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Darlington Borough Council Strategic Flood Risk Assessment Level 1 Volume II- SFRA technical report December 2009 Karen Johnson Spatial Planning Town Hall Feethams Darlington DL1 5QT 2009s0156 - DBC SFRA Vol II v2.0 i

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  • Darlington Borough Council

    Strategic Flood Risk Assessment Level 1

    Volume II- SFRA technical report

    December 2009

    Karen Johnson Spatial Planning Town Hall Feethams Darlington DL1 5QT

    2009s0156 - DBC SFRA Vol II v2.0 i

  • Revision Ref / Date Issued Amendments Issued to

    JBA Office JBA

    Warrington

    JBA Project Manager Sam Wingfield

    Revision History

    1 Digital Copy (as a pdf) to Draft report (V.1.0) Karen Johnson and

    Cameron Sked

    Final report (V.2.0) Updated following comments from EA and DBC. 1 Digital Copy (as a pdf) to Karen Johnson

    Contract This report describes work commissioned by Karen Johnson, on behalf of Darlington Borough Council. Darlington Borough Council’s representative for the contract was Karen Johnson. Sam Wingfield of JBA Consulting carried out this work.

    Prepared by ......................................Samuel Wingfield, BSc MRes Analyst

    Reviewed by .....................................Christopher Isherwood, BSc DipWEM Analyst

    Purpose This document has been prepared as a Draft Report for Darlington Borough Council. JBA Consulting accepts no responsibility or liability for any use that is made of this document other than by the Client for the purposes for which it was originally commissioned and prepared.

    JBA Consulting has no liability regarding the use of this report except to Darlington Borough Council. If the site is sold, the scope of the development changed or this report is provided to third parties then any liability or explicit or implied warranty is voided unless the consent of JBA Consulting is obtained.

    This report may be assigned by the Client by way of absolute legal assignment once only to another company taking over the whole of their interest in connection with the carrying out of the Development without the consent of JBA Consulting being required and such assignment shall be effective upon written notice thereof being given to JBA. If further assignment is required please contact JBA Consulting.

    2009s0156 - DBC SFRA Vol II v2.0 ii

  • Acknowledgments JBA would like to thank all those who provided information and data for this report. From Darlington Borough Council: Karen Johnson. From the Environment Agency: Gemma Alecks.

    Copyright © Jeremy Benn Associates 2009

    Carbon Footprint First publication of this document would result in a carbon footprint of 325g.

    256g

    Further printed copies of this questionnaire will result in a carbon footprint of 256g if 100% post-consumer recycled paper is used and 325g if primary-source paper is used. These figures assume the report is printed in black and white and in duplex.

    JBA is a carbon neutral company and the carbon emissions from our activities are offset.

    2009s0156 - DBC SFRA Vol II v2.0 iii

  • Executive Summary

    Darlington Borough Council Level 1 SFRA

    This report has been produced as a Level 1 Strategic Flood Risk Assessment (SFRA) for Darlington Borough Council, in accordance with PPS25 and its Practice Guide.

    Development and Flood Risk

    This report has been produced as a Level 1 Strategic Flood Risk Assessment (SFRA) for Darlington Borough Council, in accordance with PPS25 and its Practice Guide.

    Development & Flood Risk

    Local Planning Authorities (LPAs) have a raft of issues to consider when planning future development. These are dictated by Government Planning Policy Statements.

    Planning Policy Statement 25 (PPS25) relates to development and the constraint of flood risk, with its overarching aim of avoiding development in flood risk areas. This is achieved through PPS25 by the sequential approach to land allocation, meaning that development should be firstly avoided in flood risk areas wherever possible before considering the vulnerability of development planned or possible mitigation measures. The sequential approach is governed by two tests; the Sequential and Exceptions Test. The consideration of flood risk to people and development must be considered by the LPA at the earliest stage of spatial planning decisions and these tests allows this process to be transparent and effective.

    In order to carry out these tests a coherent understanding of flood risk is needed at a local level. High level policy and guidance documents such as Catchment Flood Management Plans (CFMPs), Shoreline Management Plans (SMPs) and Regional Flood Risk Appraisals (RFRA) have provided a good introduction in to flood risk; however they do not provide the level of detail required for the LPA to make informed spatial planning decisions.

    Strategic Flood Risk Assessments (SFRAs) offer this local level of understanding. SFRAs provide the LPA with a central source of all relevant flood risk information and the evidence base to make planning decisions and develop focused local policies required for the Local Development Framework (LDF). The SFRA therefore becomes a key planning tool that enables the LPA to select and develop sustainable site allocations.

    A Level 1 SFRA offers the foundation of this evidence base. It is based purely on the collation of existing flood risk information. The Environment Agency Flood Map is the main source of fluvial and tidal flood information across England and Wales and is the basis of PPS25 Flood Zones used in the Sequential and Exception Tests. The Level 1 SFRA must also consider flooding from all other sources (surface water, sewers, groundwater and artificial sources). This is only achievable through consulting with those stakeholders with specific interest or knowledge in other sources of flooding.

    The Level 1 SFRA is assisted greatly by the use of Strategic Flood Risk Maps providing information on flood risk factors needed to be taken into account. The PPS25 Flood Zone Map enables the LPA to carry out the first sweep of Sequential Testing. The additional maps produced as part of the Level 1 SFRA should be used during the Sequential Test ‘sieving’ process to further identify inappropriate development.

    Once the LPA has carried out the Sequential Test sieving process, they still may wish to allocate vulnerable development in high risk areas due to the wider need for economic growth and regeneration. In this case the allocations must pass the Exception Test. The evidence provided in the Level 1 SFRA is not detailed enough to justify development through the Exception Test. In order to achieve this Level 2 SFRA must be carried out.

    A Level 2 SFRA provides the LPA with a detailed understanding of flood hazard, assessing flood depth, velocity and residual risks such as flood defence breaching or overtopping. This information provided in the Level 2 SFRA will give the LPA a much more detailed understanding of flood risk at potential development sites. Although it will not provide all the information needed to apply the Exception Test, it will include the appropriateness of the development and the likelihood of it remaining safe if flooded. If the LPA has justified the development by passing parts a) and b)

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  • of the Exception Test, it must be supported by a site specific Flood Risk Assessment (FRA) in order to pass part c).

    The Three Level 1 SFRA Volumes

    The Level 1 SFRA is presented in three volumes, each with their own purpose and intended audience.

    VOLUME I: Understanding the SFRA Process

    Volume I of the Darlington SFRA introduces the SFRA process. It is an excellent reference document for current flood risk management drivers, national regional and local planning policy and introduces Environment Agency policy such as the Tees CFMPs and SMPs.

    The report also provides a brief understanding of the mechanisms of flooding and flood risk for those new to the subject. More importantly, it provides a comprehensive discussion on PPS25, the Sequential and Exception Tests and links regional and local flood risk assessments.

    Volume I holds the main ‘Consultation & Data Management’ section, identifying key stakeholders and their involvement in the SFRA process.

    This Volume should be read by:

    � The general public or those new to flood risk � Those wanting to understanding current flood risk management drivers � Those wanting to understand the sequential approach to flood risk management;

    and � Planners and Developers wanting to understand the wider constraints of

    developing in flood risk areas. VOLUME II: SFRA Technical Report

    Following on from the ‘Consultation & Data Management’ section in Volume I, Volume II provides the technical information and methods used in the assessment of flood risk across Darlington. It assesses six sources of flooding including; fluvial, tidal, surface water, sewers, groundwater and reservoirs and other artificial sources. The Volume also introduces the Environment Agency Flood Warning System and residual risks associated with flood defences.

    As discussed, flood risk has many dimensions and as a result has been presented through a suite of maps. These extend the level of detail in the Environment Agency Flood Zone maps.

    The SFRA maps include:

    SET A: PPS25 Flood Zones

    SET B: Flood Zone 3 Depths

    SET C: Tidal Climate Change Sensitivity

    SET D: Flood Risk Management Measures

    SET E: Areas Naturally Susceptible to Surface Water Flooding

    Volume II along with the suite of SFRA maps, should provide the evidence base of the Darlington Level 1 SFRA. It has been arranged in one volume to allow technical information to be easily updated when reviewed. It is only this Volume that can be updated with new flood risk information when available. Volume I and III would be difficult to update without completely revisiting the SFRA process.

    Section 4 provides the results of the first pass of the Sequential Test against Darlington Council’s proposed strategic sites and potential future allocations.

    This Volume should be read by:

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  • � Spatial Planners � Development Control � Planning consultants and agents � Developers � Emergency Planners � Key Stakeholders including the Environment Agency and Northumbrian Water

    VOLUME III: SFRA Guidance for Spatial & Development Management

    Volume III of the Darlington SFRA provides guidance and recommendations to planners, planning consultants and agents, developers and emergency planners, on how to use the flood risk information provided in Volume II and further plans which are required to improve the understanding of flood risk in Darlington.

