NOVEMBER 2009 [ISSUE 5]

Transcription

1 BRIGHTON ROAD CROYDON FLOOD RISK ASSESSMENT CLIENT: TRITHING LTD [ISSUE 5] Head Office: Unit 2 York House Edison Park Dorcan Way Swindon Wiltshire SN3 3RB Tel Fax cec@coleeasdon.com Civil Engineering ~ Highways ~ Transportation ~ Flood Risk Bristol Swindon Warrington Incorporated in UK as Cole Easdon BMP Ltd No

2 C OLE EASDON CONSULTANTS (CEC) DOCUMENT ISSUE RECORD Client: Project: Trithing Limited Proposed Residential Development at Brighton Road, Croydon Job Number: 2339 Document Title: Flood Risk Assessment Issue / Revision: Issue 1 Issue 2 Issue 3 Issue 4 Description / Status: Draft for Comment Final Planning Issue Draft for Comment Date: September 2008 November 2008 January 2008 October 2009 Prepared: S Starr BSc (Hons) S Starr BSc (Hons) S Starr BSc (Hons) S Starr BSc (Hons) Signature: Document Check: K Legg K Legg K Legg K Legg Signature: Technical Check: D Frosoni CEng MCIWEM MICE D Frosoni CEng MCIWEM MICE D Frosoni CEng MCIWEM MICE D Frosoni CEng MCIWEM MICE Signature: Authorised: D Frosoni CEng MCIWEM MICE D Frosoni CEng MCIWEM MICE D Frosoni CEng MCIWEM MICE D Frosoni CEng MCIWEM MICE Signature: File Reference: 2339 FRA Issue FRA Issue FRA Issue FRA Issue 4 Cole Easdon Consultants (CEC)

3 C OLE EASDON CONSULTANTS (CEC) DOCUMENT ISSUE RECORD Client: Project: Trithing Limited Proposed Residential Development at Brighton Road, Croydon Job Number: 2339 Document Title: Flood Risk Assessment Issue / Revision: Issue 5 Description / Status: Formal Issue Date: November 2009 Prepared: S Starr BSc (Hons) Signature: Document Check: K Legg Signature: Technical Check: D Frosoni CEng MCIWEM MICE Signature: Authorised: D Frosoni CEng MCIWEM MICE Signature: File Reference: 2339 FRA Issue 5 Cole Easdon Consultants (CEC)

4 CONTENTS SECTION HEADING PAGE NO. 1.0 Introduction The Existing Site Flood Risk Issues Discussions and Conclusions 25 Appendices Appendix 1 Figures Figure 1 Site Location Plan Figure 2 Flood Zone Map Appendix 2 Photographs Appendix 3 July 2007 Rainfall Data & Calculations Appendix 4 Surface Water Drainage Calculations Appendix 5 Borehole Data & Groundwater Monitoring Results Appendix 6 Environment Agency Correspondence Appendix 7 Drawings: Drawing No. 2339/500 - Existing Site Layout Drawing No. 2339/501 - Proposed Development Layout Falla Architects Drawing No 1671:L (--)101 Rev. B Basement Floor Plan Falla Architects Drawing No 1671:L(--) 102 Rev. B Ground Floor Plan Cole Easdon Consultants (CEC)

5 1.0 INTRODUCTION 1.1 This Flood Risk Assessment has been prepared by Cole Easdon Consultants on behalf of Trithing Limited in respect of a planning application for residential redevelopment of Brighton Road, Croydon, London CR2 6. The National Grid Reference is TQ Refer to Figure 1 - Site Location Plan [Appendix 1]. 1.2 This Study considers the issues relating to flood risk associated with the proposals. It has been prepared following discussions with officers from the Environment Agency (the EA) and the Local Planning Authority. 1.3 Note that the previous issue of this Flood Risk Assessment was approved by the Environment Agency in July Refer to Environment Agency correspondence [Appendix 6]. The amendments to the previous scheme are limited to revision of the proposed housing mix and car parking provision. These amendments will not affect the flood risk status of the development or the proposed surface water drainage strategy as presented in this report. The Sequential Test 1.4 The LPA is satisfied that the site passes the sequential test. Development Proposals 1.5 The proposed redevelopment is to comprise 86 No. flats (36 No. 1 bed; 40 No. 2 bed; 10 No. 3 bed), 10 No. 3 bed houses and a basement car park. Cole Easdon Consultants (CEC) - 1 -

