FLOOD RISK ASSESSMENT AND DRAINAGE STRATEGY

Transcription

1 FLOOD RISK ASSESSMENT AND DRAINAGE STRATEGY London Rd. WATERLOOVILE FOR McCarthy & Stone Ltd. July 2010 Such Salinger Peters Ltd

2 Flood Risk Assessment & Drainage Strategy Contents Paragraph Page 1. Introduction 3 2. Topography 3 3. Watercourses, Flood Levels & Flood Risk 3 4. Emergency Access Route 4 5. Foul Water Drainage 4 6. Surface Water Drainage 5 7. Flood Defence Consent 5 8. Code for Sustainable Housing 5 9. Conclusions & Recommendations 6 Appendices Appendix 1 Topographic Survey and Sewer Record Appendix 2 - Photographs Appendix 3 Site Plan Appendix 4 Environment Agency Pro-Forma

3 Flood Risk Assessment & Drainage Strategy 344 London Road, WATERLOOVILLE 1. Introduction: 1.1 Such Salinger Peters have been instructed to produce a site specific Flood Risk Assessment (FRA) for the proposed redevelopment of this site. This report has been prepared in accordance with Planning Policy Statement 25: Development & Flood Risk (PPS25). 1.2 The site lies in the centre of Waterlooville with a frontage onto London Road (A3). It currently comprises a Jewsons retail depot. 1.3 The proposal is to develop the site with a block of 38 Cat II Retirement Flats for the elderly. 1.4 The flood mapping published by the Environment Agency (EA) and also in the Partnership for Urban South Hampshire (PUSH) Strategic Flood Risk Assessment (SFRA) shows the site to lie in Zone 1 outside the 1 in 1000 year tidal and fluvial flood zones even when the possible impact of climate change is taken into account. 1.5 We have included the following information in this report: A topographic survey to Ordnance Datum. An assessment of possible sources of flooding. A strategy for foul and surface water drainage. A standard Environmental Agency flood risk pro-forma is attached in Appendix 3 2. Topography: 2.1 The site has been surveyed to Ordnance Datum based on GPS and shows that the site has a fall from west to east of approximately 2.8m. 2.2 A topographic survey of the site is attached to this report in Appendix The site is bounded to the north by Beaconsfield Road where the existing main site entrance is located. To the east of the site is a block of flats called Elizabeth House and to the west is London Road, a main thoroughfare (A3). South of the site is a previous McCarthy and Stone development, Homewater House. 3. Watercourses, Flood levels & Flood Risk: 3.1 E.A. flood mapping from their website, supported by the PUSH SFRA shows the site to be in Zone 1 with a Low Probability of flooding. The nearest watercourse is 250m to the east of the site 3.2 The Strategic FRA has no reports of flood incidents in this area. 3.3 Possible sources of flood risk are as follows:

4 A Surface Water: Flooding from Surface Water would take the form of excess runoff from impermeable areas. There are no separate surface water sewers in London or Beaconsfield Road. Beaconsfield Road is a cul-de-sac and falls approx 3.75m away from the junction with London Road. A drainage channel at the junction prevents runoff from London Road entering the cul-de-sac. There are a further two drainage channels at the end of the road to capture highway run-off. The front gardens of Nos. 2 & 3 Beaconsfield Rd. are laid with gravel and a gully is seen outside No. 3 indicating that these properties have suffered flooding in the past. The existing Jewsons site was almost 100 % impermeable and run-off from the concrete hard standing would have contributed to the problem, especially if the channels were not regularly maintained. The proposed Finished Floor Level for the lower ground floor of the new building is 50.69m AOD. The lowest site level on Beaconsfield Road is 49.69m AOD so ground water entering the new building is not considered a risk. B C Ground water: Flooding from groundwater could occur if the natural water table rises above ground level. Groundwater was not encountered during the site investigation s exploratory boreholes to a depth of 6.45m. Sewers: Flooding from sewers could occur through blockages, collapse or overloading. Southern Water operates the public foul sewer in Beaconsfield Road and had provided no reports of flooding incidents from local sewers at the time of writing. 3.4 We conclude that there is no significant risk of surface water, groundwater or sewer flooding the building during a 1 in 100 yr. flood event, including an allowance for climate change. 4. Emergency Access Route: 4.1 PPS 25 requires that consideration is given to the access for emergency vehicles during an extreme flood event however in this case we consider the site to be not at risk and have not pursued this issue. 5. Foul Water Drainage: 5.1 The site is served by a 150 Ø public foul sewer in Beconsfield Road, owned and operated by Southern Water. Flows to the sewer from the proposed development will increase by approx. 0.6 l/sec. based on the average occupancy of retirement flats. 5.2 The new building will discharge its foul effluent to the existing on site manhole as shown on drawing A new manhole will be built in the road to replace the existing saddle connection to the public sewer