    Initially the Volume discusses further work required such as Level 2 SFRAs and Surface Water Management Plans which have been informed by the findings of Volume II. This extra work will provide Darlington Council with a strategic and coherent framework for managing flood risk in their area.

    This Volume should be read by:

    � Spatial Planners � Development Control � Planning consultants and agents � Developers � Emergency Planners � Key Stakeholders including the Environment Agency and Northumbrian Water

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  • 2009s0156 - DBC SFRA Vol II v2.0 vii

  • Contents Executive Summary.......................................................................................................... iv

    1 Data Sources .......................................................................................................1

    1.1 Flood Zone Map...................................................................................................1 1.2 Flood Defences....................................................................................................2 1.3 Flood Risk Management and Hydraulic Modelling Studies .................................2 1.4 Topographic Data ................................................................................................4 1.5 Historical Flooding ...............................................................................................5

    2 Flood Risk in the Borough of Darlington..............................................................7

    2.1 Introduction ..........................................................................................................7 2.2 River Tees............................................................................................................8 2.3 River Skerne ........................................................................................................9 2.4 West Beck and Cocker Beck ...............................................................................11 2.5 Flooding from land ...............................................................................................13 2.6 Flooding from sewers ..........................................................................................17 2.7 Flooding from groundwater..................................................................................18 2.8 Flooding from reservoirs and other artificial sources...........................................18 2.9 Effects of Climate Change ...................................................................................18 2.10 Geology and Soils................................................................................................19 2.11 Flood Defences....................................................................................................19 2.12 Flood Warning .....................................................................................................20

    3 Strategic Flood Risk Mapping..............................................................................22

    3.1 Introduction ..........................................................................................................22 3.2 PPS25 Flood Zone Maps.....................................................................................22 3.3 Flood Zone 3 Depth Map .....................................................................................24 3.4 Climate Change Sensitivity Maps........................................................................24 3.5 Flood Risk Management Measures Map.............................................................24 3.6 Surface Water Flooding Maps .............................................................................25

    4 Site Specific Allocations.......................................................................................26

    4.1 Introduction ..........................................................................................................26 4.2 Current Development Site Sequential Test .........................................................26 4.3 Flood risk and the 2009 potential development sites ..........................................27 4.4 Preliminary Level 2 Assessment for Central Darlington ......................................34 4.5 Surface water flood risk .......................................................................................38 4.6 Individual planning applications...........................................................................39

    Appendices ....................................................................................................................... I

    A Figures ................................................................................................................. II

    B Sequential Test Table.......................................................................................... III

    List of Figures Figure 1: LiDAR Coverage in Darlington BC .................................................................... 5

    Figure 2: Darlington Borough with the allocations and rivers ........................................... 8

    Figure 3 - Possible critical drainage area in Pierremont................................................... 15

    Figure 4 - Possible critical drainage area in the centre of Darlington next to the Skerne............................................................................................................................... 16

    Figure 5 - Possible critical drainage area in Eastbourne.................................................. 17

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  • Figure 6 - Faverdale Strategic Reserve Employment site EP8 ........................................ 28

    Figure 15 - SHLAA Site 56, Salters Lane West with a high vulnerability to surface

    Figure 16 - SHLAA Site 20, Great Burdon and Yarm Road and Local Plan Employment Allocation EP2.6 areas with a high vulnerability to surface water

    Figure 7 - Muscar House Farm residential site................................................................. 29

    Figure 8 -Great Burdon residential site............................................................................. 30

    Figure 9 - Blackett Road/Red Barnes Way Employment allocation EP2.4 ...................... 31

    Figure 10 -Valley Street Employment allocation EP2.1 and Town Centre Fringe............ 32

    Figure 11 -SHLAA 60 Feethams ...................................................................................... 33

    Figure 12 -SHLAA 17 Roundhill Road, Hurworth ............................................................. 33

    Figure 13 - Flood depth grids for the 1% and 0.1% a.p. events ....................................... 36

    Figure 14 - Flood depth for the 1% and 0.1% a.p. events................................................ 37

    water flooding ................................................................................................................... 39

    flooding ............................................................................................................................. 39

    List of Tables Table 1 - DEM availability ................................................................................................. 4

    Table 2 - Recent flooding from the River Tees................................................................. 9

    Table 3 - Recent flooding from West Beck and Cocker Beck .......................................... 13

    Table 4 - Functional floodplain and candidate functional floodplain mapping.................. 23

    Table 5 - Summary of development sites at risk of fluvial flooding .................................. 27

    Table 6 - Summary of development sites at risk of surface water flooding...................... 27

    Table 8 - Flood Hazard Thresholds .................................................................................. 35

    Table 9 - Suggested screening criteria for mitigation measures ...................................... 35

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  • Abbreviations ABD Areas Benefiting from Defences A.P. Annual Probability CFMP Catchment Flood Management Plans CLG Communities and Local Government COW Critical Ordinary Watercourse CS Core Strategy DPDs Development Plan Documents EA Environment Agency EU European Union FAS Flood Alleviation Schemes FEH Flood Estimation Handbook FCERM Flood and Coastal Erosion Risk Management FRA Flood Risk Assessment FRM Flood Risk Management IDB Internal Drainage Board IDD Internal Drainage District IFM Indicative Floodplain Map LDDs Local Development Documents LDF Local Development Framework LPAs Local Planning Authorities NEA North East Assemble NFCDD National Fluvial and Coastal Defence Database NPD National Property Dataset NWL Northumbrian Water Ltd PPG Planning Policy Guidance PPS Planning Policy Statement RBD River Basin District RBMP River Basin Management Plan RFRA Regional Flood Risk Assessment RPB Regional Planning Bodies RPG Regional Planning Guidance RSS Regional Spatial Strategy RVFD Receptors Vulnerable to Flooding Database SA Sustainability Appraisal SEA Strategic Environmental Assessment SFRA Strategic Flood Risk Assessment SFVI Social Flood Vulnerability Index SMP Shoreline Management Plans SoP Standard of Protection SPD Supplementary Planning Document SUDS Sustainable (Urban) Drainage Systems SWMP Surface Water Management Plan UDP Unitary Development Plan WCS Water Cycle Study

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  • 1 Data Sources

    1.1 Flood Zone Map

    The Environment Agency Flood Zone Maps provide an overview of areas considered susceptible to flood risk in the Borough as a result of fluvial and tidal flooding. These maps have been prepared in a consistent manner across England and Wales and provide an estimation of the extent of flooding for both the 1% and 0.1% annual probability (a.p.) events.

    The Flood Zone maps were prepared using a methodology based on the national digital terrain model (NextMap), derived river flows (Flood Estimation Handbook (FEH)) and two dimensional flood routing.

    The derived Flood Zone extents have been adjusted in some locations where the results are inconsistent with historical flooding extents, where more detailed flood mapping studies have been completed, or where there are known errors in the digital terrain model used. In Darlington, the majority of fluvial and tidal Flood Zones have already been updated with the results of detailed flood mapping studies (see Section 1.3).

    The Environment Agency Flood Zone Maps are precautionary in that they do not take account of flood defences and, therefore, represent a worst-case extent of flooding. They do not consider other forms of flooding and do not take account of climate change.

    PPS25 divides the country into three basic flood zones, Flood Zones 1, 2 and 3, corresponding to areas of low, medium and high flood risk, respectively.

    1.1.1 Delineation of Low Risk Zone 1

    PPS25 considers areas within Flood Zone 1 to be at low risk to flooding. The annual probability of flooding within this zone is less than 0.1% or can be easily defined as areas within the Borough located outside either Flood Zone 2 or 3.

    1.1.2 Delineation of Medium Risk Zone 2

    PPS25 considers areas within Flood Zone 2 to be at medium risk of flooding. The annual probability of fluvial flooding within this zone is between 0.1% and 1% (or between 0.5% and 0.1% for tidal flooding). In general, Flood Zone 2 is considered suitable for most development except highly vulnerable land uses where the Exception Test is required, such as police stations, fire stations and ambulance stations.

    1.1.3 Delineation of High Risk Zone 3

    PPS25 considers areas within Flood Zone 3 to be at high risk of flooding. PPS25 splits Flood Zone 3 into two sub-zones, 3a and 3b, which correspond to high probability flooding and the functional floodplain.

    � Flood Zone 3a: High Probability In accordance with Table D.1 of PPS25 “This zone comprises land assessed as having a 1% or greater annual probability of flooding or a 0.5% or greater annual probability of sea flooding in any year.”

