Croxley Rail Link Environmental Statement. Appendix 14C. Water Environment and Drainage - Flood Risk Assessment. Appendix 14C

Transcription

1 Croxley Rail Link Environmental Statement Appendix 14C Water Environment and Drainage - Flood Risk Assessment Appendix 14C Mouchel 2011

2 Flood Risk Assessment 10 December 2011 Produced for Hertfordshire County Council London Underground Ltd. Prepared by Perrymount Road Haywards Heath West Sussex RH16 3BN T F E john.greenyer@mouchel.com Mouchel 2011

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4 Document Control Sheet Project Title Croxley Rail Link Development Report Title Flood Risk Assessment Issue E Status Final Control Date December 2011 Record of Issue Issue Status Author Date Check Date Authorised Date A Draft GN 07/09/11 JDG 07/09/11 PS 07/09/11 B Draft GN 27/09/11 JDG 27/09/11 PS 27/09/11 C Draft GN 03/10/11 JDG 03/10/11 PS 03/10/11 D Draft GN 08/11/2011 JDG 10/11/11 PS 11/11/11 E Final GN 09/12/2011 JDG 10/12/11 PR & JDG 11/12/11 Distribution Organisation Contact Copies Mouchel Tom Duckmanton 1e Mouchel Steve Parkinson 1e Mouchel Martin Morris 1e Mouchel Rob Beaumont 1e Mouchel Fiona Symes 1e Mouchel Antonios Paraskeva 1e Mouchel 2011 i

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6 Contents 1 Introduction Project Background Purpose of the Flood Risk Assessment Policy Context Planning Policy Statement 25 (2010) Site-Specific Flood Risk Assessments Climate Change Sequential Test Exception Test Adapting to Climate Change: Advice for Flood and Coastal Erosion Risk Management Authorities Thames Catchment Flood Management Plan (2009) Surface Water Management Plan for Watford Strategic Flood Risk Assessment (2007) Site Description Site Extents and Study Area Site Topography and Land Use Watercourses and Drainage Site Walk-Over Survey Site Geology and Hydrogeology Description of the Proposed Works Drainage Proposals 17 Mouchel 2011 iii

7 4 Data Collection and Historic Flooding Data Collection Consultation with Statutory Agencies Environment Agency British Waterways Thames Water London Underground Limited (LUL) Environment Agency Flood Zone Maps and River Flood Levels The EA Flood Map Flood Zone Map ( ) for Watford Borough Council from the SFRA (refer to Section 2.2) Upper Colne SFRM Study Need for the Exception Test Flooding History River Flooding Groundwater Flooding Surface Water (Land) Flooding Sewer Flooding Sea and Tidal Flooding Artificial Sources Flooding Existing Flood Defences Existing Flood Warning System Flooding Impacts Assessment On-Site Flooding Impacts...28 Mouchel 2011 iv

8 5.1.1 River Flooding Groundwater Flooding Surface Water (Land) and Sewer Flooding Artificial Sources Flooding Off-Site Flooding Impacts River Flooding Groundwater Flooding Surface Water (Land) and Sewer Flooding Artificial Sources Flooding Construction Phase Flooding Impacts Flood Risk Mitigation Floodplain Compensation Pollution Control Conclusions and Recommendations On-Site Flooding Impacts Off-Site Flooding Impacts Construction related Risk Residual Risks References...49 Mouchel 2011 v

9 Appendices Appendix A Appendix B Appendix C Appendix D Flood Risk Assessment Figures Horizontal and Vertical Alignment of Track and Permanent Way Environment Agency Flood Levels Conceptual Designs for New Track Drainage Mouchel 2011 vi

10 Figures Note: Figure numbers absent from the below list are located in Appendix A Figure 1.1: Croxley Rail Link Development Route Plan...2 Figure 3.1: Existing Outfalls into River Colne...13 Figure 3.2: Designated Source Protection Zones...15 Figure 5.1: Culvert across Existing Ordinary Watercourse...34 Figure 5.2: Conceptual Culvert Design across Ordinary Watercourse...34 Mouchel 2011 vii

11 Tables Table 2.1: Recommended National Precautionary Sensitivity Ranges for Peak Rainfall Intensities and Peak River Flows (PPS25)...4 Table 2.2: Definitions of Flood Zones (as defined in PPS25)...6 Table 2.3: Flood Risk Vulnerability Classification (as defined in PPS25)...7 Table 2.4: Flood Risk Vulnerability and Flood Zone Compatibility...8 Table 5.1: Comparison of Track Levels and Flood Levels at River Colne Crossings30 Table 5.2: Surface Water Runoff from the Proposed Scheme (Track)...40 Table 5.3: Summary of Key Construction Activities Affecting the Watercourses...44 Mouchel 2011 viii

12 List of Abbreviations AOD BGS bgl BW CC CDS CEMP CFMP CRL dia EA EIA ES FRA FWRA GUC HCC ha km LDF LPA l/s l/s/ha LUL NGR NR Above Ordnance Datum British Geological Survey below ground level British Waterways Climate Change Conceptual Design Statement Construction Environment Management Plan Catchment Flood Management Plan Croxley Rail Link diameter Environment Agency Environmental Impact Assessment Environment Statement Flood Risk Assessment Foundation Works Risk Assessment Grand Union Canal Hertfordshire County Council hectare kilometres Local Development Framework Local Planning Authority litres per second litres per second per hectare London Underground Limited National Grid Reference Network Rail Mouchel 2011 ix

13 PPS SFRA SFRM SPZ SWMP TW Planning Policy Statement Strategic Flood Risk Assessment Strategic Flood Risk Modelling and Mapping Study Source Protection Zone Surface Water Management Plan Thames Water TWA Transport and Works Act 1992 WFD Water Framework Directive Mouchel 2011 x

14 1 Introduction 1.1 Project Background Hertfordshire County Council (HCC) and London Underground Ltd (LUL) with the support of Network Rail are seeking powers for the construction and operation of a rail link (the Croxley Rail Link) between LUL s Metropolitan Line in the west of Watford and Watford Junction main line station to the north of the town centre. Powers for the proposed works are being sought via an Order under the Transport and Works Act 1992 (TWA). The proposed scheme includes the construction of a viaduct to connect the existing Metropolitan Line to the currently disused Croxley Green Branch Line and the construction of two new stations. It is intended that passenger services to the existing Watford Metropolitan Line Station will be closed though the station will continue to be used as a stabling point for LUL trains. The overall aim of the proposed scheme is to reduce congestion within Watford and improve the transport connections to Central London and the West Coast Main Line. It will provide a west to east public transport link for Watford, connecting the communities around Croxley Green with Watford town centre. It will also connect Watford Junction Station with the Underground network providing passengers wishing to travel on the West Coast Main Line with an alternative route. The location of the proposed scheme and its alignment between the Metropolitan Line and Watford Junction Station is shown in Figure 1.1. Mouchel

