodplain. Where possible SuDS should be incorporated into all new developments. However, if for instance infiltration techniques were proposed, this must first be assessed subject to the appropriate geology and ground conditions to accommodate this technique.

10.3.10 In addition, drainage of rainwater from roofs and paved areas around buildings should comply with the 2002 Amendment of Building Regulations Part H (3). The requirements are as follows:

• Adequate provision shall be made for rainwater to be carried from the roof of the building.

• Paved areas around the building should be constructed to adequately drain.

• Rainwater from a system provided pursuant to sub-paragraphs (1) or (2) shall discharge to one of the following in order of priority:

• An adequate soakaway or some other adequate infiltration system; or where that is not reasonably practicable;

• A watercourse; or where that is not reasonably practicable a sewer.

10.3.11 SuDS seek to manage storm water as close to its source as possible, mimicking storm water flows arising from the site, prior to the proposed development. Typically this approach involves a move away from conventional sewer/piped systems to softer engineering solutions inspired by natural drainage processes.

10.3.12 SuDS should be designed to take into account the surface run-off quantity, rates and also water quality ensuring their effective operation up to and including the 1 in 100 year rainfall event design standard including an increase in peak rainfall of up to 30% to account for climate change.

10.3.13 Wherever possible, a SuDS technique should seek to contribute to each of the three goals identified below with the favoured system contributing significantly to each objective. Where possible SuDS solutions for a site should seek to:

1. Reduce flood risk (to the site and neighbouring areas),



2. Reduce pollution, and,

3. Provide landscape and wildlife benefits.

10.3.14 These goals can be achieved by utilising a management plan incorporating a chain of techniques, (as outlined in Interim Code of Practice for Sustainable Drainage Systems 2004), where each component adds to the performance of the whole system:

• Prevention: good site design and upkeep to prevent runoff and pollution (e.g. limited paved areas, regular pavement sweeping)

• Source control: runoff control at/near to source (e.g. rainwater harvesting, green roofs, pervious pavements)

• Site control: water management from a multitude of catchments (e.g. route water from roofs, impermeable paved areas to one infiltration/holding site)

• Regional control: integrate runoff management from a number of sites (e.g. into a detention pond)

10.3.15 The application of SuDS is not limited to a single technique per site. Often a successful SuDS solution will utilise a combination of techniques, providing flood risk, pollution and landscape/wildlife benefits. In addition, SuDS can be employed on a strategic scale, for example with a number of sites contributing to large scale jointly funded and managed SuDS. It should be noted, each development site must offset its own increase in runoff and attenuation should not be “traded” between developments.

10.3.16 In accordance with PPS25 the local authority and Environment Agency would require any greenfield development to retain the existing greenfield runoff rate post-development and should allow for climate change. SuDS should be included in new developments where possible to manage storm water.

10.3.17 PPS25 requires the use of SuDS as an opportunity of managing flood risk, improving water quality and increasing amenity and biodiversity. SuDS should be located in accordance with the restrictions set out in Policy and Practice for the Protection of Groundwater.

10.3.18 The ground conditions of the potential development allocation have not been assessed as part of this SFRA and would be subject to the results of ground investigations for the site to determine their potential for SuDS.

10.4 Flood Mitigation 10.4.1 Flood Mitigation Policies should address the following issues:



Catchment Wide Strategies

1. Where an allocation boarders an area benefiting from flood defence, opportunities should be sought for the maintenance of these flood defences to be partly funded by the development for its lifetime;

2. Opportunities should be sought to de-culvert rivers, where possible, to return them

to a natural system, reducing back up of flows and under capacity where this does not exacerbate the flooding elsewhere;

3. River channel restoration should be undertaken where possible to return the river

to its natural state and restore floodplain to reduce the impact of flooding downstream;

4. Emergency planning strategies should be in place in order to direct people to

safety during times of flooding.


10.4.2 In order to reduce flood risk within Chelmsford town centre, the Council will need to work with the Environment Agency to put in place strategic flood defence measures upstream from Chelmsford’s Urban Area on the Rivers Can and Wid. In appropriate circumstances, local flood protection measures within development sites should be supported by planning contributions from development permitted within the Town Centre, which is supported by Policy CP4 and CP10.

10.4.3 Emergency planning should be taken into account to facilitate safe access and egress from areas identified in this SFRA to be prone to flooding. In addition, Figure B38 should be consulted to direct people to places of refuge during times of flooding. The location of any future emergency services or potential large scale refuge centres, such as hospitals, fire stations, community halls, should take into account the risk of flooding to ensure these centres remain operational during times of flooding.

