Below Ground Drainage, Statement, Description and...

Nishkam School, Hounslow. Author

S Bliss

BAM Design 1

Project: Nishkam School, Hounslow.

Job number 4467.

Below Ground Drainage,

Statement, Description and Calculation

In

Support of Planning Application

at

Nishkam School

Syon Lane

Hounslow

June 2015.


S Bliss

BAM Design 2


Job number 4467.

BAM Design

Centrium

Griffiths way

St Albans

Herts Tel No: (01727) 894200

AL1 2RD Fax No: (01727) 818850

BAM Design Technical Report.

SUBJECT

Below Ground Drainage Design.

CLIENT: BAM Construction South East.

PROJECT: Nishkam School, Hounslow.

Contract no: Job no: 4467.

Prepared by: S Bliss. Checked by: SGB.

Date: June 2015

Status of drawings / specifications: For Approval

AMENDMENTS

ref. date amendment amended

by

checked

by

AP0 June

2015

First Issue for approval. SGB SGB


S Bliss

BAM Design 3


Job number 4467.

Contents

1.0 Introduction. 2.0 Site Description. 3.0 Drainage General. 4.0 Storm Water Drainage. 5.0 Foul Water Drainage. 5.0 Summary. Appendix A. Preliminary Storm Water attenuation requirements. Appendix B. Soil Infiltration Test results. Appendix C. Soil Infiltration Test analysis. Appendix D. Storm Water Drainage Strategy. Appendix E. Foul Water Drainage Strategy.


S Bliss

BAM Design 4


Job number 4467.

Below ground drainage statement in support of planning application.

1.0 Introduction.

This drainage statement needs to be read in conjunction with the site specific Flood Risk Assessment

that has been produced by LBH Wembley Ltd. The document describes the proposed drainage system

and indicates how the storm water flows exiting the site will be managed to a level that is acceptable to

the local authority.

The calculations are indicative only at this stage and provide an indication of final pipe sizes, levels etc.

The flooding of the existing football pitch has been calculated on a discharge of 5 Litres per second per

hectare. This flow is considered to mimic the flow generated by green field sites.

Appendix A, Provides preliminary storm water attenuation volumes for differing storm events and

permitted discharge rates.

2.0 Site Description.

The new school facility will be located on land that is currently occupied by playing fields. These fields

are known as White Lodge Club, 152 Syon Lane, Hounslow, London. TW7 5PN. The approximate

National Grid reference is 515550, 177660. Access is from Syon Lane, which forms the Eastern

boundary whilst Wood Lane forms the Western boundary. TFL Piccadilly line forms the Northern

Boundary and Braybourne Drive to the South.

Approximately half of the land will be used for the new school facility.

A football pitch will be formed following the existing ground levels and it is intended to bund this area on

three sides and utilise the volume for storm water attenuation.

3.0 Drainage General.

The below ground drainage will comprise separate foul and storm water systems. The systems will be

designed in accordance with BS EN 752, The Building Regulations section H and Environment agency

Planning Policy Statement PPS25. Foul water flows will be calculated in accordance with BS EN 12056

part 3.

The Requirement H3 of the Building Regulations 2000 has been considered for this project. This

establishes a preferred hierarchy for surface water disposal. Consideration should firstly be given to

discharge to a soakaway infiltration system and secondly, to a watercourse in that priority order.

Test pits have been extended down to the Taplow gravels underlying the site. These pits were used to

perform Soil infiltration tests in accordance with BRE Digest 365. The results of these test are attached

within appendix B and C of this report. The soakage coefficient’s obtained reflect a failure which will

preclude the use of conventional soakaways and off-site disposal is likely to be required.

There are no natural water course’s adjacent to the site that could be used for storm water disposal.

This leaves the final option of discharging to the local sewer network.

Thames Water are currently investigating what capacity is available within their foul sewers adjacent to

the site. Discussions are also underway with the London Borough Of Hounslow with respect to

connection of the surface water discharge. In principle, we expect to discharge to the sewer network

within Syon Lane, although we are still awaiting confirmation of this.


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The entire drainage network will be sized and levelled using the industry accepted Micro Drainage

software suite.

4.0 Storm Water Drainage.

In accordance with current practice the storm water drainage system will be designed to prevent

flooding from a 1 in 30 year storm event. The design will then be further interrogated against a 1 in 100

year storm event + 30% for future climate change. This will be performed to ensure that any flood water

from this storm event will be contained within the boundaries of the development. Ground levels will be

set to ensure that a safe exit exists from the building during this 100 year event.