15 Proposed Section of Line & Station to be used as Stabling Point Figure 1.1: Croxley Rail Link Development Route Plan 1.2 Purpose of the Flood Risk Assessment Sections of the proposed scheme cross four watercourses and associated Flood Zones 2 and 3. This will involve works within the flood zones, modifications to existing crossings of the watercourses and works in close proximity to the watercourses. The alignment is also located within an area beneath which there is a principal aquifer and Source Protection Zone 1 (SPZ1). In light of these relationships and activities and following discussions with the Environment Agency (EA), a site-specific Flood Risk Assessment (FRA) has been undertaken in accordance with Planning Policy Statement 25: Development and Flood Risk (PPS25). The objectives have been: to establish the current nature and extent of flood risk relative to surface waters and groundwater associated with the area through which the proposed scheme will be aligned; to evaluate to what extent the construction and proposed form of the new link will have an impact on existing flood risk in terms of the extent, capacity or characteristics of existing flood zones or areas prone to localised flooding; and to establish to what extent existing and predicted flooding will be likely to affect use of the proposed scheme by trains. Mouchel

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17 2 Policy Context 2.1 Planning Policy Statement 25 (2010) Planning Policy Statements set out Government s policy in relation to various aspects of land use planning. The aim of PPS25 is to ensure that flood risk is taken into account at all stages in the planning process to avoid inappropriate development in areas at risk of flooding, and to direct development away from areas at highest risk. Where new development is, exceptionally, necessary in such areas, policy aims to make it safe without increasing flood risk elsewhere and, where possible, reducing flood risk overall Site-Specific Flood Risk Assessments PPS25 indicates that applications for development of one hectare or more in Flood Zone 1 and all applications for development in Flood Zones 2 and 3 should be accompanied by a FRA. The FRA should identify and assess the risks from all forms of flooding to and from the development and demonstrate how these flood risks will be managed, taking climate change into account Climate Change In December 2007 the Government published Planning and Climate Change as a supplement to PPS1 1. The document sets out key planning objectives relating to the delivery of spatial strategies which aim to secure new development and shape places that minimise vulnerability, and provide resilience, to climate change. The supplementary policy sets out key principles which planning authorities should apply during decision making about spatial strategies; new development should be planned to minimise future vulnerability in a changing climate; and climate change considerations should be integrated into all spatial planning concerns. PPS25 remains the key planning policy document for climate change. Tables B.1 and B.2 of PPS25 recommend sensitivity ranges for making an assessment of the impact on flooding from land, rivers and sea as part of a FRA. These sensitivity ranges are presented in Table 2.1. Table 2.1: Recommended National Precautionary Sensitivity Ranges for Peak Rainfall Intensities and Peak River Flows (PPS25) Parameter 1990 to to to to 2115 Peak rainfall intensity +5% +10% +20% +30% Peak river flow +10% +20% Mouchel

18 2.1.3 Sequential Test The aim of the Sequential Test described in PPS25 is to guide new developments to areas with the lowest probability of flooding. It is a risk based assessment to determine the suitability of land for development in areas at risk of flooding. The Environment Agency (EA) Flood Zones are the basis of application of the Sequential Test, as defined in Table 2.2. In areas at risk of river or sea flooding, preference should be given to locating development in Flood Zone 1. If there is no reasonable available site in Flood Zone 1, the flood vulnerability of the proposed development should be taken into account in locating the development in Flood Zone 2 and then Flood Zone 3. The Scheme proposals fall under Essential Infrastructure as shown in bold text in Table 2.3, vulnerability classification as defined within PPS 25. Mouchel

19 Table 2.2: Definitions of Flood Zones (as defined in PPS25) Flood Zone Definition Appropriate Uses (Note - Flood Risk Vulnerability are classified in Table 2.3 Zone 1 Low Probability Zone 2 Medium Probability Zone 3a High Probability This zone comprises land assessed as having a less than 1 in 1000 annual probability of river or sea flooding in any year (<0.1%). This zone comprises land assessed as having between a 1 in 100 and 1 in 1000 annual probability of river flooding (1% - 0.1%) or a 1 in 200 and 1 in 1000 annual probability of sea flooding (0.5% - 0.1%) in any year. This zone comprises land assessed as having a 1 in 100 or greater annual probability of river flooding (1%) or a 1 in 200 or greater annual probability of flooding from the sea (>0.5%) in any year. All uses of land are appropriate in this zone. Water-compatible, less vulnerable and more vulnerable uses of land and essential infrastructure are appropriate in this zone. Subject to the Sequential Test being applied, the highly vulnerable uses of land are only appropriate in this zone if the Exception Test is passed Water-compatible and less vulnerable uses of land are appropriate in this zone. Highly vulnerable development should not be permitted in this zone. More vulnerable uses of land and essential infrastructure should only be permitted in this zone if the Exception Test is passed. Essential infrastructure in this zone should be designed and constructed to remain operational and safe for users in times of flood. Zone 3b The Functional Floodplain This zone comprises land where water has to flow or be stored in times of flood. SFRAs should identify this Flood Zone (land which would flood with an annual probability of 1 in 20 (5%) or greater in any year or is designed to flood in an extreme (0.1%) flood, or at another probability to be agreed between the LPA and the EA, including water conveyance routes). Only water-compatible uses and the essential infrastructure that has to be there should be permitted in this zone. It should be designed and constructed to : Remain operational and safe for users in times of flood; Result in no net loss of floodplain storage Not impede water flows; and Not increase flood risk elsewhere. Essential infrastructure in this zone should pass the Exception Test. [Note: Table 2.1 defines each Flood Zone and refers to the probability of river and sea flooding only and ignoring the presence of all existing flood defences]. Mouchel