10.5 Water Environment 10.5.1 Water Environment Policies should address the following issues:

Catchment Wide Strategy

1. Development should not have a detrimental impact on the water environment through changes to water chemistry or resource.

2. All developments should incorporate water efficiency measures, not just those in

water stressed areas. The emerging East of England Plan advocated strict water efficiency levels which will have to be incorporated into new developments;



3. Any development should not be located within 9 metres of the riverbank to ensure access for maintenance but also to ensure a riparian corridor for improvement of the riverine environment.

4. River channel restoration should be undertaken where possible to return the river

to its natural state and restore the floodplain to reduce the impact of flooding downstream, by allowing the river to flood naturally. In addition, there are multiple environmental benefits to restoring the river to a natural state including encouraging fauna and flora into the area and establishing a variety of ecosystems, as well as increasing the aesthetic value of the river system.


10.5.2 Policy DC18 reinforces the need for sensitivity near watercourses stating Planning Permission will be refused for development adjacent to rivers, the Chelmer and Blackwater Navigation and other watercourses where the design and layout of the proposed development fails to be sensitive to the landscape or fails to take full advantage of the development setting afforded by open water features and their margins.

10.5.3 Where appropriate, development proposals adjoining the rivers will be required to incorporate riverside paths and open spaces.

10.5.4 Any proposals requiring the provision of a new bridge shall ensure a minimum of 2.3 metres headroom above normal water level to allow for river use and provide fauna passages suitable as wildlife corridors.



11 References Black & Veatch and Environment Agency (August 2006) River Chelmer Flood Risk Study

CIRIA C697 (2007) SuDS Manual

DCLG (2006), Circular 04/2006: Town and Country Planning (Flooding)(England) Direction 2007, The Stationary Office, London

DCLG (2006), Consultation - Planning Policy Statement: Planning and Climate Change - Supplement to Planning Policy Statement, The Stationary Office, London

DCLG (2006), Planning Policy Statement 25: Development and Flood Risk, The Stationary Office, London

Environment Agency (1998). Policy and Practice for the Protection of Groundwater (2nd Edition). The Stationery Office, London. ISBN 0 11 310145 7

Environment Agency 2006. Draft North Essex Catchment Management Plan. Environment Agency Publication

Environment Agency, Halcrow and Black and Veach, 2006. Essex Estuarine Strategies. Available via http://www.essex-estuaries.co.uk/EastAnglianStrategies/Default.htm [Accessed 17th April 2007]

Essex County Council, 2005. Roach and Crouch Estuary Management Plan

Essex Estuarine Strategies - http://www.essex-estuaries.co.uk (accessed on 14/09/07)

Scott Wilson (2007) Mid Essex Strategic Flood Risk Assessment


http://www.essex-estuaries.co.uk/EastAnglianStrategies/Default.htm

http://www.essex-estuaries.co.uk/EastAnglianStrategies/Default.htm

http://www.essex-estuaries.co.uk/


Annex A – Flood Alleviation Scheme Appraisal

Background A Strategic Flood Risk Assessment looks at current and future (i.e. with an allowance for climate change), actual flood risk. An additional component to this report has been requested by Chelmsford Borough Council to consider the future cost-benefit relationships of the potential Flood Storage Areas, examined in this chapter.

There is a prolonged history of flooding in Chelmsford Town and during October 2001 there was widespread flooding on a number of watercourses throughout Essex. During this event, water levels in the centre of Chelmsford were recorded at 100mm below the bank tops, a ‘near miss’ even (Chelmsford Flood Alleviation Viability Study: Summary Note, Sep 2007).

Following these events the Environment Agency initiated a series of flood risk studies to identify the current standards of protection and potential works to improve these standards. One of these studies was the River Chelmer Flood Risk Study, which assessed five structural options including Do Nothing and Do Minimum option. The options considered the use of upstream flood storage individually on the River Can, Chelmer and Wid, and in combination, as well as channel works through Chelmsford. This study was completed in August 2006 and was a strategic study for the Chelmer Catchment. To carry this work forward and identify if there is a viable flood alleviation scheme for Chelmsford, a viability study was commissioned. The findings of these studies are discussed below.

Chelmsford Flood Alleviation Viability Study (2007) Verbal information has been provided by the Environment Agency to assist Scott Wilson in undertaking this appraisal. Three flood storage options on River Can, River Wid the in Little Waltham, were taken forward from the River Chelmer study into the viability study.

