TECHNICAL NOTE 1

FLOOD RISK & DRAINAGE

Project Park Farm, Blyth

Document

Number PFB-BWB-EWE-XX-RP-YE-0001_Drainage Technical Note BWB Ref BMW-2322-TN1

Author Catherine Thorpe Status S2

Checked Ben Fleming Revision P01

Approved Ben Fleming Date 30/05/2018

1

INTRODUCTION

Summary Information

This Flood Risk & Drainage Report summarises a desktop study into the possible sources of flood risk to

a potential development site located at Park Farm, Blyth. It has been prepared on behalf of Planit

Planning and Development to advise on the development potential of the site from a flood risk and

drainage perspective.

This report is based on readily available information, and has not included consultation with the

Environment Agency or other flood risk consultees.

A summary of pre-mitigation sources of flood risk is available as Table 1.1.

Table 1.1– Pre-Mitigation Sources of Flood Risk

Flood Source Potential Risk

Description High Medium Low None

Fluvial X

The site is in Flood Zone 1: land

defined as being at low risk of

river/sea flooding.

Canals X The nearest canal is approximately

5km from the site.

Groundwater X

The site is not thought to be

susceptible to ground water

flooding.

Reservoirs and

waterbodies X

The site is located 0.5km away from

the nearest identified reservoir flood

risk area.

Pluvial runoff X

Surface water mapping does not

identify any risk zones within or near

to the site.

Sewers X

Due to the local topography, sewer

flooding is unlikely to pose a risk to

the site.

Site Details

The site is in Blyth, Nottinghamshire, and accessed off the Worksop Road (B6045). A site location plan

is included as Figure 1.1.

The site is bound to the north and east by east by agricultural fields and paddocks, the Worksop Road

to the south and agricultural buildings to the west.

TECHNICAL NOTE 1 FLOOD RISK & DRAINAGE

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Figure 1.1 – Site Location Plan

Relevant Policy and Guidance

Strategic Flood Assessment

The Strategic Flood Risk Assessment for North Nottinghamshire1 has been reviewed in the production

of this report. The SFRA provides information specific to the site location in the form of fluvial, surface

water and groundwater flood risk mapping, as well as records of historic flooding. The SFRA does not

make specific reference to Blyth.

Preliminary Flood Risk Assessment

The Nottinghamshire Preliminary Flood Risk Assessment2 was completed as a high-level screening

exercise to identify areas of significant flood risk within the county. The local area of Blyth is located

within a region above the flood risk threshold.

1 Bassetlaw District Council Strategic Flood Risk Assessment, July 2009 2 Nottinghamshire Preliminary Flood Risk Assessment, June 2011

TECHNICAL NOTE 1 FLOOD RISK & DRAINAGE

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SOURCES OF FLOOD RISK

Fluvial Flood Risk

Flood Zones

The Flood Map for Planning has been prepared by the Environment Agency. This identifies areas

potentially at risk of fluvial of tidal sources. An extract of the mapping is included as Figure 2.1. The

Environment Agency Flood Map for Planning shows the site to be located entirely within Flood Zone 1

(Low Probability). This Flood Zone is defined in the National Planning Policy Framework (NPPF) as land

assessed as having less than a 1 in 1000 annual probability of river or sea flooding in any year.

Figure 2.1 - Environment Agency Flood Map for Planning (Rivers and Sea)

Watercourses

The nearest Main River is the River Ryton however, as shown in Figure 1, this does not pose a significant

risk to the site.

There are no further watercourses near the study site.

TECHNICAL NOTE 1 FLOOD RISK & DRAINAGE

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Canal Flood Risk

The nearest canal is the Chesterfield Canal which is over 5km from the site and therefore does not

pose a significant risk.

Reservoir Failure Flood Risk

Reservoir failure has been prepared by the Environment Agency. This shows the largest area that

might be flooded if a reservoir were to fail and release the water it holds. This map displays a worst-

case scenario and is only intended as a guide. An extract of the mapping is included as Figure 2.2.