    � Flood Zone 3b: The Functional Floodplain In accordance with Table D.1 of PPS25 “This zone comprises land where water has to flow or be stored in times of flood”

    1.1.4 Delineation of the Functional Floodplain

    SFRAs are tasked with the responsibility of defining Flood Zone 3b. PPS25 suggests the 5% a.p. flood event for the baseline of a functional floodplain, however, a greater event can be used where appropriate, depending on catchment characteristics and on agreement between the LPA and the Environment Agency.

    SFRAs can also identify where it might be appropriate to extend the 5% a.p. flood outline to areas within Flood Zone 2 and 3 to restore or expand the functional floodplain. The ability to identify and

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  • safeguard large enough areas against redevelopment and development in both urban and rural areas means that existing open space can potentially be used for flood storage, effectively reducing flood risk downstream. This process assists Flood Zone 3 policy aims, identified in table D.1 in PPS25, which include:

    � “Reduce the overall level of flood risk in the area through the layout and form or the development and the appropriate application of sustainable drainage systems,”

    � “Create space for flooding to occur by restoring functional floodplain and flood flow pathways and by identifying, allocation and safeguarding open space for flood storage.”

    The SFRA should be fully integrated with CFMPs and other strategies that show, at catchment scale, the need to protect the floodplain and avoid inappropriate development in high flood risk areas.

    1.2 Flood Defences

    As discussed above the Environment Agency Flood Zone maps do not take account of the presence of flood defences (although defended areas and the location of raised defences are included in the suite of information provided with the maps). PPS251 states that defended areas (i.e. those areas that are protected to some degree against flooding by the presence of a formalised flood defence) are still at risk of flooding, and therefore sites within these areas must be assessed with respect to the adequacy of the defences.

    The Environment Agency’s National Flooding and Coastal Defence Database (NFCDD) has been supplied and provides information on existing defences in the area, as well as categorising them by type and providing information on who owns and maintains them. Areas Benefiting from Defences (ABDs) have also been provided. ABDs are those areas which benefit from formal flood defences in the event of flooding from rivers with a 1% a.p. event or from the sea with a 0.5% a.p. event. If the defences were not there, these areas would be subject to increased flood risk.

    1.3 Flood Risk Management and Hydraulic Modelling Studies

    Many of the main rivers through Darlington have been represented using detailed hydraulic models and the Flood Zones in these locations give a good representation of reality. However, there is no single comprehensive hydraulic model for each of the river systems. Flood Zones outside of the modelled reaches are still represented with broad scale modelling techniques used in the original Flood Zone map definition and are therefore more prone to error.

    The hydraulic models available in the area include:

    � River Tees – 1D Hydrodynamic ISIS Model updated as part of the FRM Study (2008).

    � River Skerne – Hydrodynamic model used produced for floodplain mapping (2001) and prefeasibility study (2003).

    � West Beck – Hydrodynamic Model used for floodplain mapping study (2006).

    1.3.1 Tees CFMP

    The Tees CFMP will be used for evidence of flood history, flood risk locations and sources. This information is at a broad scale nature so will need to be supplemented with other detailed information. The proposed flood risk management policy for different areas within the Borough will be noted in order to help assess if future development proposals are likely to be sustainable.

    Between June 2007 and January 2008 JBA Consulting undertook strategic catchment modelling work for the Environment Agency, primarily for the Environment Agency’s CFMPs in the North East (including the Tees CFMP). The modelling generated 19 new Flood Maps for approximately 10,000 km of watercourse, representing different flood events and assumptions about flood risk management measures.

    1 Communities and Local Government (2006) Planning Policy Statement 25: Development and Flood Risk, Annex G para G2.

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  • This catchment modelling was also developed in order to update the parts of the Environment Agency’s Flood Map for Flood Zone 3 where no detailed river modelling studies had been completed. This catchment modelling incorporates a number of improvements over the original Flood Maps, including the use of new LiDAR data in place of SAR (NextMap-Britain) data, where available. A number of different scenarios were modelled to represent when flood water would come out of bank e.g. for the 50% a.p. event. Included in these outputs are flood depth and flood hazard maps.

    The current Flood Zone maps have not been updated with this new information and it is not known whether they will be updated in the future. It was initially thought that the SFRA could use the outputs to represent Flood Zone 3b, a climate change flood outline, flood depth and flood hazard maps (where there is no detailed modelling). However, it has not been possible to find a scenario that aligns with the current Environment Agency Flood Zone maps. For example, Flood Zone 3b would be greater than Flood Zone 3a in places. This is because different modelling techniques were used for the different extents. The SFRA will use a flood depth map from this study for a broad scale overview of the potential flood depths in Darlington.

    The Environment Agency have stated that it is critical for the functional floodplain outline to be as accurate as possible. However, the Environment Agency state that as a precautionary principle, Flood Zone 3 should be used to represent Flood Zone 3b in undeveloped areas (until the functional floodplain can be accurately defined). See Section 3.2.1 for a detailed explanation of how the functional floodplain was derived.

    1.3.2 North East Regional Flood Risk Appraisal

    A scoping study for the North East Regional Flood Risk Appraisal (NE RFRA) was completed by JBA Consulting in January 2009. The primary objective of the NE RFRA is to provide an appraisal of strategically significant flood risk issues and guide strategic planning decisions. The findings of this work will form part of the Regional Spatial Strategy evidence base.

    1.3.3 2007 Tees Valley SFRA

    An SFRA was completed for the Tees Valley local authorities in 2007. This SFRA was a good example of an SFRA produced prior to the publication of the PPS25 Practice Guide, but it needs updating to reflect the new guidance.

    The majority of the flood mapping that was used in the 2007 SFRA has now been updated. However, the 2007 SFRA will be used for additional flood history and flood risk location information.

    1.3.4 River Tees Model Updates for Section 105 Mapping

    In August 2004, Atkins produced a Phase 1 study (National Section 105 Framework Agreement Phase 1 River Tees, Environment Agency) which gave recommendations for further Section 105 modelling of the River Tees. This study looked at the existing modelling and modelling outputs as well as flood history. This flood history has been integrated into this report (see Chapter 2).

    In June 2007 JBA undertook a study on the River Tees for the Environment Agency (River Tees Model Update Phase 1, Environment Agency). The aim of this study was to update the fluvial ISIS model of the River Tees originally constructed by HR Wallingford. This model extends from the A1 upstream of Darlington to the Tees barrage.

    More specifically, the study aimed to improve in-river water levels for the main part of the Tees between the A1 Bridge and the Tees Barrage. In order to improve the water level predictions, the Phase 1 study updated and re-calibrated the existing ISIS model to incorporate recent data from Broken Scar gauging station and defence crest heights and washland topography.

    In January 2008, JBA Consulting completed the project for the Environment Agency to update the existing ISIS model of the river Tees (River Tees Model Update Phase 2, Environment Agency). The existing ISIS model has a long history, as it has evolved from an original SALMON-F model developed by HR Wallingford and has undergone numerous updates (mostly by HR Wallingford and JBA Consulting.) for various projects.

    The most recent update has incorporated information from LiDAR data in the floodplain spills and floodplain units. The model was re-calibrated using recent flood data and the hydrology was updated. Results from the updated model were used to produce revised outlines and NFCDD

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  • data. It was also used to test embankment risk priorities, existing flood warning measures and to comment on implications for operations and maintenance.

    1.3.5 River Skerne

    In 1997, Section 105 modelling was undertaken on Skerne Beck in Darlington for the Environment Agency (Section 105 - 30/92 Survey, River Skerne, Environment Agency, 1999). The study area comprised 27.2km of the River Skerne, between the Woodham Burn confluence and the confluence with the River Tees. A HEC-RAS, 1D, steady state model of the River Skerne was produced for this study.

    A prefeasibility report for the River Skerne was produced for the Environment Agency in September 2003 (River Skerne Pre-Feasibility Report, Version 2, Environment Agency). This updated the 1997 report, producing modelling of the River Skerne including the production of a 1% annual probability (a.p.) flood extent. A Skerne ISIS model was built by converting the above Section 105 model to ISIS and using new survey data to include the structures and weirs. LiDAR data provided by the Environment Agency was used to extend the channel sections into the floodplains. In June 2008 JBA Consulting updated this model to produce a 0.1% a.p. flood extent for the Environment Agency.

    The outputs from the modelling (flood extents) and the flood risk information and history gathered will be used to inform this SFRA.

    1.3.6 West Beck

    In July 2001, the Environment Agency commissioned JBA Consulting to complete a detailed study of the West Beck as part of their Section 105 programme (West Beck S105 Study, Phase 2 Final Report, Volume 1 Main Report, Environment Agency). This study was recommended following the preliminary (Phase 1) studies undertaken in 1999. The Phase 1 study recommended a more detailed study, including computational hydraulic modelling in order to provide more representative flood risk maps for planning purposes. This update included a new hydrology and hydraulic study, to better understand flood risk on West Beck. The FEH rainfall runoff method was chosen as the preferred option for peak flow estimation within the West Beck Catchment. A HEC-RAS model of West Beck was also produced. This comprised of one reach extending from the confluence with the Cocker Beck upstream to Burtree Lane. The model geometry is based on the channel cross section survey carried out by Maltby Land Surveys for JBA in February 2001. Some flooding history was also included in this study.

