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  • Prepared for:

    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

    Liz Williams Senior Consultant

    Jon Robinson Associate Director

    04 April 08 Final Eleanor Cole Graduate Hydrologist

    Liz Williams Senior Consultant

    Jon Robinson Associate Director

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

  • Chelmsford Borough Council D115326 Strategic Flood Risk Assessment

    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.

  • Chelmsford Borough Council D115326 Strategic Flood Risk Assessment

    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

  • Chelmsford Borough Council D115326 Strategic Flood Risk Assessment

    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

  • Chelmsford Borough Council D115326 Strategic Flood Risk Assessment

    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

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  • Chelmsford Borough Council D115326 Strategic Flood Risk Assessment

    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;

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    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

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    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

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    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.

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    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.

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    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.

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    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.

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    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.

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    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

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    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.

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    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.

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    • 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.

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    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%

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    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

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    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.

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    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.

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    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.

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    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.

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    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.

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    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

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    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:

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    • 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.

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    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

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    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.

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    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.

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    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

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    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.

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    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.

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    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.

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    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.

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    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

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    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.

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    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

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    HAZARD ZONE RESULTS

    1 in 200 Year event The main area of high haz