˘ˇ ˆ - Central Bedfordshire Council · 11500767 Mid Bedfordshire District Council SFRA 1...

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Mid Bedfordshire District Council September 2008

QM

Issue/revision Issue 1 Revision 1 Revision 2 Revision 3 Revision 4

Remarks Stage 2B Draft FINAL-draft Final-draft Final draft Final

Date July 2007 September 2007 May 2008 August 2008 September 2008

Prepared by M.Stinton M.Stinton M.Stinton M.Stinton M.Stinton

Signature

Checked by D.Armitage D.Armitage M.Ewunetu

Signature

Authorised by S.Purcell S.Purcell S.Purcell S.Purcell S.Purcell

Signature

Project number 11500767 11500767 11500767 11500767 11500767

File reference 0707 SFRA 2B 0709 SFRA 2B 0805 SFRA 2B 0808 SFRA 2B 0809 SFRA 2B

WSP Development and Transportation Unit 9, The Chase John Tate Road Foxholes Business Park Hertford SG13 7NN Tel: +44 (0)1992 526 000 Fax: +44 (0)1992 526 001 http://www.wspgroup.com Reg. No: 2382309

Contents

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11500767 Mid Bedfordshire District Council SFRA 1

Executive Summary

Tributaries of the River Great Ouse are an important characteristic of the Mid Bedfordshire District and provide the primary source of flood risk across areas of land within the District. Flooding represents a risk to life, property and livelihood and it is essential, therefore, that future planning decisions are informed by, and take due consideration of, the flood risk posed to future development. It is also imperative that any new development does not exacerbate existing known flooding problems.

A ‘Stage 2B’ Strategic Flood Risk Assessment has been undertaken for designated study areas defined by Mid Bedfordshire District Council to provide a robust assessment of the extent and nature of the risk of flooding and its implications for land use planning.

The principal aim of the study is to set out flood risk constraints to help inform the preparation of the Local Development Framework for the District, which sets out a spatial planning strategy for the area. The study areas have been categorised into Flood Risk Zones in accordance with Planning Policy Statement 25: ‘Development and Flood Risk’.

Planning Policy Statement 25 requires Local Planning Authorities to review the variation in flood risk across their District, and to steer vulnerable development uses towards areas of lowest flood risk in the first instance. Only if it can be demonstrated that there are no suitable sites within these areas should alternative sites with medium then high flood risks be considered. This is referred to as the Sequential Test.

Consideration of information provided within this Strategic Flood Risk Assessment should allow Mid Bedfordshire District Council to undertake the Sequential Test, and allocate land for development following a risk based approach.

The Strategic Flood Risk Assessment evaluates the current (2008) flood risk situation and provides an indication of the future flood risk situation over a 107 year timeframe (2115), incorporating the impacts of climate change in line with Planning Policy Statement 25.

This report and supporting appendices provides a sound framework with an appropriate level of detail required at this stage for making consistent and sustainable future planning decisions. At this stage, the Stage 2B Strategic Flood Risk Assessment is based principally upon Flood Zone Maps published by the Environment Agency.

Refinement of the baseline information may be made at a later stage of the planning process as part of a Stage 3 study.


GLOSSARY

Core Strategy

The Development Plan Document within the Council’s Local

Development Framework which sets the long-term vision and

objectives for the area. It contains a set of strategic policies

that are required to deliver the vision including the broad

approach to development.

Defra

Department of Environment, Food and Rural Affairs

Development

The carrying out of building, engineering, mining or other

operations, in, on, over or under land, or the making of any

material change in the use of a building or other land.

Development Plan

Document (DPD)

A spatial planning document within the Council’s Local

Development Framework which set out policies for

development and the use of land. Together with the Regional

Spatial Strategy they form the development plan for the area.

They are subject to independent examination.

Drift Geology The unconsolidated sediments at or near the Earth’s surface

(overlying the bedrock formations) of Quaternary age or more

recent.

EA

Environment Agency.

Flood Routing

Direction of overland flow in event of extensive inundation of

an area.

Flood Zone Map

Nationally consistent delineation of ‘high’ and ‘medium’ flood

risk, published on a quarterly basis by the Environment

Agency.

Formal Flood Defence

A structure built and maintained specifically for flood defence

purposes.

Functional Floodplain

PPS25 Flood Zone, defined as areas at risk of flooding in the

5% AEP (20 year) design event.

Hydraulic Model A computer simulation of the stages and flows of water within

a watercourse.


LIDAR

(Light Detection and Ranging) A method of detecting distant

objects and determining their position, velocity, or other

characteristics by analysis of pulsed laser light reflected from

their surfaces.

Local Development

Framework (LDF)

Will comprise of a portfolio of local development documents

which will provide the framework for delivering the spatial

strategy for the area.

MAFF

Ministry of Agriculture Fisheries and Food

MBDC Mid Bedfordshire District Council

Planning Policy Guidance

(PPG)

A series of notes issued by the Government, setting out

policy guidance on different aspects of planning. They will be

replaced by Planning Policy Statements.

Planning Policy Statement

(PPS)

A series of statements issues by the Government, setting out

policy guidance on different aspects of planning. They will

replace Planning Policy Guidance Notes.

PPS25

Planning Policy Statement 25: Development and Flood Risk

Department of Communities & Local Government, 2006.

Previously Developed

(Brownfield) Land

Land which is or was occupied by a building (excluding those

used for agriculture and forestry). It also includes land within

the curtilage of the building, for example a house and its

garden would be considered to be previously developed land.

Reach

The extent of a watercourse.

Residual Risk

A measure of the outstanding flood risks and uncertainties

that have not been explicitly quantified and/or accounted for

as part of the review process.


SA

Sustainability Appraisal (SA) is an appraisal of plans,

strategies and proposals to test them against the four broad

objectives set out in the Government’s sustainable

development strategy.

Solid Geology (Bedrock) The consolidated soils and rock exposed at the surface of the

Earth or overlain by unconsolidated material, weathered rock

or soil.

SuDS Sustainable Drainage Systems

Supplementary Planning

Document (SPD)

Provides supplementary guidance to policies and proposals

contained within Development Plan Documents. They do not

form part of the development plan, nor are they subject to

independent examination.

Sustainable Development

“Development that meets the needs to the present without

comprising the ability of future generations to meet their own

needs” (The World Commission on Environment and

Development, 1987).

Zone 1 Low Probability

PPS25 Flood Zone, defined as areas outside of Zone 2

Medium Probability.

