technical notes - drainage

V1.0 GCE R/T

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86 Chevening Road London NW6 6EA Technical Notes- Drainage for LRS Group J002086 November 2021

www.greence.co.uk

86 Chevening Road Technical Notes- Drainage

for

LRS Group

If you require any additional information relating to this document, please contact: Anthony Cabria Green Consulting Engineers Unit 21 Bergham Mews Blythe Road Hammersmith W14 0HN United Kingdom T: +44 (0) 203 405 3120 E: [email protected]

Revision Date of issue Notes Compiled By Checked By 0 02/11/21 Preliminary Issue AC GG

Technical Notes- Drainage 86 Chevening Road J002086 www.greence.co.uk

Contents Contents .......................................................................................................................................................... 1 1. Introduction ............................................................................................................................................ 1 2. Site Description ....................................................................................................................................... 3 4. Proposed Development ........................................................................................................................... 4 5. Surface and Foul Water Drainage Proposals ............................................................................................ 5

Appendices A Site Location Plan B Environment Agency Flood Mapping C Greenfield Runoff Rates/ Qbar D Proposed Development Masterplan E Surface and Foul Water Drainage Plan F Waste Water Design Flow Calculations G Thames Water Pre-Development Enquiry


1. Introduction 1.1 This Technical Report (TN) has been produced by Green Consulting Engineers (GCE) on behalf of

the LRS Group to provide the technical documentation associated with the below-ground drainage (BGD) design and specifications for the proposed development at 86 Chevening Road, London NW6 6EA.

1.2 The site is referred to as 86 Chevening Road. Refer to Appendix A for the site location plan.

1.3 The BGD design was prepared with reference to the drainage CCTV surveys carried out for the proposed development undertaken by Quick Clear Drainage Ltd on 03 August 2021.

1.4 This TN has been prepared in accordance with the requirements of the National Planning Policy Framework (NPPF) and Planning Practice Guidance (PPG) with due regard to other relevant flood risk legislation and policy, drainage design standards and technical specifications, as well as the London Plan and the SuDS Hierarchy, the London Borough of Brent (LBB) Flood Risk Management Strategy (LBB-FRMS), and the West London Strategic Flood Risk Assessment (WLSFRA) to demonstrate that the site can be drained in an acceptable and sustainable manner.

1.5 A Basement Impact Assessment (BIA) may be separately required by the LPA/ LLFA as part of its planning validation checklist. The BIA does not form part of this TN. It is recommended that a pre-planning consultation is undertaken prior to the formal planning application in order to determine if a BIA is required and ensure greater possibility of success through the planning process.

Legislative and Planning Policy Background 1.6 GCE’s assessment of potential flood impacts and their significance, as well as the basis of the

surface and foul water design as detailed in this report are based on available baseline information references and design standards as listed below, and take into account the proposed land use, mitigation measures and design features.

1.7 The following sources of information were utilised in establishing the baseline conditions and preparing this report: ► National Planning Policy Framework (Updated February 2019);

► Flood and Water Management Act 2010 (DEFRA, 2010);

► West London Strategic Flood Risk Assessment;

► London Borough of Brent (LBB) Local Plan;

► London Borough of Brent Surface Water Management Plan (2011);

► London Borough of Brent (LBB) Flood Risk Management Strategy (Sept 2015);

► Brent Basement Supplementary Planning Document (2017);

► Planning Practice Guidance (November 2016);

► Environment Agency (EA) website www.environment-agency.gov.uk , EA Product 4 flood data, EA Pre-Development Enquiry;


► Interim Code of Practice for Sustainable Drainage Systems (DEFRA National SuDS Working Group, 2004);

► British Geological Survey website www.bgs.ac.uk;

► Proposed Site Layout (BL Architecture) Drawing P100/01 dated 25 July 2021; and

► Thames Water (TWUL) Pre-Development Enquiry;

Design Standards 1.8 The standards used for drainage design are as follows:

► BS EN 752:2008 Design of Drain and Sewer Systems Outside Buildings- Sewer Systems Management (2019) ► BS EN 16933-2:2017 Design of Drain and Sewer Systems Outside Buildings- Hydraulic

Design (2018)

