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Drainage Strategy Report

Proposed Residential Development

Land Off Woodbastwick Road

Blofield Heath

Client: David Futter Associated Limited

September 2016 – Version 1

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Land off Woodbastwick Road, Blofield Heath Drainage Strategy Report

3097 Version 1 i

Document Control Sheet

Project Name: Proposed Residential Development, Land off Woodbastwick Road, Blofield Heath, Norfolk, NR13 4QH

Project Reference: 3097

Report Title: Drainage Strategy Report

Name Position Signature Date

Prepared By: Gillian Hoy Senior Geoenvironmental Engineer

13 September 2016

Approved By: Andy Westby Director

13 September 2016

For and on behalf of Bingham Hall Associates

Disclaimer

The copyright in this report prepared by BHA Consulting Ltd is owned by them and no such report, plan or document may be

reproduced, published or adapted without their written consent. Complete copies of this report may however be made and distributed

by the Client in dealing with matters related to the brief.

The information given in this report is solely for the use of the Client noted above. BHA Consulting Ltd can accept no responsibility or

liability to any third party. The benefit of this report is not transferable to any third party except with the written agreement of the

original Client and of BHA Consulting Ltd. An extra fee will be payable for such a transfer.


3097 Version 1 ii

Contents

Executive Summary ................................................................................................................ 1

Introduction .................................................................................................................... 3

Site Appraisal ................................................................................................................... 4

Foul & Surface Water Drainage Strategy ............................................................................ 8

Appendices

Appendix A: Site Layout Plan

Appendix B: Greenfield runoff Rates

Appendix C: Drainage Strategy Plan

Appendix D: Drainage Calculations


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Executive Summary

The Site The Site is situated on land west of Woodbastwick Road in the village of Blofield Heath,

Norfolk, and currently comprises rough grass/meadow and some farm outbuildings.

The development boundary encloses an area of approximately 1.5ha. Residential

development is present to the east and south, with open farmland present to the north

and west of the Site.

Proposed Development

The proposed development comprises 4 residential properties and associated

infrastructure.

Geology The British Geological Survey (BGS) 1:50,000 map indicates the Site to be directly

underlain by superficial geology of the Happisburgh Glacigenic Formation (Diamicton –

Glacial Till). This in turn is underlain by bedrock geology of the Crag Formation (Sand

and Gravel).

Hydrology The nearest surface watercourse is a ditch located along the southern boundary, which

continues to flow southwards and westwards, eventually discharging into Witton Run

located approximately 1.3km south west of the site.

Hydrogeology The bedrock aquifer (Crag Formation) is designated as a Principal Aquifer. The

Happisburgh Glacigenic Formation (Diamicton) is designated as a Secondary

(Undifferentiated) Aquifer.

Flood Risk Zone Environment Agency Flood Map indicates the development site to be located within

FLOOD ZONE 1; an area where flooding from rivers and the sea is very unlikely. There is

less than a 0.1 per cent (1 in 1000) chance of flooding occurring each year.

Infiltration Testing Percolation testing has not been undertaken. The underlying Glacial Till is characterised

by poor drainage characteristics and infiltration drainage methods are considered highly

unlikely to be suitable.

Surface Water Drainage Strategy

A hierarchical approach to the drainage strategy has been considered for this

development in accordance with a sustainable (SuDS) approach.

As infiltration drainage methods are unlikely to be suitable for soakaway drainage, the

most appropriate form of surface water drainage for this site is sub-surface storage with

a controlled discharge to the adjacent watercourse.


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Surface water run-off from roof areas, up to the 100 year plus climate change storm

event, will be drained to individual plot cellular attenuation storage tanks. Each will

have a hydrobrake, or similar device, with the combined discharge equal to the natural

greenfield (QBAR) run-off rate.

The Shallow soils, within the top 0.5m, are likely to facilitate infiltration drainage for the

private estate road and property driveways that will be constructed of permeable

surfaces.

Water butts will be provided for garden use to reduce run-off volumes and treated

water consumption.

Surface Water

Runoff Quality

Surface water runoff from paved surfaces and roofs will receive adequate levels of

treatment to remove pollutants and to ensure there is no detrimental impact on the

water environment.

Exceedance Flows Exceedance flows will be routed via the estate road towards the existing ditch along the

southern boundary, thus ensuring both the proposed development and existing

development is protected from exceedance flows.

Foul Drainage

Strategy

The incumbent sewerage provider for the area is Anglian Water Services Ltd. There is

an existing foul sewer in Woodbastwick Road to which a gravity connection could be

made.

Summary The drainage strategy meets sustainable drainage criterion and ensures no harm to the

water environment.