Zone 2 Medium

Probability

PPS25 Flood Zone, defined as areas at risk of flooding in

events that are greater than the 1% AEP (100 year), and less

than the 0.1% AEP (1000 year) design event.

Zone 3a High Probability

PPS25 Flood Zone, defined as areas at risk of flooding in the

1% AEP (100 year) design event.

Zone 3b Functional

Floodplain

PPS25 Flood Zone, defined as land where water has to flow

or be stored in times of flooding. This land floods with an

annual probability of 1 in 20 (5%) or greater in any year. �


1 Introduction

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1.1.1 The Mid Bedfordshire District lies within a key growth area of the UK as indicated within the Draft East of England Plan (Proposed Changes, December 2006). Policy H1 of the plan, indicates that 25,400 net additional dwellings per annum are required to be built in the East of England between 2001 and 2021, a total of 508,000 dwellings.

1.1.2 Mid Bedfordshire may be required to provide for a net increase of 14,230 dwellings in the District over the Plan period.

1.1.3 In order to plan the implementation of new development in a sustainable manner, Mid Bedfordshire District Council will formulate a Local Development Framework that contains Development Plan Documents and Supplementary Planning Documents, taking into account the views of key stakeholders, and following careful consideration of sustainability issues and constraints to development. One such consideration is flood risk.

1.1.4 WSP Development and Transportation have been commissioned by Mid Bedfordshire District Council to undertake a Strategic Flood Risk Assessment for the District of Mid Bedfordshire to inform this process.

1.1.5 This Strategic Flood Risk Assessment has been carried out with the co-operation and support of Mid Bedfordshire District Council, the Environment Agency, Bedfordshire and River Ivel Internal Drainage Board, Anglian Water and the Town and Parish Councils which cover the eleven communities and other key settlements within the district.

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1.2.1 The Stage 1 Strategic Flood Risk Assessment completed by Mid Bedfordshire District Council in July 2006 identified the following 11 key settlements in the District around which it is anticipated the majority of new development will be concentrated. Refer also to the Study Area in Appendix A.

� Ampthill

� Arlesey

� Biggleswade

� Cranfield

� Flitwick

� Marston Moretaine

� Potton

� Sandy

� Shefford

� Stotfold

� Wixams (former Elstow Storage Depot, near Wilstead)


1.2.2 These communities have been identified by a settlement hierarchy which is principally a function of population, existing infrastructure, and accessibility to local facilities both by public transport and by modes of private transport. Local Planning Authorities are required to establish a settlement hierarchy which provides an apparatus for deciding where new development should be allocated through a Local Development Framework, or permitted following a planning application.

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1.3.1 The Department for Communities and Local Government aims to reduce the risks to people and the developed and natural environment from flooding by discouraging further built development within floodplain areas and by promoting best practice for the control of surface water runoff.

1.3.2 The objectives of the Strategic Flood Risk Assessment study are to:

� Provide a reference and policy document to inform preparation of the Local Development Framework.

� Ensure that Mid Bedfordshire District Council meet their obligations under the current Planning Policy Statement 25 and Local Plan Policy guidelines and standards, and future Local Development Framework policies.

1.3.3 Provide a reference and policy document to advise and inform private and commercial developers of their obligations under Planning Policy Statement 25.

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1.4.1 Since 1988 the Government has issued national planning guidance in the form of Planning Policy Guidance Notes and Planning Policy Statements.

1.4.2 The Department for Communities and Local Government released Planning Policy Statement 25: Development and Flood Risk in December 2006, which puts the focus on Local Planning Authorities to review the variation in flood risk across their district, and to seek to steer vulnerable development towards areas of lowest risk by applying the Sequential Test.

1.4.3 Another key focus of Planning Policy Statement 25 is to promote best practice for the control of surface water runoff.

1.4.4 Paragraph 10 of Planning Policy Statement 25 states:

“Flood risk assessments should be carried out to the appropriate degree at all levels of the planning process, to assess the risks of all forms of flooding to and from development taking climate change into account and to inform the application of the sequential approach.”

1.4.5 Paragraph 6 of Planning Policy Statement 25 sets out that Local Planning Authorities should prepare and implement planning strategies that help to deliver sustainable development by:

Appraising Risk

� Identifying land at risk and the degree of risk of flooding from river, sea and other sources in their areas;

� Preparing…..Strategic Flood Risk Assessments…..as freestanding assessments that contribute to the Sustainability Appraisal of their plans;


Managing Risk

� Framing policies to the location of development which avoids flood risk to people and property where possible, and manage any residual risk, taking account of the impacts of climate change;

� Only permitting development in areas of flood risk when there are no reasonably available sites in areas of lower flood risk and benefits of the development outweigh the risks from flooding;

Reducing Risk

� Safeguarding land from development that is required for current and future flood management e.g. conveyance and storage of flood water, and flood defences;

� Reducing flood risk to and from new development through location, layout, and design, incorporating sustainable drainage systems (SUDS);

� Using opportunities offered by new development to reduce the causes and impacts of flooding e.g. surface water management plans; making the most of the benefits of green infrastructure for flood storage, conveyance, and SUDS; re-creating functional floodplain and setting back defences;

A Partnership Approach

� Working effectively with the Environment Agency, other operating authorities and other stakeholders to ensure that……plans are effective and decisions on planning applications can be delivered expeditiously; and

� Ensuring spatial planning supports flood risk management policies and plans, River Basin Management Plans and emergency planning.

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1.5.1 The guidelines and standards set out within the ‘Mid-Bedfordshire District Council Local Plan’: Written Statement requires flood risk to be a material consideration for development proposals. Policy 9.7 states:

The Government looks to local planning authorities to apply a risk-based approach to development proposals within flood risk areas through a sequential test. This approach is set out in detail within Government guidance. The guidance indicates developers will be required to carry out an appropriate Flood Risk Assessment (FRA) for proposals within the designated floodplain, or where surface water drainage is inadequate. For information purposes, the 2005 Indicative Flood Map as published by the Environment Agency, is shown on the Proposals Map. The maps indicate the extent of potential flooding for a one in 100 (or one percent) and one in 1000 (or 0.1%) chance of flooding each year, ignoring the presence of defences. In some development schemes the floodplain may be capable of use for public open space or informal recreation purposes, although dense planting or landraising would be resisted by the Environment Agency or the Bedfordshire and River Ivel Internal Drainage Board. The Environment Agency must be contacted directly for the most recent version of the Flood Zones and clarification of their precise location.