► BS EN 124:1994 Gully Tops and Manhole Tops for Vehicular and Pedestrian Areas

► BS EN 12056-4: 2000 Gravity drainage systems inside buildings- Wastewater lifting plants - Layout and calculation

► BS EN 13476 Plastic piping systems for non-pressure underground drainage and Sewerage

► BS EN 1433: 2002 Drainage channels for vehicular and pedestrian areas

► BS EN 1253-1 Gullies for Buildings

► BS EN 13564-1 (2003) Anti-Flooding Devices for Buildings

► BS EN 13598 Plastic Piping System for Non-Pressure Underground Drainage and Sewerage

► BS EN 7158: 2001 Plastic Inspection Chambers for Roads and Sewers- Specifications

► BS EN 295-1 Vitrified Clay Pipes and Fittings and Pipe Joints for Drains and Sewers

► SfA8 (2019) Sewers for Adoption 8th edition

► C753 The SUDS Manual

► Building Regulations- Part H1

► The Wallingford Procedure Volume 4 – The Modified Rational Method

► NHBC Standards 2020


2. Site Description

Site Location and Description 2.1 The site is located along Chevening Road in Kensale Rise, at no. 86, off the B450 Chamberlayne

Road and B451 Harvist Road, with the centre of the site located at grid reference TQ 23503 83012. 2.2 The site is bounded to the north by Chevening Road, to the east by Milman Road and Queen’s

Park, to the west by Peploe Road, and to the south by Creighton Road . 2.3 A Site Location Plan (SLP) is appended on Appendix A.

2.4 The proposed site totals approximately 273m² (0.0273Ha) of total area and is a mix of:

Approximately 118 m² Roof area;

Approximately 92m² of landscaping/ vegetated areas – including grass, trees, and bushes; and

Approximately 55m² of external hard standing – including driveways, footpaths, and hardstanding.


3. Proposed Development 3.1 The proposed development consists of the renovations/ conversion of the existing property from

a duplex flat into a single residential dwelling house and the addition of a basement. The development proposals can be seen in Appendix D.

3.2 The proposals also include works to the external surfaces and hardstanding areas, and green landscaping.


4. Drainage Proposals 4.1 The proposed drainage network is primarily driven by the developer’s aspiration to re-use the

existing connection to the public sewer and utilising existing drainage pipe runs (lateral drain) and nodes (manholes/ chambers) if their condition and capacity warrants its re-use.

4.2 The proposed private drainage network is for a separate foul and surface water drains within the curtilage of the site and converges into a combined run (lateral drain) via the existing demarcation chamber at the proposed development before discharging to the Thames Water Utilities Ltd (TWUL) combined public sewer main along Chevening Road. The existing connections to the TWUL sewer main will be reused if suitable and in satisfactory condition.

4.3 Surface and foul water discharges from the ground floor level and above of the building will drain by gravity into the combined public sewer via the existing lateral drain and demarcation chamber. However, surface and ground water discharges from the basement will drain by pumping into the combined sewer via the existing demarcation chamber and through pressurised dual pump rising mains. Likewise, foul water discharges from appliances/ fixtures located at the basement areas will also drain by pumping into the combined sewer via the existing demarcation chamber and through pressurised foul pump rising mains. The proposed rising mains for both pump stations will converge/ discharge into an existing intermediate inspection chamber that is connected upstream of the existing demarcation chamber.

4.4 An anti-flood loop system that conforms to BS EN 12056:4 has been integrated on the drainage design proposal to prevent backflows/ protect the basement habitable or storage areas, and the pump stations from sewer flooding. The system makes discharge riser pipes being brought to a higher level above the anticipated high flood level (1% AEP) on site then it loops back into a down pipe (forming an inverted U-bend) before it runs horizontal to connect into the demarcation chamber. This will also help reduce the discharge capacity from the pump stations at the basement by limiting the pump actual discharge rate because of the higher discharge head offered by the vertically looped rising mains. Portable anti-backflow valves and foul-air traps are also proposed to be fitted at suitable locations on the below-ground drainage network.

4.5 Using the UKSUDS tool, the Greenfield Runoff Rates (GRR) for the site has also been calculated (for comparison purposes) with Qbar 0.46 li/s whilst the GRR derived for the 1 in 1-year, 30-year, and 100-year storm events are 0.39 li/s, 1.06 li/s, and 1.46 li/s, respectively. The calculations are appended on Appendix C.