    In August 2006, the Environment Agency also commissioned JBA to produce a prefeasibility study for West Beck (West Beck, Darlington, Pre-Feasibility Study, Environment Agency). This study considered the existing and historic flood risk on West Beck. In addition, the steady state HECRAS model (see above) was updated and converted to an unsteady model with new flood extents produced. This study also provides a review of the flood risk management options considered, together with a technical assessment of their viability.

    The outputs from the West Beck studies will be used for the understanding of flood risk on West Beck and potential future flood management options.

    1.4 Topographic Data

    The essential dataset required for flood modelling, mapping and general elevation information is a Digital Elevation Model (DEM). There are two main sources of DEM data for the Borough, as shown in Error! Reference source not found..

    Table 1 - DEM availability

    Type Source Grid Filtered Year

    Nextmap SAR Environment Agency 5m Filtered -

    LiDAR Environment Agency 2m Filtered & unfiltered 2000 to 2005

    LiDAR Environment Agency 1m Filtered & unfiltered 2008 to 2009

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  • LiDAR will be used in preference to Nextmap SAR data as it has a higher vertical accuracy. The coverage of the LiDAR datasets available is shown in Figure 1. The blue squares show where the high resolution 1m LiDAR data is. The red shows the extent of the lower resolution 2m LiDAR data. The white within the Darlinton BC boundary is LiDAR gaps.

    Figure 1: LiDAR Coverage in Darlington BC

    © Crown copyright. All Rights Reserved. 100023728. 2009.

    1.5 Historical Flooding

    There are a number of sources of historical flood information. The majority of historical data collected was received from key stakeholders during the SFRA consultation process or by reviewing past flood studies in the area. Studies which have provided details on flood history include:

    � 2007 Tees Valley SFRA � Tees Catchment Flood Management Plan (CFMP) � West Beck Prefeasibility Study � River Skerne Section 105 Study

    The British Hydrological Society’s Chronology of British Hydrological Events also provides some descriptive information of past flooding events in the Tees Valley. This goes back many centuries but is more consistent after 1700.

    Key holders of historical flood data are identified below. All the data has been combined to provide a historical flooding GIS based dataset. This has been mapped for the SFRA and can be used by the LPA for future development planning and can be supplied to developers for site specific Flood Risk Assessments. The historic flood risk locations are shown with the PPS25 Flood Zone maps in Appendix A, Figures A1 to A8.

    1.5.1 Environment Agency

    The Environment Agency is a key source of all flood risk information in England and Wales. As part of the Flood Map, the Environment Agency provides a national historical flood map layer. This shows the extent of major flood incidents. Those identified include:

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  • � January 1995 - overtopping of the defence on the east bank of the River Tees (just upstream of the barrage)

    � January 1995 - flooding from the fluvial River Tees in Darlington, extensive but predominantly rural locations.

    � Autumn 2000 - flooding from the River Skerne through parts of Darlington.

    1.5.2 Darlington Borough Council

    Local Authorities can be an important source of historical flood information. Unfortunately, the Council's drainage team were unable to provide any up to date flood risk data. However, during the 2007 SFRA, the Council provided a GIS list of drainage issues. This information had been archived by JBA but was made available for this study. This dataset includes 16 flood risk locations including incidents form surface water and fluvial flooding. This will be a useful data source to compare with the Environment Agency’s theoretical surface water flooding maps.

    1.5.3 County Durham and Darlington Fire and Rescue Service

    County Durham and Darlington Fire and Rescue Service have provided geo-referenced data in spreadsheet format of all water related call outs from 1995 to 2008. This has been edited down (excluding burst pipes etc) to around 290 flooding incidents and converted to GIS. These locations have also been mapped along with all other historical data collected. Again, as many of the flooding incidents are surface water related, this will be a useful data source to compare to the surface water flooding maps.

    1.5.4 Cleveland Emergency Planning Unit

    Cleveland Emergency Planning Unit (a consortium of emergency planners covering the Cleveland local authorities) provided a list of historic flooding locations. This Unit only responds to large scale events, where coordination of different emergency response organisations is required. As a result, information on only four incidents was provided.

    1.5.5 Other sources

    The Highways Agency and Northumbrian Water (NWL) were contacted for flood risk information. NWL has not been able to provide any information until recently; when it was agreed that they would send DG5 (sewer flooding data) at drainage area level. The drainage areas have also been highlighted according to the level of risk. The data is at quite a high level but NWL will provide the area and street level DG5 information at a Level 2 SFRA stage. A Level 2 SFRA should be able to identify those issues NWL have actually dealt with or those which will definitely need further work. Using this information, more precise recommendations for Surface Water Management Plans (SWMP) can be provided. These recommendations should then be use to start SWMP work. The initial DG5 data from NWL will be integrated into the final version of this SFRA.

    A-One, the Highways Agency contactor for the A1 and A66, provided a large amount of data for the area. Unfortunately, much of this is not relevant to the SFRA.

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  • 2 Flood Risk in the Borough of Darlington

    2.1 Introduction

    There is a need to understand the risk of flooding from all sources in the Borough, consider where the highest risk locations are, and plan future development and regeneration accordingly.

    This section assesses flood risk in the Borough from all sources, now and in the future. It makes use of all the data and information described in Chapter 1. It includes the fluvial Flood Zones and assesses flood risk from other sources, with the aim of providing enough information for the Council to perform the Sequential Test.

    The major watercourse in the Borough is the River Tees which originates outside of the administrative boundary. The Tees flows along the southern boundary of the Borough, mainly placing agricultural land at flood risk. A more important watercourse in terms of flood risk is the River Skerne. This flows directly through the centre of Darlington. Other watercourses of interest within the urban area include West Beck and Cocker Beck (see Figure 2).

    The historic flood risk information collated during this study also shows the distribution of other sources of flooding, the main being surface water flooding. Although the mapping shows a wide distribution of such locations (see Figures A1 to A8 in Appendix A), surface water flooding is not necessarily as significant as it may seem, due to the small scale and low hazard of the flood events. Many of the flood risk locations, for example, indicate only very localised pooling of shallow water after heavy rain.

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  • Figure 2: Darlington Borough with the allocations and rivers

    Cocker Beck

    River Tees

    River Skerne

    © Crown copyright. All Rights Reserved. 100023728. 2009.

    2.2 River Tees

    The River Tees rises in the Pennines where it passes through Cow Green Reservoir and High Force Waterfall. Further downstream, it flows through Middleton, Barnard Castle and Yarm, where it is joined by the River Leven. Ultimately, it flows through the heavily urbanised areas of Stockton-on-Tees and Middlesbrough before forming an estuary as it enters the North Sea.

    Within Darlington, the River Tees meanders through predominantly rural land. The Tees has a wide floodplain here and flood flows from the Upper Tees and River Skerne are attenuated by significant overbank flood storage. Although much of this reach has flood defences, primarily built to protect agricultural land, they are overtopped or bypassed in major floods. Some sections of the Tees are protected to a higher level in order to protect settlements, such as Hurworth and Neasham.

    2.2.1 History of flooding

    There is a history of flooding on the River Tees within Darlington that goes as far back as records began. This is to be expected from a large river which will seasonally overtop its banks and fill the floodplain. Defences have been built along the Tees to protect agricultural land, but these private defences (i.e. non Environment Agency) are built to a low standard and therefore have a history of overtopping and breaching. In general, major development has not taken place along the Tees within Darlington and future development is not expected. However, there are some existing settlements that have a long history of flooding. This includes Neasham and Hurworth Place. Some flooding dates are below.

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  • � Neasham - 1753, 1771 (15ft deep floods), 1852, 1886, 1890, 1892, 1918, 1923 (2 ft), 1924, 1927, 1928, 1963, 1967, 1968, 1995.

    � Hurworth Place - 1967, 1995, 2000.

    There is also history of flooding at the caravan park near Hurworth (1991 and 1995). There is a private embankment adjacent to the caravan park but the Environment Agency's flood defence dataset states that this offers no flood defence benefits. This caravan park is within Flood Zone 3 and the River Tees presents a significant hazard to the site.

    Table 2.1 provides details on more recent flood events on the River Tees.

    Table 2 - Recent flooding from the River Tees

    Date Description

    March 1968 30 properties were flooded at Hurworth Place. 12 properties flooded at Neasham.

