plot c, symmetry park, a41 bicester

Plot C, Symmetry Park,

A41 Bicester

Drainage Strategy

For Tritax Symmetry (Bicester Reid) Ltd

Date: 1 April 2021

Doc ref: C-11238-HYD-XX-XX-TN-0100_2

HYDROCK TECHNICAL NOTE | Tritax Symmetry (Bicester Reid) Ltd | Plot C, Symmetry Park, A41 Bicester | C-11238-HYD-XX-XX-TN-0100_2 | 1 April 2021 i

DOCUMENT CONTROL SHEET

Issued by Hydrock Consultants Limited

second Floor

172 Edmund Street

Birmingham

B3 2HB

Tel: 0121 752 5197

www.hydrock.com

Client Tritax Symmetry (Bicester Reid) Ltd

Project name Plot C, Symmetry Park, A41 Bicester

Title Drainage Strategy

Doc ref C-11238-HYD-XX-XX-TN-0100_2

Project no. C-11238

Status Planning

Date 01/04/2021

Document Production Record

Issue Number 2 Name

Prepared by A Badek

Checked by J Hayden

Approved by C Britland

Document Revision Record

Issue Number Status Date Revision Details

1 FINAL 25.03.2021 For Planning

2 FINAL 1.04.2021 For Planning

Hydrock Consultants Limited has prepared this report in accordance with the instructions of the above named

client for their sole and specific use. Any third parties who may use the information contained herein do so at

their own risk.

HYDROCK TECHNICAL NOTE | Tritax Symmetry (Bicester Reid) Ltd | Plot C, Symmetry Park, A41 Bicester | C-11238-HYD-XX-XX-TN-0100_2 | 1 April 2021 ii

CONTENTS

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

2. SURFACE WATER STRATEGY ....................................................................................................................... 1

2.1 Proposed Discharge Rates .......................................................................................................................... 1

2.2 Microdrainage Results ................................................................................................................................ 2

2.3 The Drainage Hierarchy .............................................................................................................................. 2

2.4 Suitability of Infiltration .............................................................................................................................. 3

2.5 Suitability of SUDS features ........................................................................................................................ 3

2.6 Water Quality .............................................................................................................................................. 3

3. WHOLE LIFE MAINTENANCE OF THE NETWORK ......................................................................................... 4

4. FOUL WATER STRATEGY ............................................................................................................................. 5

5. SUMMARY AND CONCLUSIONS .................................................................................................................. 5

Appendices

Appendix A Drawings

Appendix B Greenfield Runoff Calculations

Appendix C Microdrainage Calculations

Appendix D Infiltration report

Appendix E Notes from Pre-app meeting with LLFA

HYDROCK TECHNICAL NOTE | Tritax Symmetry (Bicester Reid) Ltd | Plot C, Symmetry Park, A41 Bicester | C-11238-HYD-XX-XX-TN-0100_2 | 1 April 2021 1

1. INTRODUCTION

This drainage strategy report has been produced in support of the application for full Planning Permission for

23,195sqm of logistics floor space, within Class B8 of the Town and Country Planning Use Classes Order 1987,

including ancillary Class E(g)(i) offices, 1,750 sqm, erection of security gatehouse (24sqm), security fence,

sprinkler tank and pump house, accessed from the existing Symmetry Park estate road; associated site

infrastructure including external service yard, lorry parking, landscaping, amenity open space including 10m

green corridor with 3m foot path and cycle link to wider Bicester 12 and storm water drainage infrastructure

and private sewage treatment plant.

This surface water drainage strategy has been produced in accordance with Oxfordshire Country Council

requirements and the Defra Non – Statutory Technical Standards.

This report should be read in conjunction with the Flood Risk Assessment Ref No. C11238-HYD-XX-XX-TN-C-0101

by Hydrock, Soakaway Investigation report by Listers Geo Report No. 16.02.026a dated December 2016 and the

following Hydrock drawings:

C11238-HYD-XX-XX-DR-C-0100 to 0102- Drainage Layout Sheets 1,2 & 3

C11238-HYD-XX-XX-DR-C-0105 - Catchment Plan

C11238-HYD-00-ZZ-DR-C-0150 to 0153- Drainage Standard Details

C11238-HYD-00-ZZ-DR-C-0107- Flood Extents Plan during 100yr + 40% event

C11238-HYD-00-ZZ-DR-C-0109- Drainage Maintenance Plan

1.1 Pre-Application Meeting with LLFA

A pre-application meeting was held with the LLFA's Adam Littler on the 23rd March 2021 to discuss the

proposed drainage strategy. The meeting was positive and the strategy was understood and found to be

acceptable to Adam. Notes from the meeting can be found within Appendix E.

