Flood Risk Assessment
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Flood Risk Assessment 20-22 Bridge Street, Staines, TW18 4TW Client Consulting Engineers CDP Staines Ltd GTA Civils Ltd 22 Gilbert Street Gloucester House London 66a Church Walk W1K 5EJ Burgess Hill West Sussex Ref: 6988 RH15 9AS Date: August 2017 Tel: 01444 871444 Flood Risk Assessment: 20-22 Bridge Street, Staines Index 1 Introduction 2 2 Existing Site & Flood Risk Profile 3 3 Proposed Development & Mitigation 5 Schedule of Appendices A Site Location Map & Aerial Photos B Environment Agency & SFRA Flood Maps C Architect’s Scheme Drawings D Flood Response Plan Issue Issue date Compiled Checked Preliminary Issue 22 August 2017 JP GK/MR 2nd Preliminary Issue 29 August 2017 JP MR 3rd Preliminary Issue 21 October 2017 JP MR First Issue 31 October 2017 JP MR Report by: John Pakenham BSc (Hons) Checked by: Grant Kahil BEng (Hons) Overseen by: Martin Roberts I Eng, ACIWEM, MCIHT W:\Projects\6988 FRA, Compagnie Du Parc, 22 Bridge Street, Staines TW18 4TW\2.3 Job No: 6988 Specifications & Reports\F. Flood Risk Assessments Date: August 2017 1 Flood Risk Assessment: 20-22 Bridge Street, Staines 1 Introduction 1.1 GTA Civils Ltd. was appointed by its client, CDP Staines Ltd, to provide a Flood Risk Assessment (FRA) report. This objective is to get the Environment Agency’s (EA) pre-application for the development of 20-22 Bridge Street, Staines TW18 4TW. This FRA has been written to satisfy the needs of the EA and the 2012 National Planning Policy Framework. 1.2 This report has been prepared for the Client in relation to the proposed development at the above address and no responsibility is accepted to any third party for all or part of this study in connection with this or any other development. 1.3 This report is concerned with ensuring the development will be safe, ie passing the second part of the Exception Test. W:\Projects\6988 FRA, Compagnie Du Parc, 22 Bridge Street, Staines TW18 4TW\2.3 Job No: 6988 Specifications & Reports\F. Flood Risk Assessments Date: August 2017 2 Flood Risk Assessment: 20-22 Bridge Street, Staines 2 Existing Site & Flood Risk Profile 2.1 The site is located in Staines, which is administered by Spelthorne Borough Council (SBC). 2.2 It is 100m northeast of the River Thames and 50m northwest of the River Colne, which discharges into the Thames. Currently the site comprises 2 storeys of flats (C3) over a Chinese Takeaway business (use class A5) at ground level. Site location maps and an aerial view are shown in Appendix A. 2.3 The levels to the front of the unit are higher than those to the rear (east). The external levels range between 16.76m AOD and 15.05m AOD. The existing unit’s lowest FFL is 15.12m AOD. There is a small cellar at the rear of the site whose floor level is 14.47m AOD. The site is 100% impermeable. 2.4 Fluvial Risk: The EA’s fluvial flood map in the Appendix indicates that the site has an annual probability of >1% (1 in 100 years) of flooding from rivers. The site is outside the area that benefits from defences. This places the site in undefended Flood Zone 3A, at having a high probability of flooding. (FZ3B: The cellar level of 14.47m AOD, ie less than the 20 years flood level of 14.65m AOD, is irrelevant as there is no conduit for flood waters to enter this space. This is a sufficiently watertight space). 2.5 The EA’s modelled flood data (in Appendix B) shows that the site is liable to flood to a level of 15.63m in the 1% AEP storm (100 years). This rises to 15.97m in the 1% +20% climate change AEP event (100 years + 20% CC). 2.6 The NPPF states that the predicted effects of climate change should be taken into account when determining a proposed development’s flood risk, because the flood risk has to be assessed for the likely lifetime of developments. The EA’s revised climate change allowances, introduced in 2016, should be applied. For a More Vulnerable use (dwelling) in the Thames’s basin the higher central band is to be taken (as much of this building is in place already). 