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The Whitgift Foundation

1 A Brighton Road,

Croydon

Flood Risk Assessment

Date: September 2008

Report no: 5001-GD05050-GDR-02

The Whitgift Foundation

1 A Brighton Road,

Croydon

Flood Risk Assessment

Prepared by:

T. Plessiet

MEng (Hons)

Reviewed by: D. Mardon

MScEng BScEng (Civil)

Approved by: R. Bettridge BSc BA C Eng C Env MICE FIHT MCIWEM

Report no: 5001-GD05050-GDR-02 Date: September 2008

BTP Hyder

10 Medawar Road

The Surrey Research Park

Guildford

Surrey GU2 7AR

Tel: +44 (0)870 000 3005 Fax: +44 (0)870 000 3905

www.bt-p.com / www.hyderconsulting.com

1a Brighton Road, Croydon BTP Hyder

K:\~ BTP Job Files\GD05050 - 1a Brighton Road, Croydon\I-Issued Documents\Reports\5001-GD05050-GDR-02 - 1a Brighton Road, Croydon - FRA.doc

Contents

EXECUTIVE SUMMARY.............................................................................................................3

1 INTRODUCTION..............................................................................................................5

General Information............................................................................................................. 5

Scope of Study.................................................................................................................... 5

2 SITE DESCRIPTION........................................................................................................6

Location.............................................................................................................................. 6 Sources of Potential Flooding .............................................................................................. 6

Geology and Groundwater Vulnerability ............................................................................... 7 Existing Flood Alleviation Measures..................................................................................... 9

Existing Development.......................................................................................................... 9 Topographic Survey ............................................................................................................ 9

Proposed Development ....................................................................................................... 9

3 FLUVIAL FLOODING ON SITE....................................................................................11

Fluvial Flooding................................................................................................................. 11

Historic Floods .................................................................................................................. 11 Hydrology ......................................................................................................................... 12

Structures Influencing Local Hydraulics.............................................................................. 12 Flood Level Information..................................................................................................... 12

Finished Floor Levels ........................................................................................................ 13 Flood Compensation ......................................................................................................... 13

Flood Mitigation Precautions.............................................................................................. 13

4 SITE DRAINAGE ...........................................................................................................15

Existing Drains and Sewers............................................................................................... 15 Proposed Drainage ........................................................................................................... 15

5 OTHER DESIGN CONSIDERATIONS .........................................................................16

Site Access....................................................................................................................... 16

Refuge and Escape Road in Time of Flood ........................................................................ 16 The Sequential Test .......................................................................................................... 18

The Exception Test ........................................................................................................... 18

6 CONCLUSIONS AND RECOMMENDATIONS............................................................19

APPENDIX A - DRAWINGS .....................................................................................................20

Description of referenced drawings.................................................................................... 20



APPENDIX B – FLOOD MAP...................................................................................................21

Flood map for the 1 in 100 year flood provided by the Environment Agency ........................ 21

7 APPENDIX C – List I: substances defined by EC groundwater Directive 80/68/EEC.........................................................................................................................................22

8 APPENDIX D – HISTORIC FLOODS...........................................................................23

List of Figures

Figure 1 – Site location map 6

Figure 2 – Geological map – British Geological Survey 7 Figure 3 – Environment Agency groundwater Source Protection Zone (SPZ) 8

Figure 4 – Environment Agency flood map 11 Figure 5 – Safe access to upper floors and possible escape road 17

List of Drawings

Drawing 1 – Site location plan - Howard Fairbairn & Partners 20

Drawing 2 – Existing site plan and topographical survey - Howard Fairbairn & Partners 20 Drawing 3 – Proposed ground floor plan- Howard Fairbairn & Partners 20

Drawing 4 – Proposed first floor plan - Howard Fairbairn & Partners 20 Drawing 5 – Proposed second floor plan - Howard Fairbairn & Partners 20

Drawing 6 – Proposed third floor plan - Howard Fairbairn & Partners 20 Drawing 7 – Proposed elevations - Howard Fairbairn & Partners 20

Drawing 8 – Sections A-A and B-B - Howard Fairbairn & Partners 20 Drawing 9 – TW plans - foul & surface water sewers 20

Drawing 10 – Topographic data and estimated flood levels 20



EXECUTIVE SUMMARY

BTP-Hyder was commissioned to produce a Flood Risk Assessment (FRA) in accordance with PPS 251, for the redevelopment at 1A Brighton Road, Croydon.