6 1.6 This study is based on the proposed development layout shown on Falla Associates International Architects Drawing 1671:L (--)101 Rev. B Basement Floor Plan and 1671:L(--) 102 Rev. B Ground Floor Plan (shown on Cole Easdon Drawing 2339/501 - Proposed Development Layout within Appendix 7). Copies of the abovementioned Falla drawings can be found in Appendix 7. Need for Study 1.7 The purpose of this assessment is to demonstrate to the EA that the proposals outlined above can be satisfactorily accommodated without worsening flood risk for the area, as per guidance provided within Planning Policy Statement PPS Accordingly, this study has been prepared to: i) assess flood risk to the development from fluvial sources; ii) assess flood risk to the development from other potential sources, including ditches, sewers, groundwater and overland surface water; iii) ensure that the development proposed will fully comply with the requirements of the EA's policy on the safeguarding of floodplains, as detailed within the said Planning Policy Statement 25 (PPS25). iv) Prepare a Drainage Strategy for the development 1.9 Reference has been made to the Strategic Flood Risk Assessment Level 1 Draft Report (dated December 2007) for the London Boroughs of Wandsworth, Merton, Sutton and Croydon, prepared by Scott Wilson A site topographical survey (referenced to Ordnance Datum) has been provided to Cole Easdon Consultants and has been reviewed as part of this study. Cole Easdon Consultants (CEC) - 2 -

7 Scope of Study 1.11 In Section 2.0, we describe the characteristics of the development site and surrounding area. In Section 3.0, we assess flood risk issues. Finally, conclusions are presented in Section The following abbreviations are used in this report: EA Environment Agency SUDS Sustainable Drainage Systems FZM Flood Zone Map prepared by the EA PPS25 Planning Policy Statement 25: Development and Flood Risk AOD Above Ordnance Datum Cole Easdon Consultants (CEC) - 3 -

8 2.0 THE EXISTING SITE Refer to Drawing No. 2339/500 - Existing Site Layout [Appendix 7]. 2.1 The redevelopment site is located some 4km south of Croydon town centre, on the highway known as Brighton Road. It is a brownfield site covering approximately 0.6 ha. The site is currently occupied by a car showroom, car repair workshop and multi storey car park. Access to the site is taken from Brighton Road. Refer to Figure 1 - Site Location Plan [Appendix 1] & Photographs [Appendix 2]. 2.2 The existing site area is entirely impermeable, being composed of buildings, tarmac parking/access areas, and concrete hard standings. 2.3 The site is bounded to the north by the rear gardens of residential properties on Wyche Road, and a property fronting Brighton Road. South Croydon Recreation Ground bounds the site to the east. A Honda car showroom lies immediately to the south of the site, and Brighton Road forms the site's the western margin. Land use in the immediate vicinity of the site is residential with numerous commercial properties such as shops and car dealers. 2.4 The site topography is relatively flat, with a gentle fall from west to east. Existing ground levels vary from 55.90m AOD at the western extent of the site to 55.20m AOD in the east. 2.5 Brighton Road channel levels are between 55.42m AOD and 55.56m AOD adjacent to the site; some mm lower than ground levels at the site's frontage. Brighton Road falls gently in a northerly direction. Cole Easdon Consultants (CEC) - 4 -

9 Existing Surface Water Drainage 2.6 Surface water from roofs and hard areas at the site is drained to gullies and discharged to the Caterham Bourne. The Bourne flows within a 750mm diameter public surface water sewer beneath Brighton Road, at the site's western boundary. Flows pass through a series of oil interceptor chambers located beneath the forecourt before entering the sewer. The existing connection from the site is a 150mm diameter pipe laid at a gradient of 1/20. Nearby Watercourses/Drainage Features 2.7 The Caterham Bourne, a tributary of the River Wandle, drains the local catchment. The Bourne is an ephemeral groundwater fed stream and as such flows only when sustained rainfall saturates the underlying chalk strata. The EA relates that on average the Bourne flows once in every 7 years. The Bourne is designated a Main River. 2.8 The Bourne rises from springs near Caterham, some 8km south of the site. It then follows a north easterly route as an open channel punctuated by numerous culverts and crossings, for approximately 7km to Purley. 2.9 In Purley, the watercourse enters a public surface water sewer. The sewer flows in a northerly direction through South Croydon to Croydon centre, where it joins the River Wandle Flows entering the sewer are controlled at Purley. The Bourne flows into a soakage/balancing pond located at the Council's Purley Oaks Depot. Outflow from the pond is pumped to the public surface water sewer in Brighton Road at a controlled rate. Discharge is restricted by a motorised Penstock linked to a sensor in the public sewer. Cole Easdon Consultants (CEC) - 5 -

