PMG

Radyr Court RoadDanescourt

Drainage Strategy

June 2018

REPORT CONTROL

Document: Drainage Streategy

Project: Radyr Court Road, Drainage Strategy

Client: PMG

Job Number: 173674

File Origin: /tt/file_convert/5bb6f22409d3f2f06e8cb9b0/document.docx

Document Checking:

Primary Author Keith Mortimer Initialled: KM

Contributor Initialled:

Review By Leigh Fotiadis Initialled: LF

Issue Date Status Checked for Issue

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Contents

1 INTRODUCTION............................................................................................................1

2 EXISTING SITE...............................................................................................................2

Site Location................................................................................................................2Site Description..................................................................................................2Existing Drainage Network..................................................................................3River and Watercourses......................................................................................3

3 SURFACE WATER DRAINAGE STRATEGY........................................................................4

Discharge to a Watercourse.........................................................................................4Discharge to a Storm Sewer.........................................................................................4

4 PROPOSED FOUL WATER DRAINAGE STRATEGY............................................................5

Proposed Foul Flow......................................................................................................5Foul Calculations..........................................................................................................5Foul Water Drainage Strategy......................................................................................5

5 CONCLUSIONS AND RECOMMENDATIONS....................................................................6

Figures

Figure 1.1 - Site Location

Appendices

Appendix A - Illustrative MasterplanAppendix B - Welsh Water Maps and LetterAppendix C - NRW Indicative Flood Maps (including Q100 and Q1000 Flood Extent)

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1 INTRODUCTION

1.1 Vectos has been commissioned by PMG to undertake a High-Level Drainage Strategy for the

proposed development at Radyr Court Road, Danescourt.

1.2 The site is currently greenfield. Development proposals include the construction of circa 40

residential dwellings, all necessary highways, private drives, gardens and an area to be

transferred to Cardiff County Council to be used for informal open space.

1.3 This report considers the risk of flooding at the application site and assesses any impact to

the surrounding catchment resulting from proposed developments. The report also

considers viable methods for the disposal of both surface and foul water.

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2 EXISTING SITE

Site Location

2.1 The site is located within Danescourt, an outer suburb of western Cardiff. It is some 2.4km to

the south of Radyr and some 2.8km to the north of Fairwater. The site is also located approx.

5.3 km north of the centre of Cardiff.

2.2 The site is bound to the west by residential properties at Danescourt, to the north and east

by the River Taff, Taff Trail, railway line and green land and to the south by further residential

properties and open green land.

2.3 Figure 1.1 indicates the site location in a local context.

Figure 1.1 – Site Location

Site Description

2.4 The site is currently Greenfield. The topographical survey shows that the site falls from north

to south with levels ranging from approximately 34m AOD to the south and 24m AOD to the

north.

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Existing Drainage Network

2.5 The Welsh Water sewer Maps confirm that there is 4 sewers flowing from north to south

within the site. These are grouped in two with a single foul and a single storm sewer forming

each group of sewers. The most southerly group of sewers consists of a 225 mm foul and a

450mm diameter storm. The second most northerly group of sewers consists of a 525mm

storm sewer and a 375mm foul. Both foul sewer runs lead into a common foul system south

of the development. Similarly, this also is the case for the surface water sewers. The

proposals would be to converge and combine the two foul and the two surface water sewers

under a diversion with DCWW. This will be done under a Section 185 of the water industry

act.

2.6 DCWW have provided a connection point for the foul sewers only. Refer to Appendix B.

River and Watercourses

2.7 The TAN 15 DAM flood map indicates that the River Taff is located along the north-western

boundary of the site. The site boundary is delineated by the railway to which this main river

runs parallel to. The River Taff is categorised as a main river by Natural Resources Wales and

the Welsh Government.