The site is currently occupied by a garage formerly in use for retail motorcycles sales with frontage along Brighton Road. The proposal is to provide a block of mixed residential and commercial units of up to 4 floors. The development consists of 2 shops units at the ground floor providing approximately 463m2 of commercial space as well as 14 two-bedroom flats on the upper floors.

The site lies inside the 1 in 100 year floodplain indicated by the floodplain maps held by the Environment Agency. No flood levels are available for this area.

The Thames Water plans show that a 1050mm diameter culvert runs in a northern direction along Brighton Road, conveying one of the intermittent sources of the River Wandle and runoff generated in the surroundings. The capacity of this sewer is thought to be insufficient, and out of manholes flooding is expected to occur during major rainfall events. The Croydon Council has confirmed that flooding along Brighton Road has occurred several times in the past, although they were unable to provide any precise details.

Based on the Environment Agency flood map, the 1 in 100 year flood water level is estimated to vary between 49.04m AOD and 49.19m AOD. 300mm should be added to take climate change into account. However as no hydraulic (sewers) modelling has been undertaken as part of this flood risk assessment, uncertainties regarding the accuracy of the flood water levels remain. With the current flooding information available, the finished floor levels of the ground floor commercial units of the new buildings are proposed to be the same as the original ones (49.03m AOD) and not out character with the neighbouring commercial properties.

The footprint of the proposed building will be reduced smaller than the existing one (reduction of 45%) and on-site storage is expected to increase as water will be allowed to flow freely into the new rear parking area in the event of an extreme flood. Therefore no floodplain compensation is expected to be necessary. The development proposals for the site include an extensive planted green roof on the third floor (area of approximately 45m2). The existing site is completely hardstanding and a slight decrease in runoff generated on site is expected.

Sleeping accommodation is only proposed on the first floor and above. Two single storey retail units are proposed on the ground floor. Internal access to upper floors will be available for the shop unit located in the northern part of the proposed development. Both shops have a rear entrance from the car parking area. Shops occupiers will be able to safely gain upper floors if necessary during extreme floods via the car parking area.

The proposed redevelopment will look to implement flood resilience measures in the construction in order to reduce the consequences of a flood and enable a swifter

1 Planning Policy Statement 25 – Development and flood risk. Department for Communities and

Local Government, December 2006



recovery. Flood proofing measures include (but are not limited to) raising electrical connections, using flood resistant materials and investigating the introduction of flood barriers for the commercial units.



1 INTRODUCTION

General Information 1.1 The proposed development is located at a site on 1A Brighton Road

in Croydon. The proposed redevelopment of the existing garage is a mix of residential and commercial units, together with associated amenity spaces and car parking.

1.2 The site is shown to lie within Flood Zone 3 of the culverted River Wandle.

1.3 The proposed mixed residential and commercial development falls under the “more vulnerable” category of development as defined in PPS 25 Table D2.

1.4 The Environment Agency was consulted to obtain relevant flooding information.

1.5 This Flood Risk Assessment (FRA) has been compiled to assess the impact on the risk to life and property due to flooding that will result from the proposals, in respect of both future residents of the development and third party landowners.

Scope of Study 1.6 The main objectives of this study are to:

• Provide a flood risk assessment of the site, compliant with the guidelines set out in PPS 25, to accompany any application for planning permission.

• Provide advice on the site layout and design that will ensure safe operation of the site in an extreme flood event.



2 SITE DESCRIPTION

Location 2.1 The proposed redevelopment is located on a 0.1ha (approximately)

site off Brighton Road, Croydon. The proposed site is bounded by the Brighton Road to the west and residential and commercial properties to the north, east and south. Figure 1 shows the location of the site on Brighton Road in Croydon, and also the indicative course of the River Wandle as it flows in an underground culvert near the site.

Figure 1 – Site location map

Sources of Potential Flooding 2.2 The primary source of potential flooding on site is fluvial in nature,

from the River Wandle that flows past site within an underground culvert.

2.3 Secondary sources of potential flooding may include ground water or surface water flooding. These issues will be addressed later within this assessment.