10 2.11 The Bourne flows within a 750mm diameter Thames Water surface water sewer located beneath Brighton Road and approximately 3.5m from the site's western boundary. The sewer is at a depth of some 2.3m (to invert level) in the proximity of the site. As mentioned in paragraph 2.6 above, surface water from the site drains to this sewer/watercourse. Existing Ground Conditions [Refer to Appendix 5] 2.12 The British Geological Survey map (Sheet 270, scale 1:50 000) of the area indicates that the site is underlain by Hackney Gravel substrata, beneath which lies chalk of the Upper Chalk Strata An intrusive site investigation has been undertaken by RPS Health, Safety & Environment in February The investigation comprised 12 No. boreholes established to a maximum depth of 15m Made ground was encountered across the site to a depth of between 0.9m to 4m. It consisted of asphalt, hardcore or brick paving underlain by various fill materials including gravel, silt and brick Hackney Gravel was encountered beneath the made ground, to a maximum depth of 4m. These strata comprise silty sandy flint gravels with occasional cobbles Upper Chalk strata was present to depth across the site. This material comprises soft to firm chalk containing occasional flint gravels and cobbles Groundwater was observed at between 9.2 and 10.3m below ground level. No perched water was present. Cole Easdon Consultants (CEC) - 6 -

11 2.18 A programme of groundwater monitoring was also undertaken by RPS. Groundwater standpipe readings have been recorded in May 2005, September 2005, May-June 2007 and October Maximum levels of up to 3.6m below ground level have been recorded in October The EA's Groundwater Source Protection Zone Map indicates that the site is within Zone 2, the outer protection zone. The RPS report relates that the site is underlain by a major aquifer relating to the Upper Chalk. The report identifies a potable water abstraction point 1.4 km north west of the site Level data presented within the RPS report is allegedly referenced to Ordnance Datum. However, comparison of these levels with the topographical survey for the site suggests that the report is in error. Surveyed levels at the site are some 1m higher than those included in the RPS report. Cole Easdon Consultants (CEC) - 7 -

12 3.0 FLOOD RISK ISSUES 3.1 This section presents an assessment of flood risk to the development from: a) external sources; b) surface water discharge from the proposed development. 3.2 Recommended flood risk mitigation measures appropriate to the level of perceived risk are included in the assessment. The mitigation measures are summarised in Table 3.1 on page 23. A) Assessment of Flood Risk to the Development Site from External Sources Ai) Flood History 3.3 The SFRA for Wandsworth, Merton, Sutton and Croydon relates that the study area experienced significant fluvial flooding during 1937 and However, these events were confined to the Borough of Merton (flooding of Pill Brook and Beverley Brook) and Sutton (flooding of Pill Brook). The SFRA also contains records of smaller isolated historical flood events in Croydon, none of which affected the development site. 3.4 As previously mentioned in paragraph 2.7, the Caterham Bourne is a groundwater fed ephemeral stream. When rainfall coincides with high groundwater levels, surcharging of the watercourse can lead to flooding. The most serious flood event associated with the Caterham Bourne occurred in December 2000, when flooding affected land from Caterham to Purley, some 2km south of the site. The site was not flooded during this event. Cole Easdon Consultants (CEC) - 8 -

13 July 20 th Much of Croydon experienced significant flooding in July The flooding was caused by a short period of intense rainfall. 3.6 Rain gauge data from within some 2km of the site has been provided by Croydon Council. The data indicate that 55.6mm of rain fell during 2 hours between 9am and 11am on 20 th July. According to Flood Estimation Handbook software this rainfall represents a circa 1:144 year storm. Refer to Appendix Witnesses recall that floodwaters in the vicinity of the site were largely confined to Brighton Road, and followed the gradient of the highway, flowing to the north. The crown line of the cambered highway was visible throughout the event. This suggests that flood flows on the highway adjacent to the site attained a maximum level of 55.70m AOD in the south and 55.60m AOD in the north. Water attained a maximum depth of 200mm within the highway channels. 3.8 Flood flows on Brighton Road were relatively short lived. Witnesses relate that the duration of highway flooding was around 2 hours. Localised flooding remained in low lying areas for some time. 3.9 Flood waters from Brighton Road entered Wyche Grove, 20m north of the site. Staff of a business located on the corner of Brighton Road and Wyche Grove observed that the water drained away rapidly following rodding of a blocked highway gully The accounts of the July 2007 Brighton Road flood suggest that it occurred as a result of a brief period of intense rainfall (a 1:144 year event) which overwhelmed the local drainage system. Brighton Road acted as a conduit for floodwaters unable to enter the drainage system. Cole Easdon Consultants (CEC) - 9 -

