Drainage Statement

8/7/2019 Drainage Statement

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FINAL

DRAINAGEASSESSMENT

for

LAND TO THE WEST OFPUFFIN ROAD,

HERNE BAY

on behalf of

KITEWOOD ESTATESLTD

OH/JSM/D/TRN1027911 April 2011

Regent House, Hubert Road, Brentwood, Essex CM14 4JE

t 01277 238 100 f 01277 238 150 e [email protected]

Belfast - Brentwood - Bristol Cardiff - Dublin - Edinburgh - Glasgow - Leeds Manchester - Nottingham

Registered in England Number 4100666 Registered Office: Pickfords Wharf, Clink Street, London SE1 9DG


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Final - Drainage Assessment Waterman Boreham LtdLand at the West of Puffin Road, Herne BayD/TRN1027911 April 2011

Contents

Page

Executive Summary 41. Introduction 52. Scope of the Assessment 63. Existing Situation 74. Topographical Survey and Southern Water Records 95. Proposed Development 116. Surface Water Drainage 127. Foul Drainage 148. Conclusion 15

Appendices

A Location of Site Photographs Drawing TRN10279_design01

B Photographs

C Southern Water Records

D Southern Water Correspondence

E Outline Development Plan

F As Built Drainage Drawing for Puffin Road Drawing 57400-1683-2000

G Proposed Drainage Strategy Drawing TRN10279_design02

H Microdrainage Report


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EXECUTIVE SUMMARY

Surface Water

There is no surplus capacity within the drains in Rosebery Avenue,

Rowland Crescent and Richmond Drive to the north

Puffin Road surface water discharges to the adjoining ditches which are

within the control of the applicant

Therefore, the use of the existing surface water drains in Puffin Road is

the preferred method of disposal subject to on site water attenuation

(approximately 250m3) under the 2 main car parks.

Foul Water

It is proposed to discharge to existing foul water drain in Puffin Road,

which are yet to be adopted, and which a right of connection has been

agreed.



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1. INTRODUCTION

1.1 Waterman Boreham (WB) has been instructed by Kitewood Estates Ltd to carry out a surface and

foul drainage assessment for a proposed housing development of 40 dwellings at land to the

West of Puffin Road, Herne Bay, CT6 6RT.

1.2 The housing development is proposed to be built on an existing agricultural field, which is located

on the east of Herne Bay, immediately to the west of Puffin Road and to the south of Richmond

Drive.

1.3 The area of the site to be developed is approximately 1.091 hectare. Its boundaries are formed by

an existing housing development to the north and east; on the south, a natural boundary of

woodland and a ditch separate the site from an agricultural land.



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2. SCOPE OF THE ASSESSMENT

2.1 The scope of the assessment is to carry out a Surface Water & Foul Drainage Assessment for the

proposed site and include the following:

Visit the site.

Liaison with Southern Water and obtain their up to date Surface and Foul

Water records for the area.

Obtain a topographical survey and check the levels of the site and

determine where the proposed development is likely to drain to.

Determine the impermeable area for the proposed development.

Calculation of the existing greenfield runoff rate (rate for undeveloped

land) which will determine the limited discharge rate of water to be

discharged to Southern Water sewers or local ditch.

Carry out some surface and foul drainage calculation based on

impermeable areas and number of dwellings.

Propose and discuss a mean of storage for surface water.

Propose and discussed discharge points for both foul and surface water.



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3. EXISTING SITUATION

3.1 Waterman Boreham attended site on the 8th March 2011 and walked the entire perimeter of the

site with good views of the land. Below is a summary of the observations made:

3.2 It was noted that although there had not been major rain over the past week the ground condition

was generally saturated in water with two areas on the east side of the site being flooded.

3.3 Although weather was sunny and dry on the day of visit it was noticed a smell of stagnant water

on the approach of the site.

3.4 The entire area of the site was planted with the exception of the two eastern corners; crops were

approximately 10cm high.

3.5 The site can be physically accessed from three existing dead end roads: Rosebery Avenue,

Rowland Crescent and Richmond Drive. In the future, the site could also be accessed from Puffin

Road assuming that the natural hedge fence is cut down and metal mesh and posts removed.

3.6 There is however no vehicular access at present from any of these 3 dead end roads. The farmer

accesses the site through the fields on the southern side.

3.7 At the end of these roads, some utility chambers are present as well as foul and surface drainage

manholes. These manholes appear to be the top of runs for both foul and surface drainage.