20 Table 2.3: Flood Risk Vulnerability Classification (as defined in PPS25) Essential Infrastructure Highly Vulnerable More Vulnerable Less Vulnerable Watercompatible Development Essential transport infrastructure (including mass evacuation routes) which has to cross the area at risk, strategic utility infrastructure, including electricity generating power stations and grid and primary substations. Police stations, Ambulance stations and fire stations and Command Centres and telecommunications installations required to be operational during flooding. Emergency dispersal points. Basement dwellings. Caravans, mobile homes and park homes intended for permanent residential use. Installations requiring hazardous substances consent Hospitals. Residential institutions such as residential care homes, children s homes, social services homes, prisons and hostels. Buildings used for: dwellings houses, student halls of residence, drinking establishments, nightclubs and hotels. Non-residential uses for health services, nurseries and educational establishments. Landfill and sites used for waste management facilities for hazardous waste. Sites used for holiday or short-let caravans and camping, subject to a specific warning and excavation plan. Buildings used for: shops; financial, professional and other service; restaurants and cafes; hot food takeaways; offices; general industry; storage and distribution; non-residential institutions not included in more vulnerable ; and assembly and leisure. Land and buildings used for agriculture and forestry. Waste treatment (except landfill and hazardous waste facilities). Minerals working and processing (except for sand and gravel working). Water treatment plants. Sewage treatment plants (if adequate pollution control measures are in place). Flood control infrastructure. Water transmission infrastructure and pumping stations. Sewage transmission infrastructure and pumping stations. Sand and gravel workings. Docks, marinas and wharves. Navigation facilities. MOD defence installations. Ship building, repairing and dismantling, dockside fish process and refrigeration and compatible activities requiring a waterside location. Water-based recreation (excluding sleeping accommodation). Lifeguard and coastguard stations. Amenity open space, nature conservation and biodiversity, outdoor sports and recreation and essential facilities such as changing rooms. Essential ancillary sleeping or residential accommodation for staff required by uses in this category, subject to a specific warning and excavation plan. Mouchel

21 2.1.4 Exception Test The Exception Test should be applied by decision-makers, if after the Sequential Test has been applied it is unable to deliver acceptable sites located in areas with a lower probability of flooding (i.e. Zones 1 and 2). The Exception Test can be applied as a method of managing flood risk while still allowing necessary development to occur. The Exception Test should not be used to justify highly vulnerable development in the high risk flood zones. It should be applied when more vulnerable development and essential infrastructure cannot be located in Flood Zones 1 or 2 and highly vulnerable development cannot be located in Zone 1. The circumstances where the Exception Test may be applied are shown in Table 2.4. Table 2.4: Flood Risk Vulnerability and Flood Zone Compatibility Flood Zone Essential Infrastructure Water Compatible Highly Vulnerable More Vulnerable Less Vulnerable Zone 1 Zone 2 Exception Test required Zone 3a Exception Test required Exception Test required Zone 3b Exception Test required 2.2 Adapting to Climate Change: Advice for Flood and Coastal Erosion Risk Management Authorities The EA issued updated advice in relation to adapting to climate change in September This advice is based on the UKCP09 research and data and provides a range of estimates in relation to climate change with regard to both location (Thames Region with regard to CRL) and climate change scenario (low emissions (Lower end estimate), mid emissions (Change factor) and high emissions (Upper end estimate). The extreme scenario is also included, this is beyond the likely range but within the physically plausibility. This scenario is referred to as the H++ scenario. No H++ scenario is provided in relation to rainfall. The external briefing note provided by the EA in association with the updated advice comments that: The advice has not yet been tailored for the land use planning system. Until then, we should advise local planning authorities and developers to continue to use the allowances found in Department of Communities and Local Government's Planning Policy Statement 25, PPS25. Mouchel

22 2.3 Thames Catchment Flood Management Plan (2009) The Thames Catchment Flood Management Plan (CFMP) helps us understand the scale and extent of flood risk in the present and in the future and sets policies for managing flood risk within the catchment. The Thames CFMP helps stakeholders including the EA and regional planning bodies, water companies, transportation planners, etc. in making planning decisions. The Thames CFMP outlines six major policies to manage flood risk within the catchment: Policy 1: Continue to monitor and advise on flood risk in areas of little or no flood risk Policy 2: Reduce existing flood risk management actions in areas of low to moderate flood risk Policy 3: Manage existing flood risk effectively in areas of low to moderate flood risk Policy 4: Take necessary actions to keep pace with climate change in areas of low, moderate and high flood risk which are already being managed for flood risk Policy 5: Take necessary action in areas of moderate to high flood risk Policy 6: Take necessary action with others to store water or manage runoff in locations that provide overall flood risk reduction or environmental benefits in areas of low to moderate flood risk In accordance with the Thames CFMP, Watford town is identified as one of the towns in the central Thames catchment where Policy 4 would apply. 2.4 Surface Water Management Plan for Watford The Surface Water Management Plan (SWMP) for Watford town is currently under review prior to publication. Surface water flood risk information and recommendations should be incorporated into this FRA when the SWMP is released. 2.5 Strategic Flood Risk Assessment (2007) The Strategic Flood Risk Assessment (SFRA) 2 for Dacorum Borough Council, St Albans City and District Council, Three Rivers District Council and Watford Borough Council forms a part of the Core Strategy of the Local Development Framework (LDF) 3 for Watford Borough. The relevant flood risk policies which form a part of the Councils flood policy documents are listed below: Protect the functional floodplain from development; Direct vulnerable development away from flood affected areas; Ensure all development is Safe, meaning that dry pedestrian access to and from the development site is possible without passing through the 1 in 100 year plus climate change (CC) floodplain, and emergency vehicular access is possible; Mouchel

23 Promote the use of sustainable urban drainage systems in all flood zones to achieve greenfield discharge rates on both greenfield and brownfield sites; and Support flood alleviation measures under consideration by the EA by safeguarding possible sites for flood storage and other channel works. Mouchel