14 3.11 It is unlikely that the capacity of the Bourne was exceeded during this event. It is more likely that the capacity of road gulley grilles to accept highway runoff was exceeded. At the storm's peak, 49mm of rain fell in 45 minutes. This equates to a rainfall intensity of some 62mm/hour. Road gully gratings are designed to accept flows up to 50mm/hour. Under these storm conditions, runoff would have been unable to enter the sewer rapidly enough, resulting in surface water flood flows arising. This situation would not have been aided by potential gully blockages that may have existed prior to the storm event. The ease with which flooding in Wyche Grove was relieved by unblocking a gully as well as the limited flood duration suggests that the source of flooding can be attributed to exceedance of gully grating design capacity, resulting in overland flood flows along Brighton Road As such, fluvial flooding of the Caterham Bourne is not considered to have been a contributing factor in the flooding of Brighton Road in July Staff of the BMW dealership presently located at the site relate that floodwaters flowing along the highway did not enter the site, with the exception of 'bow waves' caused by large vehicles. On the basis of the abovementioned estimated flood levels of 55.60m AOD to 55.70m AOD, runoff would have been precluded from entering the site by the raised back of footway and higher area at the site's frontage, where levels are between 55.75m AOD and 55.88m AOD. Officers of Croydon Borough Council have confirmed that flows were largely contained within the highway corridor in the vicinity of the site Notwithstanding that flows were contained within the highway corridor during the July 2007 event, flooding did occur at the site. Staff at the site relate that due to the intensity of the rainfall, low-lying parts of the site were inundated when the capacity of the on site surface water drainage system was exceeded. Cole Easdon Consultants (CEC)

15 3.14 Contd. Floodwaters issued from the private surface and foul water drainage system. Significant quantities of water also entered the building though existing faults in the roof and ceilings. Refer to Photographs [Appendix 2] Water moved through the site, gravitating towards low-lying areas. Localised ponding occurred in these areas. Water levels within the building reached a depth of 50mm - 75mm. The site was closed for 3 days as a result Flooding of the development site was caused by a combination of leaking roofs and ceilings and failure of the on site drainage which, in turn, lead to ponding of surface water in low lying areas. The flooding of Brighton Road had a negligible direct effect upon the site due to raised existing ground levels at the site's western extent It should be emphasised that the July 2007 flooding was an extreme event. The site's maintenance manager has been working at the premises for the past 15 years. With the exception of the 2007 event, the site has not flooded during his tenure. Aii) Assessment of Flood Risk from Fluvial Sources 3.18 The Flood Zone Map (FZM) for the locality as produced by the EA is shown on Figure 2 within Appendix 1. The FZM shows that the site and Brighton Road lie within Flood Zone 3 (High Risk) The FZM is indicative only. It provides an estimate of the extent of the fluvial floodplain, derived from J FLOW modelling. This modelling is based on generalised topographical data and open channel flow. The influence of on line structures and controls such culverts and orifices are not considered. Cole Easdon Consultants (CEC)

16 3.20 In this case, the watercourse is an ephemeral groundwater fed stream. Fluvial flows generated by groundwater ingress are present only infrequently. In the proximity of the site, the watercourse is not an open channel; it is within a public sewer Furthermore, fluvial flows entering the piped section of the Bourne are controlled at Purley Oaks Depot, upstream of the site. The Bourne enters a balancing/soakage pond at the Depot. The pond outfalls to the piped section of the Bourne. Discharge from the pond is restricted by an automatic penstock, which is connected to a sensor within the sewer. Should the Bourne experience significant groundwater ingress or in the event of surcharging of the sewer, discharge from the pond is restricted As such, it is highly unlikely that the J FLOW generated flood plain, as shown on the EA FZM, can be considered accurate in this location As previously mentioned in paragraph 3.11, the flooding of Brighton Road in July 2007 is not considered a fluvial event; it is likely that due to the high intensity of rainfall, surface water entry into the Bourne was limited by the design capacity of road gully gratings, leading to flooding of the highway This scenario is supported by an account of floodwaters draining freely following the clearance of a blocked gully at the junction of Brighton Road and Wyche Grove, and by the short duration of the flooding (2 hours) In addition, the extensive flooding caused by the Bourne in Year 2000 was restricted to land south of Purley, some 2km south of the site, where the watercourse flows as an open channel. This suggests that fluvial flows arising from groundwater discharge to the Bourne are unlikely to enter the piped section of the Bourne, which is routed north to Croydon, passing the site. Cole Easdon Consultants (CEC)