Figure 2.2- Risk of Reservoir Flooding Mapping

The site is located approximately 0.5km from the closest inundation area from reservoir failure/breach.

It is therefore considered to not pose a significant risk to the site.

Surface Water Flood Risk

Surface water (pluvial) flooding can occur during prolonged or intense storm events when the

infiltration potential of soils and/or the capacity of drainage infrastructure is overwhelmed, leading to

the accumulation of surface water and generation of overland flow routes.

TECHNICAL NOTE 1 FLOOD RISK & DRAINAGE

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Risk of surface water flood mapping has been prepared by the Environment Agency. This shows the

potential flooding which could occur when rainwater does not drain away through the normal

drainage systems or soak into the ground, but lies on or flows over the ground instead. An extract of

the mapping is included as Figure 2.3.

Figure 2.3 - Environment Agency Surface Water Flood Risk Map

The mapping does not identify any areas within the site or in its immediate vicinity at risk from pluvial

flooding.

Groundwater Flood Risk

Online British Geological Survey (BGS) geological mapping shows the geology of the site to comprise

of the Chester Formation (sandstone, pebbly) with no superficial deposits. The soil is reported to be

freely draining and sandy in texture.

There is no groundwater flood risk to Blyth identified within the SFRA.

Sewer Flood Risk

Sewer assets near the site are owned and maintained by Severn Trent. Sewer asset plans are included

for reference in Appendix 1.

TECHNICAL NOTE 1 FLOOD RISK & DRAINAGE

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There is a 150mm diameter foul sewer running along the Worksop Road in an easterly direction and

the storm water sewer runs in a northerly direction along the High Street.

Due to the topography of the surrounding ground, the absence of overland flow routes and the flow

direction of the sewers, sewer flooding is determined to pose a low risk to the development site.

TECHNICAL NOTE 1 FLOOD RISK & DRAINAGE

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SURFACE WATER DRAINAGE

Drainage Requirements

The Lead Local Flood Authority (LLFA) as statutory consultees have a duty to ensure that surface water

flood risk is not increased because of the development.

The LLFA does not have any specific SuDS guidance to inform planning applications and in the

absence of further information, the Non-Statutory Technical Standards for Sustainable Drainage

Systems3 (as published by DEFRA) should be utilised to inform a drainage strategy.

If discharging to an existing sewer or drain, evidence will be required to demonstrate that the owner

of the sewer or drain (sewerage undertaker for sewers, highways authority for highway drains, or

private owner) has accepted the point and rate of discharge, and that there is capacity to

accommodate these flows. The owner may request improvements to the sewer or drain, or for

discharge rates to be reduced below the Greenfield rate or other rate agreed by the LLFA.

If discharging to the wider network, surface water attenuation will be required at the site so that water

can be stored and released at a controlled rate. Storage volumes should be designed so that the 100-

year storm can be accommodated within the site, including a 40% allowance for climate change.

Source control measures should be implemented as far as possible with the appropriate levels of water

treatment provided prior to water leaving the site.

Drainage Hierarchy & Potential Storm Water Drainage Strategy

The drainage hierarchy outlined in statutory guidance states that surface water should be discharged

from development sites to a receptor in the following order of preference:

(i) Soakaway/infiltration; or

(ii) Local watercourse; or

(iii) Sewer network.

In view of the underlying ground conditions, it is likely that the existing building drains via infiltration

because there are no localised watercourses near the site or an adjacent surface water sewer.

It is therefore considered that infiltration can provide a solution to the disposal of surface water from

the proposed development. Conducting future soakaway tests would confirm that the site is suitable

for disposal of surface water via infiltration. It should be noted that the proposed conversion would

result in a reduction in the area of current built development (through the introduction of garden

green space) and subsequently there would be a decrease in surface water runoff generated.

Based upon an assumed impermeable area of 0.08ha (30% of the total site area) approximately 38m³

of soakaway storage would be required to accommodate the 1 in 100 year + 40% climate change

scenario. This could be achieved using a 13m x 8m x 0.4m storage crate with an assumed 95% void

ratio. This is a conservative estimate based upon an assumed permeability of 1x10-5m/s. There is room

within the proposed layout (Appendix 2) to locate this crate within the green space of the

development.

