Crossrail Line 1 Mott MacDonald Assessment of Water Impacts Technical Report Cross London Rail Links Limited Appendix H: Flood Risk Assessment 203357/31/Final/February 2005 Appendix dividers H.doc Crossrail Line 1 Mott MacDonald Flood Risk Assessment - Surface Water Crossings Cross London Rail Links Limited Cross London Rail Links 1 Butler Place London SW1H 0PT United Kingdom Crossrail Line 1 Flood Risk Assessment – Surface Water Crossings Crossrail Reference: 1E0321-GOE00-00006 February 2005 Mott MacDonald Demeter House Station Road Cambridge CB1 2RS UK Tel : 44 (0)1223 463500 Fax : 44 (0)1223 461007 203357/31/Final/February 2005 AppH_1.doc Crossrail Line 1 Mott MacDonald Flood Risk Assessment - Surface Water Crossings Cross London Rail Links Limited Summary The proposed Crossrail route extends from Maidenhead in the west to Shenfield and Abbey Wood in the east. The route crosses a number of watercourses ranging from minor brooks to substantial main rivers. There are concerns that the proposed works may increase the potential for flooding at the crossing sites and in surrounding areas. There are also concerns that the Crossrail route may itself be affected by flood events at the crossings. Flood Risk Assessments have therefore been carried out in accordance with the procedures set out in the Government Planning Policy Guidance Note 25 (PPG25). These have been set out in this report, with the exception of Pudding Mill Lane (Route Window C13) where, because of the extent of the works proposed within watercourses, a more detailed Flood Risk Assessment has been carried out and is reported separately. In carrying out the required Flood Risk Assessments, reference has been made to the ‘Flood Map’, which is shown on the Environment Agency (EA) website as of 7 October 2004, and is the primary source of publicly available flood risk information. At those locations where there is shown by the EA Flood Map to be a potential impact on flood risk, a more detailed assessment has been undertaken, and a more detailed determination of flood risk established. These local Flood Risk Assessments have made use of any available detailed modelling, backed up by analysis using Flood Estimation Handbook (FEH) procedures. Flood levels have been taken from existing models where possible. Detailed topographic data gathered for the Crossrail alignment has also been used where possible. For the locations where there was assessed to be a risk of exacerbating flooding and/or encroachment on the floodplain, further study has been carried out to determine how such risks may be mitigated. The conclusions of the Flood Risk Assessments are summarised in Table S.1 which shows that there is no significant residual risk at any of the sites. It has been assessed that there is no risk of flooding of the Crossrail line at any of the named sites. The only exception to this is Gidea Park where levels from the most recent EA modelling indicate that there is a small risk of flooding to the existing railway. The proposals for Crossrail would not increase this risk. 203357/31/Final/February 2005 AppH_1.doc Crossrail Line 1 Mott MacDonald Flood Risk Assessment - Surface Water Crossings Cross London Rail Links Limited Table S.1: Summary Conclusion of Flood Risk Assessment Route Watercourse Residual risk Ref Flood related impacts window after mitigation NE4 Ilford Station Alders Brook • None Not Significant NE9 Romford River Rom • None Not Significant Station & Depot (East) NE11 Gidea Park River • There may be minor filling of the floodplain and displacement of flood Not Significant Stabling Ravensbourne storage. This is subject to model output verification and detailed Sidings design. • There would be a small headloss (afflux) due to the increase in culvert length. This can be mitigated by improvements to the entry and exit conditions to the culvert. W13 West Drayton Fray’s River • Construction of the new road bridge would have an impact on upstream Not Significant Station water levels. This can be mitigated by provision of hydraulic streamlining at the new bridge and between the new and existing bridge. 3 • Reduction in available flood storage of up to 150 m during the 100- year+20% flood event. This can be mitigated by providing compensation storage. • Temporary encroachment of floodplain during construction workswould need to be managed to minimise the volume of storage taken up and the duration of the works. 