The viability study included undertaking further detailed topographic and channel surveys, alongside a refinement of the existing hydraulic modelling to confirm both the current existing standard of protection to Chelmsford and the increased standard of protection that could be provided using a preferred option selected under the DEFRA guidance.

The schemes were appraised using current government guidance (Defra PAG3), which identified that there would be little additional economic benefit for the River Can and Little Waltham flood storage sites, although these schemes would reduce flood water levels. Therefore, in accordance with Making Space for Water, these sites should be safeguarded for potential future storage areas.

The revised hydraulic modelling as part of the viability study shows that a standard of protection between 1 in 75 and 1 in 200 years could be provided to existing properties in Chelmsford through the construction of a storage area on the River Wid and additional flood defence works within Chelmsford town centre (Figure B0-3).



The study identified additional improvements to the flood defences that could provide an increased standard of protection for nine proposed development sites in the town centre from 1 in 60 to 1 in 200 (equivalent to 1 in 100 plus the effect of 100 years climate change).

An addendum to the viability study is currently being completed, which aims to identify additional flood defence options to bring the standard of protection, for the remaining proposed future development sites in Chelmsford town centre, up to the 1 in 200 year standard. This aims to look at opportunities to deliver the schemes in conjunction with Chelmsford Borough Council. Potential additional works that are under further investigation include:

• optimising the flow control structure and storage capacity in the River Wid Flood Storage Area;

• combining the River Wid and Little Waltham Flood Storage Areas with minor bank works along a short stretch of the River Chelmer;

• combining the River Wid and Little Waltham Flood Storage Areas, (Chelmsford Flood Alleviation Viability Study: Summary Note, Sep 2007).

The current preferred option involves an on-line Flood Storage Area on the River Wid and a 2m high earth bund situated close to the Sewage Treatment Works. This will provide protection to the town of Chelmsford. This scheme has a Benefit Cost Ratio of 5.0 and a DEFRA Priority Score of 13.0. This Priority Score is below the current threshold required for Environment Agency internal approval and DEFRA Grant Aid, most projects currently receiving funding through DEFRA Grant in Aid have achieved a priority score of significantly over 20 (this is subject to change at the next funding review). As such, the project will not at present receive funding. Other funding schemes will therefore need to be considered unless the funding threshold reduces or the score of the scheme increases. (Chelmsford Flood Alleviation Viability Study: Summary Note, Sep 2007).

Flood Storage Areas & Breach Scenarios The flood storage areas discussed in the alleviation schemes have the potential to become reservoirs, as defined by the Reservoirs Act 1975. A reservoir is classified as such where the volume of water impounded above adjacent natural ground levels is over 25,000 cubic metres. The preferred option of a flood storage area on the River Wid could yield a storage capacity of 1.5 million m3, and would therefore be classified as a reservoir under this act. Whilst other flood storage areas are possible in the future, only the preferred option on the River Wid has been considered further with respect to residual risk.

Policy

The safety operation and management of reservoirs is essential as a dam failure could have major consequences, including loss of life. The Reservoirs Act 1975 is enforced by the Environment Agency and strict standards are in place to ensure reservoir safety is highest possible and safety inspections are carried out by experienced, appropriately qualified engineers.

Section 77 of the Water Act, 2003 commences:



• ‘(1) The Secretary of State may, by written notice served on the undertakers in relation to a large raised reservoir, direct them to prepare a plan (a "flood plan") setting out the action they would take in order to control or mitigate the effects of flooding likely to result from any escape of water from the reservoir.

• (2) A direction may in particular-

• (a) Specify the matters to be included in the flood plan;

• (b) Require the flood plan to be prepared in accordance with such methods of technical or other analysis as may be specified by the Environment Agency” (The Enforcement Authority for the Reservoirs Act 1975, in accordance with Clause 74 of the Water Act);

Reservoir emergency planning is increasingly recognised as one of the measures, which when applied systematically, may be used to manage (and reduce) the risk from high hazard installations. Emergency planning may be subdivided into:

• a) Assessing the potential consequences of failure (the “impact assessment”);

• b) Actions the owner of the high hazard installation takes on his land to prevent or mitigate a failure (the “on-site” plan); and

• c) Measures taken on third party and public land to mitigate the effects of a failure (the “offsite” plan).