3.8 From a general view, the site is falling slowly toward the south at a gradient of 1 in 135 and more

generally to the southeast. On the south east corner the ground tends to fall quickly over a

distance of approximately 15 meters at a gradient of 1 in 50 and this is where a considerable

volume of water is trapped (approximately 300mm deep). The same situation applies for the

northeast corner of the site but this is not as severe.

3.9 A wide hedge strip is located along the south part of the side.

3.10 Immediately on the south of the hedge, a shallow ditch runs toward the east where it discharges

into another ditch heading south.

3.11 Although it cannot be confirmed, because of the amount of vegetation, it is possible that the

proposed site was originally linked to these ditches through the hedge. This blocked link would

explain why water is ponding in the southeast corner of the site.



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3.12 A plan and photographs illustrating the above can be found in Appendix A and Appendix B.



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4. TOPOGRAPHICAL SURVEY AND SOUTHERN WATER RECORDS

4.1 The levels of the existing site on the topographical survey confirm that the site falls toward the

south and more generally toward the southeast corner.

4.2 The OS map seems to indicate by the word DRAIN that an existing ditch is present along the

south boundary of the land.

4.3 The most obvious outfall for the proposed foul and surface drainage will be to connect to one or

all of the 3 existing roads around the proposed site (Rosebery Avenue, Rowland Crescent,

Richmond Drive and Puffin Road).

4.4 Some drainage manhole information is shown on the topographical survey for most of these

roads, unfortunately, they are very shallow manholes.

4.5 Southern Water records received in February 2011 (Appendix C) show the following information

for the 4 following roads:

Rosebery Avenue:

Surface water manhole 3950: Cover Level (CL): 35.772

Invert Level (IL): 35.002

Depth: 0.77m

Foul water manhole 3901: Cover Level (CL): 35.694


Depth: 1.25m

Rowland Crescent:



Depth: 0.72m



Depth: 1.05m



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Richmond Drive:



Depth: 1.17



Depth: 1.17

Puffin Road:

No information available. Southern Water Records indicate that sewers in this area are not

adopted.

4.6 As part of a previous planning application for this site, Southern Water advised in a letter dated

12/11/2003 (Appendix D) that there is not enough capacity within the adjacent sewerage system

to serve the proposed development as a single point flow. It is advised that the proposed foul flow

should be distributed, equally as far as possible, between manhole references 3901, 3904 and

4901.

4.7 With regards to the surface drainage, the same letter indicates that there is insufficient capacity

within the surface water sewers, downstream of the application site to accommodate the

proposed development.

4.8 Although this letter is dated 2003, it has been assumed that the situation remains unchanged.



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5. PROPOSED DEVELOPMENT

5.1 The proposed development consists of approximately 40 dwellings and the main vehicular access

will be from Puffin Road.

5.2 A masterplan no. 02-263-102 for the proposed development can be found in Appendix E.



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6. SURFACE WATER DRAINAGE

Impermeable areas

6.1 As stated previously, 40 dwellings are proposed to be constructed on a site of 1.091 hectare. This

is considered as a medium density development.

6.2 From the masterplan it has been determined that 5740m2

of the 1.091 hectare will be

impermeable.

6.3 This impermeable area of 5740m2

includes houses, driveways, carriageways and footways. The

rest is considered as permeable and will therefore drain naturally.

Greenfield runoff rate

6.4 In line with current drainage policies and legislation, the Environmental Agency (EA) will require

that the outflow from the proposed site does not exceed the outflow generated by the existing

agricultural/undeveloped site (greenfield runoff rate).

6.5 In order to achieve the above, surface water will have to be stored on site and discharged at a

limited rate to a local sewer or ditch. Some negotiation with the relevant Water Authority will have

to be undertaken to agree the discharge rate and location of discharge.

6.6 With the existing site being agricultural land and fully permeable, a greenfield runoff (IH124) has

been calculated using microdrainage software.

6.7 The Microdrainage result for a greenfield runoff for an area of 5740m2

is 2.4 litres per second (l/s).

6.8 However the code for sustainable homes, technical guidance Note 001 issued 16 December 2009

states that Where this calculation results in discharge flow rates of less than 5l/s this rate may be

increased up to a level of no more than 5l/s at the point of discharge from the site to reduce the

risk of blockage.

6.9 It is therefore assumed that the discharge for the proposed site is increased to 5l/s.

Storage Calculation

6.10 To achieve this limited discharge rate some sort of storage will have to be provided. The EA will

usually require that water is infiltrated into the ground if possible by the means of soakaways

(soakage test will have to be carried out) or stored on site by the means of balancing ponds or

underground tanks.