'© Crown copyright. All rights reserved.

Licence No 100018322'

Site

River Wandle

(culverted)



Geology and Groundwater Vulnerability 2.4 The strata underlying the site are river gravel deposits (Hackney

Gravel Formation), as shown is Figure 2.

Figure 2 – Geological map – British Geological Survey

2.5 The site is located in the large chalk aquifer of the London Basin (defined as a major aquifer) which is heavily exploited for the water supply of London area.

2.6 Major aquifers are typically highly permeable formations with known or probable presence of significant fracturing. They may be highly productive and able to support large abstractions for public water supply and other purposes.

2.7 Examination of the Environment Agency (EA) website indicated that the site is situated within Source Protection Zone I (SPZ1: Inner Zone1) as can been seen in Figure 3.

1 Catchment within 50 day travel time of an abstraction point or circle of radius 50m centred on an

abstraction point

Site



Figure 3 – Environment Agency groundwater Source Protection Zone (SPZ)

2.8 The document Groundwater Protection: Policy and Practice (GP3) Part 41 summarises the legislation relevant to the management and protection of groundwater. Particular attention to the following policies is required:

2.9 P4-4 Regulatory/Planning: “Inside SPZ1 we will object to any new trade effluent, storm sewage or other significantly contaminated discharges to ground. Where such discharges already exist if necessary we will use prohibition notices to control this activity.”

2.10 P4-5 Planning: “The discharge of clean roof water to ground is acceptable both inside and outside SPZ1 provided that all roof water down-pipes are sealed against pollutants entering the system from surface run-off, effluent disposal or other forms of discharge. This must not create new pathways for pollutants to groundwater.”

2.11 P4-8 Planning: “We [Environment Agency] will object to new developments that pose an unacceptable risk of pollution to groundwater from sewage effluent, trade effluent or contaminated surface water, either from an individual discharge or the combined effects of several discharges.”

2.12 P4-11 Planning: “we [Environment Agency] will object to developments involving discharge of surface water run-off to ground

1 Groundwater Protection: Policy and Practice (GP3) Part 4: legislation and policies (public consultation

2007) - Environment Agency, March 2007

Inner zone

Outer zone

Total catchment

Special interest

Site



from areas subject to contamination. This applies especially to those involving the storage, handling and use of List I substances1 (such as garage forecourts, lorry parks, scrap yards) unless the site has been subject to a risk assessment and an acceptable level of treatment of the effluent is provided”.

2.13 Soakage methods that form most of the Sustainable Drainage Systems (SuDS) favoured by the Environment Agency are not suitable.

Existing Flood Alleviation Measures 2.14 At present it is understood that there are no flood alleviation

measures in the area.

Existing Development 2.15 The site is currently occupied by a garage formerly in use for retail

motorcycles sales with frontage along Brighton Road.

Topographic Survey 2.16 The topographic survey for the site is provided as Drawing 2. The

survey shows that:

• The external pavement levels at the location of the proposed redevelopment range from 48.86m AOD to 48.97m AOD along Brighton Road.

• Levels in the small yard north-east of the site with access to South End range from 48.72m AOD to 48.98m AOD.

• Levels in the concrete yard south-east of the site with access to South End range from 48.60m AOD to 49.29m AOD.

• The current Finished Floor Level (FFL) of the existing buildings is 49.03m AOD.

Proposed Development 2.17 The proposed development layout is shown in Appendix A (Drawing 3

to Drawing 8).

2.18 The proposal is to provide a block of mixed residential and commercial units of up to 4 floors. The development consists of 2 shops units at the ground floor providing approximately 463m2 of commercial space as well as 14 two-bedroom flats on the upper floors.

2.19 Off-street parking is provided for 14 cars, as well as storage areas for cycles.

1 See Appendix C



2.20 Pedestrian and vehicular access to the block will be via Brighton Road. A rear pedestrian access to the south-east part of the parking is also provided via South End.

2.21 Amenity spaces consisting of terraces at the first and third floors are also provided.

2.22 The development proposals for the site include an extensive planted green roof on the third floor (see Drawing 6), which will cover an area of approximately 45m2.