If the proposed Flood Storage Areas were to become reservoir areas, reservoir safety modelling must be undertaken in order to assess the impacts of reservoir failure and assess dam spillway capacity. Spillway capacity should be capable of withstanding flows up to the 1 in 1,000 year and 1 in 10,000 year level. Revised guidance to determine spill capacity states that rainfall estimates should be assessed for the 1 in 1,000 year event using the Flood Studies Report (FSR) Volume 2 and the Flood Estimation Handbook (FEH), but the more extreme estimate of the two should be used. However, for the 1 in 10,000 year rainfall return period the FEH should not be used as it significantly over estimates, therefore only FSR should be used in this scenario. The local hydrology should be investigated and used by the panel engineer undertaking the risk assessment to hydraulically model the dam spillway capacity.

A strategic assessment of the potential flood risk areas and potential flood flow paths as a result of a potential breach failure has been undertaken for each of the three areas identified as possible Flood Storage Areas. The assessment of the flow paths is strategic, and has been based on contours from ordnance survey maps.

River Wid Reservoir Breach

The River Wid Flood Storage Area extends from mid way between Hope Bridge and Whites Bridge downstream of Margetting. The Flood Storage Area is approximately 3.5-4km in length and appears to act as an online reservoir. The downstream limit is at the upstream face of the A414 embankment.



The Flood Storage Area is fragmented into sub sections by existing road embankments and rail embankments. The River Wid runs through the valley floor of the rural area and the settlements, which are generally at a reasonable distance from the proposed storage area. One settlement area is near Highlands Mill and the housing estate on Butts Way (569300, 203500) and as such could potentially lie within an area at risk from a failure of the flood storage area defences.

Interim development period for Chelmsford It is important for the economic growth of Chelmsford that flood defence protection is provided up to at least the 1 in 100 year fluvial standard of protection, inclusive of the climate change, as per the requirements of PPS25 (indicated in the Chelmsford Flood Alleviation Viability Study: Summary Note, September 2007).

Prior to the construction of an Alleviation Scheme for Chelmsford, future development proposed within Flood Zones 2 or 3 should be subject to a detailed Flood Risk Assessment as per the requirements of PPS25.

It is anticipated that prior to development of the alleviation scheme, future development within Flood Zones 2 and 3 will need to provide flood compensation storage on a level-for level basis for any increases in the existing building footprint. Site planning, storage, mitigation and defence measures will be determined on a site-by-site basis, as part of a site-specific flood risk assessment.

This SFRA should be reviewed on the eventual implementation of the above flood storage scheme and advice outlined with respect to flood compensation guidance should take into account the improved defence scheme.



FSA Appraisal TABLE B0-1: RISK FROM FLOODING TO UNDEFINED FLOOD CELLS AT RISK FROM FLOODING UNDER 2 MAINTENANCE SCEANRIOS (EXTRACT FROM THE RIVER CHELMER FLOOD RISK STUDY)

Do Nothing Do Minimum Flood

Cell ID Location Indicative Std

Approximate Standard of Protection

1 East of A12 <10 1 year 1 year 2 Chelmer Village 50 – 200 1 year 1 year 3 A138 to Chelmer Village LB <10 2 years 2 years 4 A138 to A12 RB <10 >500 years >500 years

5A Downstream Chelmsford LB 50 – 200 10 years 25 years 5B Chelmer town centre LB 50 – 200 10 years 75 years 6A Downstream Chelmsford RB 50 – 200 >500 years >500 years 6B Can town centre 50 – 200 2 years 25 years 7A Can town centre LB 50 – 200 10 years 25 years 7B Chelmer town centre RB 50 – 200 10 years 50 years 8 Can Prykes Drive 50 – 200 2 years 10 years 9 Can Rainsford Avenue 50 – 200 25 years 100 years 10 Can Beach’s Drive 50 – 200 1 year 500 years 11 Can u/s railway RB 50 – 200 2 years 25 years 12 Can and Wid u/s Chelmsford <10 10 years 10 years 13 Chelmer u/s of railway RB 10 200 years 200 years 14 Chelmer u/s of railway LB 10 100 years 100 years 15 North of A138 <10 100 years 100 years 16 Chelmsford to Little Waltham <10 5 years 5 years 17 Great Waltham <10 1 year 1 year 18 Gt Waltham to Gr Dunmow <10 1 year 2 years 19 Gt Dunmow 50 – 200 5 years 10 years 20 Gt Eastern <10 25 years 75 years 21 Duton Hill <10 >500 years >500 years 22 Thaxted <10 >500 years >500 years

Note:

Downstream Chelmer

Upstream Chelmer Can and Wid

LB – Left Bank RB – Right Bank

As a means of assessing current risk to properties, the flood study divided the geographic area of the catchment into flood cells in order to assess flood risk. The details of these areas can be found in Figure B-0-1 and the geographic areas are described in the table above (Table B0-1) with the indicative standard of flood risk to that area under the two scenarios ‘Do Nothing’ and ‘Do Minimum’ scenario provided, where the ‘Do Nothing’ scenario is where there are no new flood alleviation schemes promoted and no maintenance of existing flood defence structures, and the ‘Do Minimum’ scenario involves the maintenance of existing flood defences.