2.8 The site is not affected by fluvial and/or tidal flooding and therefore is considered as flood

free from these sources of flooding. The site is considered to have been flooded in the past

but only from surface water flooding. The TAN 15 categorisation of this site is zone B areas

to be known to have flooded in the past. However, the development proposals are

considered acceptable under TAN 15. Any surface water flooding will be managed at detail

design and will be catered for by the drainage proposals. These proposals might not alleviate

the issue fully but will provide a betterment in the general vicinity and will not pose a risk to

the new development proposals whilst also ensuring that the flood risk is not increased on

any of the neighbouring and existing developments.

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3 SURFACE WATER DRAINAGE STRATEGY

3.1 The site is currently greenfield. All flows generated from single or multiple dwellings would

drain naturally and will be managed at source. It is suggested in the Site Investigation report

undertaken by IG that suitable infiltration is achieved and depth. Therefore, in order to deal

with surface water at source it is proposed that soakaways are used.

3.2 Should these methods above not be achievable, a hierarchy would be followed and where

practical follow the discharge of surface water to the Watercourse and then lastly to a sewer.

3.3 Detailed soakaway testing should be carried out to determine the exact depth and locality of

these soakaways.

3.4 Soakaway testing should be undertaken in accordance with BRE365.

Discharge to a Watercourse

3.5 The river Taff flows from north to south along the eastern boundary of the application site.

3.6 There is an existing storm drain constructed leading towards the River Taff past the

application site, it is assumed that this storm sewer flows into the River Taff.

3.7 If it is found that this sewer does not connect into the River Taff or does not have the

capacity to accommodate the flows from the proposed development should this be required,

then a connection directly to the watercourse at the south east of the site should be used. It

is worth noting that due to the railway it is required to go through the pedestrian underpass.

Discharge to a Storm Sewer

3.8 As a last resort and if the two previously mentioned options are not feasible a connection to

the sewer will be considered. Any surface water flows that flow through this pipe will need

to be restricted to the Qbar rate of 5l/s via a hydrobrake or similar. Attenuation will then be

required to contain the volume for each rainfall events.

3.9 Discharge to this pipe and then into the DCWW sewer will require their permission.

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4 PROPOSED FOUL WATER DRAINAGE STRATEGY

Proposed Foul Flow

4.1 The proposed development will comprise 40 new residential dwellings. Based upon Sewers

for Adoption 7th Edition, the foul flow has been calculated as 1.85 l/s.

Foul Calculations

4.2 Proposed Foul flow calculations are based on 4000 litres per dwelling per day, which is then

divided by 86400 (24 hours x 60 minutes x 60 seconds) to give a foul flow rate in litres per

second.

Foul Water Drainage Strategy

4.3 A foul only existing network is located within site. DCWW have confirmed connection for foul

only flows to this sewer. Therefore, it is accepted that capacity is available to receive this

amount of foul flow generated by the new development. The proposed development will

utilise a newly built gravity sewer which will discharge to a newly constructed foul pumping

station. This pumping station will then discharge the flows via a newly built rising main to

the connection point given by DCWW in their correspondence.

4.4 Sewers for Adoption 7th Edition confirms that storage for foul pump stations must be

160l/day. Based on this requirement, the pump station requires circa 9.6m3 of storage,

which includes 50% extra storage for emergency purposes should there be a failure of the

pumps.

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5 CONCLUSIONS AND RECOMMENDATIONS

5.1 This report concludes that the proposed development is at no risk of fluvial or tidal flooding.

5.2 Soakaways will be utilised and if this is not proven viable then a watercourse connection and

a sewer connection will be sought in that order.

5.3 Foul water connection to the existing sewer has been granted by DCWW. This will allow for

the pumping station flows to be connected to it at the given location.

5.4 A diversion of the existing sewer will be proposed at detailed design to combine the two foul

and the surface water sewers. This will be done under a S185 agreement.

IMPORTANT SECTION BREAK NEXT – DO NOT DELETE PAST THIS CHEVRON >

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APPENDIX A

APPENDIX B

APPENDIX C