1.5.2 Section 9.7.4 of the Local Plan goes on to state in Policy DPS17:

Developers must take full account of the impact of their proposals on surface water drainage and infrastructure and incorporate appropriate controls as necessary. The Council will refuse proposals for development that would:

(i) intensify the risk of flooding; or

(ii) be at an unacceptable risk of flooding; or

(iii) prejudice existing flood control and maintenance works; or

(iv) adversely affect wildlife habitat in the floodplain.

Planning applications may be required to include a levels survey of the proposal site. Any compensatory works associated with development proposals will be considered against other Local Plan policies as appropriate.

1.5.3 The ‘Mid-Bedfordshire District Council Local Plan’ is current planning policy, though it is due to be superseded by the Mid Bedfordshire Local Development Framework. This framework will include policies incorporating the impact and risk of flooding on development including an assessment of climate change over the lifetime of the future development. The results of this Strategic Flood Risk Assessment are aimed at informing the Council in preparing its development strategy to 2021.


2 Study Area

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2.1.1 The District of Mid Bedfordshire occupies an area of approximately 500 square kilometres and is bounded by North Bedfordshire and Cambridgeshire to the north and east, South Bedfordshire, Luton and North Hertfordshire to the south and Buckinghamshire and Milton Keynes to the west. Refer to Appendix A for details of the extent of the Mid Bedfordshire District Council boundary.

2.1.2 Mid Bedfordshire is predominantly rural; interspersed by towns and villages.

2.1.3 This Strategic Flood Risk Assessment focuses on 11 key settlements (refer to Section 1.2.1) in the District. Refer to the Study Area in Appendix A.

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2.2.1 The principal feature of the Mid Bedfordshire landscape is the raised Greensand Ridge which runs from Ridgmont and Ampthill in a predominantly north easterly direction towards Sandy. Either side of the ridge, the general topography is gently undulating. The topography of the Mid Bedfordshire District Council can be split into distinct areas (see below for details and indicative typical ground levels).

� The western region of the Mid Bedfordshire District falls in all directions from a raised plateau at Cranfield having an elevation of 110m Above Ordnance Datum (AOD) overlooking the western slopes of the Marston Vale. Land falls sharply to the south east of Cranfield to Marston Moretaine (40m AOD) which lies within a natural valley. Wixams New Settlement (35m AOD) also lies within the flat low-lying valley floor to the south of Bedford. Ampthill (90m AOD) lies on the Greensand Ridge, with land falling predominantly southwards towards a natural valley which separates Ampthill from Flitwick (80m AOD). Ground levels at Flitwick fall predominantly southwards towards the adjacent Flit valley (60m AOD).

� The central region of the Mid Bedfordshire District follows the low-lying Flit valley and comprises settlements such as Clophill (50m AOD) and Shefford (40m AOD).

� Settlements across the eastern half of the Mid Bedfordshire District are predominantly located adjacent to the River Ivel valley, with ground levels falling gently northwards from Stotfold (50m AOD) through Biggleswade (35m AOD) to Sandy (30m AOD). Arlesey lies adjacent to the River Hiz valley at a typical elevation of 40m AOD. Potton is separated from Sandy by intervening higher ground, but it lies in a natural valley at a typical elevation of 35m AOD, with ground levels generally falling in a south westerly direction towards the River Ivel.

2.2.2 Ordnance Survey Contour Mapping information for the study area has been presented graphically within Appendix B.

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2.3.1 There are three principal drainage catchments within the study area; these are the Rivers Ivel, Hiz and Flit, which form tributaries of the River Great Ouse. Refer to Watercourse Plan in Appendix C.

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2.3.2 The River Ivel flows from its headwaters at Baldock (North Hertfordshire) to Stotfold before heading in a northerly direction to Langford, Biggleswade and Sandy to its confluence with the River Great Ouse at Tempsford adjacent to the northern boundary of the District.

2.3.3 The River Hiz flows from its headwaters south of Hitchin (North Hertfordshire) in a northerly direction to Arlesey and on towards its confluence with the River Ivel to the north east of Henlow. A tributary of the Hiz, the Pix Brook, rises in Letchworth (North Hertfordshire) before flowing in a northerly direction to Stotfold, then north west to its confluence with the Hiz to the north of Church End village.

2.3.4 The River Flit flows from its headwaters north of Luton in a northerly direction to Flitwick where it merges with a significant tributary (see 2.3.5 below) before heading in a north easterly direction to Shefford. At Shefford, the Flit merges with a significant tributary (see 2.3.5 below) before flowing north eastwards as the River Ivel Navigation, a historic man made watercourse, which outfalls to the River Ivel to the north of Langford.

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2.3.5 In addition to the three Main Rivers, there are intricate networks of watercourses and drains throughout the district. The most significant in terms of the study area are the following:

� Watercourse No. 163 which bisects Ampthill and Flitwick, flowing eastwards to its confluence with the Flit near Flitton.

� Watercourse No. B1(4) which flows from Lidlington to Marston Moretaine before becoming Watercourse B40 which flows in a north easterly direction from Marston Moretaine to Stewartby Flood Balancing Reservoir.

� Watercourse B3(1) ‘Harrowden Brook’ which flows along the western and northern boundaries of The Wixams new settlement before flowing in a predominantly north easterly direction to its confluence with Elstow Brook at Harrowden, on the outskirts of Bedford.

� Watercourse No. 151 (the River Hit) which accepts flows from several small tributaries at Upper Gravenhurst before flowing in a north easterly direction towards Shefford to its confluence with the River Ivel Navigation.

� Watercourse No. 88A which flows from Northill in a north westerly direction to its confluence with the River Ivel downstream of Beeston.

� Potton Brook which flows through Potton in a combination of open channel and culverted sections before heading in a south westerly direction towards the River Ivel.

� Watercourse No. 74 which flows northwards along the eastern fringes of Biggleswade to its confluence with Potton Brook, eventually outfalling to the River Ivel upstream of Sandy.

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2.3.6 There are understood to be 28 ‘Award Drains’ within the district’s operating area totalling 8.7km in length. All of these drains have recently been inspected and mapped in accordance with the Environment Agency’s National Flood Coastal Defence Database guidelines; they have also been rated in order of flood risk. This information is available to developers from Mid Bedfordshire District Council and will allow the flood risks associated with each drain to be assessed in relation to adjacent proposed development sites on a site specific basis.

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2.4.1 Main Rivers within the district generally fall under the jurisdiction of the Environment Agency (Anglian Region) Central Area Office, based in Brampton, Huntingdonshire.