24 3 Site Description 3.1 Site Extents and Study Area As shown in Figure 1.2 and detailed in Figure 3.1 and Figure 3.2 in Appendix A the proposed scheme corridor extends west to east from the existing Metropolitan Line immediately south of Baldwins Lane to Watford Junction Station. There are three principal sections which constitute the corridor: A western 0.5km section linking the Metropolitan Line and the disused Croxley Green Branch Line at old Ascot Road. The section comprises earthworks to tie in the existing LUL line and proposed extension, a 440m nine-span viaduct, and earthworks and three-span bridge crossing the Ascot Road dual carriageway and old Ascot Road; A central 2.5km section comprising the disused Croxley Green Branch Line between Ascot Road and the existing London Overground line at Wiggenhall Road ; and An eastern section comprising the existing London Overground line between Wiggenhall Road and Watford Junction Station. The FRA has been focused on the western and central sections where construction with the potential to affect established flood zones and areas prone to local flooding is proposed Site Topography and Land Use Site topographic data is provided in Appendix B. General levels across the western section of the proposed scheme corridor between Baldwins Lane and old Ascot Road fall from 56 to 57m Above Ordnance Datum (AOD) to 52 to 53m in the vicinity of the GUC and River Gade and then rise to 57-58m east of old Ascot Road. Continuing east, neighbouring areas of housing and open space occupy a landform at the crest of cutting slopes associated with the disused branch line which falls gently from west to east. Levels at the crest of the cutting slopes fall from approximately 62-63m AOD in the vicinity of Tolpits Close to 60-58m in the vicinity of Stripling Way. Within the River Colne valley the disused branch line is located on low embankments which are slightly elevated above general floodplain levels which vary between 51-54m AOD. Approximately 650m of the disused branch line is on embankment, 1250m is in cutting and the remaining 600m is at-grade; Embankment heights vary from 8.0m west of the Ascot Road dual carriageway, to 5-5.5m immediately east of Old Ascot Road and Stripling Way and 1-1.5m across the Cone Valley. The maximum depth of the cutting is about 6.5m mid-way between Tolpits Lane and Vicarage Road. Mouchel

25 The cuttings and embankments which frame the ballast and single track line between Old Ascot Road and Stripling Way are generally covered in maturing woodland, scrub and coarse grasses. East of Stripling Way there are areas of scrub on low embankments and open sections of coarse grassland. Land use adjacent to the proposed scheme corridor comprises a mix of residential, development, commercial and industrial use, schools, areas of amenity grassland and allotments Watercourses and Drainage There are four watercourses which cross the proposed scheme corridor; the Grand Union Canal (GUC), the River Gade, the Beggars Bush Lane Drain, the River Colne side channel and a small un-named brook at Fisher s Industrial Estate. The GUC and River Gade (a main river) run parallel to each other. They follow a north to south course which crosses the proposed scheme corridor towards the eastern end of the proposed viaduct. There is a disused railway bridge to the south of the proposed viaduct which spans both watercourses (Figure 3.1) The Beggars Bush Lane Drain (an ordinary watercourse) flows east of and parallel to River Gade (Figure 3.1) through a masonry arch culvert underneath the existing embankment supporting the disused railway bridge. The River Colne side channel is a main river. It crosses beneath two bridges on the disused branch line. The first, U/BR.1 (Colne bridge 1), is located east of Stripling Way where the side channel flows south beneath the disused branch line and through the Lairage Lands Local Nature Reserve. The second, U/BR.3 (Colne Bridge 3), is located to the west of the Cardiff Road Industrial Estate. The location of both is shown in Figure 3.1. The un-named brook flows west from the Fisher s Industrial Estate beneath a bridge on the disused branch line (U/BR.2 Colne bridge 2) on the disused branch line. It then runs alongside and adjacent to the north-western boundary of the disused branch line up to its confluence with River Colne side channel upstream of Colne bridge 3. It was noted during the site walk-over survey that the brook carries runoff from the hardstandings and parking areas at the industrial estate. However, the extent of the sub-catchment contributing runoff to the brook are unknown. Drainage of the disused branch line appears to be limited to discontinuous lengths of unlined ditches and the track drainage Site Walk-Over Survey A site walk-over survey was undertaken in April Observations and notes were made relating to topography, potential sources of flooding to and from the site, overland flow paths, existing drainage features and outfall points in the vicinity of the disused line. There were no outfalls identified at the GUC and the River Gade. Three Mouchel

26 outfalls to the River Colne were identified as described below and indicated in Figure 3.1. None of the outfalls identified incorporate pollution control devices. Outfall 1 discharges to the un-named brook on the south side of the disused branch line in the vicinity of U/BR.2. Outfall 2 is an uncontrolled discharge into the River Colne side channel on the north side of the disused branch line in the vicinity of U/BR.3. It is an open ditch carrying runoff through a pipe under the adjacent car park. Outfall 3 discharges an open ditch, which runs parallel to the disused branch line, into the River Colne side channel on the north side of the disused branch line in the vicinity of U/BR.1. Figure 3.1: Existing Outfalls into River Colne Site Geology and Hydrogeology The British Geological Survey (BGS) Geological maps for the area (Beaconsfield Sheet 255 and North London Sheet 256) demonstrate the proposed scheme corridor and surrounding area is underlain by Upper Chalk bedrock of Cretaceous age. Superficial deposits locally overlie the chalk (although it outcrops at the surface at a number of locations). These comprise cohesive and granular alluvium, glacial sands and gravels. The alluvium is associated with the Rivers Gade and Colne. The glacial sands and gravels are generally located between old Ascot Road and the River Colne. Elsewhere the chalk outcrops at the surface. Where gravels do occur they are less than 9.0m thick. Mouchel

27 The cuttings located along the disused branch line appear to be formed through glacial sands and gravels with the Upper Chalk at depth. Embankments associated with the disused branch line in the vicinity of the River Colne appear to be formed of alluvium. The proposed scheme corridor between the Metropolitan Line and the London Overgound line is underlain by a Principal Aquifer and Source Protection Zone 1 (SPZ1) as shown in Figure 3.2. An SPZ1 is defined by a travel time of 50 days or less for a pollutant to reach an abstraction point, around a public abstraction borehole. The GUC, River Gade and River Colne all pass under the proposed alignment and it is likely the rivers (and potentially the canal if hydraulically connected) are receiving water from the underlying chalk and gravel aquifers. The EA data, which is available in Appendix C, indicates that groundwater levels vary between 0.0m to 5.0m below ground level (bgl) in the immediate vicinity of the River Gade and River Colne. Along the remaining sections of the corridor, the groundwater levels range between 5.0m and 20.0m bgl. The EA have also clarified that these levels indicate generalised regional maximum groundwater contours only and discrepancies at map boundaries have been smoothed out. The contours are for major, bedrock aquifers only and do not reflect gravels groundwater. Groundwater levels are likely to be a combination of groundwater in both the superficial deposits and the chalk, which appear to be in hydraulic continuity, i.e., where groundwater is very shallow (near the river, as indicated by contours and borehole monitoring), the gravels appear to be saturated. This is confirmed (at the western end of the site) by groundwater monitoring data from a dual installation borehole in the superficial deposits and the chalk during the ground investigation (Soil Mechanics, 2011). In addition, the borehole log information (Costain, 2003) indicates shallow groundwater depths (1.0mbgl and 1.5mbgl) within the superficial deposits at this location. Data supplied by the EA indicates there are two groundwater abstractions within 10m of the disused branch line as shown in Figure 3.2. A further four abstractions are located between m of the proposed scheme corridor. It is assumed they continue to be licensed for the abstraction of potable water supplies. Mouchel