Surface and Ground Water Discharge Strategy

4.6 The proposed development is located on a brownfield site of area approximately 0.0273 hectares. The proposed development impermeable areas (roof) will increase by around 8 sq.m. if compared to the roof area of the existing building footprints. The estimated existing surface water runoff discharge from the site to the TWUL combined sewer is approximately 3.20 li/s for a 50mm/ hour storm (this is generally the accepted suitable rainfall design intensity for sizing surface water drains in accordance with the Building Regulations Approved Document Part H).


4.7 The above surface water (SW) runoff coming from the ground floor level and above will discharge via gravity drains and will connect into the existing demarcation chamber. The additional 8 sq.m. impermeable area will generate roughly 0.14 li/s of extra flows, or a total discharge rate of 3.34 li/s.

4.8 The ground investigation report (by Hertz and Essex SI, July 2021) has revealed that the site sits on clay superficial geology deposits and underlain by the London Clay bedrock formation. Ground water (GW) was not encountered during the drilling of the 10-m deep borehole thus it is assumed that the basement will sit above the groundwater table.

4.9 Ground water ingress is therefore assumed at a rate of 0.001 li/s/sq.m. of basement slab and walls. A factor of safety of 5 was assumed for design purposes. The ground water flow rate is not expected to exceed 1.0 li/s.

4.10 The lightwell areas are not expected to exceed 12 sq.m. If this is exceeded, an additional dedicated surface water pump station will be required.

4.11 It is assumed that a proprietary cavity membrane waterproofing system will be used at the basement/ subterranean development in conjunction with the surface/ground water pump station.

4.12 The pumped surface and ground water discharge rate from the proposed basement is 1.65 li/s.

4.13 The pump station design has assumed a 5-minute storm event and a 24-hour storage requirement. A 500-year (0.20% AEP) return period is assumed for basement applications. Backup battery for pumps can be alternatively used instead of physical storage.

4.14 The proposed GW/ SW pump station specifications is:

Make and Model: Delta Dual V3 Sump Pump Operating Duty: 1.65 li/s at 4.5m head Pump Type: Submersible vortex Min. Storage Capacity: 0.304 cu.m. Pump Chamber Size: 660mm dia. x 655mm deep Access Hatch Size: 310mm x 310mm opening (hatch cover by others) RC Box Size: 1.2m x 1.2m (refer to the Structural Plans/ RC Detail for the depth/details)

4.15 Anti-backflow valves may be fitted within the private drainage network in addition to the anti-flood loop system, if desired.

4.16 Gullies located within light well areas at the basement level and those exposed externally at ground floor level to be of anti-flooding type. Foul air traps (FAT) may be optionally fitted, if desired.

4.17 The surface and ground water drainage plan is shown on Appendix E.


Foul Water Drainage Discharge Strategy

4.18 The foul water (FW) runoff coming from the ground floor level and above will discharge via gravity drains and will connect into the existing demarcation chamber. The foul/ waste water flows from the proposed development (except the basement area) have been calculated according to the Sewers for Adoption (SfA) 8th edition methodology as follows: ► 4000 litres per day x 1 Dwellings = 4000 l/day

► 4000 litres per day / 24 Hours = 167 l/hour

► 167l/hour / 60 minutes = 2.8 l/min

► 2.8 l/min / 60 seconds = 0.463 l/s.

4.19 The pumped foul/waste water peak discharge from the proposed basement area is estimated at 1.14 li/s (Refer to Appendix F). This was calculated to BS EN 12056:2 and BS EN 752 using the Wastewater Design Flow Calculator to derive the theoretical peak design wastewater flow.

4.20 The proposed FW pump station specifications is: Make and Model: Delta Foul V3 Sump Pump D10SA Operating Duty: 4.5 li/s at 5m head Pump Type: Submersible vortex Min. Storage Capacity: 0.410 cu.m. Pump Chamber Size: 660mm dia. x 800mm deep Access Hatch Size: 310mm x 310mm opening (hatch cover by others) RC Box Size: 1.2m x 1.2m x 800mm deep (refer to the Structural Plans/ RC Detail for the details)

4.21 The FW pump station is not designed to collect ground and surface water from a cavity

membrane system. 4.22 Therefore, the estimated peak foul flow discharge from the site is approximately 1.603 l/s. A

pre-development enquiry has been sent to Thames Water (TWU), and TWU have confirmed that there is sufficient capacity within the network to accommodate these calculated flows. This response can be seen in Appendix G.