This report demonstrates that the proposed development would not increase flood risk

to the Site, its occupants or the surrounding existing properties.

This Executive Summary contains an overview of the key findings and conclusions in this report. No reliance

should be place on any part of the Executive Summary until the whole of this report has been read. Other

sections of the report may contain information which puts into context the findings summarised in the

Executive Summary.


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Introduction

1.1 Bingham Hall Associates (BHA) has been commissioned by David Futter Associates Ltd (DFAL),

to prepare a Foul & Surface Water Drainage Strategy Report for land off Woodbastwick Road,

Blofield Heath, to be referred to hereafter as ‘the Site’. The proposed development comprises 4

residential properties and associated infrastructure. The Site Layout Plan is provided in

Appendix A.

1.2 The report follows the guidance set out in:

National Planning Policy Framework 2012 (NPPF)

National Planning Policy Guidance.

SuDS Manual 2015 (C753)

1.3 The following data was reviewed as part of this assessment:

British Geological Survey’s (BGS) ‘Geology of Britain’ on-line maps

We have also utilised our knowledge of the local area and experience in dealing with

similar related matters.


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Site Appraisal

Site setting

2.1 The Site is situated on land west of Woodbastwick Road in the village of Blofield Heath, Norfolk,

and currently comprises rough grass/meadow and some farm outbuildings.

2.2 The development boundary, as identified in Figures 2.1 and 2.2, encloses an area of

approximately 1.5ha. Residential development is present to the east and south, with open

farmland present to the north and west of the Site.

Figure 2.1: Site Location Plan

Site


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Figure 2.2: Site Location Plan: Aerial View

Topography

2.3 The topographic survey, included within the Site Layout Plan (Appendix A), indicates that the

Site slopes towards the south, from an elevation of approximately 23.00mAOD down to

21.75mAOD nearest to the drain along the southern boundary.

Geology

2.4 The British Geological Survey (BGS) 1:50,000 map indicates the Site to be directly underlain by

superficial geology of the Happisburgh Glacigenic Formation (Diamicton – Glacial Till). This is

turn is underlain by bedrock geology of the Crag Group (Sand and Gravel).

Site


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Hydrology

Surface water features

2.5 There are no surface water features on Site.

Surface watercourses

2.6 The nearest surface watercourse is a ditch located along the southern boundary, which

continues to flow southwards and westwards, eventually discharging into Witton Run located

approximately 1.3km south west of the site.

Site Sensitivity

Hydrogeology - Superficial Aquifer

2.7 The Happisburgh Glacigenic Formation (Diamicton – Glacial Till) is designated as a Secondary

(Undifferentiated) Aquifer. This is assigned in cases where it has not been possible to attribute

either category A or B to a rock type. This means that the layer in question has previously been

designated as both minor and non-aquifer in different locations due to the variable

characteristics of the rock type.

Hydrogeology - Bedrock Aquifer

2.8 The bedrock aquifer (Crag Group – Sand and Gravel) is designated as a Principal Aquifer (i.e.

high intergranular and/or fracture permeability - meaning they usually provide a high level of

water storage. They may support water supply and/or river base flow on a strategic scale).

Groundwater Vulnerability

2.9 The groundwater vulnerability mapping indicates the Site is underlain by high leaching potential

soils.

Source Protection Zone

2.10 The Site is not within a Source Protection Zone.

Infiltration Testing

2.11 Percolation testing has not been undertaken. The underlying Glacial Till is characterised by

poor drainage characteristics and infiltration drainage methods are considered highly unlikely to

be suitable.

Flood Risk Zone

2.12 The Environment Agency (EA) Flood Map indicates the development site to be located within

FLOOD ZONE 1; an area where flooding from rivers and the sea is very unlikely. There is less

than a 0.1 per cent (1 in 1000) chance of flooding occurring each year.


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Natural (Greenfield) Run-off Rate

2.13 The natural (greenfield) runoff rate for a 1.0ha area for the 1 year, QBAR, 30 year, and 100

year, storm events has been calculated in accordance with the ICP SuDS Mean Annual Flood

methodology (refer to CIRIA C753 - SuDS Manual) as noted in Table 2.1. Calculations are

shown in Appendix B.

Natural Run-Off rates (l/s/ha)

Return period (yrs) 1 (QBAR) 30 100

2.5 2.8 6.8 10.1

Table 2.1: Natural Run-Off Rate

Sewerage

2.14 There is an existing Anglian Water foul water sewer along Woodbastwick Road. There are no

surface water sewers.

Climate Change

2.15 The importance of climate change in regard to flooding and development is highlighted in the

NPPF and the impacts of climate change will be taken into account when designing the new Site

drainage infrastructure.