17 3.26 It is therefore suggested that the 1:100 year floodplain (Flood Zone 3) in this area is less extensive than as depicted on the EA FZM, and is restricted to Brighton Road. As such, the site in fact lies within Flood Zone 1 (Low Risk). Indeed, it is likely that Brighton Road itself may also lie within Flood Zone 1 on the basis that highway flooding is not caused by the Bourne itself but is due to the limited capacity of road gullies to accept runoff during extreme rainfall events Accordingly, fluvial flood risk to the site is considered low. Fluvial Sources Flood Risk Mitigation Measures Refer to Drawing 2339/501- Proposed Development Layout [Appendix 7] 3.28 Floor slab levels will be raised by a minimum of 600mm above existing ground level to approx. 56.3m AOD. Finished floor levels and external levels will be set at 1.5m above existing ground level throughout most of the development. This will preclude internal property flooding. Floodwaters on Brighton Road reached a maximum depth of 55.70m AOD during the July 2007 flood The proposals include a raised footpath linking the site with South Croydon Recreation Ground, to the east of the site. Ground levels in the Recreation Ground are approximately 1m higher than those on Brighton Road. In the event of flooding, the footpath could be utilised as a dry access and egress route, albeit that based on the July 2007 event, flood levels at the frontage of the site are unlikely to prevent pedestrian movement along Brighton Road Emergency vehicular access can be taken from Brighton Road. Based on the July 2007 event floodwaters in the highway are not anticipated to reach sufficient depth to impede emergency vehicles. Cole Easdon Consultants (CEC)

18 Basement Car Park 3.31 The proposals include a basement car park. The basement will be accessed from Brighton Road via a new road and ramp to be located in the south of the site. Although the existing topography will offer some protection, flooding in excess of the July 2007 event, and 'bow waves' could cause flooding of the basement In order to prevent surface water entering the basement, a raised area can be installed on the access road. This could take the form of a speed hump or raised table arrangement An appropriate number of gullies can also be positioned on the proposed access road to ensure that rainfall does not runoff the road and enter the basement. A linear drainage channel, such as an Aco drain will be located at the top of the ramp As the ramp is partly uncovered, rain falling directly upon its surface will drain into the basement. A second Aco type system will be installed at the base of the ramp to accommodate this runoff. This drain will be connected to the basement drainage system, as discussed below A surface water drainage system will be required within the basement to deal with any surface water infiltration. Surface water runoff from the proposed ramp access will also drain to this system. A gravity connection to the existing sewer network will not be possible from the basement. Flows will therefore be pumped to the surface water sewer in Brighton Road. Runoff will be directed to an oil interceptor prior to discharge. This will remove hydrocarbon contaminants resulting from fuel or oil spillages The pump can also be employed to drain the basement in the event of flooding caused by an extreme event or a water mains pipe burst. Cole Easdon Consultants (CEC)

19 3.37 Plant situated in the basement will be set upon raised plinths The basement is to be served by 3 No. stairwells, which provide access to the ground floor. The stairwells will remain dry during a flood event, as the ground floor is to be raised between 600mm and 1.5m above existing ground level. Should floodwaters enter the basement via the proposed vehicle access ramp, the stairways will offer a safe escape route Signs directing car park users to the stairways during flooding will be prominently displayed throughout the car park. Aiii) Assessment of Flood Risk from Existing Sewers 3.40 The Caterham Bourne is contained within a 750mm diameter public surface water beneath Brighton Road. 2 No. public foul sewers (600mm dia. & 900mm dia.) are also located in Brighton Road The SFRA contains Thames Water Records of sewer flooding. There have been no recorded incidents of sewer flooding in the vicinity of the site within the past 10 years Flooding from the Caterham Bourne sewer is discussed in detail in paragraph Aii) above In isolation, the existing foul sewers in Brighton Road are unlikely to flood; flows within foul sewers do not fluctuate greatly. Cole Easdon Consultants (CEC)