28 Figure 3.2: Designated Source Protection Zones 3.2 Description of the Proposed Works The proposed scheme comprises some 3.0km of new dual track extending from the existing Metropolitan Line in the vicinity of Baldwins Lane to the London Overground line at Wiggenhall Road and 1.8km of the London Overground line between Wiggenhall Road and Watford Junction Station. The connection between the Metropolitan Line and the currently disused rail corridor east of Ascot Road dual carriageway will be achieved by the widening of the south east embankment slope supporting the existing dual track Metropolitan line to accommodate the transfer onto a new dual track line via a 400m long, curved ninespan viaduct. The viaduct will cross over Baldwins Lane, an existing storage compound, the A412 Rickmansworth / Watford Road, an existing recreation ground, the Grand Union Canal, Beggars Bush Lane and the River Gade. A reinforced concrete abutment will provide the support for the western end of the viaduct and will be integrated with wing walls which will tie the abutment to the modified embankment slope. The eastern end of the viaduct will terminate at the abutment of a reinforced earth wall which will be located between the River Gade and the small watercourse close to and west of the river. The existing bridge crossing the Grand Union Canal, Beggars Bush Lane and the River Gade. will be retained. Mouchel

29 The eastern abutment to the reinforced earth wall will be located at the back of the verge to the west of the Ascot Road dual carriageway. A new three-span bridge supported on the earth wall abutment will provide for the crossing of the dual carriageway and the old Ascot Road. The existing bridge over the old Ascot Road will be demolished. A new station will be built at Ascot Road between the dual carriageway and old Ascot Road. East of the old Ascot Road, the three-span bridge will be supported on a reinforced concrete abutment at the back of the verge to the old road. The existing embankment, which carried the disused branch line onto the old Ascot Road bridge from the east, will be raised to provide for increased vehicle clearance beneath the new bridge and will be reinforced to accommodate the new station platforms which will extend to the east from the new station. A new 1.8m high culvert will be constructed through the new reinforced earth wall, to the west of the Ascot Road dual carriageway. The culvert will be built on the line of the existing culvert to enable the course of the Beggars Bush Lane Drain to be maintained. Continuing east, the new dual-track will be aligned along the currently disused branch line as far as a reinstated connection with the existing operational London Overground line east of Wiggenhall Road. New track levels will be maintained close to existing levels A new electricity sub-station will be constructed adjacent to the disused line, off Tolpits Lane. Along this section, lengths of existing cutting slopes will be re-profiled and sheet piling retaining walls will be introduced at the base of the cutting to achieve current design and safety standards adopted by LUL for the slopes and provide for the introduction of the proposed dual track on a widened formation in the bottom of the cutting. Existing bridges crossing roads and watercourses will be modified to accommodate the additional track. This work will include widening and repairs to the two existing railway bridges across the River Colne side channel (U/BR.1 and U/BR.3) and existing railway bridge (U/BR.2) over the un-named brook at the Fisher s Industrial Estate. The soffit levels of these bridges will be maintained at the existing levels. A second new station will be built on the west side of Vicarage Road where the existing bridge currently carries the road over the disused branch line. The station will effectively replace the former Watford West Station (located to the west, off Tolpits Lane) and former Watford Stadium Halt (located just to the east of Vicarage Road). The platforms associated with both the former station and halt will be removed or modified to accommodate the two new tracks. Mouchel

30 Gabion toe-walls to embankments will be provided within the floodplain associated with the River Colne in the vicinity of Stripling Way. East of Wiggenhall Road, the new dual track will merge with the existing dual track London Overground line which will be jointly used by underground and overground trains to the termination of the proposed scheme at Watford Junction Station Drainage Proposals The approach to the drainage design for the proposed scheme has been informed by its location in relation to the Source Protection Zone 1 which extends across a substantial part of the underlying chalk aquifer associated with central and west Watford. The design allows for the capture of surface water runoff from the viaduct, the platforms and station buildings and sub-station via a combination of gullies, slit drains, guttering and down pipes. Along the proposed dual-track section of the corridor between old Ascot Road and the merge with the London Overground line, a system of perforated pipes will capture the runoff from the track and associated cutting and embankment slopes. Runoff from the viaduct and bridges will be discharged to the existing foul water sewer network at old Ascot Road. Drainage from the station and car park will be discharged to the local surface water and foul sewer network as appropriate. The runoff from the proposed electricity sub-station and station at Vicarage Road will be discharged to the local surface water and foul sewer network as appropriate. Runoff captured along the length of new dual track east of Ascot Road as far as the London Overground line will be collected via non-perforated carrier drains along each side of the upgraded rail corridor. The drainage catchments for this section of the proposed scheme will comprise: a catchment draining west from the western end of Tolpits Close and discharging to the foul water sewer network at old Ascot Road; a catchment draining east from the western end of Tolpits Close and discharging to the foul water sewer network at Stripling Way; a catchment draining east from the bridge north of Striping Way with an outfall to the River Colne side channel at the Lairage Lands LNR south of the bridge over the side channel; a catchment draining east from the bridge over the River Colne side channel at the Lairage Lands LNR to an outfall on the western bank of the side channel south of the north bridge over the side channel; Mouchel

31 a catchment draining west from the bridge over the brook at the Fisher Industrial Estate to an outfall on the east bank of the River Colne side channel south of the north bridge over the side channel; and a catchment draining west from Wiggenhall Road to an outfall on the east bank of the brook at the Fisher Industrial Estate and south of the line. The specific connection points into the Thames Water sewer network will be determined in consultation with Thames Water during detailed design. The track drainage scheme has been designed in accordance with LUL Gravity Drainage Systems which recommends a 1 in 5-year design storm without surcharging and a 1 in 50-year design storm without causing the water levels to rise within 50mm below the underside of the sleeper for LUL drainage infrastructure. EA requirements, that the design return period for sections of the track discharging into River Colne should be 1 in 100-year+CC and that surface water discharges into River Colne should be restricted to 5 litres per second per hectare (5l/s/ha) have also been adopted. The non-perforated carrier drains have accordingly been sized to provide online attenuation storage and ensure that the EA design requirement of 5l/s/ha wil be met. Oil interceptors and silt traps have also been provided for at the discharges to the river and un-named brook. The EA have recommended an additional 30% allowance in peak rainfall intensities for a project with life after However, in accordance with the LUL Gravity Drainage Systems criteria, the drainage assets should have a design life of 60 years. This accounts to a project life of 2071 for the drainage assets, which falls within the 20% allowance band as per the climate change criteria in Table 2.1; Mouchel