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6.11 At this stage, the ground infiltration option has not been considered. Some soakage tests will

have to be undertaken to demonstrate that water can be infiltrated into the ground.

6.12 Utilising Microdrainage software, the amount of storage required for this site is calculated

between 274m3

and 376m3

, giving an average 325m3

. This volume of 325m3

will be used as aguide when a drainage strategy for the site is proposed.

Drainage strategy

6.13 As stated previously, Southern Water indicates in their letter that they have no capacity in their

sewers on Rosebery Avenue, Rowland Crescent and Richmond Drive. Moreover, with the depth

of their manholes being very shallow and the proposed site falling toward the south east, it would

be almost impossible to connect to their manholes without major earthworks taking place and

levels of the site being raised locally.

6.14 It will possible to attenuate the surface water from the proposed development to 5l/s and

discharge into Puffin Roads sewers which currently discharge water into the drainage ditches

adjoining the site. It is understood that the connection and discharge rate to Puffin Road surface

water sewer have been agreed by the developer for the scheme. An as built drainage drawing is

attached to the report, refer to Appendix F.

6.15 As surface water from Puffin Road discharges into the adjoining drain, the simplest option is to

connect the drainage network for the proposed development to surface water manhole SWMH1 tothe far east of Puffin Road refer to drawing TRN10279_design02 in Appendix G.

6.16 It is therefore proposed to store the surface water under the 2 main car parks of the proposed

development using 2 cellular storage systems, cellular storage 01 and 02. These 2 storage

systems will be linked to a pipe network located under the main road down to Puffin Road. To limit

the outflow to Puffin Road surface water sewer to 5l/s, a hydrobrake will be installed in manhole

MH7. A proposed drainage layout, (drawing TRN10279_design02), is shown in Appendix G.

6.17 A microdrainage model has been undertaken and simulated for a 100 year storm return period

and 30% increase for climate change.

6.18 The sizes for cellular storage 01 and cellular storage 02 have been calculated at respectively

250m2

and 260m2

in area with a depth of 500mm giving a volume of storage of approximately

120m3 and 124m3. Levels and location for these storage systems are shown on drawing

TRN10279_design02 (Appendix G).

6.19 A microdrainage report for the drainage model can be found in Appendix H.



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7. FOUL DRAINAGE

7.1 With regards to foul drainage, Sewer for Adoption 6th

Edition, 2006 states that the design flows for

gravity sewers for residential developments should be 4000 litres / unit dwelling / per 24 hours.

7.2 In line with the above, the foul flow generated from the proposed housing development will be:

40 dwellings x 4000 l / day = 160,000 l / day

= 1.86 l/s

7.3 Similarly, to the surface drainage, an application will have to be made to the Local Water Authority

Southern Water in order to connect and discharge into their sewers.

7.4 To equally drain the foul water to Rosebery Avenue, Rowland Crescent and Richmond Drive,

level difference will be an issue and foul water will have to be pumped. This represents additional

costs and ongoing maintenance liability.

7.5 It might be possible to discharge into Puffin Road foul sewer under gravity. Indeed Southern

Water records do not show any information for this area as the sewers in this area have not

been adopted pending the completion of a deed of consent for Discharge to the adjoining

drainage ditches which is within the control of the applicant.

7.6 In view of the total flows to the foul water treatment works at Sweetbridge Road, the additional

flows from 40 dwelling will be negligible.



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8. CONCLUSION

8.1 A drainage assessment for the proposed development has been carried out in order to determine

the volume of storage required for the surface water, the foul flow generated by the development

as well as the location of discharge for both foul and surface water drainage.

8.2 With regards to surface water, the preferred option is to store approximately 300m3

of water on

site. To achieve this, cellular storage systems have been proposed under the 2 main car parks. A

pipe network will drain the water toward the east of Puffin Road. At this location, a hydrobrake

manhole will limit the flow from the proposed site at 5 l/s.

8.3 As far the foul drainage is concerned, the preferred option is to drain the proposed site by gravity

and discharge into the existing foul sewer at Puffin Road. This option is believed to be the

cheapest and easiest and a right of connection has been agreed by the developer.


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APPENDICES


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APPENDIX A

Location of SitePhotographs Drawing

TRN10279_design01


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APPENDIX B

Photographs


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Photograph 01:

Photograph 02:


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Photograph 03:

Photograph 04:


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Photograph 05:

Photograph 06:


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Photograph 07:

Photograph 08:


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Photograph 09:

Photograph 10:


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Photograph 11:

Photograph 12:


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Photograph 13:

Documents

Drainage Statement