20 3.44 During unusually intense rainfall, should overland flow enter the foul water system flooding could occur. During the July 2007 event, it is possible that effluent mixed with the floodwaters on Brighton Road. However, with the exception of 'bow waves' caused by large vehicles, floodwaters were contained within the highway Due to the magnitude of the July 2007 event (a 1:144 year storm), both the private surface and foul water drains at the site flooded. Floodwaters and effluent accumulated in low-lying areas throughout the site, including within the existing buildings. Note that with the exception of this extreme event, the site has not experienced flooding with in the past 15 years Accordingly, flood risk to the site from existing sewers is deemed to be medium. Sewer Flood Risk Mitigation Measures 3.47 Raised slab levels, as described in paragraph 3.28 above, will afford protection from sewer flooding The mitigation measures discussed with reference to safeguarding the basement car park from fluvial flooding will also mitigate sewer flood risk. Refer to paragraphs 3.31 to 3.39 above Additionally, the existing private foul and surface sewers serving the site will be replaced under the proposals The surface water control strategy discussed in Section 3b below will reduce flows entering the existing public surface water sewer. This will reduce the probability of sewer surcharging and flooding both to the development site and to the locality. Cole Easdon Consultants (CEC)

21 Aiv) Assessment of Flood Risk from Overland Flow 3.51 Existing overland flow routes are shown on Drawing No. 2339/500 - Existing Site Layout [Appendix 7] Due to its position in a dry river valley, the SFRA identifies Brighton Road as an area potentially at risk of surface water flooding. The existing topography of the locality is such that surface water flows are directed towards the highway, which acts as a conduit, conveying floodwaters north to Croydon Centre The July 2007 event demonstrated this overland flow mechanism in action. Brighton Road experienced significant overland flows during the July 2007 event. Water up to 200mm in depth followed the grade of the highway to the north According to employee accounts, with the exception of bow waves created by large vehicles, floodwater from the highway did not enter the site. The topographical survey confirms that existing ground levels at the rear of Brighton Road footway and at the western extent of the site's forecourt are some 200mm - 400mm higher than the adjacent highway. This precluded the ingress of floodwaters Overland flow is unlikely to move onto the site from the north. The existing topography is such that surface water is expected to flow in a northerly direction, away from the site. The existing workshop/office building and the forecourt boundary wall would also act as a barrier to any runoff approaching the site from the north. Cole Easdon Consultants (CEC)

22 3.56 South Croydon Recreation Ground, situated at the site's eastern boundary, is over 1m higher than the site. A brick retaining wall separates the Recreation Ground from the lower lying development site. Although there is potential for overland flow to enter the site from this direction, flows are anticipated to be limited to minor greenfield runoff from the grassed areas within the park It is possible that overland flow could enter the site from the Honda Garage situated at the site's southern boundary. The topographical survey indicates that at some locations the adjacent land is at a higher level higher than the site. In other locations, the site is higher than the neighbouring land. Flows from the Honda premises could cause localised ponding near the boundary between the two sites There are numerous low-lying areas throughout the site. During intense rainfall surface water falling on the site will collect in these lower areas. Similarly, any overland flow entering the site will gravitate to these areas Accordingly, flood risk to the site from overland flow is considered medium. Overland Flow Flood Risk Mitigation Measures 3.60 Raised slab levels, as described in paragraph 3.28 above, will also afford protection from overland flow. Flood risk to the existing Honda site will not be increased in terms of flood depth or frequency as a result of the development. Although the available ponding storage area will be reduced by the development, so too will the catchment; runoff will be contained within a formal drainage system instead of accumulating as surface water and overland flow. Cole Easdon Consultants (CEC)

23 3.61 The maximum possible depth of ponding in the adjacent Honda premises will also remain at a similar level. At present, water can attain a maximum level of 55.75m AOD before flowing off site into Brighton Road. Under the proposals, ponding on the Honda site reaching a level greater than 55.73mAOD will enter the basement parking area. Flood risk to the Honda site from ponding/overland flow will not therefore increase as a result of the proposed development The previously mentioned measures designed to prevent basement flooding will provide effective protection from overland flow flooding. Refer to paragraphs 3.31 to Av) Assessment of Flood Risk from Groundwater 3.63 As discussed in Section 2.0, a programme of groundwater monitoring has been undertaken at the site. Groundwater levels have been recorded in Spring and Autumn 2005, Spring 2006, and Spring and Autumn The highest groundwater level encountered was 3.6m below ground level. Recorded levels fluctuated between 3.6m and 10.6m below surface level. Refer to Appendix 5 for borehole logs and ground monitoring results The SFRA identifies the location of groundwater flooding incidents reported between 2000 and It contains no record of groundwater flooding at the site Officers of Croydon Borough Council relate that the area is not considered prone to groundwater flooding. Cole Easdon Consultants (CEC)