APPENDIX A – SITE LOCATION


APPENDIX B – ENVIRONMENT AGENCY FLOODING MAP


APPENDIX C – GREENFIELD RUN-OFF RATE/ Qbar RESULTS


APPENDIX D – PROPOSED DEVELOPMENT MASTERPLAN


APPENDIX E – SURFACE AND FOUL WATER DRAINAGE PLAN


APPENDIX F – WASTE WATER DESIGN FLOW CALCULATIONS


APPENDIX G – THAMES WATER PRE-DEVELOPMENT ENQUIRY

Thames Water Utilities Limited – Registered Office: Clearwater Court, Vastern Road, Reading RG1 8DB

Company number 02366661. VAT registration no GB 537-4569-15

Mr Anthony Cabria Green Consulting Engineers Ltd Unit 21 Bergham Mews Blythe Road Hammersmith W14 0HN

13 October 2021

Pre-planning enquiry: Confirmation of sufficient capacity

Site Address: 86 Chevening Road, London, NW6 6EA

Dear Anthony,

Thank you for providing information on your development.

Proposed site: Two flats conversion to one house and drainage is like for like except depaved areas. The foul and seepage water from basement will be pumped into the existing demarcation chamber, then drains by gravity through existing lateral to the combined sewer. Foul and surface water discharges from the ground floor level and above will drain by gravity into the existing combined public sewer. Foul pumped at 2l/s, and total surface water at 1.65l/s. We have completed the assessment of the foul water flows and surface water run-off based on

the information submitted in your application with the purpose of assessing sewerage capacity

within the existing Thames Water sewer network.

Foul Water

If your proposals progress in line with the details you’ve provided, we’re pleased to confirm that

there will be sufficient sewerage capacity in the adjacent combined sewer network to serve your

development.

This confirmation is valid for 12 months or for the life of any planning approval that this

information is used to support, to a maximum of three years.

You’ll need to keep us informed of any changes to your design – for example, an increase

in the number or density of homes. Such changes could mean there is no longer

sufficient capacity.

Surface Water When developing a site, policy 5.13 of the London Plan and Policy 3.4 of the Supplementary

Planning Guidance (Sustainable Design And Construction) states that every attempt should be

made to use flow attenuation and SuDS/Storage to reduce the surface water discharge from the

site as much as possible.

In accordance with the Building Act 2000 Clause H3.3, positive connection of surface water to a

public sewer will only be consented when it can be demonstrated that the hierarchy of disposal

methods have been examined and proven to be impracticable. Before we can consider your

DS6087771

surface water needs, you’ll need written approval from the lead local flood authority that you

have followed the sequential approach to the disposal of surface water and considered all

practical means.

The disposal hierarchy being:

1) rainwater use as a resource (for example rainwater harvesting, blue roofs for irrigation)

2) rainwater infiltration to ground at or close to source

3) rainwater attenuation in green infrastructure features for gradual release (for example

green roofs, rain gardens)

4) rainwater discharge direct to a watercourse (unless not appropriate)

5) controlled rainwater discharge to a surface water sewer or drain

6) controlled rainwater discharge to a combined sewer.

Where connection to the public sewerage network is required to manage surface water flows we

will accept these flows at a discharge rate in line with CIRIA’s best practice guide on SuDS or

that stated within the sites planning approval.

If the above surface water hierarchy has been followed and if the flows are restricted to a total of

1.65 l/s then Thames Water would not have any objections to the proposal.

Please see the attached ‘Planning your wastewater’ leaflet for additional information.

What happens next?

Please make sure you submit your connection application, giving us at least 21 days’ notice of

the date you wish to make your new connection/s.

If you’ve any further questions, please contact me on the number below

Yours sincerely

Kind Regards

Natalya Collins

Developer Services – Adoptions Engineer Mobile: 07747 641 932

Clearwater Court, Vastern Road, Reading, RG1 8DB

Find us online at developers.thameswater.co.uk Get advice on making your sewer connection correctly at connectright.org.uk

https://developers.thameswater.co.uk/

http://www.connectright.org.uk/

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