3 FLUVIAL FLOODING ON SITE

Fluvial Flooding 3.1 The Environment Agency flood map of the area is shown in Figure 4

and indicates the potential risk of fluvial flooding at the site. The blue area indicates the extent of the 1 in 100 year fluvial flood event and the light blue shows the additional extent of an extreme flood (1 in 1000 year fluvial flood event).

3.2 As can be seen, the site lies in the 1 in 100 year floodplain of the culverted River Wandle.

Figure 4 – Environment Agency flood map

Historic Floods

3.3 A list of flooding events that have recently occurred in the site area has been provided by the Croydon Council and is presented in Appendix D. The characteristics of these flooding events (exact extent, depths, duration, velocities) are unknown.

Possible fluvial flooding (1 in 100 year flood)

Extent of extreme flood (1 in 1000 year flood)

Site



3.4 Thames Water have also been contacted however they were unable to provide any further information regarding flooding in the area.

Hydrology

3.5 The River Wandle catchment upstream of the site, located in the North Downs, is large (area of approximately 106km2 - taken from the Flood Estimation Handbook CD) and predominantly Chalk. The two principal sources of the River Wandle are located downstream of the site in Waddon and Carshalton.

3.6 Since 1840, small intermittent springs in the neighbourhood of Croydon have progressively been culverted and conveyed in a north-west direction to the source in Waddon.

3.7 The mean discharge and the 10% exceedance discharge at the gauging station 39004 (Wandle at Beddington Park – flow data recorded from 1936 to 2004) approximately 3.5km downstream of the site are respectively only 0.19m3/s, and 0.35m3/s, and illustrate the low volume of flow compared to the topographic catchment size. This is likely due to the chalky nature of the area.

3.8 This does not give an indication of the flood flows but rather that a gauge exists on the watercourse that could be used in future estimates of the catchment hydrology.

Structures Influencing Local Hydraulics 3.9 At site, the watercourse flows through a 1050mm diameter circular

culvert under Brighton Road, as can be seen on Thames Water plans (Drawing 9).

3.10 The capacity of this circular culvert at site is estimated to be approximately 1.19m3/s.

Flood Level Information 3.11 The Environment Agency are unable to provide flood level information

and have recommended that further investigation such as sewers modelling is carried out. This is currently outside the budgeted scope of this assessment.

3.12 Modelling of the River Wandle has not been undertaken as part of the flood risk assessment.

3.13 The flood levels relevant to the site have been estimated from the existing flood map and the topographic data available (Drawing 2 and Thames Water manholes cover levels).

3.14 The indicated 1 in 100 year flood water levels at site is estimated to potentially vary between 49.04m AOD and 49.19m AOD. Estimated flood water levels and the corresponding floodplain are shown in Drawing 10.



3.15 A nominal 300mm above the 1 in 100 year level is commonly taken as being equivalent to the 1 in 100 year flood level including climate change where, as in this case, the climate change level is not available. Therefore taking climate change into account the 1 in 100 year flood water levels would vary between 49.34m AOD and 49.49m AOD.

3.16 These flood levels are at the moment only indicative and subject to changes. Should hydraulic (sewers) modelling be required, this would be carried out at a later stage.

Finished Floor Levels 3.17 At this stage of knowledge concerning the flood water levels, the

finished floor levels of the redevelopment are proposed to be maintained as existing (49.03m AOD), including a standard allowance to restrict surface water from entering the buildings. This is considered acceptable since the ground floor is for commercial use in nature. Sleeping accommodation is only proposed on the first floor and above.

Flood Compensation 3.18 The proposed development lies within an area indicated as the 1 in

100 year floodplain. The proposed building and the bins shelters would occupy a total area of 545m2.

3.19 The existing garage covers an area of 1042m2. Therefore the footprint of the buildings following the proposed redevelopment will be significantly reduced (reduction of 45%), hence increasing the volume of the floodplain. This offers an improvement over the existing site.

3.20 Therefore it is expected that floodplain compensation would not be necessary.

Flood Mitigation Precautions 3.21 No residential accommodation is proposed on the ground floor. It will

remain commercial in nature with 2 shop units.

3.22 Flood resilient construction techniques will be considered in order to reduce the consequences of flooding and enable a swifter recovery. Flood resilient measures are outlined in Preparing for Floods1.

• Demountable flood boards and airbrick covers that may be considered across the doorways in particular along Brighton Road.