    1991 Caravan park flooded at Newbus Grange

    1995 There was a flood event on the River Tees which lasted from 31 January to 1 February. At the onset of the event, the upland catchment was already saturated and further heavy rainfall together with mild weather conditions meant that snowmelt was also a significant factor. The average flow was estimated as having a 1.3% annual probability. Flooding of isolated properties in the upper and middle reaches occurred, and a total of 72 properties were reported as affected by flooding.

    The banks of the River Tees were overtopped at Hurworth, the caravan park, Newbus Grange and Neasham causing significant flooding to properties.

    2.2.2 Kent Beck

    Kent Beck enters the River Tees just upstream of Neasham. During the January 1995 and June 2000 flood events, the Neasham flood defence scheme prevented flooding from the River Tees, but flooding still occurred to properties in Neasham. This flooding originated from Kent Beck, which backed up behind the flap/penstock at the Neasham defences. 55 properties were flooded during the 1995 event but since then, the defence at Willow Garth (Neasham) has been raised.

    The 2000 event occurred due to seepage from through holes in the left flood defence bank, flooding 13 properties (steel piling has since been installed here and the bridge parapet strengthened). It was suggested in the Tees Model Update Report (Phase 2, Environment Agency, January 2008) that the flow in Kent Beck during the June 2000 event had a 1.6% annual probability.

    2.3 River Skerne

    The River Skerne catchment is divided into two distinct reaches. The upper reach is predominantly rural, consisting mostly of farmland. The lower reach comprises the urban conurbation of Darlington.

    Compared to the River Tees, the Skerne is a dry lowland catchment and has extensive areas of very flat floodplains. The catchment is relatively dry because of the drier climate and the greater permeability of soils and bedrock. The volume of flow is also relatively low as the channel has artificially enlarged to convey the flood volume carried. However, when there is heavy rainfall or snowmelt, the channel has insufficient capacity and floodwater can spill out over a wide area. The areas around the upper (rural) floodplain are flooded nearly every winter and the land remains poorly drained throughout the year.

    The Tees Valley CFMP states that the Skerne is affected by two conflicting influences in its flood generating characteristics:

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  • 1. Percentage storm runoff is generally less than 20 percent (Archer, 19842), due to low rainfall and high soil storage capacity and soil moisture deficits. These characteristics mean that summer floods are rare (Archer, 19813) despite the maximum storm rainfall occurring in summer. Instead, most floods occur towards the end of the winter when soils are approaching saturation.

    2. The catchment has extensive areas of very flat land adjacent to the main channel and tributaries. These wetland ‘carrs’, notably at Mordon, Preston, Bradbury and Mainsforth, are the original floodplain of the natural river and were regularly flooded in winter. Drainage and embankment of the carrs for agricultural use means they are not flooded from the river by small events, although ponding occurs from incident rainfall. There is still substantial flooding in major events.

    The main areas of flood risk from the 1% a.p. flood event, identified in the River Skerne Section 105 studies (1999) are:

    � Skerne Park in Darlington, � the housing area at Riverside in Haughton-le-Skerne, � the agricultural land at Great Burdon Bridge in Darlington, � Ketton Lodge � Coatham Mundeville

    The Skerne Prefeasibility Study (2003) concludes that the following locations are most at risk of flooding from the 1% a.p. event on the Skerne.

    � Aycliffe Village � Coatham Munderville � Skerningham Plantation � Barmpton � Great Burdon � Haughton Road � Cleveland Industrial Estate � Victoria Embankment

    From this flood event, there are only pockets of properties at risk from the Skerne and this most recent modelling (Skerne Prefeasibility, 2003) shows no serious flooding in Darlington town centre itself during the 1% a.p. flood. The Skerne Prefeasibility Study concludes that the floodplain storage upstream of Darlington, combined with the improvement works to the channel itself between 1952 and 1972, have succeeded in protecting Darlington town centre from widespread damage for all but very extreme fluvial flood events. The only history of flooding in Darlington town centre was in 1952 and 1967 (pre flood alleviation works).

    The Skerne Prefeasibility Study also stated that through Darlington town centre, between John Street Bridge and South Park, the Skerne’s hinterland is significantly lower than the bank top levels. To protect the town, continual engineering of the channel over the past century has increased the river’s capacity and seemingly resolved much of the flooding problems. Although the standard of protection is now in excess of the 0.5% a.p. event through much of Darlington town centre, there remains a large defended area which, if breached or bypassed, could result in flooding of a significant number of properties. This can be seen in the difference between the Flood Zone 3 and 2 maps (see Figures A1 to A8 in Appendix A).

    The Skerne Section 105 reports state that during more frequent flooding events (than the 1% a.p. event), water levels in the River Skerne may affect land and urban drainage networks. Backing up of discharges could result in surface water flooding from drains and manholes, leading to ponding in low lying areas. Historic flooding records have provided evidence for this, for example, in the north of the Borough there is evidence of storm drains overflowing around Coatham Mundeville.

    2 Archer, D. (1984) The estimation of seasonal probable maximum flood. British National Committee on large dams Conference, Cardiff, Technical

    Papers. 1-20

    3 Archer, D. (1981) The seasonality of flooding and the assessment of seasonal flood risk. Proceedings Institution of Civil Engineers, Part 2, 1023

    1035.

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  • 2.3.1 History of flooding

    Historic flooding information reveals that the Skerne has flooded in the following years: 1771, 1852, 1856, 1875 (twice), 1876, 1878, 1880. 1886, 1892, 1895, 1900, 1903, 1924, 1928, 1948, 1967 (twice), 1979, 2000 and 2001.

    The Skerne Prefeasibility Study (Environment Agency, September 2002) provides a review of recent flood history on the Skerne. Some of this is summarised below.

    The first records of flooding along the Skerne are for an event in 1771, which caused widespread flooding to properties within the centre of Darlington. Flooding occurred twice in the same year in 1875 due to heavy rainfall. Flooding occurred from the gasworks at the upstream end of Darlington through to South Park.

    In 1876 work to improve the capacity of the Skerne channel downstream from Victoria Road Bridge was undertaken. As a result of these works, flooding during the twentieth century seems to have been less frequent.

    By 1966, a flood alleviation scheme designed to improve the capacity of the Skerne was completed. The works included:

    � Weir at outlet to South Park lake was remodelled as a gauging station � Russell Street weir lowered � Cocker Beck was diverted into the Tees via the Baydale Beck � Regrading of the Skerne channel

    Since the completion of the scheme, flooding on the Skerne has greatly reduced, however, other events have occurred. For example, in November 1967, 54 properties along Valley Street, John Street, Oxford Street, Mount Street and Parkgate were all flooded. The severity of the flooding was attributed to the laying of a temporary sewer within the river channel which had obstructed the flow. This event prompted further improvement works between 1970 and 1972, which lowered the weir again at Russell Street and widened the channel between Chesnut Street and John Street from twenty-five to thirty-five feet.

    In March 1979, the strength of the flood defences were tested. A heavy snowfall on the 17th March had begun to melt and swell the river, when on the 28th March 40-50 millimetres of rain fell on the saturated catchment. The peak flow at South Park for this event was 60m3/s, a flow rate probably not reached since the flood of 1875. Within the centre of Darlington the bridge at Priestgate was overtopped and the river came out of bank on to the Ring Road. However no properties were flooded. It therefore appeared that all the engineering works carried out over the previous 50 years had succeeded in preventing widespread flooding damage to domestic and commercial properties.

    The most recent flood events were in November 2000 and February 2001 where a number of properties were flooded in Darlington. The Environment Agency's historic flood outlines show that in November 2000, the River Skerne overtopped its banks at: Barmpton, Great Burdon, HaughtonLe-Skerne and the land on the opposite side of the bank to Skerne Park.

    2.4 West Beck and Cocker Beck

    West Beck is a tributary of Cocker Beck, which in turn flows into the River Skerne (which is a tributary of the River Tees). West Beck originates north of Darlington, in the triangle of land between the A1 and the Darlington-Bishop Auckland railway line. The northern part of the West Beck catchment is almost entirely rural and is currently used for a mixture of arable and livestock farming. The remaining part of the catchment between Faverdale (north west Darlington) and Cockerton (further south in the urban area) is essentially urbanised. West Beck converges with Cocker Beck in Cockerton just upstream of the B6279.

    Several small drains flow into West Beck. In addition, two un-named watercourses can be identified as tributaries of West Beck. These include a small watercourse which joins West Beck just upstream of the A68 Road Bridge and the second just upstream of High Faverdale Farm, both of which drain from the left bank. No further significant tributaries are present within the catchment.

    West Beck was culverted in 1974-5, downstream of the former Faverdale Chemical works site (now West Park), by the Northumbrian Water Authority (NWA). Between Newton Road and the confluence with the Cocker Beck, West Beck remains open channel.