24 3.67 Small quantities of groundwater have been observed to reach ground level during sustained rainfall. Groundwater ingress has occurred through damaged tarmac surfacing and in pits used to work on the underside of vehicles. This indicates that groundwater levels can reach higher levels than standpipe readings suggest. Groundwater Flood Risk Mitigation Measures 3.68 Raising finished floor levels will prevent groundwater flood risk to the proposed dwellings The proposed basement areas should be tanked waterproof construction. The basement drainage system will remove any groundwater infiltration The previously mentioned measures designed to prevent basement flooding will provide effective protection from groundwater flood risk. Refer to paragraph 3.31 to B) Assessment of Flood Risk Arising from Surface Water Discharge from the Proposed Development Refer to Drawing No. 2339/501 Proposed Development Layout [Appendix6] 3.71 In order to mitigate flood risk posed by post development runoff, adequate control measures will be required within the site. Bi) Surface Water Runoff Control 3.72 Surface water runoff from roofs and hard areas will be disposed of to the Caterham Bourne public sewer, as per the existing arrangement A 150mm pipe laid at 1/20 is the only existing surface water connection from the site discharging to the public surface water sewer in Brighton Road. The maximum capacity of this pipe has been calculated as 39l/s. Refer to Appendix 4. Cole Easdon Consultants (CEC)

25 3.74 In order to relieve pressure on the existing drainage network and provide betterment, post development discharge will be reduced by a third to 26l/s. Discharge will be restricted by means of a Hydrobrake, orifice or similar flow control device. A nonreturn valve should be installed to prevent backflow from the existing sewer from entering the onsite system On site attenuation will be provided within a box culvert located beneath the proposed access road. In accordance with PPS25 requirements, the proposed culvert will be sized to accommodate flows resulting from events up to and including the 1:100 year + 30% storm Surface water will be conveyed to the attenuation culvert in pipework slung from the basement ceiling The proposals also include green roof systems to the new buildings. While it is acknowledged that green roofs have limited attenuation capabilities during extreme events, they will provide further attenuation during lower magnitude storms. Note that any storage afforded by green roofs has not been considered when sizing the proposed culvert storage This surface water drainage strategy represents significant betterment over the existing situation Proposed discharge rates and storage requirements are summarised in Table 3.2 on page 24. All surface water attenuation calculations have been undertaken using MicroDrainage software. Refer to Appendix 4. Cole Easdon Consultants (CEC)

26 Bii) Adoption 3.80 It is proposed that all on-site SUDS facilities, sewers and flow controls will be maintained privately. Biii) Soakaway Drainage 3.81 Site investigation indicates that permeable strata, comprising an upper layer of Hackney Gravel underlain by Upper Chalk exist beneath the site. Although these strata are suitable materials in which to site soakaway devices, employee accounts indicate that groundwater levels are higher than the groundwater monitoring programme would suggest. A high water table will prevent soakaways functioning effectively. For this reason, soakaway drainage is not promoted in this report. Biv) Water Quality 3.82 All Surface water discharge from basement parking areas will pass through an interceptor before disposal to the watercourse/public sewer. This will prevent hydrocarbon spills entering the surface water system. Bv) Design Exceedence 3.83 Under extreme conditions, or in the event of a pipe blockage, the proposed surface water drainage system could fail. Under such circumstances, floodwater is expected to issue from the control manhole located in the proposed access road. It will then flow onto Brighton Road, and proceed in a northerly direction following the grade of the existing highway As such, in accordance with guidance presented in PPS25, the proposed development will remain safe should the capacity of the proposed drainage system be exceeded. Cole Easdon Consultants (CEC)

27 Table Summary of Assessment of Flood Risk to the Development Site Arising from External Sources Source of Potential Flooding to the Development Site Fluvial Sewers Groundwater Overland Flow Flood Risk Low. Caterham Bourne contained within public sewer. Fluvial flows entering the sewer are controlled upstream of the site. Low/Medium. Existing drainage may cause flooding during heavy rainfall Low/Medium Medium. Mitigation/Comments Finished floor levels will be set a minimum of 600mm above existing ground level. Speed ramp will prevent surface water entering basement. Gullies/Aco drains will be installed on the basement access ramp. Finished floor levels should be set at a minimum of 600mm above existing ground level. Speed ramp will prevent surface water entering basement from highway. Gullies/Aco drains will be installed on the basement access ramp. Existing private on-site drainage will be replaced by a managed runoff disposal system Basement will be tanked waterproof construction. Finished floor levels should be set at a minimum of 600mm above existing ground level. Speed ramp will prevent surface water entering basement from highway. Gullies/Aco drains will be installed on the basement access ramp. Pumped basement drainage system to be implemented Finished floor levels should be set at a minimum of 600mm above existing ground level. Speed ramp will prevent surface water entering basement from highway. Gullies/Aco drains will be installed on the basement access ramp Reduction in catchment area and formal drainage system will prevent an increase in flood risk to the adjacent Honda site. Cole Easdon Consultants (CEC)