In the unlikely event that infiltration is proved unviable for the site, and in the absence of any

appropriate local watercourses, there is the option to attenuate surface water from the proposed

3 Non- Statutory Technical Standards for Sustainable Drainage Systems, DEFRA, 2015

TECHNICAL NOTE 1 FLOOD RISK & DRAINAGE

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development within the grounds of the new properties before discharging at a controlled rate to the

storm water sewer to the east the site.

A Developer Enquiry submitted to Severn Trent would confirm capacity within the surface water sewer,

or alternatively highlight any required improvement works necessary to accommodate surface water

flows from the proposed development.

TECHNICAL NOTE 1 FLOOD RISK & DRAINAGE

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CONCLUSIONS & RECOMMENDATIONS

BWB Consulting have been instructed by Planit Planning and Development to undertake a desk-based

appraisal to advise on the development from a flood risk and drainage perspective. The identified

risks are summarised within Table 5.1.

Table 0.1 - Summary of Flood Risk Mechanisms

Flood Source Risk Description

Fluvial The site is identified to fall within Flood Zone 1, land defined as being at low

risk of river/sea flooding.

Canals The Chesterfield Canal is more than 5km from the site, therefore does not

pose a risk to the site.

Groundwater

In view of the underlying conditions, groundwater flooding is unlikely. If

groundwater is encountered during the construction phase, it is

recommended that an appropriately qualified groundwater contractor is

employed, and the appropriate mitigation measures are employed.

Reservoirs

and

waterbodies

The site is located approximately 0.5km from the closest inundation area from

reservoir failure/breach. It is therefore considered to not pose a risk to the site.

Pluvial runoff The site is not shown to be at risk from pluvial runoff.

Sewers Sewer flood risk is determined to be low.

This summary should be read in conjunction with BWB’s full report. It reflects an assessment of

the desk-based information available to BWB at the time of production.

The proposed method of drainage is soakaways however, infiltration tests should be carried out to

confirm that this is a viable drainage option.

Surface water runoff from the development should be appropriately managed and designed up to

the 100-year storm event, including an 40% allowance for climate change. SuDS should be

incorporated within the development layout and provide a suitable level of on-site storage.

It is recommended that a Developer Enquiry should be submitted to Severn Trent to confirm whether

there is capacity within the local surface water, should soakaways not be viable, to accommodate

flows from the proposed development.

APPENDIX 1

Severn Trent Sewer Records

Abandoned Gravity Sewer

Private Combined Gravity Sewer

Private Foul Gravity SewerPrivate Surface Water Gravity SewerPublic Combined Gravity SewerPublic Foul Gravity SewerPublic Surface Water Gravity SewerTrunk Combined Gravity SewerTrunk Foul Use Gravity SewerTrunk Surface Water Gravity Sewer Combined Use Pressurised SewerFoul Use Pressurised SewerSurface Water Pressurised SewerHighway DrainCombined Lateral Drain (SS)Foul Lateral Drain (SS)Surface Water Lateral Drain (SS)

All Private Sewers are shown in magenta All section 104 sewers are shown in greenAll Sewers that have been transferred to Severn Trent Water after the 1st October 2011, but have not been surveyed and confirmed by Severn Trent Water are shown in orange