• Water resistant materials for floor, walls and fixtures.

1 Preparing for Floods - DTLR, February 2002



• Practical techniques improving the water proofing of the ground floor to prevent groundwater flooding.

• Electrical controls, cables and appliances should be raised within reason.



4 SITE DRAINAGE

Existing Drains and Sewers 4.1 Both foul and surface water onsite are currently discharged to the

existing Thames Water network as shown in Drawing 9.

Foul water

4.2 A 750mm diameter adopted foul sewer is shown to be running northwards along Brighton Road.

Surface water

4.3 A 1050mm diameter adopted surface water sewer conveying one of the intermittent sources of the River Wandle and runoff is shown to be running down the length of Brighton Road. The capacity of this sewer is thought to be insufficient, and out of manholes flooding is expected to occur during major rainfall events.

Proposed Drainage 4.4 The surface and foul water drainage systems will remain essentially

unchanged by the development.

4.5 The current site is completely hardstanding and no increase in impermeable area is expected to result from the proposed development.

4.6 An extensive green roof is proposed on the third floor as can be seen in Drawing 6 (total area of approximately 45m2). This green roof will aid in reducing the peak runoff rate.

4.7 A slight increase in foul flows due to the change in use from commercial to residential is expected and a capacity enquiry should be carried out at the detailed design stage.

4.8 Given the large proposed car parking area the development has the potential to include additional onsite surface water storage, in the form of underground storage tank. The details of this may be agreed with the Environment Agency and Thames Water during the detailed design process.



5 OTHER DESIGN CONSIDERATIONS

Site Access 5.1 The main access to the shops and flats will be from Brighton Road.

This road is prone to flooding, and dry access in time of a major flooding event will not be available.

5.2 Depths, velocities and duration of flooding are unknown at this stage. Therefore the hazard to people and buildings in time of flood can not be determined at the moment with certainty.

5.3 Rear access to the shop units and flats is provided from the car parking.

Refuge and Escape Road in Time of Flood 5.4 Internal access to upper floors is provided for the northern proposed

ground floor single storey retail unit, as shown in Drawing 3.

5.5 The second proposed ground floor retail unit is also single storey and internal access to the upper floors is not available.

5.6 The car parking area is detached from the main overland flow paths as it is surrounded by buildings, and therefore water velocities in this area are expected to be low.

5.7 Access to the yard south of the site where higher grounds leading to South End stand should be available. The access gate to this yard is only 11m from the rear entrance of the southern shop unit. It is expected that this escape road would not pose a significant risk to people wishing to evacuate through it as water (if any) is likely to be flowing slowly.

5.8 Furthermore the site and its surroundings are expected to be flooded for short durations (in the order of few hours although this can not be confirmed at this stage). For short durations of flooding, it is likely that residents or shops occupiers who do not wish to enter flood water of any depth could remain safe in the proposed building until the water recedes.



Figure 5 – Safe access to upper floors and possible escape road

Access to upper floors for dwellers and shops

occupiers available from the car park

Possible escape road (to South End) in time of

extreme flood

Access should be available (gate)

Internal access to upper floors to be provided for the

northern shop unit

N

Rear entrances from the car park



The Sequential Test 5.9 The site lies in the Flood Zone 3a (High probability – 1 in 100 or

greater annual probability of flooding in any given year).

5.10 The ground floor will remain commercial in nature and residential accommodation is proposed for upper floors. The development is classed as ‘More Vulnerable’ according to the Table D.2 in appendix D of PPS25.

5.11 PPS25 states that residential development within Flood Zone 3a is appropriate should it be shown that the Exception test is passed.

The Exception Test 5.12 The proposed development is on previously developed commercial

and residential area.

5.13 No sleeping accommodation is proposed at the ground floor and therefore below the likely 1 in 100 year flood water level (including climate change).

5.14 No increase in surface water runoff is expected. The proposed development is not expected to increase the flood risk elsewhere. The proposed car parking area will be freely floodable and flood water storage on site is expected to increase.

5.15 It is our opinion that the requirements set by the Exception Test (PPS25 Appendix D, paragraph D9) have been satisfied. Therefore satisfying the Sequential Test and showing the development to be appropriate within Flood Zone 3a.