35% climate change should be applied, which means that the flood level rises to 35/20 x (15.97-15.63) + 15.63 = 16.225m AOD. 2.7 Recorded flooding between 11-13 February 2014 affected a large extent of Staines. It is contended that the return period was in excess of 1 in 100 years: it was reported in the media that these floods were the worst recorded in the last 250 years. It is understood that this area was unaffected, however. Photographs were taken of the site next door at Provident House, which is at the same elevation, broadly speaking as this site – see Appendix E. It would appear that the area benefitting from the defences extends to this site – not stopping short outside Provident House next door, as shown on the EA’s main flood map. W:\Projects\6988 FRA, Compagnie Du Parc, 22 Bridge Street, Staines TW18 4TW\2.3 Job No: 6988 Specifications & Reports\F. Flood Risk Assessments Date: August 2017 3 Flood Risk Assessment: 20-22 Bridge Street, Staines 2.8 Surface water flooding occurs when excess rainwater does not infiltrate into the ground, or is not intercepted. The EA’s Online ‘Surface Water Depth - Low Risk Scenario’ (1 in 1000 years) Flood Maps (in Appendix B) show that there is a very shallow and small area outside the unit on the east side that is affected in this scenario. Other sources of flooding 2.9 Sewer flooding occurs due to limited capacity or blockage in the sewer system causing backing up of storm water and effluent. There is no indication in SBC’s Strategic Flood Risk Assessment (SFRA) – or from the EA – that this site is prone to flooding from sewer failure. 2.10 Groundwater flooding occurs when water levels in the ground rise above surface elevations. Groundwater flooding is the least understood of all the sources of flooding and more research is needed. Low lying areas of permeable strata overlying an impermeable stratum are typical places where this can occur. As the proposals do not extend below ground level it is contended that this risk is very low. 2.11 Artificial sources: flooding from reservoirs, canals and docks. The EA reservoir flooding map shows the site is liable to flood to a depth of approximately 2m (as the shaded areas bisect the site). The ramifications of this will be discussed in the next section. It must be stressed that reservoirs are extremely well maintained and there are very few instances of such failure flooding in the last 150 years. There are no docks or canals in this vicinity. 2.12 In summary, the flood risk profile of this site is ‘High’: the main risk of flooding is from the river (Thames) but the risk of reservoir failure flooding cannot be discounted. W:\Projects\6988 FRA, Compagnie Du Parc, 22 Bridge Street, Staines TW18 4TW\2.3 Job No: 6988 Specifications & Reports\F. Flood Risk Assessments Date: August 2017 4 Flood Risk Assessment: 20-22 Bridge Street, Staines 3 Proposed Development & Mitigation 3.1 The proposed development is for refurbishment, extension (and conversion at ground floor to residential apartments) resulting in a 5 floor block of 9 apartments. All units shall be single storey. 3.2 Sequential / Exception Tests: as summarised in section 2.9 the flood risk is ‘High’. 3.3 Vulnerability and the Sequential Test: the vulnerability classification of any new development should be considered. According to Table 2 of the NPPF’s TG, dwelling units are rated as ‘More Vulnerable’. Hot food takeaways and offices are classed as ‘Less Vulnerable’. There is an increase in vulnerability to More Vulnerable, therefore. 3.4 Table 3 of the NPPF states that More Vulnerable uses, such as dwellings, are subject to passing the Sequential and Exception Tests in Flood Zone 3A and Less Vulnerable uses are ‘appropriate development’ in FZ3A. Figure 1 below shows this table: Figure 1: Table 3 of the NPPF’s Technical Guidance W:\Projects\6988 FRA, Compagnie Du Parc, 22 Bridge Street, Staines TW18 4TW\2.3 Job No: 6988 Specifications & Reports\F. Flood Risk Assessments Date: August 2017 5 Flood Risk Assessment: 20-22 Bridge Street, Staines 3.5 New buildings’ floor levels in FZ3A have to be set at least 0.3m above the critical flood level over its expected lifetime. The ground floor’s level will be set at the compliant level of 16.525m AOD, ie 0.3m above the flood level of 16.225m AOD – see section 2.6 above. 3.6 The building’s footprint (roofed) area will be reduced by 10.2m2. Appendix C contains a ‘built area comparison roof plan’ layout, which shows not only the footprint area reduced to 164.2m2 from 174.4m2, but also the area of permeable surfaces increasing from zero to 52.4m2. The table on this drawing shows both 34.5m2 of permeable paving at ground level and 17.9m2 of rain gardens at upper levels including roof level. 3.7 Safe access to dry land: because of the possibility of reservoir failure flooding and because occupants of all the flats would not be able to make their way unassisted to FZ1 (dry ground) during an intense storm - a Flood Response Plan is proposed. 3.8 It is proposed that a Flood Response Plan is incorporated in each unit - see Appendix D.