DRAINAGE STRATEGY REPORT For Proposed … rate by gravity into the public sewer based upon 4000...

Stilwell Limited Consulting Engineers Satelliet House 2 Nexus Park Lysons Avenue Ash Vale GU12 5QE Telephone: 01276 700 400 Fax: 01252 544 934 E-mail: [email protected] Web Site: www.stilwell-ltd.co.uk Version No: 1.0

Stilwell Partnership ISO 9001:2008 Certificate No: LRQ 4004624

DRAINAGE STRATEGY REPORT

For Proposed Development at: Lonach, Horley Lodge Lane, Salfords, Reigate, Surrey, RG1 5EA

For Brookworth Developments Limited

June 2013

Brookworth Developments Limited Lonach, Horley Lodge Lane Drainage Strategy Report

Version 1.0 Page 2 Version Date: 14/06/2013 Date Printed: 14/06/13 Author: David Brooke Owner: The Stilwell Partnership

S:\Clients\BrookworthHomesLtd\Lonach, Horley Lodge Lane, Salford - P2456\Reports\Drainage Strategy V1.0.doc

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Revision History This document has the following history:

Version No. Version Date Summary of Changes Changes marked

1.0 14/06/2013 N/A N/A

Approvals This document requires the following approvals:

Name Title & Organisation

David Brooke Director – Stilwell Limited

Distribution This document has also been distributed to:

Name Organisation

Oliver Watkins Brookworth Homes Ltd

Reigate and Banstead Borough Council




CONTENTS

1.0 INTRODUCTION ................................................................................................ 4

2.0 SITE LOCATION ................................................................................................ 4

3.0 SITE DESCRIPTION AND CONTEXT ............................................................... 4

4.0 DEVELOPMENT PROPOSALS......................................................................... 5

5.0 DRAINAGE PROPOSAL ................................................................................ …6

6.0 FLOODING INFORMATION .............................................................................. 7

7.0 SUMMARY AND CONCLUSIONS ..................................................................... 8

APPENDICES

A Site Location Plan B Topographical Survey C Proposed Site Layout Plan D Drainage Layout E Attenuation Calculation

F Flood Map (EA)

G Map of Flood Risk (Area 2 Wealden Greensand Ridge) of SFRA




1.0 INTRODUCTION

1.1 The Stilwell Partnership has been instructed by Brookworth Developments Limited, to undertake

a Drainage Strategy Report and Flood Risk Statement to support a Planning Application for the

proposed development at Lonach, Horley Lodge Lane, Salfords, RH1 5EA.

1.2 The Stilwell Partnership has been instructed by Brookworth Developments Limited to carry out a

drainage assessment to determine how both foul and surface water, from the site, can be

satisfactorily drained and not to increase the likelihood of flooding.

2.0 SITE LOCATION

2.1 The site is located on the western side of the A23 Bonehurst Road, just south of the dual

carriageway. Approximately just 800m from Salfords (Surrey) Train Station. The exact location is

shown in Appendix A. The site is bounded by Oak Lodge Drive along the western boundary with

residential properties to the northern boundary and the western boundary. The access is via

Horley Lodge Lane with runs along the southern site boundary.

2.2 The Local Authority is Reigate and Banstead Borough Council.

3.0 SITE DESCRIPTION AND CONTEXT

3.1 The site is currently occupied by a large detached house. The topographical survey of the site is

included in Appendix B.

Horley Lodge Lane

3.2 Horley Lodge Lane is a narrow residential no through road, being just 4.5m wide at the junction

with Bonehurst Road and just 3m wide adjacent to the site at Lonarch.




4.0 DEVELOPMENT PROPOSALS

4.1 The proposal is to redevelop the land and provide detached housing containing 4 dwellings. The

properties will be oriented towards Horley Lodge Lane and each will have garden areas at the

front and rear of the properties. All properties will have level access.

4.2 A proposed site layout is included at Appendix C.

4.3 The existing vehicular access will continue to be used, although an internal layout will be

designed in accordance with Design Manual for Roads and Bridges and Manual for Streets which

will serve each property within the site and associated parking.

4.4 The parking areas and turning areas will be formed with porous surfacing to reduce the runoff. If

the underlying soil doesn’t have any infiltration capacity then a partial infiltration pipe will connect

to the surface water system.