2.16 In accordance with the NPPF, the climate change allowance is dependent upon the

development lifespan. This development falls within the upper end limit as described in Table 2

(https://www.gov.uk/guidance/flood-risk-assessments-climate-change-allowances) requiring an

allowance of 40% for the increase in rainfall rates.

https://www.gov.uk/guidance/flood-risk-assessments-climate-change-allowances


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Foul & Surface Water Drainage Strategy

Existing Surface Water Drainage

3.1 The topography dictates that surface water falls naturally to the ditch along the southern

boundary of the site. It is understood that the existing farm buildings are undrained and surface

water falls direct onto the surrounding ground.

Surface Water Drainage Strategy

3.2 The NPPF requires the surface water drainage strategy to follow a sustainable (SuDS) approach.

The emphasis of SuDS is to first consider source control (i.e. disposal of runoff within the plot

boundary, followed by site control (site wide disposal) and then regional control (appropriate for

larger development with strategic drainage infrastructure). Where possible the drainage

strategy should target a zero runoff from the immediate development area.

3.3 A hierarchical approach to the drainage strategy has been considered for this development in

accordance with Table 4.1. Each method is assessed to reach a conclusion appropriate to the

development.

Hierarchical SuDS Assessment

Comment Issues To Consider

Appropriate To

Development

Green roofs Can be used on suitable low

rise buildings

Safe maintenance access

In keeping with character/appearance of adjacent buildings

Unlikely to be

appropriate due to appearance of adjacent dwellings and ‘traditional’ house type design

Rainwater Harvesting

Rainwater harvesting reduces the total runoff volume from the developed Site, particularly first 5mm of rainfall, and reduces treated water consumption

Economics for full rainwater harvesting systems

Full rainwater harvesting unlikely to be economical for this Site

Water butts for roof runoff appropriate to development

Plot infiltration Reduces total runoff volume from the developed Site

Underlying geology is likely to have poor drainage characteristics, therefore, unsuitable for infiltration drainage

Not appropriate

Permeable Paving & Sub-Base Drainage

Can be used to provide pavement runoff water quality enhancement (treatment train) and sub-base attenuation

Permeable paving proprietary sub-base storage reduces requirements and

Shallow soils within the top 0.5m are likely to be suitable for infiltration drainage via permeable surfaces

Impact of saturation on pavement stability to be considered possibly requiring extensive use of impermeable

Appropriate


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volumes of storage structures

membranes and under-drainage Specialist maintenance

equipment

Swales Swales provide above ground flood conveyance routes

Water quality enhancement is provided by filtration through vegetation and stone media

Swales are reliant on ground porosity for infiltration

Underlying geology is likely to have poor drainage characteristics, therefore, unsuitable for infiltration drainage

No space available for open basins in agreed site layout.

Not appropriate

Infiltration

Basins

Infiltration basins can be

located within landscaped areas to attenuate storm flows up to the 100 year plus climate change event

Underlying geology is likely to

have poor drainage characteristics, therefore, unsuitable for infiltration drainage

Health and safety issues for potential open water bodies in residential areas.


Not appropriate

Detention Ponds

Detention basins can be located within landscaped areas to attenuate storm flows up to the 100 year plus climate change event

Health and safety issues for potential open water bodies in residential areas.


Not appropriate

Sub-Surface Storage (Tanks & Pipes)

Suitable for use where there is insufficient space for an

open basin Suitable for use where other

factors prevent the use of open basins

Below ground tanks or oversized pipes can be

considered as part of a controlled discharge system

Appropriate to development

Table 3.1: SuDS Assessment

3.4 In terms of a means of disposal of surface water NPP Guidance (Paragraph: 080 Reference ID:

7-080-20150323) requires a Surface Water Hierarchy to be considered. This requires

consideration to be given in order of priority to:

a. disposal to the ground

b. disposal to a watercourse

c. disposal to a public surface water sewer

d. disposal to a combined sewer


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3.5 Based on the assessment provided in Table 4.1, and in accordance with the Surface Water

Hierarchy, the most appropriate method of surface water management and disposal is as set

out below. The Drainage Strategy Plan is provided in Appendix C.

Rainwater harvesting using water butts for roof water run-off to reduce the total

volume of water discharged

Permeable paving of private drives and private estate road

A controlled discharge to the watercourse with the discharge rate controlled to the

natural greenfield (QBAR) run-off rate. The QBAR run-off rate to be applied to the

design impermeable area – see Table 3.2, below:

Development Discharge Rate

QBAR Natural discharge rate

4.2l/s 3.6l/s

Table 3.2: Development runoff rate

3.6 Each property will have an attenuation storage tank to accommodate up to the 100 year plus

climate change storm event. Attenuation volumes will be based on the design impermeable

area of each plot plus 10% allowance for development creep in accordance with CIRIA C753 –

see Table 4.3. Attenuation storage calculations are provided in Appendix D.

