DRAINAGE IMPACT ASSESSMENT

Tridax Ltd Version 1.0

DRAINAGE IMPACT ASSESSMENT

for

Proposed Residential Development at

‘Old Rectory Court’ The High Street

Burwash Etchingham East Sussex

TN19 7EW

on behalf of

MH Architects


Job No. EMC-2017-129 2 July 2017

Document Control Sheet

Project Title Proposed Residential Development

Demolition and redevelopment of ‘Old Rectory Court’ to provide

residential accommodation with associated parking and refuse/bin

stores

Document Title Drainage Impact Assessment

Job No. EMC-2017-129

Revision 1.0

Status Draft – Issue to LPA

Issue Status Author Date Check Date Authorised Date

1.0 Draft S. Carr 19.07.17 P. Lavender 19.07.17

Distribution List

Version Issued to Purpose Date

1.0 MH Architects (via email) 1st Draft 19.07.17


Job No. EMC-2017-129 3 July 2017

CONTENTS

1.0 Status………….…………………………………….. 4

2.0 Introduction………………………….……………....

Background………………………………………….

Site Location…………………………………………

Existing Site Layout…………………………………

Proposed Development…………………………….

5

5

5

6

8

3.0 Foul Water Drainage……………………………......

Existing Discharge…...……………………………...

Proposed Discharge……………….……………….

Consents……………………………………………..

12

12

13

13

4.0 Surface Water Drainage……….………………......

Existing Discharge…...……………………………...

Proposed Discharge……………….……………….

Consents……………………………………………..

15

15

16

16

5.0

6.0

Flood Risk………………........................................

Conclusions…………………………………………

17

18

Appendix A Tridax Drawings EMC-2017-129-01 – Site Location EMC-2017-129-02 – Existing Site Drainage EMC-2017-129-03 – Outline Drainage Strategy

Appendix B Calculations Greenfield Runoff Quick Storage Calculation

Appendix C Correspondence


Job No. EMC-2017-129 4 July 2017

1.0 STATUS

1.1 This Report is prepared for the sole use of MH Architects and their agents in

connection with the proposed forthcoming planning application. No responsibility

can be assumed for the Report if used by others.

1.2 For the purposes of the Contracts (Rights of Third Parties) Act 1999, nothing in

this Report shall confer on any third party any right to enforce or benefit from any

term of this Report


Job No. EMC-2017-129 5 July 2017

2.0 INTRODUCTION

Background

2.1 Tridax Ltd have been commissioned by MH Architects on behalf of Amicus

Horizon (Optivo) and requested to prepare a Drainage Impact Assessment for

the proposed demolition and redevelopment of ‘Old Rectory Court’, High Street,

Burwash, Etchingham to provide residential accommodation consisting of 4 two

bed houses and 15 flats with associated car parking.

2.2 This Report is in accordance with the requirements of the Technical Guidance to

the National Planning Policy Framework (NPPF).

2.3 The Report details the observations, calculates the probable flows that may be

generated by the development and makes recommendations for the disposal of

the foul and surface water.

2.4 Discussions have previously been undertaken with the Local Authority Planning

department and a copy of correspondence received 11 December 2015 is

included in Appendix C. This report seeks to address the issues raised in respect

to the disposal of foul and surface water drainage.

Site Location

2.5 The development is located adjacent to the The High Street (A265), as indicated

on the extract as Frames 1 and 2 below. The total site area is approximately

2,480m² and is centred at Ordnance Survey reference (567521mE, 124700mN)

on sheet TQ675247. The site boundary is shown edged red.


Job No. EMC-2017-129 6 July 2017

Frame 1 – Site Location Plan Extract

Frame 2 – Existing Site Layout Plan

Existing Site Layout

2.6 The location of the proposed development is a ‘Brownfield’ site that is currently

occupied with residential buildings known as ‘Old Rectory Court’. Approximately

1190m2 of site is composed of impermeable surface finishes. The site is

currently laid out as twenty-three 1-bed flats. Extracts of the public sewer records

are shown as frame 3 below.


Job No. EMC-2017-129 7 July 2017

Frame 3 – Existing Site Drainage

2.7 Inspection of the sewer records indicates that there is a 175mmØ public foul

water located in the highway to the front of the existing property (High Street)

which is approximately 3m deep. There is also a 100mmØ public foul sewer

located to the rear of the site but no details are currently available. There are no

surface water sewers identified in close proximity to this site. There may be

other private drainage crossing this site that will require abandoning in order to

carry out the construction work. A survey will be required to confirm the current

on-site drainage arrangements. An application will be required under Section

106 of the Water Industry Act 1991 for the connecting the new developments

drainage to the public sewer.

2.8 Inspection of the British Geological Survey map for this area indicates the site to

be underlain with a bedrock geology of either Wadhurst Clay Formation,

consisting of Mudstone, or Tunbridge Wells Sand Formation, consisting of

Sandstone. A Ground investigation will be required on site to confirm the

geology.


Job No. EMC-2017-129 8 July 2017

Proposed Development

2.9 The proposed development comprises the demolition of the existing property,

‘Old Rectory Court’, and the construction of fifteen No 1-bed flats and four No 2-

bed houses with associated car parking. Refer to the Architects planning

drawings and the extract of the proposed site plan as Frames 4, 5, 6, 7, 8 and 9

below.