5.0 DRAINAGE PROPOSAL

Surface Water

5.1 Currently the site is an existing property situated on a large plot, which is used as gardens and

driveway access. The total site area is 3285m2 and 193m

2 of the total is impermeable. The

proposed layout for the development will include an area of hardstanding which will be an area of

approximately 597m².

5.2 The NPFF recognises that increased run-off rate and volume of surface water can be managed

by minimising changes in the volume and run-off rate from the development sites, and

recommends that the priority is given to use of Sustainable Drainage Systems (SUDs) in new

development, this being complementary to the control of the development within the floodplain.

For the drainage proposals we propose to provide a sustainable surface water drainage scheme,

inclusive at many measures such as porous paving, water butts, and an underground attenuation

system with a restricted outfall. The design will focus to capture and manage all surface water

within the site boundary to prevent any likelihood of flooding to adjacent properties.

5.3 The primary sustainable drainage technique is infiltration drainage. A detailed soil investigation

has not been undertaken at this preliminary stage, however information of the geology and the

ground water can be checked through the British Geological Survey (BGS) website. There are

available borehole records in the proximity of the site. Boreholes around the site show that the

soil is silty and sandy clay with scattered chalk fragments. And the ground water level is high and

approximately 1.50m below the surface, therefore the infiltration methods may not viable for the

site (subject to a full site investigation).

5.4 All surface water from the roof area and remaining areas will be discharged into the conventional

surface drainage system. The surface water running off from the roof area will be connected into

the rainwater butts; there will be an overflow connection between rainwater butts and the surface

water system. The water then will be gravitated into the existing surface water sewer through an

underground attenuation system. The water will be discharged into the public sewer at a limited

rate subject to Thames Water’s (TW) approval. The discharge rate will be discussed in detail with

Thames Water.




5.5 The runoff rate for different return periods are calculated for the existing site. The results are

presented as follow.

1 in 2 year return event plus CC 1.3 litres / second



5.6 The discharge rate of the proposed site has been limited to 3 litres/second. This rate has been

chosen as lower rates may create maintenance issues. The surface run-off rate of the existing

site has been calculated and 1 in 2 year return period peak flow for the existing site is 1.3

litres/second, therefore it is not possible in practice to limit the discharge to this level. We

propose to limit the discharge to 3 litres/second by introducing a hydro-brake flow control unit.

5.7 The underground attenuation system (aqua cells crates, or similar) as part of surface water

drainage system will be designed to accommodate for a 1 in 30 year storm event, plus 10% for

climate change. Catch pit manholes will be provided to limit any debris entering the system. The

storage tanks will be sealed to avoid any water infiltration. A copy of the Schematic Drainage

Layout can be seen in Appendix D.

5.8 As stated above, the size of the storage tank has been set for a 1 in 30 year storm event plus

10% climate change, with a discharge rate of 3 litres per second subject to TW approval. A copy

of the attenuation calculations is enclosed in Appendix E. There is a need to store at least

12.32 m3. This has been accommodated in the attenuation tanks.

5.9 By introducing the drainage scheme shown on drawing TSP/BRO/P2456/01 in Appendix D, the

proposed site will have the benefit of managing the surface water on the site. The surface water

discharge will be controlled and discharged over in longer period so this will not cause any risk of

flooding at any downstream site. If found at the detailed design stage that infiltration into the

subsoil can be achieved then infiltration techniques will be introduced to reduce runoff.

Foul Drainage

5.10 The nearest public foul sewer is in Bonehurst Road. The foul water sewer will be connected into

the public sewer via the existing foul connection. The foul water connection is shown on the

Schematic Drainage Layout in Appendix D. The foul water will be discharged at a 0.185

litres/sec rate by gravity into the public sewer based upon 4000 litres/per unit per day. This is

again subject to TW approval but 0.185 litres/second should not cause any capacity issues for

175mm diameter foul public sewer. Indeed only 0.139 litres / second would be new flows as the

site already has one unit.




6.0 FLOODING INFORMATION

6.1 The initial phase in identifying if the site is potentially at risk of flooding is to check the

Environment Agency Flood Zone maps (web based). According to EA’s website the site is not

within a Flood Zone. Therefore there are no modelled flood level data available for the site. A

Flood Map provided by the Environment Agency can be found at Appendix F.