32 4 Data Collection and Historic Flooding 4.1 Data Collection Baseline data has been obtained from a number of sources as listed below: existing and proposed track designs and topographic information (Appendix B); geotechnical desk studies and investigations undertaken for the project; proposed design information provided by the engineering design team relating to the track, viaduct, stations, bridges, earthworks and drainage; floodplain information relating to flood maps, flood extents and flood levels for the River Colne and River Gade, provided by the EA; hydraulic models for the River Colne; BGS soil maps and borehole logs; TW sewer records; flooding records from the EA, HCC and TW; and observations from site walk-over surveys. 4.2 Consultation with Statutory Agencies The statutory agencies maintaining and/or monitoring the water bodies within the study area were consulted at various stages to inform them of the proposed scheme and seek advice on flooding aspects Environment Agency The EA has been consulted throughout the planning, design and assessment of the proposed scheme (May 2010, April 2011 and October 2011)). The main points raised are summarised below. 1) The drainage strategy must outline a sustainable approach to surface water runoff and must be based on a critical 100 year+cc rainfall event; 2) The runoff rates to watercourses must be restricted to equivalent greenfield runoff rates of 5l/s/ha; 3) Suitable bank protection measures should be incorporated in the design; 4) Deck drainage for the viaduct collected in carrier drains must be designed to contain critical 1 in 100-year+CC rainfall events and discharges must be limited to greenfield rates; Mouchel

33 5) The impact of widening of any of the embankments should be considered on overland river flows and flood storage; 6) The existing alignment of the culvert along the Beggars Bush Lane Drain should be maintained, additional culverting should be avoided and, where possible, ecological enhancements on either side of the culvert should be considered; 7) The viaduct piers adjacent to the River Gade and the GUC should be set back as far as possible from the banks of the watercourses to limit impacts on biodiversity and flood risk; 8) The design should make a 30% allowance in rainfall intensities for any project with a life after 2085 (see 3.2.1). 9) The outfall structures should be maintained sympathetic to the surrounding area; 10) Pollution control devices on all discharge points to the River Colne or existing sewer network should be adequately sized and maintained; other discharge points or infiltration drainage should be the subject of further consultation with the Agency; 11) There should be a limit of 200ppm on hydrocarbon runoff into the surrounding water environment and other pollution mitigation aspects from the proposed scheme; and 12) Foundation Works Risk Assessment (FWRA) methodology to be undertaken for the piling foundations supporting the proposed viaduct British Waterways British Waterways (BW) was consulted in April 2011 for advice on managing flood risk along the proposed viaduct and potential discharges into the River Gade. BW recommended that discharge of runoff into the GUC and the River Gade should be avoided due to the complex hydraulic arrangements across the two channels and the risk of impacting the existing public amenity features in the vicinity of the site Thames Water Thames Water (TW) was consulted in May 2011 to seek advice on the potential connection points into their public sewers and allowable discharges into the system. Information provided by TW has demonstrated that the proposed scheme is generally on the upstream end of the local sewer network and that there are two trunk sewers which cross the proposed scheme corridor near old Ascot Road and Station Approach Road (Figure 4.1 and Figure 4.2). Mouchel

34 CRL Route Existing surface water sewers Existing foul sewers Figure 4.1: Thames Water Public Sewers near Ascot Road CRL Route Existing surface water sewers Existingfoul sewers Figure 4.2: Thames Water Public Sewers near Station Approach Road London Underground Limited (LUL) LUL was consulted in June The company suggested that consideration should be given to the use of filter drains and infiltration drainage rather than the proposed Mouchel

35 piped system. The proposal was raised in discussions with the EA. The Agency confirmed that any form of infiltration to the ground would be unacceptable due to the presence of the highly sensitive SPZ1 underlying the proposed scheme and the risk of contamination associated with infiltration drainage. 4.3 Environment Agency Flood Zone Maps and River Flood Levels Flood Zone data has been derived from the EA Flood Map (which provides the flood extents for Flood Zones 1, 2 and 3) and a more detailed Flood Zone Map from the SFRA for Flood Zones 3a and 3b The EA Flood Map The EA Flood Map which is reproduced in Figure 4.3 illustrates the extent of Flood Zones 1, 2 and 3 in relation to the proposed scheme corridor and surrounding area. The mapping demonstrates that the section of the proposed scheme within the River Colne valley is located within medium probability Flood Zone 2 and high probability Flood Zone 3 lands. Each of these flood zones appear to be spread out on either sides of the bridge crossings U/BR.1 and U/BR.3. The high probability Flood Zone 3 of the River Gade and the GUC are quite narrow floodplains and appear to be contained within the channel. Grand Union Canal (GUC) CRL Route River Gade River Colne Figure 4.3: Environment Agency Flood Map, 2011 Mouchel

36 4.3.2 Flood Zone Map ( ) for Watford Borough Council from the SFRA (refer to Section 2.2) The Flood Zones ( ) for Watford (Figure 4.4) (Appendix A) illustrate the predicted flood risk arising from the surface watercourses within the Borough (this map was developed based on higher resolution photogrammetry data for the River Gade and a hydraulic model for the River Colne, which is an improvement over the existing EA Flood Zone Map (Figure 4.1). Based on this map, a small section of the proposed scheme falls within Flood Zone 3a (high probability) of the River Colne and Flood Zone 3b (functional floodplain) of the River Gade Upper Colne SFRM Study In addition to the Flood Zone Maps, the EA has provided flood levels for the River Colne and River Gade from the recently completed Upper Colne SFRM Study Hydraulic Modelling and Mapping 4. CRL Route Figure 4.4: River Colne Flood Outlines from Upper Colne SFRM Study Mouchel

37 CRL Route GUC River Gade Figure 4.5: River Gade Flood Outlines from Upper Colne SFRM Study Need for the Exception Test A Sequential Test has been undertaken for the proposed scheme in accordance with the criteria stated in the PPS25 (Section 2.1.4), which are summarised below: 1. The majority of the proposed scheme falls within the low probability Flood Zone 1, except the section of the route crossing the River Colne (Flood Zone 3a) and the proposed viaduct which spans over the River Gade and the GUC (Flood Zone 3b of River Gade) 2. Croxley Rail Link is identified as a major transport project within the Core Strategy of the LDF for Watford Borough and can be classified as essential infrastructure. 3. Due to limited options for connection of the disused line, it was required to connect it to the existing Met Line Based on the above criteria, PPS25 indicates that an Exception Test should be carried out for sites located within high probability Flood Zones 3a and 3b. The following criteria apply to qualify for an exception test: It must be demonstrated that the development provides wider sustainability benefits to the community that outweigh flood risk, informed by a SFRA where one has been prepared A section of the rail route falls within high probability Flood Zones 3a and 3b. However, it is being re- Mouchel