Frame 4 – Proposed Site Location Plan


Job No. EMC-2017-129 9 July 2017

Frame 5 – Proposed Ground Floor Plan - Housing Block

Frame 6 – Proposed First Floor Plan - Housing Block


Job No. EMC-2017-129 10 July 2017

Frame 7 – Proposed Lower Ground Floor Plan – Flats

Frame 8 – Proposed Upper Ground Floor Plan – Flats


Job No. EMC-2017-129 11 July 2017

Frame 9 – Proposed First Floor Plan – Flats


Job No. EMC-2017-129 12 July 2017

3.0 FOUL WATER DRAINAGE

Existing Discharge

3.1 ‘Old Rectory Court’ is currently laid out as eighteen 1-bed flats. Assuming each

flat has a kitchen sink, washing machine, bath, wc and washbasin a conservative

existing peak discharge in order of 6.4 litres/second could be expected

(Q=(0.7)kDU√(∑(DU)).

Peak Flow Rates to BS EN 752: Part 4

kDU frequency factor (0.7 for dwellings)

Sanitary No of DU ∑DU Appliance Features Existing WC's 1 x 18 1.20 21.6 Roselands Washbasin 1 x 18 0.30 5.4 Bath 1 x 18 0.80 14.4

Kitchen Sink 1 x 18 0.80 14.4

Washing Machine 1 x 18 1.50 27.0

(Q=(0.7)kDU√(∑(DU)) 82.8 Q= 6.4 l/s

3.2 The roof water from the existing buildings, hardstanding areas and path

(1190m²) are assumed to be disposed of via infiltration techniques within the site

such as soakaways although this is yet to be confirmed. In order to determine

potential impacts upon the public system it is considered that some or all of this

surface water runoff may actually have been combined with the sites foul

drainage system and connected to the foul water sewer. Enclosed within

Appendix B are the results from MicroDrainage Windes indicating the following

peak discharges;

1 year return ~ 2.1 l/s



3.3 The existing peak discharge to the foul water sewer is assumed to be

6.4litres/second which could rise to approximately 11.0 litres/second taking

into account a 30 year return period should it be confirmed that surface water

runoff combines with the foul drainage system from this site.


Job No. EMC-2017-129 13 July 2017

Proposed Discharge

3.4 The proposed peak foul water discharge from the residential development

calculated in accordance with BS EN 752: Part 4 is 4.9 litres/second, as per

the table below;

Peak Flow Rates to BS EN 752: Part 4

kDU frequency factor (0.5 for dwellings)

Sanitary No of DU ∑DU Appliance Features Block A WC's 2 x 4 1.20 9.6 4 No Houses Washbasin 2 x 4 0.30 2.4 Bath 1 x 4 0.80 3.2



Block B WC's 1 x 15 1.20 18.0

15 No Flats Washbasin 1 x 15 0.30 4.5

Showers 1 x 15 0.30 4.5



Q=kDU√(∑DU)

85.9 Q= 4.63 l/s

3.5 The design flow from the proposed development using ‘sewers for adoption’ 7th

Edition is calculated as 0.97 litres/second as below;

6DWF = 4,000 litres/dwelling/day x 19No Dwellings = 0.88 l/s

24hours

3.6 The replacement development will result in a reduction of the foul flow being

generated and discharged to the public foul drainage of 1.77 litres/second (6.4 -

4.63l/s). If it is determined that surface water does actually currently discharge to

the public system, then its potential disconnection from the foul water sewer will

create a spare peak discharge capacity of 4.6/s. An indicative foul water

drainage plan is shown on the drainage strategy drawing EMC-2017-129-03 is

enclosed within Appendix A.

Consents

3.7 A Section 106 Water Industry Act application to connect to the public foul water

sewer will be required to be made to and approved by Southern Water Services.


Job No. EMC-2017-129 14 July 2017

3.8 Southern Water’s letter of 6 November 2015 (Ref Plan-011498) states that there

is inadequate capacity available within the public foul drainage system to provide

additional foul sewage disposal service to a proposed development. As the new

development represents a reduction in foul flows entering the public foul system

this will increase available capacity within the public system and there is no

requirement to submit a formal application for a capacity check to Southern

Water Authority. However, Southern Water have requested that should an

application receive planning approval then the following condition is attached to

the consent: ‘Construction of the development shall not commence until details

of the proposed means of foul and surface water sewerage disposal have been

submitted to, and approved in writing by, the Local Planning Authority (LPA) in

consultation with Southern Water.’ A detailed foul and surface water drainage

design will need to be submitted to the LPA for approval to discharge such a

planning condition.


Job No. EMC-2017-129 15 July 2017

4.0 SURFACE WATER DRAINAGE

Existing Discharge

4.1 The requirement of NPPF is that the run-off from the development proposals

replicates the natural drainage characteristics of the pre-developed site. In the

case of ‘Brownfield’ development, the drainage proposal will be measured against

the existing performance of the site, although it is preferable for solutions to

provide characteristics similar to ‘Greenfield’ development wherever possible.