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  • The current Flood Zone maps show widespread flooding of agricultural land at the upstream study extent. The flood risk maps also indicate that approximately 78 properties in the West Beck catchment are at flood risk from the 1% a.p. flood event. This includes 19 properties in the recent housing development in Faverdale, 50 properties in the vicinity of Spring Court, 7 properties in the Newton Lane and Cockerton Green area and 2 properties adjacent to the Cocker Beck confluence.

    The prefeasibility study for West Beck (August 2006) summarised that for National Funding, 'Do Minimum' is the preferred flood risk management option to reduce flood risk. This would entail:

    � Keeping all culverts and bridges clear of blockage � Maintaining vegetation in the channel and floodplain � Monitoring of silt levels at West Bridge

    However, the study also stated that there is a viable locally funded scheme for West Beck which would provide a standard of protection (SoP) up to the 1.3% a.p. event. The preferred option for this locally funded scheme includes the following works:

    � Construction of new flood wall along the right bank upstream of West Auckland Road

    � Construction of a new flood wall along the right bank upstream of the rail embankment adjacent to West Auckland Road tying into natural high ground

    � Construction of new flood walls either side of West Beck between West Bridge and Prior Street.

    In addition, after an internet search, it was shown that Cocker Beck has recently been desilted, adjacent to Westbrook Terrace by the Environment Agency. It had been noted that the silt and debris which had built up in the beck prevents the drains from discharging effectively during heavy rain, which may lead to flooding on North Road.

    2.4.1 History of Flooding

    There are records of flood problems along West Beck and Cocker Beck dating back to 1975. These include 3 incidents of flooding recorded in the last 25 years in October 1976, March 1979, and June 1982 involving surcharging of the Newton Lane culvert and flooding of the road and a small number of residential properties. Flooding of fields at the Cocker Beck confluence was also recorded in October 1976. The watercourse has been identified as a problem watercourse by Darlington Borough Council and the Environment Agency as a result of the flooding at Newton Lane. The main cause has been attributed to a lack of capacity in the culvert and channel downstream, owing to inadequate size and siltation.

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  • Table 3 - Recent flooding from West Beck and Cocker Beck

    Date Description

    1875 Cockerton Green, below the School House was submerged and several houses were flooded.

    1903 The bridge at Cockerton washed away and houses on the south side of the village were flooded.

    1976 Heavy localised rainfall resulted in flooding at West Beck and Cocker Beck confluence and of Newton Lane on West Beck.

    1979 Flooding at Newton Lane.

    1982 Severe storms over West Darlington caused overtopping of West Beck at Newton Lane culvert. Two residential properties were flooded (Numbers 90 and 111 The Green). Four others only saved by having raised door thresholds. The new developments of sheltered flats at Newton Court surrounded by water but not flooded.

    2000 Despite widespread flooding in the Tees catchment in June and October 2000 there appears to be no record of flooding in the West Beck catchment.

    2.5 Flooding from land

    2.5.1 Historical flooding data

    Many of the historical flooding locations collated from the Environment Agency, Darlington BC and Cleveland Fire Brigade are located outside of the fluvial and tidal Flood Zones. This indicates that these flooding locations are due to other sources, such as surface water flooding directly from the land or drainage system. The distribution of these locations can be seen in Figure A, Appendix A1 to A8.

    The historic flooding locations data shows that overflowing of stormwater drains has occurred around Coatham Mundeville. This data also shows that Coatham Mundeville has a history of surface water flooding from land runoff after a heavy rainfall event. The surface water flooding maps also tie in with this location.

    Other flooding incidents caused by surface water runoff have been recorded at:

    � Redworth � Heighington � Burdon Hall � Petty's Nook � Souith of Bishopton � Skip Bridge � Low Middleton

    These locations also tie in to some extent with the surface water maps but they are small-scale, isolated and predominantly outside the urban area.

    The Tees CFMP states that surface water flooding is a particular problem in Darlington where surface water is unable to discharge to drains and rivers. The CFMP does not go into any detail as to why there is this surface water issue in Darlington.

    2.5.2 Surface water maps

    The Areas Naturally Susceptible to Surface Water Flooding Maps, based on the Environment Agency data, is discussed in Section 2.4.1 of this report. The surface water maps for Darlington are included in Appendix A (Figures E1 to E8). These maps show the surface water flood extent and variation in susceptibility due to an extreme rainfall event. These maps should be used to assist the strategic consideration of the impacts of surface water flooding but also the sequential approach, the production of Surface Water Management Plans (SWMPs), and detailed FRAs for specific development proposals.

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  • The distribution of surface water flooding ties in with the fluvial flooding locations and other galleys and low areas. This is where surface water will flow after heavy rainfall. The built up locations with significant surface water flooding extents are:

    � Middleton St George � Around the airport � Coatham Mundeville

    Within the Darlington urban area: � Eastbourne area � Lingfield area � Pierremont area � Town Centre Fringe around the fire and police stations

    Potential development sites with significant surface water flooding extents are:

    � West Park � EP2.6 employment site - Lingfield

    These locations, where surface water can pool due to topography, do not always present significant problems. This type of surface water flooding can be managed for new development and in developed areas through surface water drainage improvements and the application of SUDS.

    However, pathways where surface water will naturally flow should be noted. These can potentially be hazardous and can damage property, but it should be possible to control this with surface water management techniques such a sequential approach to site layouts or adopting SUDS. These locations can be described as critical drainage areas (CDA) and are discussed below.

    2.5.3 Screening of Critical Drainage Areas (CDA)

    SFRAs provide the opportunity for local authorities to assess, at a strategic level, the risk from multiple sources of flooding, which can then feed into more detailed assessments where appropriate by both themselves and other operating authorities. This includes the identification of Critical Drainage Areas which are those areas identified from historical flood events and/or modelled data as having a significant risk from surface water flooding. Recommendations can then be made for the future provision of Surface Water Management Plans (SWMPs) in high risk locations or areas of significant development for which an integrated drainage solution is possible that can reduce flood risk to both the current community and new development (SWMPs guidance is discussed in Volume III).

    Screening for CDAs within the Borough were undertaken using data from the following sources:

    � Fire Brigade flood incident locations � The national Surface Water Map (SWM) � Local authority flood risk locations � Flooding records from previous studies

    The area was assessed to try and identify any potential CDAs. This was done by finding any clusters of historical flooding locations. If incident clusters and mapped surface water flow pathways overlap, this would be an indication that there is a CDA.

    It will be important to get feedback from Northumbrian Water (and the Council's drainage team) on whether the locations identified:

    � are actual problems, � were existing problems but a scheme has been completed to deal with the

    problem. � are existing problems but a scheme will be completed in the future

    Pierremont

    As highlighted in Section 2.5.2, the Pierremont area of Darlington has a particularly extensive surface water flooding area. Some parts of this include the intermediate and high susceptibility to

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  • surface water flooding maps. The three areas circled in Figure 3, show where historic flooding locations overlap the SWM. The northwest area may be linked to Cocker Beck (which is to the north). This could be a natural flow pathway for surface water making its way to Cocker Beck. Backing up could occur which would increase risk to the Pierremont area. The other two locations appear to be low areas where surface water can pool. However, the whole area has a number of historic flooding locations (unrelated to fluvial flooding) and SWM extents.

    In addition to this, Section 2.6 describes a scheme that Northumbrian Water is to undertake in the Pierremont area. Information on this scheme from Northumbrian Water's website states that during storms, rainfall entering the sewerage network has resulted in flooding to the properties on Pierremont Crescent. It is not known whether this scheme will rectify all the potential problems in the Pierremont area.

    Figure 3 - Possible critical drainage area in Pierremont

    © Crown copyright. All Rights Reserved. 100023728. 2009.

    Purple - high susceptibility to surface water flooding, orange = intermediate, green = low, green dots = historic flooding locations

    Town Centre Fringe

    In the centre of Darlington there is a cluster of historic flooding incidents (see Figure 4). However, not all of these locations tie in with the SWMs. Directly to the west of the River Skerne, there are around 10 historic flooding incidents. Also within this area, there are a number of outfalls into the Skerne. There is little descriptive data on the historic flooding locations but it may be that there is an issue with outfalls into the Skerne backing up. It was mentioned in Section 2.5.1 that the Tees CFMP recognises surface water flooding as a problem in Darlington, in particular, surface water being unable to discharge to drains and rivers. In addition, the Skerne Section 105 report state that during flooding events on the Skerne, water levels may affect land and urban drainage networks. Backing up of discharges could result in surface water flooding from drains and manholes, leading to ponding in low lying areas. Due to this evidence, this area has been highlight for surface water flooding issues, possibly due to backing up of culverts flowing into the Skerne.

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  • Figure 4 - Possible critical drainage area in the centre of Darlington next to the Skerne

    Crown copyright. All Rights Reserved. 100023728. 2009.