2.4.2 The Bedford Group of Internal Drainage Boards control the remaining drains and watercourses within the district. The River Flit upstream of Shefford, falls under the jurisdiction of the Bedfordshire and River Ivel IDB. Refer to the watercourse plan in Appendix C.

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2.4.3 Public sewerage within the Mid Bedfordshire District falls under the jurisdiction of Anglian Water.


3 General Approach & Methodology

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3.1.1 This Strategic Flood Risk Assessment has been conducted in line with the Environment Agency’s Guidance for Strategic Flood Risk Assessments, the Department for Communities and Local Government’s Practice Guide Companion to PPS25 ‘Living Draft’ and Mid Bedfordshire District Council’s Brief to Consultants dated November 2006.

3.1.2 The study has been carried out in 3 stages, namely:

� Stage 1 which comprised an initial review of allocated and proposed development sites alongside existing EA Flood Zone Mapping (completed July 2006).

� Stage 2A focused on data collection and review, and included the identification and assessment of the principal sources of flood risk associated with the study area in relation to existing, proposed, and potential key development proposals. It also highlighted the limitations associated with existing flood mapping and technical data available and set out recommendations for refinement and enhancement of hydraulic modelling data currently available (completed February 2007).

� Stage 2B (this report) focuses on providing a strategic overview of flood risk constraints and strategic opportunities within the District (based upon existing hydraulic modelling data and EA flood mapping), together with an assessment of flood risk issues concentrating on likely future growth areas and potential development sites across the 11 key settlements.

3.1.3 An additional stage could be undertaken in the future. This would effectively concentrate upon the refinement and enhancement of existing hydraulic modelling data, and the re-definition of flood outlines including an assessment of climate change impacts and the definition of the functional floodplain.

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3.2.1 The principal sources of flooding within the study area that have been focussed upon within the Stage 2B Strategic Flood Risk Assessment include:

� Fluvial (river) flooding resulting from ‘out of bank’ flows from rivers and watercourses;

� Surface water runoff from new development;

3.2.2 Fluvial flooding is the dominant source of flood risk within the district and will clearly have the greatest influence upon land-use planning. It would, therefore, be appropriate to make strategic allocations of land based upon this data set at this stage.

3.2.3 Fluvial flood risk assessment has been based primarily upon the flood outlines indicated upon the Environment Agency’s Flood Zone Maps; supplemented by hydraulic modelling data results and more site specific information where applicable and available.


3.2.4 Other potential sources of flooding within the district which have been considered on a more holistic basis as part of the Stage 2B study, include the following:

� Overtopping of flood defence structures including flood storage facilities;

� Breach (failure) of flood defences or flood alleviation schemes;

� Groundwater flooding, including groundwater-fed watercourses;

� Localised surface water flooding, including from highway drainage;

� Overland flow; and

� Drainage infrastructure including sewers, pumping stations, water mains etc.


4 Data Collection And Review

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4.1.1 A considerable range and breadth of data was gathered and collated as part of Stages 2A and 2B of the study. In summary, key sources of data include (but are not limited to):

� Environment Agency Flood Zone Maps, Environment Agency publications and archive reports;

� Reports and studies by consultants;

� Hydraulic modelling data;

� Topographical survey data via Ordnance Survey mapping and LiDAR information;

� Historical flood outlines;

� Flood defence and key asset information;

� Archive and Internet research;

� Local knowledge (including Parish and Town Council questionnaires);

� Local Plan policy documents and Settlement Hierarchy;

� Geological data;

� Anglian Water and East of England Plan sewerage infrastructure capacity study data.

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4.2.1 The Environment Agency publish Flood Zone Maps, which show areas potentially deemed to be at risk of fluvial (river) flooding. The Flood Zone Maps have been produced using appropriate good quality mapping and modelling data, where available, supplemented with data derived from national generalised modelling and appropriate good quality local data which conform to the Agency’s acceptable criterion. The nationally generalised modelling utilises a Digital Terrain Model which excludes the presence of man-made features such as flood defences and road and rail embankments. Fluvial flood zone outlines were produced using a 2-dimensional raster floodplain model (Jflow) and show the probability of flooding without the presence of defences.

4.2.2 Whilst the modelling methodology used to produce Flood Zone Maps excludes the presence of flood defences, in order to ensure that the extent of the functional floodplain is delineated, the Flood Zone Maps show areas that benefit from defences that provide protection against the 1:100 year flood event. The maps also show areas deemed to be at risk of flooding for the 1 in 100 year return period event (Flood Zone 3), and 1 in 1000 year return period event (Flood Zone 2) for all watercourses with a catchment area greater than 3 km2 in the UK (Flood Zone 2 also includes historic flood extents where these are greater than the 1 in 1000 year modelled extent). Areas outside the 1 in 1000 year flood extent are classified as being in Flood Zone 1.

4.2.3 Flood Zone Mapping data has been provided by the Environment Agency, in electronic shape file format, for the study area. Flood Zone Maps, supplemented with hydraulic modelling data where applicable, provide the basis for figures provided in Appendix E.


4.2.4 Across the study area, Flood Zone Mapping data would generally appear to shadow the routes of the rivers and watercourses, with no significant off-setting of the mapping layers noted, with the following exception:

� River Ivel to the north of Malthouse Lane, Stotfold

4.2.5 There would, however, appear to be no significant deficiencies in the graphical representation of flood risk areas and Flood Zone Maps currently available for the study catchment would appear to be fit for purpose.

4.2.6 It is important to note that in order to apply a precautionary principle, the EA have requested that all areas shown in Flood Zone 3 should be considered as Flood Zone 3b (functional floodplain) unless otherwise proven. This also applies to the areas covered by detailed hydraulic modelling along the River Ivel and River Flit. However, this situation may change as additional hydraulic modelling is undertaken along these watercourses.

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Fluvial

4.3.1 Historic flooding information has principally been obtained from desk studies and archive research as only sparse records were available from the Environment Agency. Historic fluvial flooding locations have been shown graphically in Appendix E.

4.3.2 Details of selected historic fluvial flooding records gathered during the study have been tabulated below. Note that these focus upon the key settlements forming the study area and not the entire district.

Location Address Date Source of Flooding / Details

Data Source

Ampthill Holland Road 2002 Sweet Briar Brook

Mid Bedfordshire District Council and Ampthill Town Council

Ampthill Land between Sweet Briar Ditch and Alameda middle school

2002 Sweet Briar Brook

Ampthill Town Council

Ampthill Ashburnham Road

2002 Sweet Briar Brook

Mid Bedfordshire District Council

Ampthill Elms Close 2002 Highway drainage


Arlesey Stotfold Road 2002 Pix Brook Mid Bedfordshire District Council


Biggleswade Osprey Road,

Kittiwake Close, Avocet Close, Kingfisher Close.