The retained football pitch has a slope of approximately 850mm across its diagonal. It is therefore

proposed to bund the low corner of this pitch utilise the volume for storm water attenuation and flood

alleviation measures. This will be achieved by providing a gravel filled trench at the base of this bund.

This trench will have a piped connection to the storm water drainage system. The top level (ground

level) of the trench will be the lowest ground level connected to the storm drainage network. The

network will be provided with a vortex flow control device located immediately downstream of the gravel

filled trench. At times of very high flow, the water that is restricted by the flow control will back up the

system. When this water reaches the low trench ground level, the water will flood onto the football field

and be contained by the bund. Once the storm has rescinded, the water will continue to discharge off

site at the controlled rate and the water level within the network will drop, this will allow the stored water

on the pitch to re-enter the drainage network and discharge off site. The gravel filled trench will act as a

filter and prevent any silts / debris entering the drainage network.

In accordance with Environment Agency Planning Policy Guidance 3 (PPG3) car parks with more than

50 spaces or 800m² will require a petrol interceptors. We are proposing that the car park is provided

with a separate drainage system. The parking bays will have porous surfaces over a layer of

approximately 400mm deep granular fill containing a minimum of 30% voids ratio. These voids provide

storm water attenuation. Water will be routed through the attenuation layer by a series of perforated

pipes. The system will operate similar to the main network whereby water will back up from a flow

control device and flood / drain the voids via the perforated pipe. The complete granular fill and all

perforated pipes will be wrapped with Inbitex Geotextile fabric. This is a polypropylene and polyethylene

non-woven fabric. During the manufacturing process, small dishes are created on the surface of the

hairs from which the geotextile is manufactured. This allows an early establishment of a microbial

biomass which is responsible for eating and degrading oils in the water. It is also important during dry

periods when the microbial population can retreat into the dishes where moisture is to be found. Thus

when it rains again the microbial biomass is quickly re-established. This therefore negates the need for

traditional petrol interceptors.

We believe all of the above is in accordance current SuDS legislation and the requirements of the

“Technical Guidance to the National Planning Policy Framework”.

Our Storm Water Drainage strategy is attached under Appendix D of this report.

5.0 Foul Water Drainage.

Thames Water Asset records indicate that there are public foul water sewers adjacent to the site within

Syon Lane. A Section 106 consent for a new foul water connection to this sewer will be made once

Thames Water have confirmed that it has adequate capacity.

The new school facility will be provided with a gravity foul water drainage system which will collect all

foul water discharges generated by the new development. The system will convey the sewage to a

below ground foul water pumping chamber. This chamber will contain duty and standby sewage pumps


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Job number 4467.

which will pump the effluent to the sites Syon Lane boundary. The effluent will discharge to a new

manhole constructed with the site boundary. From this manhole the effluent will discharge by gravity to

the Thames Water sewer network.

Access Chambers and rodding points will be provided to allow future maintenance of the system.

Adequate ventilation pipes and trapped gullies will be provided to prevent the build-up of foul smells in

or around the building.

The buildings final sanitary ware provision is awaited before foul water drainage calculations can be produced. In an effort to reduce water usage, wash hand basins will be provided with push button spray taps whilst WC’s will have 6 – 3 litre duel flush mechanisms. Our Foul Water Drainage strategy is attached under Appendix E of this report.

6.0 Summary. Storm Water attenuation will be provided within the granular base below roadway and car parking areas. The water will enter this attenuation medium via porous surfaces. This system improves the quality of water discharged off site and negates the need for petrol interceptors. The same method of storm water attenuation will also be provided to the multi-use games area, (MUGA). Again offering an improvement to the quality of the water being discharged. Storm water run-off from the building’s roof will be stored at ground level using the natural contours of the ground and a bund to manage the temporary storage of the storm run-off. This will omit the need for large underground attenuation tanks, excavations, pumping etc. Low flow water fittings will be specified to reduce water consumption and foul water discharge volumes.


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BAM Design 7


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Appendix A: Preliminary storm water attenuation for differing storm events and permitted discharge

rates.


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Appendix B: Soil Infiltration Test Results.


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BAM Design 12


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Appendix C: Analysis of Infiltration test results.


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Appendix D: Storm Water Drainage Strategy.


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Appendix E: Foul Water Drainage Strategy.

Below Ground Drainage, Statement, Description and...

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