2. SURFACE WATER STRATEGY

2.1 Proposed Discharge Rates

Symmetry Park, Bicester comprises of two (2) zones, Zone 1 and Zone 2. Zone 1 is the eastern portion of the site

comprising of Units A1 and A2 which have already been constructed. These drain in an easterly direction to an

existing ditch via a swale.

Zone 2 is the larger portion of the site (the western portion), comprising of Plot B and C -this zone drains to the

NW corner of the site to a different existing ditch via a swale.

The surface water drainage strategy for Plot C has been undertaken to comply with the drainage strategy for

Zone 2. Due to level constraints, site discharge from the site will be via a pumped system. This is due to the

maximum floor level of the site and the existing level of the ditch which prevents the site from being able to

discharge via gravity.


Discharge from the site will be via a pumped system to a proposed swale which shall then drain to the existing

ditch in the northwest corner of the site. The existing infrastructure from Plot B is to remain, with the swale on

the north western corner of the site enlarged to cater for Unit C. Please refer to drainage drawings located

within Appendix A.

In reference to DEFRAs Non-Statutory technical Standards for SUDS Design Part S6 due to the lack of infiltration

potential the runoff volume must be discharged at a rate that does not adversely affect flood risk. In accordance

with the Environment Agency Rainfall Runoff Management for Developments Report (Report SC030219) where

infiltration or long-term storage is not possible QBAR or 2l/s/ha (which ever is greater) should be used for the

total allowable discharge rate from the site for events up to the 100yr event.

Greenfield runoff rates, including QBAR (2.3year event), based on an area of 4.99ha were calculated using the

UK SuDS drainage tool, which utilises the Wallingford procedure. Full results are contained within Appendix B,

QBAR is 20.8l/s or 4.2l/s/ha.

The surface water run off from external HGV areas shall be collected by means of external gullies and drainage

channels, before draining to an interceptor and the below ground attenuation systems.

The car parks shall drain onto permeable paving system within the car park aisles. Attenuation shall be provided

for the car parks within the lined subbase of the permeable paving. The surface water from the roofs shall drain

directly to the below ground attenuation. Levels (and volume requirements) prevent the roof drainage and

service yard drainage being attenuated within the permeable paving.

2.2 Microdrainage Results

The surface water drainage network has been modelled using Microdrainage. The network has been modelled

for a 1 in 1 year, 1 in 30 year and 1 in 100 year + 40% return period storms. The model has allowed flooding

during a 1 in 100 year event and above.

The flood water shall be retained within the plot that it is generated as stated within the results. The flood

water shall be stored above ground in locations where property is not affected until the storm subsides, at

which point it shall re-enter the network. A majority of the flood water shall be stored within the ‘Dock Leveller’

areas of the units, car parks or landscaping. Refer drawing C11238-HYD-00-ZZ-DR-C-0107 for Flood Extents

Plan.

2.3 The Drainage Hierarchy

The drainage hierarchy sets out the preference for managing surface water run-off.

The hierarchy and reasoning for using or not using each level is as follows:

Level Hierarchy Description Included?

1 Store Water for reuse or infiltration Infiltration testing has taken place on site. Results show

the permeability to be negligible therefore Infiltration has

been ruled out. Refer 2.4

2 To a surface water body It is proposed to discharge to an existing ditch via a swale.

3 To a surface water sewer There are no surface water sewers in the vicinity

4 To a public sewer There are no public sewers in the vicinity

It is proposed to comply with level 2.


2.4 Suitability of Infiltration

Soakaway testing was undertaken to confirm whether there was any potential for infiltration. The testing

carried out by Listers Geo and detailed in Report No: 16.02.026a Dated December 2016 shows permeability

rates to vary between 8x10-8 m/s and 9 x 10-7 m/s. These results confirm that the permeability of the ground

is low and not suitable for infiltration methods

2.5 Suitability of SUDS features

The following SUDS features have been considered during the design process, the table states which features

have been incorporated and which have been excluded and reasons for their exclusion.

SUDS feature Incorporated

within design

Reasoning

Swales Yes The pumped discharge drains to a swale to allow

for further water quality enhancement prior to

entering the ditch.

Attenuation Pond (above ground) No Due to level constraints and the requirement of a

pumping station above ground attenuation ponds

are not feasible.

Attenuation (below ground) Yes Below ground attenuation is proposed to

supplement the permeable paving.

Rainwater Harvesting No Rainwater harvesting is not effective for

warehousing where there are a limited number of

toilets/ uses for harvested water

Infiltration methods No As stated above infiltration tests have proved the

site to be unsuitable for infiltration

Permeable paving Yes Permeable paving has been provided where

possible, where ground levels permit. Due to the

extensive quantities of attenuation required it is

not feasible to achieve the entire storage volume

within the subbase of permeable paving alone,

hence this has been supplemented with below

ground crates.