6.2 The nearest watercourse to the site is Salfords Stream which is 410 metres to the north of the

site.

6.3 There are no historical records of flooding at the site. The Strategic Flood Risk Assessment of

Reigate and Banstead Borough Council shows observed flooding events on Flood Risk Map (See

Map of Flood Risk in Appendix G) and according to the map there is no historical flooding at the

proposed site.




7.0 SUMMARY AND CONCLUSIONS

7.1 Brookworth Developments Ltd is in the process of submitting a Planning Application for 4 No.

residential houses at Lonach, Horley Lodge Lane, Salfords, Reigate, RH1 5EA.

7.2 The Stilwell Partnership has been instructed by Brookworth Developments Limited, to produce a

Drainage Strategy Report and Flood Risk Assessment under NPFF to support the planning

application.

7.3 The Environment Agency has confirmed that the site is not within a Flood Zone. No documentary

or anecdotal evidence has been found to show previous flooding events for this site. This report

has demonstrated that this site is not at risk from any form of flooding.

7.4 The report has demonstrated that a Sustainable Drainage systems (SUDs) can be introduced to

minimize flood risk.

7.5 This Flood Risk Assessment has confirmed that the proposed development is appropriate and

sustainable.

APPENDIX A

Site Location Plan

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APPENDIX B

Topographical Survey

APPENDIX C

Proposed Site Layout Plan

APPENDIX D

Drainage Layout

APPENDIX E

Attenuation Calculation

P2456 / 01

Prepared by : G. Ozkan, 14.06.2013

Impermeable area m2

597

Design return period in years 30

Permitted rate of discharge litressec

3

Duration of storm (mins) 5 10 30 45 60 120 180 210 240 300 360 480 600

Area (m2) 597 597 597 597 597 597 597 597 597 597 597 597 597

Rainfall (Bilham's Formula )(mmhr

) 189.2 118.3 55.6 41.9 34.3 21.1 15.9 14.3 13.0 11.1 9.8 8.0 6.8

Storm Volume (m3) 9.4 11.8 16.6 18.8 20.5 25.2 28.5 29.8 31.0 33.1 34.9 38.0 40.6

Allowable discharge (m3) 0.9 1.8 5.4 8.1 10.8 21.6 32.4 37.8 43.2 54.0 64.8 86.4 108.0

Balance to store (m3) 8.5 10.0 11.2 10.7 9.7 3.6 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Time to empty (hrs) 0.8 0.9 1.0 1.0 0.9 0.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0

The computation is based on the assumption that the storm volume is generated at the attenuation storage vessel in the time given. In practise, there is a

delay, depending on the distance from the storage vessel to the most distant point in the catchment, and also the rate at which the water will flow to it. This

is known as 'The time of concentration'. From the above figures, it can be seen that the critical time is the 10 minute event, and the solution is to provide a

storage volume of 12.32m³ including 10% climate change.

Project: Lonach, Horley Lodge Lane, Salfords

Surface Water Attenuation Calculations

The computation of storage volume is based on BS EN 752-4:1997, and the flood design frequency is 10 years for Rural areas, 20 years for Residential

areas, 30 years for City centres/Industrial/Commercial areas and 50 years for Underground railways/Underpasses

APPENDIX F

Flood Map (EA)

APPENDIX G

Map of Flood Risk (Area 2 Wealden Greensand ridge) of SFRA

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Reigate & Banstead BoroughStrategic Flood Risk Assessment

(SFRA)

Maps produced by Reigate & Banstead Borough Council to support the SFRA document. Compiled using informationsupplied by the Environment Agency, Surrey County Counciland incidents of flooding reported to Reigate & Banstead Borough Council

Scale @ A1 1:20,000

© Crown Copyright and database right 2012Ordnance Survey licence number 100019405

Area 2Wealden

Greensand Ridge

Area 1

Area 2

Area 3

LegendMain RiversReservoirIncidents of observed floodingFlood Zone 3Flood Zone 2AStSWF* - MoreAStSWF* - IntermediateAStSWF* - LessBorough boundaryExtent Area 2 (between lines)

* Area Subject to Surface Water Flooding** Number relates to table in Appendix 6

¯

Date: May 2012

47**

Reservoir - The LakeGatton Park

Reservoir - Mill Pond & Boating LakeEarlswood Common

Map of Flood Risk

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