5 6. Surface Water Drainage: 6.1 The surface water run-off from the proposed development will be designed in accordance with Sustainable Urban Drainage (SUDS) guidelines. Guidelines are published by CIRIA and the EA. 6.2 The existing site has an impermeable area of 3000 m 2 from a total of 3209 m 2 (i.e. 93%) The proposed development reduces the impermeable area to 1557 m 2 (48%). 6.3 The existing site produced a total peak flow of 41.7 l/sec. from a 50mm intensity storm. The surface water drained to a 150 Ø off site sewer and to the road. The slotted concrete drainage channels used on the boundary are not very efficient in heavy storms and it is assumed that this would not have helped the flooding issues at the bottom of the road. This possible issue will be reduced as all rainwater on site will be captured and not allowed to run to the highway. 6.4 A small area of roof drained to a soakaway that had failed and was recorded to be full of water many days after any rainfall. The site investigation records the sub strata as London Clay where soakaways will not be an efficient form of drainage. 6.5 We recommend that a positive drainage system should be designed to deal with a storm of a 1 in 30 year return period, plus an allowance for climate change as per Sewers for Adoption 6 th Edition but with additional storage to cater for a 100 year event by allowing temporary ponding of the car park. 6.6 The discharge will be controlled by reducing the flow to 5 l/sec. and providing an Attenuation Tank to store the excess. Based on calculations for the worst storm scenario (short, sharp summer storm) a storage tank of 24 m 2 is required. This will be placed under the car park as shown on drawing The volume of water discharged will be further reduced by the addition of a rainwater harvesting tank that will allow external water usage for the development. 7. Flood Defence Consent 7.1 The development does not require any works, in or close to a listed water course; therefore Flood Risk Consent is not required. 8.0 Code for Sustainable Housing 8.1 The site lies in Flood Zone 1 and should therefore qualify for two credits under the Code for Sustainable Homes

6 9. Conclusions & Recommendations: 9.1 We have demonstrated that the developed site is in Zone 1 with a low probability of surface water flooding. We have also considered other possible sources of flooding and concluded that the risk is low. 9.2 The foul water drainage from the new development will discharge to the 150 Ø public sewer in Beaconsfield Road owned and operated by Southern Water. 9.3 The new development will have a surface water system designed in accordance with Sustainable Urban Drainage Principles.The solution is likely to include an attenuation tank and a control to 5 l/sec. before draining to the existing surface water sewer running through the site. 9.4 The annual peak surface water flow rate will be reduced from 41.7 l/sec. to 5 l/sec. and the discharge volumes halved. G. Fairbrass

7 Appendix 1 Topographic Survey Sewer Record & EA Map

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10 Appendix 2 Photographs Junction London Rd./ Beaconsfield Rd. showing Drainage Channel Second Channel at lower site boundary

11 Channel between site & highway Failed Soakaway

12 Appendix 3 Site Plan

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14 Appendix 3 E.A Pro-Forma 1 Development description and location 1a. What type of development is proposed and where will it be located? Retirement Flats in a residential area 1b. What is its vulnerability classification? MORE VULNERABLE 1c. Is the proposed development consistent with the Local Development Documents? YES 1d. Please provide evidence that the Sequential Test or Exception Test has been applied in the selection of this site for this development type? 2. Definition of the flood hazard 2a. What sources of flooding could affect the site? SEE SECTION 3.0 2b. For each identified source, describe how flooding would occur, with reference to any historic records wherever these are available. SEE SECTION 3.0 2c. What are the existing surface water drainage arrangements for the site? Unrestricted run-off to surface water sewer & Highway 3. Probability 3a Which flood zone is the site within? ZONE 1 3b If there is a Strategic Flood Risk Assessment covering this site, what does it show? NO RISK OF FLOODING 3c What is the probability of the site flooding taking account of the contents of the SFRA and of any further sitespecific assessment? LOW 3d What are the existing rates and volumes of run-off generated by the site? 4. Climate change 4a How is flood risk at the site likely to be affected by climate change? NOT APPRECIABLY 5. Detailed development proposals 5a Please provide details of the development layout, referring to the relevant drawings. See appendix 2 5b Where appropriate, demonstrate how land-uses most sensitive to flood damage have been placed in areas within the site that are at least risk of flooding. N/A 6. Flood risk management measures 6a. How will the site be protected from flooding, including the potential impacts of climate change, over the development s lifetime? SUDS SYSTEM TO BE WELL MAINTAINED 7. Off site impacts 7a How will you ensure that your proposed development and the measures to protect your site from flooding will not increase flood risk elsewhere? SURFACE WATER RUN-OFF MANAGED BY SUDS SCHEME 7b How will you prevent run-off from the completed development causing an impact elsewhere? SUDS SCHEME WILL NOT ALLOW ANY SURFACE WATER RUNOFF FROM SITE 8. Residual risks 8a What flood-related risks will remain after you have implemented the measures to protect the site from flooding? NONE 8b How, and by whom, will these risks be managed over the lifetime of the development? N/A

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