2.6 Water Quality

With reference to the SUDS Manual 2016, the water treatment benefits offered by the components provided

within the proposed network have been considered in parallel with the risk of pollution imposed by the nature

of the site and its usage.

To mitigate the effects of any pollution generated by trafficked areas, surface water from the loading docks shall

pass through Class 1 full retention bypass separators, which will capture gross pollutants, sediments and oils.

The car parks shall drain through permeable paving which is known to offer water quality benefits.

Surface water from the roof and external areas not trafficked by vehicles shall pass directly into the attenuation

systems.

With surface water run off collected by means of external gullies, drainage channels and catchpits which shall

have sumps further treatment is provided close to source for the removal of silts.


In addition, the surface water is also passed through a swale prior to it entering the existing ditch which

provides additional water quality benefits.

3. WHOLE LIFE MAINTENANCE OF THE NETWORK

To enable the water treatment process to occur as intended the drainage network shall require maintenance.

The maintenance strategy and responsibilities are as set out in the table below. This should be read in

conjunction with the Drainage Maintenance plan within Appendix A.

Component to

be maintained

Actions Frequency Responsibility of:

Site wide

external areas

Site to be generally kept free from litter and debris which

may enter the drainage system.

On-going Site management

team/tenants

Roof gutters Check roof gutters for sediment, plant debris etc and clean

as necessary

Annually Site management

team/tenants

On site below

ground

attenuation

Check inlets /outlets for sediment/debris build up and clear

if needed

Check and maintain crates and base of crates. Ensure there

are no blockages, geo membrane and lining is intact and

there is no other damage.

Survey and inspect the below ground crates, remove

sediment and any litter as required

Annually

Annually

Every 5 years

Site management

team

General

network inc.

pipes/

manholes/

gullies/channels

Manhole, gully and channel covers/grates to be lifted and

inspect for sediment build up, clean as required. Any pipe

blockages to be cleared, catch pits and sumps to be cleared

of any debris and sediment

Annually and after

each heavy rainfall

event

Site management

team

Flow control

manhole

(including

pump stations)

Manhole cover to be lifted and inspect, remove litter/

debris and sediment. Manhole base, sump, any debris

screen, orifice opening, hydrobrake control, pump

components to be cleaned as required. Check operation of

pivoting by-pass door/remote access cable and fix if

required.

Check operation and mechanics of flow control device(s)

and any mechanics of pumps in accordance with

manufacturer specifications.

6 monthly

Annually or as

recommended by

manufacturer

Site management

team

Permeable

paving

Clean surface with brush to remove any build up of

sediment and litter and remove any weeds. Inspect outs

and control structures for any blockages

Note: If loss of permeability suspected (indicated by

ponding waters on the surface) use jetting or agitation

brushing (manually or mechanically with a suction brush) to

dislodge the affected jointing material and refill with clean

material to manufacturers specification.


team

Swales Any plants/shrubs/grass to be watered and maintained in

accordance with landscaping requirements. Surface to be


team


inspected regularly for any sediment, weeds and litter

removed.

Check headwalls for sediment/debris build up and clear if

needed

Full Retention

Separator

(high level alarm to be also used as back up but does not

remove the need for maintenance shown)

Inspect, remove litter/ debris and sediment

Change filter

Remove oils/ grease etc.

6 monthly, more

regularly for first 6

months to establish

sediment build up

rate

As recommended by

manufacturer

6 monthly or after

spill. Frequency may

be varied dependant

on finding of

inspections.

Unit occupier/

Site management

team

As required the Class 1 full retention Separator shall have a closure device fitted that automatically prevents

flow passing through once the quantity of oil within the separator reaches the limit. A high level alarm fitted

within will alert the site to the immediate maintenance requirements. The alarm shall be audible and visible.

The separator shall be inspected every six months as per the manufacturer’s instructions.

4. FOUL WATER STRATEGY

Foul water from the site will discharge to an onsite wastewater treatment system. Treated effluent will then be

discharged to the surface water system. Details of the size and treatment capacity will be finalised at detailed

design.

Please refer to the drainage drawings within Appendix A showing the proposed foul strategy for the site.

5. SUMMARY AND CONCLUSIONS

In summary the surface water drainage strategy complies with Oxfordshire Country Council requirements and

the Defra Non – Statutory Technical Standards.

The Surface water run-off from Plot C shall be discharged at Greenfield (QBAR) flow rate for events up to the

100yr event + 40% climate change. Attenuation storage is provided via permeable paving and below ground

attenuation crates.

Foul drainage shall discharge to waste water treatment works within the site before drainage to the surface

water system.


Appendix A

Drawings

plot c, symmetry park, a41 bicester

Documents