FIGURE B-0-1 FLOOD CELL LOCATIONS – FROM THE RIVER CHELMER FLOOD RISK STUDY



The Flood Risk Study further details the number of properties within these flood cells that are at risk from flooding during the 1 in 100 year event (Table B0-2).

TABLE B0-2: PROPERTIES AT RISK FROM FLOODING IN THE RIVER CHELMER CATCHMENT (EXTRACT FROM RIVER CHELMER FLOOD STUDY)

No. Properties within 100 year flood outline

Location River Flood Cell ID

Indicative Std

Residential Commercial

Current SoP*

East of A12 Chelmer 1 <10 23 0 1 year Chelmer Village Chelmer 2 50 – 200 84 1 1 year A138 to Chelmer Village LB

Chelmer 3 1.25 – 10 4 0 2 years

A138 to A12 RB Chelmer 4 1.25 – 10 0 0 >500 years Downstream Chelmsford LB

Chelmer 5A 50 - 200 1 31 25 years

Chelmer town centre LB

Chelmer 5B 50 – 200 0 9 75 years

Downstream Chelmer RB

Chelmer 6A 50 – 200 0 0 >500 years

Can town centre RB Can 6B 50 – 200 145 16 25 years Can town centre LB Can 7A 50 – 200 7 40 25 years Chelmer town centre RB

Chelmer 7B 50 – 200 0 10 50 years

Can Prykes Drive Can 8 50 – 200 68 3 10 years Can Rainsford Avenus

Can 9 50 – 200 0 0 100 years

Can Beach’s Drive Can 10 50 – 200 0 0 500 years Can u/s railway RB Can 11 50 – 200 38 1 25 years Can and Wid u/s Chelmsford

Can 12 1.25 – 10 5 0 10 years

Chelmer u/s of railway RB

Chelmer 13 10 0 0 200 years

Chelmer u/s of railway LB

Chelmer 14 10 0 0 100 years

North of A138 Chelmer 15 1.25 – 10 0 0 100 years Chelmsford to Little Waltham

Chelmer 16 1.25 – 10 1 4 5 years

Great Waltham Chelmer 17 1.25 – 10 0 0 1 year Gt Waltham to Great Dunmow

Chelmer 18 1.25 – 10 2 0 2 years

Gt Dunmow Chelmer 19 50 – 100 8 0 10 years Gt Easton Chelmer 20 1.25 – 10 1 0 75 years Duton Hill Chelmer 21 1.25 – 10 0 0 >500 years Thaxted Chelmer 22 1.25 – 10 0 0 >500 years * where standard of protection relates to the flood event return period above which significant damage and possible failure of the flood defences could occur. The definition of ‘Standard of Protection’ varies depending on the presence of flood defences. Where flood defences are present, SoP is defined as ‘the return period at which defences are overtopped or fail’. Where there are no flood defences present, SoP is defined as ‘the return period at which flooding first causes significant damage or disruption in residential or industrial areas or causes main roads to become impassable’, (Black and Veatch and The Environment Agency 2006)




The table above (Table B0-2) demonstrates the standard of protection and the number of properties at risk in areas of the River Chelmer catchment by showing the Standard of Protection and the number of properties at risk in specific areas.

An appraisal of the Flood Storage Areas is required to understand whether the proposed flood storage requirements to manage fluvial flood risk would protect existing assets and improve conditions for future development in Chelmsford to take place.

The potential Flood Storage Areas that are currently being considered as part of a flood defence scheme for Chelmsford could increase the standard of protection for the town centre areas up to the 1 in 200 year standard of protection (equivalent to the 1 in 100 year event plus climate change). This study is currently being undertaken by the Environment Agency and, at the time of writing this report, was still in draft form.