6 CONCLUSIONS AND RECOMMENDATIONS

6.1 The site is currently occupied by a garage formerly in use for retail motorcycles sales with frontage along Brighton Road. The proposal is to provide a block of mixed of commercial (approximately 463m2) and residential units (14 two-bedroom flats).

6.2 The site lies inside the 1 in 100 year indicative floodplain of the culverted River Wandle. However the Environment Agency do not hold precise flooding information and flood water levels are unavailable.

6.3 The Croydon Council has indicated that flooding in the site area has occurred recently (in particular on the Brighton Road carriageway). One of the sources of the River Wandle flows under this road through a 1050mm diameter circular culvert whose capacity is thought to be exceeded during extreme rainfall events.

6.4 The Environment Agency have recommended that further modelling is carried out to determine floods characteristics. Based on the Environment Agency flood map, the 1 in 100 year flood water level is estimated to vary between 49.04m AOD and 49.19m AOD.

6.5 With the current flooding information available, the finished floor levels of the renovated buildings are proposed to be no lower than existing (49.03m AOD).

6.6 Flood proofing measures should be incorporated, in particular to prevent any water ingress from Brighton Road.

6.7 No residential accommodation is proposed on the ground floor that will remain commercial in nature with two single storey retail units. Internal access to upper floors will be available for the shop unit located in the northern part of the proposed development.

6.8 Both shops have a rear entrance from the car parking area. Shops occupiers will be able to safely gain upper floors if necessary during extreme floods via the car parking area.

6.9 Depths, velocities and duration of flooding are unknown at this stage. However flow velocities in the car parking area are expected to be low as it is detached from the main flow path (Brighton Road).

6.10 The surface water drainage system and foul sewage will remain essentially unchanged.

6.11 The existing site is completely hardstanding, and the development proposals for the site include an extensive planted green roof on the third floor which will cover an area of approximately 45m2. Therefore a slight decrease in surface water runoff is expected.

6.12 Water will be allowed to flow freely in the car-parking and floodplain storage will be increase. Consequently the proposed development is not expected to increase the flood risk elsewhere.



APPENDIX A - DRAWINGS

Description of referenced drawings

Drawing 1 – Site location plan - Howard Fairbairn & Partners

Drawing 2 – Existing site plan and topographical survey - Howard Fairbairn & Partners

Drawing 3 – Proposed ground floor plan- Howard Fairbairn & Partners

Drawing 4 – Proposed first floor plan - Howard Fairbairn & Partners

Drawing 5 – Proposed second floor plan - Howard Fairbairn & Partners

Drawing 6 – Proposed third floor plan - Howard Fairbairn & Partners

Drawing 7 – Proposed elevations - Howard Fairbairn & Partners

Drawing 8 – Sections A-A and B-B - Howard Fairbairn & Partners

Drawing 9 – TW plans - foul & surface water sewers

Drawing 10 – Topographic data and estimated flood levels



APPENDIX B – FLOOD MAP

Flood map for the 1 in 100 year flood provided by the Environment Agency



7 APPENDIX C – List I: substances defined by EC groundwater Directive 80/68/EEC

These substances should be prevented from being discharged into groundwater due to the risk of toxicity, persistence and bio-accumulation. List I contains the individual substances which belong to the families and groups of substances specified below.

1) Organohalogen compounds and substances which may form such compounds in the aquatic environment.

2) Organophosphorous compounds.

3) Organotin compounds.

4) Substances which possess carcinogenic, mutagenic or teratogenic properties in or via the aquatic environment.

5) Mercury and its compounds.

6) Cadmium and its compounds.

7) Mineral oils and hydrocarbons.

8) Cyanides.



8 APPENDIX D – HISTORIC FLOODS

Historic floods in the neighbourhood of the site

Source: Croydon Council

NB: This list might be non exhaustive

Pub "Swan & Sugar loaf" (1 Brighton Road) October 1976

(possibly inside the cellar)

October 1984 Pub "Swan & Sugar loaf" (1 Brighton Road)

Flooding from Upland Road to the pub "Blue Anchor" June 1985

(78 South End)

December 1985 Brighton Road & Nottingham Road junction

October 1990 Brighton Road (carriageway)

August 1997 Brighton Road (carriageway)

July 2007 Brighton Road (carriageway)