4.2 The existing site characteristics are summarised as below;

Total Site Area 0.248 hectares

Current Site Condition Brownfield

Existing Impermeable Area 1,190m²

Proposed Impermeable Area 1,430m²

Existing Surface Water Potentially 1190m² to surface water Discharge Method run-off to foul sewer TBC (Section 3.0)

4.3 Enclosed within Appendix B are the results from Micro Drainage Windes for the

existing surface water system of the existing site indicating the following peak

discharges;





Job No. EMC-2017-129 16 July 2017

Proposed Discharge

4.4 Considering the underlying geology of the site Tunbridge Wells Sand

Formation, it is possible that the use of on-site filtration may be appropriate. This

will be subject to completion of a Geotechnical Investigation which will need to

confirm the ground water table level and sub-strata details with ground infiltration

rates provided to enable the design of suitable infiltration techniques. If found to

be Wadhurst Clay Formation, it is highly unlikely that infiltration techniques will

be an acceptable method for surface water disposal. There are no public surface

water sewers identified in close proximity to this site. There are however two

public foul sewers adjacent the site which are only small bore pipes (175mm and

100mm Ø) so it is unlikely that these will have been designed to accept surface

water discharge although it may be that there is some surface water that is

connected without permission. It is unlikely that an alternative solution to

attenuated surface water storage system with controlled discharge will be

permitted by Southern Water to connect to the public foul water sewer. Due to the

lack of public surface water sewers at this location together with the high volume

of properties discharging surface water runoff it is assumed that infiltration

techniques do currently work at this location although this will require confirming

by further ground investigations as mentioned above.

4.5 An outline drainage strategy drawing EMC-2017-129-03 is enclosed within

Appendix A

Consents

4.6 A detailed foul and surface water drainage design will need to be submitted to the

LPA for approval to discharge any relevant planning condition prior to

commencing work on site. The responsibility of the management and

maintenance will need to meet the requirements of the Flood and Water

Management Act, however it is currently unclear when the Adopting SUDS

Authority will be empowered (East Sussex County Council). Until such time the

responsibility for any storage structure and/or flow control device will remain with

the property owners.


Job No. EMC-2017-129 17 July 2017

5.0 FLOOD RISK

5.1 Inspection of the Environment Agency Website identifies the proposed site to be

within the extent of the Flood Zone 1 (Low Probability). This zone comprises

land assessed as having a less than 1in1000 annual probability of river or sea

flooding (<0.1%). All uses of land are appropriate within this zone. Refer to

Frame 10 below.

Frame 10 - Extract of Environment Agency Flood Map

5.2 Reference to Table 1 of the NPPF states the policy aim to reduce the overall

level of flood risk by the appropriate application of sustainable drainage systems.

As the proposed site is below one hectare in size, no separate flood risk

assessment is required.


Job No. EMC-2017-129 18 July 2017

6.0 CONCLUSION

6.1 Tridax Ltd have been commissioned by MH Architects on behalf of Amicus

Horizon (Optivo) and requested to prepare a Drainage Impact Assessment for the

proposed Residential Development on the former ‘Old Rectory Court’ site at The

High Street, Burwash, Etchingham, East Sussex.

6.2 The new development will reduce the quantity of foul water being discharged to

the public foul water sewer from 6.4l/s to 4.63l/s therefore creating additional

available capacity of 1.77l/s within the public foul water sewer.

6.3 Should it be confirmed that the existing buildings surface water runoff does enter

the foul drainage system, then the potential disconnection from the foul water

sewer and disposal via infiltration techniques within the development site will

create a spare peak discharge capacity of 4.6l/s within public foul water sewer.

6.4 The intention for the disposal of the surface water runoff from the new

development will be to discharge within the development site via suitable SUDs

infiltration techniques.

6.5 A Section 106 Agreement will be required with Southern Water Authority for

connections to the public foul sewer.

6.6 An outline drainage strategy drawing EMC-2017-129-03 is enclosed within

Appendix A

6.7 The site is indicated to be within the extent of the Flood Zone 1 (Low Probability).

This zone comprises land assessed as having a less than 1in1000 annual

probability of river or sea flooding (<0.1%). All uses of land are appropriate within

this zone

6.8 In accordance with the requirements of the Technical Guidance to the National

Planning Policy Framework (NPPF), we conclude that the site can be adequately

drained and is not at risk from flooding or increases the risk of flooding elsewhere.


Job No. EMC-2017-129 19 July 2017

APPENDIX A

Tridax Drawings EMC-2017-129-01 – Site Location EMC-2017-129-02 – Existing Site Drainage EMC-2017-129-03 – Proposed Drainage Strategy


Job No. EMC-2017-129 20 July 2017

APPENDIX B

Calculations Greenfield Runoff Quick Storage Calculation


Job No. EMC-2017-129 21 July 2017


Job No. EMC-2017-129 22 July 2017


Job No. EMC-2017-129 23 July 2017

APPENDIX C

Correspondence

DRAINAGE IMPACT ASSESSMENT

Documents

Transcript of DRAINAGE IMPACT ASSESSMENT