Dwn: EC App: LW Chk: LW Rev: 1

Scale at A3: NTS

FIGURE B0-2 Legend: See above

Project Mid Essex Strategic Flood Risk Assessment, Chelmsford Borough Council

Preferred Option from: Chelmsford Flood Alleviation Viability Study: Summary Note, September 2007Figure

Dwn: EC App: LW Chk: LW Rev: 1

FIGURE B0-3

S

Scale at A3: NT

Legend: See above

Project Mid Essex Strategic Flood Risk Assessment, Chelmsford Borough Council

Existing and Scheme 1 in 100 year Flood Event outlines from: Chelmsford Flood Alleviation Viability Study: Summary Note, September 2007

Figure


Annex B – Halcrow Methodology for River Chelmer Modelling Updates Project River Chelmmer

Flood Outlines Date 14 September 2007

Note Approach Adopted Ref WN/CBAC/008/D4546 Author Alamgir Kabir

Introduction Halcrow was requested by Scott Wilson to carry out the production of flood outlines and flood water depth information for the River Chelmer catchments. These consist of following flood event scenarios:

• 1 in 20 year flood event

• 1 in 20 year flood event plus climate

• 1 in 100 year flood event plus climate change

• 1 in 1000 year flood event plus climate change

Overview of Hydraulic Model This study uses an up-to-date ISIS hydraulic model (updated by Halcrow in 2007). This model was originally developed during the River Chelmer Flood Risk Study by Black & Veatch (B&V) in November 2006. Halcrow reviewed the B&V model during the Chelmsford Flood Alleviation Scheme Viability Study in May 2007. The following key points were noted:

• The original B&V model consists of river sections and structures to model the in-bank and lateral spill/floodplain sections and storage cells (reservoir units based on LiDAR) to simulate floodplain flow and water levels.

• The inflow hydrographs were based on the Rainfall-Runoff model using Flood Estimation Handbook (FEH). The downstream boundary condition was set in-bank at Beeleigh Falls for all return periods.

• In general, the model represents the flow paths well. However, the storage cells (reservoir units) were poorly distributed and connected by floodplain sections using adjacent ground elevations along their separation line instead of defining high ground elevations that would prevent water spilling from one cell to another.



• The model simulation shows instability and consequently results in higher computational time. However, the flow and water level hydrographs appeared to be acceptable following a review of the results.

Halcrow made a substantial improvement of the B&V ISIS hydraulic model during Chelmsford FSA study in August 2007. The following key points were noted:

• Inclusion of the surveyed sections to the ISIS model and connection of sections with the adjacent flood storage cells using additional floodplain units. Along the River Wid, 19 nos. river survey sections were added to extend the model 4.5km upstream A414 Bridge. Along the River Chelmer, 18 nos. river survey sections were added to improve the resolution and the stability;

• 6 new bridge units, 6 spills and 18 floodplain sections were also added to the B&V model. The spill levels were taken from LiDAR data. Details of the bridge units and spill levels for floodplain sections were taken from survey data;

• The initial conditions (river level) for the new river sections were adjusted using the data related to adjacent upstream and downstream sections;

• Along the River Wid, the surveyed sections were extended using the LiDAR data to include the full extent of the proposed Wid FSA (Flood Storage Area). Four additional interpolated sections were used to improve the stability of the model as well as to represent the storage volumes within the FSA;

• The sinuosity of the River Wid within the proposed FSA was ignored and the distance between cross-sections was adjusted in order to ensure that the actual storage volume compared well with the modelled storage volume;

• The Flood Estimation Handbook (FEH) inflow hydrographs were reviewed and found to be acceptable for this stage of the study (i.e. no update required).

Methods and Assumptions The catchment floodplain flows and water levels were simulated (maximum 55hrs) using the up-to-date River Chelmer ISIS model for the desired flood event scenarios (1 in 20 year, 1 in 20 year plus climate change, 1 in 100 year plus climate change and 1 in 1000 year plus climate change). The FEH hydrological model was used to define the flood event scenarios.

It was assumed that a 20% increase in peak flows would take into account climate change.

For the 1 in 1000 year flood event plus climate change simulation, the model was run up to 28hrs to reach the peak water levels to avoid instability and excessive computational time.



MapInfo GIS was used to create a water depth triangular irregular network (TIN) using the ISIS model results and through comparison with the LiDAR data used to prepare a range of return period flood outlines.

The flood outlines were reviewed and manual adjustments were made by discounting the hydraulically disconnected flood areas based on the survey and LiDAR data. On the contrary, the B&V flood maps were based on broad scale analysis including the hydraulically disconnected areas.

The above explains that why the 1 in 100 year flood extents produced by B&V is significantly wider than the 1 in 100 year flood extents produced by Halcrow.

Outputs The final flood outlines and water depth grids were produced in MapInfo format.


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