28 Table 3.2 Summary of Proposed Surface Water Drainage Strategy Impermeable Area (ha) Discharge Rate (l/s) On Site Attenuation (m 3 ) Existing Proposed Cole Easdon Consultants (CEC)

29 4.0 DISCUSSION AND CONCLUSIONS Assessment of Flood Risk from External Sources 4.1 Flood risk to the proposed development from various sources including fluvial, sewers, groundwater and overland flow has been considered in this study. 4.2 On July 20 th 2007, the locality experienced an extreme storm event. Using Flood Estimation Handbook software CEC has calculated that the storm return period was 1:144 years for this event. Refer to Appendix Based on the events of July 20th 2007, overland flow and flooding of the private drainage system have been identified as potential risks to the site. Overland Flow 4.4 During heavy rainfall, Brighton Road acts as an overland flow path for surface water, conveying floodwater north, to Croydon Town Centre. The SFRA confirms that, due to the existing topography of the area, Brighton Road is considered prone to surface water flooding. 4.5 During the July 2007 event, floodwaters reached a depth of up to 200mm in the road channel adjacent to the site. However, with the exception of bow waves caused by large vehicles, floodwaters from Brighton Road did not enter the site. 4.6 Existing ground levels at the site's frontage to Brighton Road are some mm higher than the highway channels. This prevented overland flow inundating the site. 4.7 Although overland flow from Brighton Road did not enter the site, finished floor levels will be set a minimum of 600mm above existing ground level in order to mitigate any residual risk. Cole Easdon Consultants (CEC)

30 4.8 Flood risk to the proposed basement car park will be mitigated by means of a speed hump to be located on the proposed access road. A surface water drainage system comprising gullies on the access road, and Aco channels at the top and bottom of the basement access ramp will provide further protection. Refer to Drawing No. 2339/501 Proposed Development Layout [Appendix 7]. Sewer Flooding 4.9 During the July 2007 event, the capacity of the on-site private drainage system was exceeded. Flooding due to failure of the drains was exacerbated by the existing topography of the site; numerous low-lying areas are present, in which ponding occurred Finished floor levels will be a minimum of 600mm above existing ground levels under the proposals. Much of the development will be 1.5m above existing levels. This will prevent flooding and will largely eliminate any low-lying areas where floodwaters can accumulate Furthermore, the existing site drainage system will be replaced. The disposal of surface water arising from the development will be managed through the provision of on-site attenuation. Refer to Drawing No. 2339/501 Proposed Development Layout [Appendix 7]. The Sequential Test 4.12 The LPA is satisfied that the site passes the sequential test. Cole Easdon Consultants (CEC)

31 Assessment of Flood Risk Arising from Surface Water Discharge from the Proposed Development Refer to Drawing No. 2339/501 Proposed Development Layout [Appendix 7] 4.13 Surface water will be discharged to the existing public surface water sewer (Caterham Bourne) beneath Brighton Road, as per the current arrangement Post development discharge will be restricted to 26l/s (two thirds of the existing discharge rate). This represents significant betterment over the current situation, and will reduce flood risk to the locality Discharge will be restricted by means of a Hydrobrake, orifice or similar flow control device. A non return valve will be installed in order to prevent backflow from entering the on site drainage system On site storage will be provided in the form of a box culvert to be located beneath the proposed access road. In accordance with the requirements of PPS25, the culvert will be sized to accommodate runoff from the 1:100 year + 30% event Any surface water ingress into the basement car park will be pumped to the existing public sewer via a petrol interceptor Green roofs will also be incorporated into the development as part of the overall management of surface water This study has been undertaken in accordance with the principles set out in PPS25. We can conclude that providing the development adheres to the conditions advised in paragraphs 4.7 to 4.17 above, the said development proposals can be accommodated without increasing flood risk within the locality in accordance with objectives set by Central Government and the EA. Cole Easdon Consultants (CEC)