38 developed to improve the transport connectivity between Watford and Central London and West Coast Main Line; The development should be on developable, previously-developed land, or it is not on previously developed land, that there are no reasonable alternative sites on developable previously developed land. The site is an existing disused railway line which is being re-developed to improve the transport connectivity within Watford. The site can be categorised as an essential transport infrastructure; and The FRA must demonstrate that the development will be safe, without increasing flood risk elsewhere, and, where possible, will reduce flood risk overall: This FRA aims to demonstrate that the development will be safe, without increasing flood risk elsewhere, and where possible, will reduce flood risk overall. 4.4 Flooding History A number of sources including the HCC, EA and TW were reviewed to study the flooding history in the vicinity of the Scheme. In addition, the SFRA provides an overview of various forms of flood risk within Watford and covers River Colne and River Gade catchments River Flooding Based on the EA flood history map, Fishers Industrial Estate and the area between the River Colne side channel and the main river were recorded as being subject to river flooding in 2000 (Figure 4.5 in Appendix A). The SFRA indicates that Watford has been affected by river flooding many times during the past century. In particular, properties along Lower High Street, Bushey Mill Lane and Water Lane have a long history of flooding Groundwater Flooding The SFRA identifies three areas in the vicinity of the proposed scheme where minor groundwater flooding incidents have been reported in the past. These include Kingsfield Road and Cedar Road in Oxhey (Figure 4.6 GW1) and Gade Avenue (Figure 4.6 GW2) Surface Water (Land) Flooding The SFRA highlights three areas in the vicinity of the proposed scheme where major surface water flooding incidents have been reported in the past. Lower High Street (Figure 4.6 WD SW9) is flooded when levels in the River Colne rise and prevent discharges from the surface water drainage system; Properties along Riverside Road experience frequent surface water flooding as a result of the low-lying nature of the area (Figure 4.6 WD SW5); and Mouchel

39 Properties along Wiggenhall Road experience frequent surface water flooding due to the low-lying nature of the area and an under capacity open culvert which runs parallel to Fishers Industrial Estate (Figure 4.6 WD SW4). The culvert is prone to flooding when the water level rises in the River Colne. CRL Route Figure 4.6: Historic Groundwater and Surface Water Flooding Locations (SFRA) Sewer Flooding The four Boroughs (Dacorum, St Albans, Three Rivers and Watford) are sub-divided into sewer flood risk zones in the SFRA. The sub-divisions are based on sewer flooding records from TW. The proposed scheme corridor and wider area, as indicated in Figure 4.6 are within a high sewer flood risk zone, high risk being zones where more than 15 properties have been flooded in 10 years. TW records report sewer flooding at 22 properties in the past 10 years within the Watford postcode area WD18. All instances related to foul sewer overloading. The records for the postcode do not report any incidents involving overloading of surface water or combined sewers Sea and Tidal Flooding The study area is not located in coastal location or within a tidal flood zone Artificial Sources Flooding The GUC has an interaction point with the River Gade in close proximity to the proposed scheme. Based on the information provided in the SFRA, there are no records of canal overtopping or breach. There area number of factors which explain why such events have not occurred and would be unlikely to occur. Mouchel

40 The water levels in the canal are controlled through locks which prevent overtopping; due to the hydraulic connections between the River Gade and the GUC, the water levels within the River Gade are maintained at the same level as the GUC; Along most of its course the canal is raised slightly above the river and therefore any water overtopping from the canal would be discharged into the river; At locations where the rivers and canal are at similar levels and are in close proximity, the area between them is naturally wet encouraging the growth of watercress, which provides storage for any water discharged from either water body; There are no canal embankments with a high probability or consequence of failure as defined by British Waterways. All canal embankments in the study area have a designated low/medium risk of failure; The canal is fairly self supporting (fixed lateral weirs) given that there are no major control sluices other than a large flood control structure at Batchworth (approximate NGR , ); and There are a large number of locks and fairly short pounds throughout the canal system. As such any break outs would be expected to be contained within the canal. Significant flooding could only occur if two lock gates were to fail simultaneously which is unlikely given that no such failures have occurred in the UK over the last 200 years. 4.5 Existing Flood Defences Linear structures such as road and rail embankments are considered as informal flood defences which act as water retaining structures in addition to their primary function. The SFRA identifies approximately 18 road and railway embankments which cross the River Colne and provide a barrier to flow resulting in upstream storage. The railway embankment crossing the River Colne side channel acts as an informal flood defence. The EA Flood Map identifies the area between the two River Colne crossings as area benefiting from flood defence. 4.6 Existing Flood Warning System The current flood warning system for the study area is operated by the EA which monitors rainfall and river levels 24 hours a day at a number of Flood Warning telemetry stations throughout the study area and uses the information to forecast the probability of flooding. The Croxley railway site is located within two Flood Warning Areas: The River Colne (and its tributaries) extending between North Mymms to Maple Cross; and The River Gade and Bulbourne from Great Gaddesdon to Rickmansworth Mouchel

41 5 Flooding Impacts Assessment 5.1 On-Site Flooding Impacts This section considers various sources of flooding associated with the wider area within which the proposed scheme is located and the likelihood of impact from each of these sources on the future operation of the proposed scheme River Flooding River Colne Crossings The section of the proposed scheme which will be at risk of flood-related inundation and could in turn lead to the temporary suspension of services is located within the River Colne floodplain east of Stripling Way and the track merge with the existing London Overground line. Throughout this section, the existing earthworks associated with the disused branch line will generally remain unaltered whilst track levels will be maintained at existing levels or marginally higher. Two sections of gabion retaining wall will be introduced immediately east of Stripling way to enable the dual track to be introduced in this location without the need to extend the works beyond the disused branch line boundary. Proposed improvements to the two existing bridges crossing over the River Colne side channel will involve repairs to the masonry substructure, extension of the existing decking to accommodate maintenance walkways and, treatment of corrosion to several areas of the main girders. There will be no modification to the clear span over the watercourse or changes in the decking soffit levels. The design approach has accordingly ensured there will be no material change in the relationship between the form of the disused branch line and this section of the proposed scheme and the River and Colne and its floodplain. The potential for flood risk inundation will accordingly not be altered. Comparison has been made between the existing and proposed track levels and their relationship to flood risk events of varying return period to establish if the LUL 1 in 50-year operating objective will be met. The comparison has been undertaken at the two bridge locations, UB/R.3 representing the low point in the existing and proposed vertical alignment. The data related to existing track levels, proposed track and sleeper levels and flooding levels for short and long-term flood events is scheduled in Table 5.1. The table also details the difference in the flood levels for the 1 in 50-year flood event and the 1 in 100-year+CC flood event. The table further details the anticipated difference in level between the anticipated water levels and LUL s operating design standard that water levels should not encroach to within 50mm of the underside of sleeper levels during a 1 in 50-year flood event. Mouchel