2003 River Ivel Mid Bedfordshire District Council and Bedfordshire and Luton Flood Response Plan.

Biggleswade Extensive 1998 River Ivel Mid Bedfordshire District Council

Biggleswade Extensive 1947 River Ivel Environment Agency

Cranfield High Street, Lodge Road and Court Road

Unknown Blocked culvert and adjacent ditches

Cranfield Parish Council

Flitwick Templefield School, Steppingley Road

2002 Highway drainage


Flitwick Ampthill Road Ongoing since 1996

Culverted watercourse with insufficient capacity


Marston Moretaine

Bedford Road Frequent Blocked grille at back of gardens


Marston Moretaine

Station Road 2003 Unknown Bedfordshire and Luton Flood Response Plan

Potton Sutton Mill Road 2005 Highway drainage


Potton Brook End Unknown Highway drainage

Potton Town Council

Sandy Unknown 1875 River Ivel Hydrochronology website

Sandy New Road 1907 Thawing snow and heavy rainfall

Bedford Mercury

Sandy Extensive 1947 River Ivel Environment Agency

Sandy Bedford Road, Willow Farm, Ivel Road, Holiday Inn and Yew Tree Cottage

2003 River Ivel Bedfordshire and Luton Flood Response Plan, Mid Bedfordshire District Council


Sandy Bedford Road 2006 River Ivel Sandy Town Council

Sandy Riddy Nature Reserve

Unknown River Ivel Sandy Town Council

Shefford Unknown 1841

1947

River Ivel Hydrochronology website

Shefford Maple Drive 1993 Excess overland flow from fields affected 26 properties


Shefford Manor Cottage, Beadlow Manor, The Swan Public House, Stanford Road, Purcell Way, Clifton Road and Ampthill Road

2003

River Flit

Bedfordshire and Luton Flood Response Plan and Mid Bedfordshire District Council and Shefford Town Council.

Shefford Main streets 1918 Unknown Hydrochronology website

Shefford Tippet Drive 2003 Tree roots blocking privately owned watercourse.

Mid Bedfordshire District Council.

Stotfold Brook Street Unknown Pix Brook Stotfold Town Council

Wixams No Data Available No Data Available

No Data Available

No Data Available

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4.3.3 Information relating to return periods of historic events is inherently subjective, largely anecdotal, and scarcely available. The Environment Agency can provide historic flood levels for certain areas of the Mid Bedfordshire district.

Groundwater

4.3.4 Historic groundwater flooding locations within the district were provided by the Environment Agency. No recorded events were noted within the study area.

4.3.5 Natural springs are known to be prevalent within Cranfield and Ampthill, which may pose localised flood risks when activated by high groundwater conditions.


Sewerage

4.3.6 Historic flooding information for the majority of the study area has been obtained from the DG5 ‘At Risk’ Register provided by Anglian Water, together with archive research.

4.3.7 The DG5 register sets out property addresses which:

� have been affected by flooding due to hydraulic deficiency on two or more occasions within the last ten years; or

� are protected from internal property flooding by non return valves.

4.3.8 Further information on sewer flooding has been obtained from Mid Bedfordshire District Council officers and from research.

4.3.9 Details of historic flooding records gathered during the study have been tabulated below and presented graphically in Appendix F:

Location Address Details Data Source

Biggleswade Boddington Gardens

Unknown Anglian Water (DG5)

Flitwick Fir Tree Close Unknown Anglian Water (DG5)

Marston Moretaine

Lower Shelton Road


Sandy Market Square Poor highway drainage Sandy Town Council

Shefford Ampthill Road and Stanford Road


4.3.10 The DG5 register is maintained by the sewerage undertaker’s network management team from information gathered from verified complaints made by the public, observations made by operational staff during flood events and, to a lesser degree from hydraulic modelling studies.

4.3.11 It should be noted that properties may not appear on the DG5 register, or be removed from the register, if:

� the frequency of recurrence of flood events is less than 1 in 10 years;

� flood alleviation schemes have been implemented;

� insufficient significant rainfall events have occurred within the 10 year timeframe.

4.3.12 Historic sewer flooding locations have been shown graphically in Appendix F in order to ascertain any clear patterns or trends within the data set. Based upon the limited records identified, sewer flooding in the district may be deemed not to influence land use allocations at this time.


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LiDAR Data

4.4.1 Topographical data points in the form of LiDAR data has been made available for the study area from the EA’s Data Management Team based in Twerton. Sufficient data was obtained in order to complement any future hydraulic modelling and flood mapping.

4.4.2 The vertical tolerance of LiDAR data typically ranges between +/- 0.2m, whilst that of SAR (surface model) data ranges between +/- 0.5m. This level of accuracy is often unsuitable for detailed hydraulic modelling but is highly suitable for strategic assessments and for the assessment of overland flood routing whereby the gradient of the terrain is the key requirement.

Topographical Survey Data

4.4.3 Contour mapping information provided on 1:10,000 scale Ordnance Survey maps for the entire district has been provided by Mid Bedfordshire District Council in digital format. This has been used to describe the topography of the study area within Section 2.2.

4.4.4 No detailed topographical survey data was made available for the study.

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4.5.1 Refer to the Watercourses Plan provided within Appendix C detailing the location and jurisdiction of watercourses within the study area.

4.5.2 Stewartby Flood Balancing Reservoir near Marston Moretaine provides the most significant strategic open water body within the study area, providing substantial attenuation of flood flows being conveyed to the downstream reaches of Elstow Brook.

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National Flood and Coastal Defence Database (NFCDD)

4.6.1 Details of hydraulic structures have been provided by the Environment Agency in electronic graphics compatible format for the study area and presented upon the Ordnance Survey Contour Mapping in Appendix B.

4.6.2 Structure details from the Environment Agency’s NFCDD records for the River Ivel show a range of raised embankments and hydraulic structure arrangements along the Ivel Valley. The level of information provided for the Ivel is relatively good but the Environment Agency have advised that data fields pertaining to ‘Standard of Protection’ and ‘Crest Levels’ should be treated with caution, but they give an initial source of data to work with.

4.6.3 The NFCDD is often incomplete for Main River reaches, and does not include hydraulic structures along Ordinary Watercourses, land drainage, or under private ownership.

4.6.4 Detailed surveys of key hydraulic structures should be undertaken as part of any future detailed hydraulic modelling and hydraulic assessments.