    Purple - high susceptibility to surface water flooding, orange = intermediate, green = low, green dots = historic flooding locations

    Eastbourne

    Figure 5 shows significant intermediate (and some high) surface water susceptible locations. A number of historic flooding locations tie in with these areas. The topography seems to show this area as a shallow basin, where surface water flow from the north could flood the lower flatter area in the centre (see Figure 5). The area susceptible to surface water flooding generally covers Eastbourne Park, the play area to the south and the school playing fields further south. These fields should be kept undeveloped and, subject to more detailed studies, opportunities to open up areas further south could be sought.

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  • Figure 5 - Possible critical drainage area in Eastbourne

    © Crown copyright. All Rights Reserved. 100023728. 2009.

    Purple - high susceptibility to surface water flooding, orange = intermediate, green = low, green dots = historic flooding locations

    2.6 Flooding from sewers

    Northumbrian Water is currently unable to provide information on flooding from sewers and any planned improvement schemes. As a result, it has not been possible to consider the potential foul and surface water sewer risk to Darlington in any detail.

    The River Skerne Prefeasibility study noted that within the centre of Darlington, there are a number of drainage outfall pipes discharging to the Skerne, which could convey flood waters and place properties within the centre of Darlington at risk. However, we do not have any flood history information on the backing up of sewers within Darlington.

    The West Beck Prefeasibility Study states that considerable parts of the West Beck catchment are sewered, with all new developments in Faverdale and northwards being drained on a separate surface/foul system. Parts of Cockerton, however, remain on a combined system.

    The older, combined sewer systems can be prone to flooding due to capacity issues. In addition, as the storm and foul sewers are combined, flooding would pose a human health and pollution hazard.

    A search of Northumbrian Water's website has enabled this study to highlight some upcoming sewer improvement schemes. This identifies sewer flooding problem locations but also summarises works to reduce the risk. Northumbrian Water is to invest £3 million to reduce the risk of flooding to fourteen homes in Pierremont Crescent. In times of heavy rainfall the quantity of rainwater entering the sewerage network has resulted in flooding to the properties on Pierremont Crescent. To prevent this from happening, 650 metres of sewer pipe is to be upgraded, and

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  • increased in size. A new combined sewer overflow will also be built in Brinkburn Dene Park. The work to build a screened, combined sewer overflow will also improve the water quality of Cocker Beck and the River Skerne.

    The fire brigade flooding incident dataset shows that this location also has a history of flooding. The fire brigade were needed to pump flood water out of a number of houses. The surface water flood maps also show this location at risk of surface water flooding. The scheme currently being completed should alleviate the surface water sewer flooding problems. However, the combined sewer overflow will enter Cocker Beck, which already has a history of flooding. This increased discharge could have the potential to increase flood risk on Cocker Beck.

    A £4 million sewerage scheme is to be built at Middleton One Row, south east of Darlington. Northumbrian Water will lay 4km of pipes to transfer waste water to Stressholme treatment works on the outskirts of Darlington. The works include building a new pumping station and storm water tank at Middleton One Row. The work is needed due to an expansion in housing in the last few years, and the system is designed to cater for population growth. The work is in tandem with a £17m upgrade of the Stressholme treatment works to enable it to deal with 70m litres of waste water per day once it is complete.

    2.7 Flooding from groundwater

    The Draft Tees CFMP states that there is little documented evidence of groundwater flooding in the Tees catchment. However, with the cessation of minewater pumping and reductions in abstractions on the western edge of the Magnesian Limestone Escarpment, water table levels have risen and are thought to have caused flooding. Groundwater levels in the Skerne catchment are continuing to rise as a result of mine water rebound.

    It is suspected that groundwater flooding occurs regularly in the Skerne catchment, but since the events often result in surface water flooding, they are recorded as such in the records. There is also the possibility that mine subsidence could cause flooding but there are no recorded instances in the catchment.

    2.8 Flooding from reservoirs and other artificial sources

    There are no major reservoirs or canals in Darlington.

    2.9 Effects of Climate Change

    UKCIP02 scenarios suggest that winters will become wetter over the whole of England, by as much as 20% by the 2050s. A shift in the seasonal pattern of rainfall is also expected, with summers and autumn becoming much drier than at present. Snowfall amounts will decrease significantly throughout the UK, the number of rain-days and the average intensity of rainfall is expected to increase. This could have significant implications for surface water flooding and should be considered when designing drainage systems for new developments. An increase in peak flow by around 20% over the next 50 years will translate into higher water levels.

    Figures C1 to C8 in Appendix A shows the difference in extent between the 1% a.p. flood event with 1% a.p. event plus climate change. This shows an increase in flood extent from the River Tees in the Hurworth and Middleton St George areas. However, significant numbers of properties are not put at risk as a result (just isolated farm properties). The River Skerne to the north of South Park shows a small increase in extent, potentially putting more properties at risk. The flood extent of the River Skerne also increases by a small degree from Coatham Mundeville to the B6279 in Darlington (in mainly rural locations) The flood extent from Cocker Beck around Cockerton increases in this built up area, potentially putting more properties at risk.

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  • Extracted from PPS25

    2.10 Geology and Soils

    The geology and soils of the Borough were investigated using a strategic scale (1:250,000) map available from the National Soil Research Institute and can be viewed at: http://www.landis.org.uk/soilscapes/

    According to the soils map the majority of the Borough is covered by grass, arable land and some woodland. This is slowly permeable seasonally wet loam and clay soils. This will impede natural drainage. The area around the River Tees is composed of grassland and some arable land. This loamy soil is freely draining.

    Unfortunately the scale of this data does not make it particularly relevant at a local level and should only be used as an indication of the potential for groundwater and surface water flooding and as a generalised dataset for the implementation of source control and infiltration sustainable drainage techniques (SUDS).

    Therefore, geology and soils should also be investigated at a site level during a FRA. Their characteristics are not the only considerations when designing SUDS. It is recommended (refer to Volume III of this SFRA) that the application of SUDS should be explored at an early stage of new development projects and design requirements documented within any FRA produced. More detail on the application of SUDS and the SUDS “Management Train” is provided in Volume III.

    2.11 Flood Defences

    2.11.1 River Skerne

    The River Skerne has had flood defences built in the past to reduce flood risk in Darlington. From 1952 to 1967 a comprehensive scheme was constructed to improve flood capacity. Further work was carried out in 1972 following the floods of 1967. As stated in section 2.3, since these defences were completed there have been no flooding incidents in Darlington.

    In the Environment Agency's National Flood Defence Dataset (NFCDD), the only raised flood defence on the River Skerne are in the centre of Darlington, stretching from where the railway crosses the Skerne (south of Cleveland Street) to the Chesnut Street Bridge (although there is another small section adjacent to St Cuthberts Way). These consist of a mixture of private and Environment Agency owned concrete flood walls. NFCDD does not show the standard of protection (SoP) for these defences.

    Current maintenance for the Skerne consists of twice yearly inspections of known hard defences, intermittent inspection of watercourse for tree removal and large urban debris and reactive inspection when required.

    The River Skerne Prefeasibility Study (2003) assessed the economic viability of a flood alleviation scheme. The conclusion of the study was that there was no economic justification for a scheme. The recommended option was to 'do-minimum'; continuing with maintenance and accepting that there are pockets of properties at risk from flooding which should be integrated into the flood warning procedures.

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  • 2.11.2 River Tees

    Much of the River Tees reach within Darlington has flood defences, primarily built to protect agricultural land. However these are overtopped or bypassed in major floods. Some sections of the Tees are protected to a higher level in order to protect settlements, such as Hurworth and in Neasham, where the defences protect the properties up to the 1% a.p. event but some flooding would occur from Kent Beck at this return period

    The River Tees Model Update Phase 2 report (Environment Agency, January 2008) assessed the impact of removing defences on the River Tees. This study used the 1996 HR Wallingford report entitled ‘Lower River Tees: Impact of flood banks on flood water levels”. This used the ISIS model to identify the standard of the defences and to investigate effects of changing embankment levels on flood levels at Yarm. It was found that the existing standard of defence varied and in some cases was less than 5 years (rural defences). The minimum standard of protection was less than 100 years for about 75% of the washlands. The River Tees Model Update Phase 2 report built upon this and provided a risk category for each section of flood defence on the River Tees. Of the 43 sections of defences assessed, 20 (46%) are classified as low risk and maintenance could be safely withdrawn without impacting on flood risk at the 50-year return period.

    2.12 Flood Warning

    The Environment Agency has the lead role in providing the flood warning service in England and Wales. The aim of the flood warning service is to reduce risk to life, distress to people and damage to property caused by flooding by providing accurate, timely flood warnings to residents within the floodplain of rivers, estuaries and coasts; to the media and partner organisations.