3.7 Plots 1-3 will each have a storage tank with dimensions of 3m x 4.5m x 0.8m deep. Plot 4 will

have a storage tank with dimensions of 3m x 3.5m x 0.8m deep. Discharge from each tank will

be controlled by a hydrobrake, or similar device, with the combined discharge equal to the

natural greenfield (QBAR) run-off rate.

Development Attenuation Volumes

Plot (s) Impermeable Area

ha

Plot Area With Allowance For

10% Creep ha

Discharge Rate

Total Attenuation

Storage Volume

1-3 0.027 0.03 1.1l/s 12.0m3

4 0.021 0.023 0.9l/s 9.4m3

Combined

Development Totals

- 0.113 4.2l/s -

Table 3.3: Development attenuation volumes

3.8 Each attenuation storage tank will remain in the private ownership of the respective properties

served.


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Surface Water Runoff Quality

3.9 In accordance with NPPF (Section 110) the development should not have a detrimental impact

on the environment, including the water environment. NPP Guidance (Paragraph: 016

Reference ID: 34-016-20140306) provides advice on the benefits of ensuring runoff quality is to

an appropriate standard to enhance the water environment.

3.10 The SuDS Manual 2015 provides guidance on the treatment of surface water runoff for the form

of land use and sensitivity of receiving water body. Table 3.3 (SM – 2015) rates the pollution

hazard from residential roof water runoff as VERY LOW. The pollution hazard for individual

property driveways and low traffic roads (i.e. cul-de-sacs) as LOW. The requirement for roof

water runoff is the removal of gross solids and sediments only. Runoff from LOW hazard areas

requires a simple index approach. The method of water quality treatment may require a single

mitigation system, or combination of mitigation components depending on the pollution hazard

rating. A single SuDS component with a high capacity for removal of pollutants can be suitable.

3.11 Roof water runoff will be treated through the provision of trapped gullies and water butts to

intercept gross solids and sediment, guidance will be provided to householders on appropriate

maintenance requirements including regular cleaning of gullies.

3.12 The simple index approach requires that the mitigation indices for the mitigation proposed

exceeds the pollution hazard indices for the pollution hazard level. Table 4.2 provides

information on hazard indices and mitigation indices.

Land Use

Total Suspended

Solids (TSS) Metals Hydrocarbons

Pollution Indices Low traffic roads (LOW hazard

rating) 0.7 0.6 0.7

Mitigation Indices

Constructed Permeable Pavement (where a suitable filtration layer is included that provides treatment, and including a geotextile at the base separating the

foundation from the subgrade) underlain by a soil with a good contaminant attenuation potential of at least 300mm in depth.

Table 3.4: Runoff mitigation assessment

3.13 It is therefore demonstrated that roof water runoff can be adequately treated through the use

of trapped gullies and water butts, to intercept gross solids and silt, and a constructed

permeable pavement, for which the run-off percolates, will provide adequate water quality

treatment for the private estate roads and property driveways.


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Exceedance Flows

3.14 Exceedance flows are those in excess of the design storm event (i.e. > 1 in 100+CC).

Exceedance flows will be routed via the estate road to Flegg Green and to the offsite

watercourse. Exceedance flow flood routes are shown on the Drainage Strategy Plan at

Appendix C.

Foul Drainage Strategy

3.15 The incumbent sewerage provider for the area is Anglian Water Services Ltd (AWS).

3.16 There is an existing foul sewer in Woodbastwick Road to which a gravity connection could be

made.

Flood Risk Management Measures

3.17 Finished floor levels of the residential properties will be set at least 150mm above the prevailing

ground levels in Flood Zone 1. All external hard standing areas will be designed to fall away

from the properties.

3.18 In accordance with the NPPF, access and egress to the Site during all storm events should be

considered with preference being over dry land. As the Site is wholly in Flood Zone 1 the

access routes to and from the properties are naturally above the 1 in 100 year plus climate

change event.

3.19 All on-site foul and surface water drainage infrastructure will be constructed to current building

regulation standards.

Residual Risks

3.20 Whilst the on-site drainage will be constructed to current building regulation standards, there

always remains a small residual risk of flooding due to blockage or failure of on-site private

drains.

3.21 There are also further residual risks related to the maintenance of off-site drainage assets by

third parties, such as highway drainage, foul sewers and drainage ditches.

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