203357/31/Final/February 2005 AppH_1.doc Crossrail Line 1 Mott MacDonald Flood Risk Assessment - Surface Water Crossings Cross London Rail Links Limited Route Watercourse Residual risk Ref Flood related impacts window after mitigation W15 Dog Kennel Horton Brook • Reduction in available flood storage of up to 715 m³ during the 100- Not Significant Bridge year+20% flood event due to widening of the embankment. This can be mitigated by providing compensation storage. • Minor headloss (0.08 m) due to increase in culvert length of 7 m. This can be mitigated by provision of streamlining at the entry and exit transitions. C11 Isle of Dogs Docks • Available flood storage volume within the dock system would be Not Significant Station reduced, with potential for minor increases in the risk of flooding elsewhere in the vicinity. Effective mitigation should be possible, but cannot be guaranteed at this stage. SE7 Church Wickham • None Not Significant Manorway Valley Bridge Watercourse NE8 Romford Two non-main • None Not Significant Depot (West) rivers Various Other sites River Thames • As the stations and shafts are within the Thames protected floodplain, Not Significant located within which is protected to the 1000 year event, they are considered to have the Thames no flood related impacts for the purpose of this study. Protected Floodplain • At Maidenhead there may be a minor temporary impact due to the construction works. This should be managed to minimise the volume of storage taken up and the duration of the works. 203357/31/Final/February 2005 AppH_1.doc Crossrail Line 1 Mott MacDonald Flood Risk Assessment - Surface Water Crossings Cross London Rail Links Limited List of Contents Page Chapters and Annexes 1 Introduction 1 2 Route Window NE4 – Ilford Station 2 2.1 Existing Conditions 2 2.2 Proposed Works 3 2.3 Flooding Assessment 3 2.4 Mitigation 6 2.5 Conclusions 6 3 Route Window NE9 – Romford Station & Depot (East) 7 3.1 Existing Conditions 7 3.2 Proposed Works 9 3.3 Flooding Assessment 10 3.4 Mitigation 11 3.5 Conclusions 11 4 Route Window NE11 – Gidea Park Stabling Sidings 12 4.1 Existing Conditions 12 4.2 Proposed Works 13 4.3 Flooding Assessment 14 4.4 Mitigation 15 4.5 Conclusions 16 5 Route Window W13 – West Drayton Station 17 5.1 Existing Conditions 17 5.2 Proposed Works 19 5.3 Flooding Assessment 19 5.4 Mitigation 21 5.5 Conclusions 21 6 Route Window W15 – Dog Kennel Bridge 22 6.1 Existing Conditions 22 6.2 Proposed Works 24 6.3 Flooding Assessment 24 6.4 Mitigation 26 6.5 Conclusions 26 i 203357/31/Final/February 2005 AppH_1.doc Crossrail Line 1 Mott MacDonald Flood Risk Assessment - Surface Water Crossings Cross London Rail Links Limited 7 Route Window C11 – Isle of Dogs Station 27 7.1 Existing Conditions 27 7.2 Proposed Works 27 7.3 Flooding Assessment 28 7.3.1 EA’s Concerns 28 7.3.2 British Waterways Concerns 28 7.3.3 Temporary Works 29 7.3.4 Permanent Works 29 7.4 Mitigation 30 7.5 Conclusions 30 8 Route Window SE7 – Church Manorway Bridge 31 9 Romford Depot (West) 32 10 Other Thames Sites 33 10.1 Tidal Floodplain 33 10.2 Fluvial Floodplain 34 11 Conclusions 35 Annex A: Topographic Survey Data A ii 203357/31/Final/February 2005 AppH_1.doc Crossrail Line 1 Mott MacDonald Flood Risk Assessment - Surface Water Crossings Cross London Rail Links Limited Table of Figures Figure 2.1: Route Window NE4 – Ilford Station 2 Figure 2.2: Alders Brook as it passes under existing tracks 3 Figure 2.3: EA Flood Map – Ilford Station 4 Figure 2.4: Approximate Dimensions of Alders Brook at site 5 Figure 2.5: Alders Brook Barrage 5 Figure 3.1: Route Window NE9 – Romford Station & Depot (East) 7 Figure 3.2: Area to West of Romford Station 8 Figure 3.3: River Rom Culvert Beneath Railway Tracks 8 Figure 3.4: Position of Platform Extension at River Rom Culvert Entrance 9 Figure 3.5: EA Flood Map – Romford Station & Depot (East) 10 Figure 3.6: Approximate Dimensions of River Rom Channel at Culvert 11 Figure 4.1: Route Window NE11 – Gidea Park Stabling Sidings 12 Figure 4.2: River Ravensbourne near Crossrail Route 13 Figure 4.3: Ravensbourne Culvert at Gidea Park Sidings 13 Figure 4.4: EA Flood Map – Gidea Park Stabling Sidings 14 Figure 5.1: Route Window W13 – West Drayton Station 17 Figure 5.2: Fray’s River passing under railway tracks 18 Figure 5.3: Area Proposed for West Drayton Stabling 18 Figure 5.4: Lower Colne Model Flood Outline 19 Figure 5.5: Approximate Dimensions of Fray’s River 20 Figure 6.1: Route Window W15 – Dog Kennel Bridge 22 Figure 6.2: Horton Brook near Chequer Bridge 23 Figure 6.3: Chequer Bridge 23 Figure 6.4: EA Flood Map – Dog Kennel Bridge 24 Figure 6.5: Approximate Dimensions of Horton Brook 25 Figure 6.6: Approximate Dimensions of Horton Brook Culvert 25 Figure 7.1: Route Window C11 – Isle of Dogs Station 27 Figure 8.1: Route Window SE7 – Church Manorway Bridge 31 Figure 9.1: Route Window NE8 – Romford Depot (West) 32 Table of Tables Table 5.1: West Drayton Sidings Flood Levels and Topographic Information .......................20 Table 11.1: Summary of Flood Risk Assessments.................................................................35 iii 203357/31/Final/February 2005 AppH_1.doc Crossrail Line 1 Mott MacDonald Flood Risk Assessment - Surface Water Crossings Cross London Rail Links Limited 1 Introduction The proposed Crossrail route extends from Maidenhead in the west, through Whitechapel to Shenfield in the north east and Abbey Wood in the east.