    It is crucial that people at risk receive appropriate flood warnings and take action to protect themselves and their property. Within the Environment Agency corporate plan “Creating a Better Place4” the Agency has highlight three main targets:

    � To have 80% of properties at risk in the floodplain in England and Wales receiving an appropriate flood warning service;

    � 75% of people who live in flood risk areas take appropriate action by 2011; and � To have major incident plans in place for high flood risk areas.

    Currently the Environment Agency operates a flood warning service in specific locations known as “Flood Warning Areas” where “Flood Warning Codes” are assigned based on the overall impact of flooding within an area. These codes include:

    “flooding of low-lying land and roads is Flood Watch expected”

    “ flooding of homes and businesses is Flood Warning expected”

    Severe Flood Warning “severe flooding is expected”

    All Clear “all clear or receding floodwaters”

    The Environment Agency’s Floodline Warnings Direct service provides flood warnings direct to people by telephone, mobile, email, SMS text message, fax or pager. There are a number of Flood Warning and Flood Watch areas that cover the Borough, some of which cross over administrative boundaries. Figure D in Appendix A show the Flood Warning Areas covered by Floodline Warnings Direct. They include:

    4 Environment Agency (2006) Creating a Better Place: Corporate Strategy 2006-2011

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  • Skerne Properties in Darlington Town Centre, and businesses on Valley Street North.

    Tees Caravan Park at Newbus Grange Neasham village, from Kent Bridge, following Teesway, to Dibdale Road

    2.12.1 Flood warning improvements

    There are currently no flood warnings for West Beck and Cocker Beck. Recent studies on these two watercourses recommended that Darlington Borough Council and the Environment Agency take account of the identified flood risk areas and the findings of this report in their Emergency Planning.

    For Cocker Beck it was highlighted that the installation of a telemetry rain gauge within the catchment to generate forecast for use in flood warning would be beneficial. Consideration should also be given to the modification of the model to run ‘real-time’ flood forecasting.

    The flood warning procedures for the Skerne are based on trigger levels at Bradbury and Preston-le-Skerne Gauging Stations. Using the hydraulic model during the Skerne Prefeasibility Study, revised levels have been provided at Preston-le-Skerne and South Park Gauging Stations when flooding first commences at the four flood risk areas. This study recommended that that the current flood warning procedures be reviewed to consider these levels.

    The River Tees Model Update Phase 2 report also recommended that the River Tees flood warning areas are updated to match the latest modelled outlines to eliminate inconsistencies and ensure that all people at risk are offered a warning.

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  • 3 Strategic Flood Risk Mapping

    3.1 Introduction

    The investigation and identification of the extent and level of flood risk to an area is assessed primarily geographically. Whilst the Environment Agency’s Flood Maps are very useful in this respect in showing indicative land use planning zones as required by PPS25, they are only a starting point in the consideration of flood risk in a particular area.

    PPS25 Flood Zone Maps should be used primarily to enable the Sequential Test to be carried out, firstly in avoiding inappropriate development and then secondly, to seek compatibility between flood risk vulnerability and Flood Zones as required in Table D3 of PPS25.However, more detailed analysis is often needed to gain a greater understanding of the varying degree of flood risk at a district level.

    At this SFRA level, it is not appropriate to look at flood risks in detail for individual development proposals, as this is a requirement of the site specific FRA and will be undertaken by developers in respect of a specific development proposal and prior to submitting a planning application. However, there is a need to undertake a broad assessment of flood risk issues to assist the LPA in making appropriate spatial planning decisions. This will enable a degree of certainty that the proposed strategic sites and allocations put forward in future DPDs, allow compliance with the Sequential and Exception Tests in PPS25 and importantly provide information to test whether the developments should be safe for occupants and users.

    This broad assessment is assisted greatly by a suite of Borough wide flood risk information including the PPS25 Flood Zone Maps. No one map should be considered in isolation without reference to the others. The set of Strategic Flood Risk Maps provided in the Darlington BC Level 1 SFRA can be found in Appendix A and include:

    SET A: PPS25 Flood Zones

    SET B: Flood Zone 3 Depths

    SET C: Climate Change Sensitivity

    SET D: Flood Risk Management Measures

    SET E: Areas Naturally Susceptible to Surface Water Flooding

    After the PPS25 Flood Zone Map has been used to carry out the first sweep or Sequential Testing for various proposed development locations, all sets of maps need to be interpreted consistently in order to complete the second or third pass of the sequential approach sieving process. They can also be used “outside” of the development planning process to gain an understanding of various flood risk factors in other areas of interest across the Borough.

    The detail provided in the Strategic Flood Risk Maps may also facilitate the application of the Exceptions Test where applicable. These maps should be used in sequence as shown in the Sequential Test sieving process as shown in Volume I of the SFRA.

    3.2 PPS25 Flood Zone Maps

    The PPS25 Flood Zones have been produced on a set of eight maps covering the Borough (see Appendix A Figures A1 to A8). The fluvial Flood Zone maps are based on information provided in the Environment Agency Flood Map. Version 3.14 of the Environment Agency Flood Zones issued in June 2009 has been used as the latest flood zones for the Borough. Flood Zones 2 and 3 were checked against all the most recent modelling outputs to see if they had all been integrated. Both Flood Zones 2 and 3 appear to have been updated with all the currently available hydraulic models.

    As stated in PPS25, an aim of SFRAs is to define Flood Zone 3b: the functional floodplain. In this SFRA, this has been delineated using the method outlined in section 3.2.2.

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  • These key maps should be used for facilitating the undertaking of the Sequential Test by planners and developers according to PPS25, as discussed previously in Volume I and illustrated within stage 1 of the Sequential Test sieving process.

    3.2.1 Functional Floodplain

    The Functional Floodplain (Flood Zone 3b) has been defined using modelled 4% a.p. outlines where available. The modelled outlines were then edited using the following methodology:

    � Inclusion of land which provides a function for flood conveyance or flood storage (e.g. washlands)

    � Removal of areas benefitting from defences (ABDs) � Removal of developed (Brownfield) land � Removal of major transport infrastructure (e.g. motorways and railways) � Removal of ‘dry islands’ defined using the ‘size standards’ within the Environment

    Agency SFRM Specification for Flood Risk Mapping5

    For those watercourses that have not been modelled or where a 4% a.p. outline is available, “Candidate Flood Zone 3b” areas have been identified based on the Environment Agency Flood Zone 3 outlines. Greenfield areas within Flood Zone 3 have been identified which should be safeguarded from future development. Storing flood water in these areas during an event could potentially reduce risk downstream in urban areas in the future.

    However, as these areas have not been explicitly modelled (or have used a 1% a.p. modelled outline) and are partly based on professional judgement, it is important that they are assessed in more detail at a site-specific FRA level if development is planned in the future. Nevertheless it is recommended in this SFRA that they are left as open greenfield for future flood storage or as flood compensation needed to allow other development.

    The data used to define the functional floodplain and “Candidate Flood Zone 3b” for each watercourse is summarised in Error! Reference source not found..

    Table 4 - Functional floodplain and candidate functional floodplain mapping

    Watercourse Extent Data source

    River Tees (east of A1) 4% a.p. River Tees ISIS model (2008)

    River Skerne 4% a.p. River Skerne floodplain mapping "001 and 2003)

    West Beck 4% a.p. West Beck floodplain mapping (2006)

    River Tees (west of A1) 1% a.p. Candidate Flood Zone 3b Flood Zone 3 Version 3.14

    Cocker Beck 1% a.p. Candidate Flood Zone 3b

    Flood Zone 3 Version 3.14

    Cree Beck (north of Hurworth) 1% a.p. Candidate Flood Zone 3b Flood Zone 3 Version 3.14

    Neasham Stell (north of Neasham)

    1% a.p. Candidate Flood Zone 3b

    Flood Zone 3 Version 3.14

    Coatham Beck 1% a.p. Candidate Flood Zone 3b Flood Zone 3 Version 3.14

    Carcut Beck 1% a.p. Candidate Flood Zone 3b

    Flood Zone 3 Version 3.14

    Billingham Beck 1% a.p. Candidate Flood Zone 3b Flood Zone 3 Version 3.14

    5 Environment Agency (2006) Strategic Flood Risk Management Specification for Flood Risk Mapping release 1.2

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  • Halliwell Beck 1% a.p. Candidate Flood Zone 3b

    Flood Zone 3 Version 3.14

    Corner Beck 1% a.p. Candidate Flood Zone 3b Flood Zone 3 Version 3.14

    Dyance Beck 1% a.p. Candidate Flood Zone 3b

    Flood Zone 3 Version 3.14

    Summerhouse Beck 1% a.p. Candidate Flood Zone 3b Flood Zone 3 Version 3.14

    3.3 Flood Zone 3 Depth Map