WSP Datasheets

4.6.5 A site walkthrough along the River Ivel, Potton Brook, River Flit, River Hiz and Pix Brook, coupled with visits to Cranfield, Marston Moretaine and The Wixams, was undertaken as part of the Stage 2A study, in order to view key hydraulic structures. Datasheets have been prepared for each hydraulic structure, which comprise a photograph and key information relating to the structure. These will be issued to Mid Bedfordshire District Council under separate cover to supplement their asset management data. No detailed dimensional survey work was carried out as part of this site walkthrough. These key hydraulic structures have been shown in Appendix C.

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Flood Alleviation Schemes

4.7.1 Stewartby Flood Balancing Reservoir near Marston Moretaine provides substantial volumes of attenuation storage, allowing continuation flows to the downstream reaches of Elstow Brook to be regulated via a sluice arrangement. In the unlikely event of a breach (failure) or overtopping of the reservoir, which is considered to be a residual risk, water would tend to flow in all directions due to the relatively level topography in this area.

4.7.2 Pix Brook Flood Storage Reservoir, located to the north of Letchworth in North Hertfordshire, provides a significant quantum of flood storage and regulation of continuation flows thus helping to safeguard areas downstream along the Pix Brook at Stotfold, within the Mid Bedfordshire District. In the unlikely event of a breach (failure) or overtopping of the reservoir, water would tend to be contained within the valley possibly flowing in a north to south direction as opposed to east and west. Under these circumstances, this may pose a threat of flooding to communities such as Stotfold if the reservoir was full.

4.7.3 The Bedfordshire and River Ivel Internal drainage Board have stated that both Stewartby and Pix Brook reservoirs are registered under the 1975 Reservoirs Act. As both these reservoirs are inspected and regularly maintained, the likelihood of failure is considered to be very low.

Flood Defences

4.7.4 Main River flood defence details have been provided by the Environment Agency (see Appendix C); throughout the district these take the form of raised embankments as opposed to formal defences. Following a technical review, the presence of the raised embankments generally enhance conveyance capacity within the River Ivel and do not formally provide flood protection to adjacent development. Under such circumstances, channel capacity will afford a certain level of protection as opposed to formal defences.

4.7.5 The extent of raised flood embankments within the study area is largely confined to the River Ivel along the following reaches:

� North west of Biggleswade, Ivel Gardens to Fairfield Cottage;

� Lower Caldecote to Mill Lane, Sandy;

� West of Sandy, Girtford Bridge to Sewage Works.


Scheme Feasibility Reports

4.7.6 No relevant scheme feasibility reports have been identified within the study.


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Existing Studies

4.8.1 The Stage 2A assessment identified the extent of hydraulic modelling data available for the study area. Existing model coverage has been set out within the hydraulic modelling data review in Appendix J.

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Sewer Records

4.9.1 Sewer records and network plans for the Anglian Water region, have been obtained via Anglian Water’s Regional Mapping Service in order to view the extent and layout of the public sewerage network, and to assess the likely impact of future growth upon the system.

4.9.2 Details of existing discharge consents, capacity, and scope for expansion of local Sewage Treatment Works were discussed with Anglian Water during the stakeholder engagement.

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Geological Maps

4.10.1 British Geological Survey maps were obtained for review during the Stage 2B study as part of the SUDS viability assessment.

4.10.2 Refer to the tables in Section 8.1 for an overview of the geology underlying the study area.

Source Protection Zone Maps

4.10.3 Source Protection Zone boundaries, in electronic graphical compatible format, were provided by the Environment Agency.

Hydrometric Data

4.10.4 Limited hydrometric data appears to be held by the Environment Agency. Formal data requests should be made upon commencement of any supplementary hydraulic modelling studies.

Contaminated Land Issues

4.10.5 Generic advice on the treatment of contaminated land issues was sought from Mid Bedfordshire District Council as part of the Stage 2A study to inform the SUDS viability assessment.

4.10.6 Due to the ongoing release of new studies and data, together with the potentially sensitive nature of this type of information, it was deemed appropriate to assess SUDS viability independently of this data source, but make reference to its presence as a consideration of overall SUDS viability as part of this Level 2B study.

Ecological Issues

4.10.7 No specific principles, requirements, or criteria for the provision of strategic ecological and landscaping buffers within development proposals were highlighted by Mid Bedfordshire District Council as being required to feed into the Strategic Flood Risk Assessment.


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4.11.1 Within the Mid Bedfordshire District, as elsewhere in England, the responsibility for flood warning rests primarily with the Environment Agency. The Environment Agency provides flood warnings for designated Flood Warning Flood Risk Areas across the Mid Bedfordshire District; these warnings will only cover fluvial flooding.

4.11.2 Based upon information gained from the Environment Agency’s published records, flood warning coverage extends along a reach of the River Ivel, incorporating only the following key settlements within the study area:

� Biggleswade

� Sandy

� Shefford

4.11.3 According to the Bedfordshire and Luton Flood Response Plan flood warning procedures cover the River Ivel including the Flit, Hit and Hiz tributaries in the Mid Bedfordshire district.

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4.12.1 In line with the draft East of England Plan (Proposed Changes), Mid Bedfordshire District Council, will be required to accommodate 14,230 dwellings within the plan period to 2021.

4.12.2 Potential Local Development Framework growth areas have been provided by Mid Bedfordshire District Council and have been included in Appendix G, which have been developed as part of an ongoing iterative process by Mid Bedfordshire District Council taking into account a wide range of sustainability factors, one of which includes flood risk.

4.12.3 Sufficient information on potential development and growth areas is available to assess its corresponding impact upon flood risk and capacity of local sewerage infrastructure.

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4.13.1 Stakeholder involvement as part of the Stage 2A Strategic Flood Risk Assessment was obtained from officers from the Mid Bedfordshire District Council within the following disciplines; Planning, Engineering, Building Control, Emergency Planning and Land Drainage.

4.13.2 Meetings have been held with Mid Bedfordshire District Council to determine pertinent information held with respect to the key development sites and in an effort to determine where proposed development sites are likely to be located.

Environment Agency

4.13.3 Meetings have been held with the Environment Agency to establish contact and to set out a schedule of data requirements. Extensive liaison with the Environment Agency’s Development Control, Planning Liaison, Flood Risk Mapping, and Asset Management teams has been undertaken in order to obtain, or confirm the availability of, relevant data for the study.


Bedfordshire and River Ivel Internal Drainage Board

4.13.4 Meetings were held with the Internal Drainage Board in order to obtain relevant data and historical records for the study.