42 The data sources used include: topographic survey detail and proposed design levels provided by the engineering design team; The bridge soffit levels were estimated from bridge deck depths provided by the engineering design team. A review of the River Colne hydraulic model indicates that the bridge soffit levels match closely with the soffit levels estimated from the scheme bridge designs; and flood levels for various return periods were taken from the Upper Colne hydraulic model (Upper Colne SFRM Study). The flood level information at the River Colne crossings is included in Appendix C. It is to be noted that the flood levels used in the current assessment are indicative and were used for preliminary comparison purposes. The EA have recommended further surveys and hydraulic modelling to further refine the detailed design of the scheme. Mouchel

43 Table 5.1: Comparison of Track Levels and Flood Levels at River Colne Crossings Chainage/ Location Existing Levels Proposed Levels River Colne Side Channel Levels Diff. in Levels (50-Yr) Diff. in Levels (100-Yr+cc) Rail to Soffit Depth Soffit Level Diff. in Soffit and River Levels Track Track Sleeper 2-Yr 5-Yr 10-Yr 20-Yr 50-Yr 100-Yr 100-Yr+cc Track Sleeper Track Sleeper m m 50-Yr 100-yr+CC CH R.Colne ROB CH U/BR.1 R.Colne CH R.Colne LOB CH R.Colne LOB CH U/BR.3 R.Colne CH R.Colne ROB Note: All levels are in metres Above Ordnance Datum (AOD) Mouchel

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45 Flood Risk Assessment (U/BR.1) The lowest proposed track level at Bridge U/BR.1 is 55.1m AOD. The 1 in 50-year and 1 in 100-year+CC flood levels are 51.6m and 51.8m, respectively, which are approximately 3.4m and 3.2m below the permissible flood level from the underside of the sleeper. As such the railway track and the permanent way are not at risk of river flooding during each of these events. The maximum flood levels for the 1 in 100-year+CC rainfall event are 2.3m below the bridge soffit level of 54.1mAOD, which indicates that there is sufficient freeboard under the bridge deck for the flood waters to pass. U/BR.3 The proposed track level at Bridge U/BR.3 is 53.6m and the 1 in 50-year and 1 in 100-year+CC flood levels are 52.2m and 53.2m, respectively. LUL s design parameter relative to the 1 in 50-year flood event will accordingly be met. The data does indicate that water levels during a 1 in 100-year+CC rainfall event will be above the base of sleeper levels, at 53.3m. As such there may be occasions when temporary suspension of service may be required whilst flood waters recede. The comparison of the bridge soffit level with the river flooding levels for the various return periods indicates that the bridge is currently surcharged for all of the return periods considered. This is a situation which will continue there being no change to the bridge soffit levels and bridge span over the watercourse or predicted flood levels for the various events as a result of the implementation of the proposed scheme. U/BR.2 Modifications to Bridge U/BR.2 include construction of walkways using longitudinal girders spanning over the abutment walls 2m beyond the walls in both directions, treatment of corrosion in accordance with the steelwork repairs, repairs to the substructure and track re-alignment to match the proposed horizontal and vertical alignments of the permanent way; and The proposed modifications will not alter the waterway clearance for the ditch. In addition, the ditch falls in the low probability Flood Zone 1 and as such is not at risk of river flooding. The backflow from River Colne during high flood levels will be taken into account during detailed design. Viaduct The track level on the viaduct at the western end of the proposed scheme corridor will be in the order of 65.25m AOD as it crosses over Baldwins Lane and 60.87m AOD inn the vicinity of old Ascot Road. As shown in Figure 5.1 in Appendix A piers 7 and 8, the closest to the GUC and River Gade, are located outside of Flood Zones 2 and 3 associated with the two watercourses. Mouchel

46 Flood Risk Assessment Flood and bank levels for the River Gade for the 1 in 100-year+CC event based on information in the River Colne SFRM indicate a maximum flood level of 52.8 AOD within the watercourses and bank levels of 53.7m AOD. The data demonstrates that the track will be in excess of 8m above the level for the 1 in 100-year+CC event and that there will accordingly be no risk of inundation during all of the flood event return periods considered. Beggars Bush Lane Drain - Culvert and Embankment The Beggars Bush Lane Drain is culverted through the base of the existing embankment located between the Ascot Road dual carriageway and the eastern bank of the River Gade. As shown in Figure 5.2, the culvert comprises a masonry arch structure and is heavily vegetated. The hydraulic details of the culvert are not known. The masonry arch structure will be replaced with a larger concrete section with sufficient headroom and clearance to allow for access and maintenance works. It will be approximately 50% shorter than the existing structure. The newly opened section of the watercourse to the south of the reinforced earth wall through which the new culvert will be aligned will be profiled and the margins planted to enhance the local ecological value of the watercourse. In common with the relationship between the track on the viaduct and the River Gade, the track on the reinforced earth wall be in excess of 5m above the watercourses such that there will be no risk of inundation of the track during flood events associated with long-tern return periods. Based on the information provided by the EA, the ordinary watercourse receives overflows from the River Gade when the river levels rise. It is also likely that the watercourse drains the surrounding developments into the River Gade. The hydrological and hydraulic details of the watercourse will be investigated in greater detail during detailed design to ensure that flow rates within the channel are regulated and to secure downstream flow characteristics similar to current characteristics. These are measures which will be likely to improve channel conveyance and storage capacities and which will be likely to improve an area of localised flood risk. Mouchel

47 Flood Risk Assessment. Figure 5.1: Culvert across Existing Ordinary Watercourse Figure 5.2: Conceptual Culvert Design across Ordinary Watercourse Mouchel