Culverted WatercourseCable, EarthingCable Junction

Cable, Optical Fibre/Instrumentation

Cable, Low Voltage

Cable, High VoltageCable, Other

Housing, Building

Housing, Kiosk

Disposal Site

Sewage Treatment Works

Housing, Other

Pipe Support Structure

Sewage Pumping Facility

Sewer Facility Connection Inlet / Outlet

Blind Shaft

Combined Use Manhole

Flushing Chamber

Foul Use Manhole

Grease Trap

Head Node

Hydrobrake

Lamphole

Outfall

Overflow

Penstock

Petrol Interceptor

Sewer Chemical Injection Point

Sewer Junction

Sewerage Air Valve

Sewerage Hatch Box Point

Sewerage Isolation Valve

Soakaway

Surface Water Manhole

Vent Column

Waste Water Storage

Pre-1937 Properties

MATERIALS- - NONEAC - ASBESTOS CEMENTBR - BRICKCC - CONCRETE BOX CULVERTCI - CAST IRONCO - CONCRETECSB - CONCRETE SEGMENTS (BOLTED)CSU - CONCRETE SEGMENTS (UNBOLTED)DI - DUCTILE IRONGRC - GLASS REINFORCED CONCRETEGRP - GLASS REINFORCED PLASTICMAC - MASONRY IN REGULAR COURSESMAR - MASONRY RANDOMLY COURSEDPE - POLYETHLENEPF - PITCHPP - POLYPROPYLENEPSC - PLASTIC STEEL COMPOSITEPVC - POLYVINYL CHLORIDERPM - REINFORCED PLASTIC MATRIXSI - SPUN (GREY) IRONST - STEELU - UNKNOWNVC - VITRIFIED CLAYXXX - OTHER

SHAPE PURPOSEC - CIRCULAR C - COMBINEDE - EGG SHAPED E - FINAL EFFLUENTO - OTHER F - FOULR - RECTANGLE L - SLUDGES - SQUARE S - SURFACE WATERT - TRAPEZOIDALU - UNKNOWN

CATEGORIESW - WEIRC - CASCADEDB - DAMBOARDSE - SIDE ENTRYFV - FLAP VALVEBD - BACK DROPS - SIPHONHD - HIGHWAY DRAINS104 - SECTION 104

Severn Trent Water LimitedAsset Data ManagementPO Box 5344CoventryCV3 9FTTelephone: 0845 601 6616

SEWER RECORDO/S Map scale:

Date of issue:

This map is centred upon:O / S Grid reference:

x :

y :

Disclaimer Statement:1. Do not scale off this Map.2. This map and any information supplied with it is furnished as a general guide, is only valid at the date of issue and no warranty as to its correctness is given or implied. In particular this Map and any information shown on it must not be relied upon in the event of any development or works (including but not limited to excavations) in the vicinity of Severn Trent Water’s assets or for the purposes of determining the suitability of a point of connection to the sewerage or distribution systems.3. On 1 October 2011 most private sewers and private lateral drains in Severn Trent Water's sewerage area, which were connected to a public sewer as at 1 July 2011, transferred to the ownership of Severn Trent Water and became public sewers and public lateral drains. A further transfer takes place on 1 October 2012 (date to be confirmed).Private pumping stations, which form part of these sewers or lateral drains, will transfer to the ownership of Severn Trent Water on or before 1 October 2016.Severn Trent Water does not possess complete records of these assets.These assets may not be displayed on this Map.4. Reproduction by permission of Ordnance Survey on behalf of HMSO. © Crown Copyright and database right 2004. All rights reserved. Ordnance Survey licence number 100018202. Document users other than Severn Trent Water business users are advised that this document is provided for reference purpose only and is subject to copyright, therefore, no further copies should be made from it.

21.05.18462464

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APPENDIX 2

Proposed Site Layout

Scale on A3:

Checked by:

Drawn by:

Date:Project no:

Revisions The copyright of this drawing and design is vested in the Architects and must not be copied or reproduced without their written consent.All contractors must visit site and be responsible for taking and checking all necessary dimensions relative to this work.

Do not scale: Figured dimensions only to be used. If in doubt ASK

T: 01386 555 166 E: studio@poolephillips.co.uk W: www.poolephillips.co.ukRegistered No: 3820127

Drawing Number:Title:Project:

for

1 : 500

38011 Issue Date

JD

VAP102Class Q Conversion - Blyth

-

Existing Site Plan

No. Date Description

1 : 500

Site Plan - Proposed

North

VISUAL SCALE 1:500 @ A3

25m5m 20m15m10m0m