Anglian Water

4.13.5 A meeting was held with Anglian Water to seek to establish the potential impact of proposed future growth upon the public sewerage system.

Other Stakeholders

4.13.6 Completed flooding questionnaires have been received from the Clerk of the Parish Councils for Cranfield, Henlow, Campton, Chicksands, Clifton, Langford, Astwick and Marston Moretaine in order to obtain further historic and anecdotal information relating to flooding in the study area.

4.13.7 Completed flooding questionnaires have been received from the Clerk of the Town Council for Shefford, Flitwick, Stotfold, Arlesey, Biggleswade, Potton, Sandy and Ampthill in order to obtain further historic and anecdotal information relating to flooding in the area.

Media

4.13.8 A copy of a recent (2006) Anglia TV programme entitled ‘Secret Rivers’ which followed the route of the River Ivel, from its headwaters at Baldock through the Mid Bedfordshire District Council study area, to its confluence with the River Great Ouse was obtained. Information from this programme has been useful for assessing the impact of hydraulic structures, including mill structures, and would be useful as part of any future hydraulic modelling studies.

Records Search

4.13.9 A variety of other data sources have been investigated as part of the Stage 2A and Stage 2B study. These include:

� Hydrochronology Database

� Website Search

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4.14.1 Following a review of baseline information collected during Stage 2A of this Strategic Flood Risk Assessment, it was concluded that there was sufficient information currently available, in terms of quantity and quality, for significant areas of the study catchment to progress to the Stage 2B assessment (refer to Appendix J).

4.14.2 Recommendations were set out within the Stage 2A report, primarily relating to areas for further investigation to complete the Stage 2B process. These principally focussed upon refinements to the existing hydraulic modelling data and included the following key elements:

� Address deficiencies within the River Ivel hydraulic modelling where flood extents may be over-cautious due to crude modelling techniques being used to assimilate the presence of raised embankments;

� Extend the River Ivel model to incorporate Stotfold and the lower reach of the River Hiz at Arlesey;


� Run hydraulic models available within the study catchment for a 1 in 20 year return period event in order to better define the functional floodplain (Flood Zone 3b), as required by Planning Policy Statement 25;

� Run hydraulic models available within the study catchment for a 1 in 100 year return period event, incorporating an allowance for climate change (typically a 20% increase in fluvial flow), in order to better define the predicted flood extents for the lifetime of the proposed developments, as required by Planning Policy Statement 25;

� Run hydraulic models available within the study catchment for a 1 in 1000 year return period event, in order to better define Flood Zone 2, as required by Planning Policy Statement 25;

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4.15.1 Due to time constraints it was agreed that the Stage 2B assessment be undertaken based upon existing data, comprising a combination of hydraulic modelling data and Environment Agency Flood Zone Maps.

4.15.2 The proposed approach clearly introduces some limitations to the study; however, the appraisal of potential land allocations based on the Sequential test, represents an appropriate methodology for district-wide land-use planning and is fully in line with the Level 1 methodology set out within Department for Communities and Local Government’s Practice Guide Companion to Planning Policy Statement 25 ‘Living Draft’.


5 Strategic Flood Risk Assessment (Stage 2B)

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5.1.1 The Strategic Flood Risk Assessment gives, as its name implies, a strategic overview of flood risk in the Mid Bedfordshire District. It should be noted that:

� this Strategic Flood Risk Assessment reflects planning policies in place at the time of writing;

� policies may change; and

� flood levels / flood zone classifications may change.

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5.2.1 A strategic assessment of the principal sources of flood risk within the district has been made based upon the data collected within the Stage 2A and Stage 2B studies. The sections below set out the findings from the Stage 2B assessment for each fluvial (river) catchment, highlighting the likely constraints to future development growth arising from the alternative flood risk sources, management and control of surface water runoff, and in terms of sewerage infrastructure.

5.2.2 Potential outline strategic flood mitigation opportunities have been identified, along with ways in which residual flood risks may be managed, either strategically or on a site specific basis, as appropriate.

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5.3.1 Refer to flood mapping and historic flooding locations depicted graphically in Appendix E.

5.3.2 Predicted flood extents and areas designated as being at risk of fluvial flooding generally correspond well with historic flooding locations highlighted during the study.

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5.4.1 While groundwater flooding is not deemed to pose a significant risk within the study area, natural springs and groundwater-fed streams are known to be prevalent in Ampthill, and to a lesser degree in Cranfield.

5.4.2 It is imperative, however, that careful consideration be given to this source of flooding by developers at the planning stage as part of any site specific Flood Risk Assessment prepared to support their proposals. The impact of this source of flooding was not deemed sufficiently significant to affect district-wide land-use planning.

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5.5.1 Other potential sources of flood risk from overland flow, sewers, water mains would need to be assessed by developers at the planning stage as part of a site specific Flood Risk Assessment prepared to support their proposals. The impact of these other sources of flooding were not deemed sufficiently significant to affect district-wide land-use planning.


6 Assessment Of The Impact Of Climate Change

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6.1.1 Annex B of Planning Policy Statement 25 takes into account the impacts that climate change may have on flooding issues.

6.1.2 Within the study area all main rivers and watercourses are fluvial and not tidally influenced in any way.

6.1.3 Planning Policy Statement 25 recommends that climate change impacts for fluvial catchments be represented as an increase in flood levels and water level rise within the study catchment over the lifetime of the development, which for residential development is to 2115. Planning Policy Statement 25 Annex B (Table B.2) recommends a peak fluvial flow increase for the critical 1 in 100 year return period event of 20%, and is a figure widely accepted by the Environment Agency.

6.1.4 For the purposes of this Stage 2B study, none of the hydraulic modelling work undertaken has taken account of climate change impacts, thus a visual interpretation of the impact of climate change upon the 1 in 100 year flood outline has been taken as being the midpoint between the extent of Flood Zone 3 (1 in 100 year) and Flood Zone 2 (1 in 1000 year).

6.1.5 Where doubt remains, the precautionary approach should be taken whereby the extent of Flood Zone 2 should be taken as being the extent of the 1 in 100 year outline incorporating climate change.


7 Flood Management Areas

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7.1.1 As set out in Section 4.7, the extent of informal flood defences in the form of raised flood embankments, within the study area is largely confined to the River Ivel. It has been ascertained that the presence of the raised embankments generally enhance conveyance capacity within the River Ivel and do not formally provide flood protection to adjacent development. As previously stated, channel capacity will afford a certain level of protection as opposed to formal defences.

7.1.2 Standards of protection offered by the existing informal defences are, therefore, likely to relate to standards required for agricultural land uses as set by Defra or MAFF and not to a level of protection that would be appropriate to safeguard existing or new development in line with Planning Policy Statement 25 criteria.

7.1.3 For accurate assessment of the standard of protection offered by existing flood defence arrangements, hydraulic modelling will be required in order to run a range of return period events to assess the performance level of sections of flood defence.

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7.2.1 No detailed hydraulic modelling has been undertaken to assess the reliance placed upon infrastructure such as raised road and railway embankments acting as a flood defence within the study area. Raised transport infrastructure acting as a defence, also falls outside of the Environment Agency’s remit, so little technical information was available for use within the Stage 2B study.

7.2.2 From site visits and perusal of flood mapping it is apparent, however, that the following areas may benefit from the presence and function of such defences:

� Arlesey (potentially benefits from presence of adjacent railway embankment);

� Flitwick northern fringe (potentially benefits from presence of adjacent railway embankment);

7.2.3 The function, performance and integrity of any defence from transport infrastructure, would need to be assessed by developers at the planning stage, as part of a site specific Flood Risk Assessment prepared to support their proposals. It would need to be demonstrated as part of any proposals that any such defences, having an important hydraulic function, were deemed fit for purpose. Planning consent would need to be conditional upon a formal inspection and maintenance programme being implemented, to help safeguard the structure for the lifetime of the development.

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7.3.1 Due to the lack of areas that rely upon flood defences within the Mid Bedfordshire District Council study area, there are no significant corresponding zones of rapid inundation of floodwater in the event of flood defence breach (failure).

7.3.2 Raised flood embankments within the study area (refer to Section 4.7.5) do not act as formal flood defences and no areas of development are currently indicated as benefiting from, or being reliant upon, these structures. It may, therefore, be concluded that any zone of rapid inundation resulting from a flood embankment failure would not extend beyond the current 1 in 100 year flood outline (Flood Zone 3a) and would be highly unlikely to impact upon any existing development, or any future built development.


7.3.3 Only two hydraulically significant impounding structures have been identified as part of the Stage 2B study, namely:

� Stewartby Flood Balancing Reservoir

� Pix Brook Flood Storage Reservoir

7.3.4 Although the probability of occurrence is low, there is the potential for either impounding structure to fail suddenly, releasing significant volumes of floodwater within a short duration towards downstream areas. In the case of Stewartby Reservoir, the consequences to downstream areas are relatively high as areas immediately downstream comprise existing residential and commercial development; the terrain in this area is low lying in all directions. In the case of Pix Brook, consequences are lower as areas immediately downstream are currently rural; however, due to the topography of the valley, flows would be swiftly routed to the urban area of Stotfold further downstream, albeit at a controlled rate due to the presence of a culvert beneath the A507 carriageway (see Appendix D).

7.3.5 Due to the significant volumes of floodwater held within each facility there is a potential risk of rapid inundation of downstream areas in the event of a structural failure, operational error, or act of vandalism. Whilst a highly unlikely scenario, siting of any built development downstream within close proximity (circa 1000 metres) should be avoided unless it can be demonstrated that flood risks due to rapid inundation may be eliminated or adequately mitigated.

7.3.6 Siting of any built development upstream of Stewartby Reservoir should also be avoided unless it can be demonstrated that flood risks due to rising water levels within the impounding facility may be adequately mitigated.

7.3.7 As stated in section 4.7.3, the Bedfordshire and River Ivel Internal drainage Board have stated that both Stewartby and Pix Brook reservoirs are registered under the 1975 Reservoirs Act. As both these reservoirs are inspected and regularly maintained, the likelihood of failure is considered to be very low.


8 SUDS Viability

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8.1.1 The British Geological Survey 1:50,000 Solid and Drift editions 204, (Biggleswade) 220, (Leighton Buzzard) and 221, (Hitchin), the 1:63,560 Drift edition 203, (Bedford) and the NRA 1:100,000 Groundwater Vulnerability edition 31, (Bedfordshire) have been consulted to give the geological summary of the site area.

8.1.2 The area is underlain by the Lower Chalk formation in the south east, with the Gault, the Woburn Sands, the West Walton and Amptill Clay and the Oxford Clay formations subsequently outcropping towards the north west of the site. The Lower Chalk is exposed in the south east of the study area, whilst the Gault Formation is more locally exposed in the south west, north east and east of the site. The Woburn Sands are exposed centrally, and in the north east and south west of the site. The Oxford Clay outcrops within the northwest of the study area, south/south west of Bedford.

8.1.3 The drift deposits are more varied across the site and are predominantly deposits from the Anglian Glaciation, however Alluvium and River Terrace Deposits are recorded adjacent to the rivers in the study area. Table 8.1 and Table 8.2 overleaf give a general description of each of the strata encountered in the study area, and of the strata’s drainage potential. Descriptions of the geology across the eleven identified settlements within the Mid Bedfordshire study area follows the tables within Section 8.2.


Table 8.1: Drift Geology

Table 8.2: Solid Geology

Geological Name

Generic Description Soakaway Potential

Alluvium Soft to firm consolidated, compressible silty clay, but can contain layers of silt, sand, peat and basal gravel. Groundwater is likely to be encountered at shallow depth.

NO

Head deposits Variable clay, sand and gravel, poorly sorted and poorly stratified.

Uncertain

Glaciofluvial Deposits

Undifferentiated; sand and gravel with lenses of silt, clay or organic matter.

Uncertain

Glacial Sands & Gravels Sand and gravel locally with rare clay interbeds.

YES

Glacial Till Chalky, sandy, stony clay. NO River Terrace Deposits

Silt, sand and gravel. YES

Valley Gravels Sand and gravel locally with lenses of silt or clay. YES

Geology name

Generic description Soakaway potential

Lower Chalk Formation

A grey marly chalk with marl content decreasing upwards. No flint. Comprises a thin basal bed of glauconitic marl (Cambridge Greensand) overlain by more typical Lower Chalk sequence that is usually divided into a lower "Chalk Marl" with rhythmic alternations of chalk and marl, and an upper "Grey Chalk" separated by a distinctive hard band. (Totternhoe Stone).

NO

Gault Formation

Pale to dark grey or blue-grey clay or mudstone, glauconitic in part, with a sandy base. Discrete bands of phosphatic nodules (commonly preserving fossils), some pyrite and calcareous nodules. Thin variable junction beds at base have some limestones.

NO

Woburn Sands Formation

Sandstone (or loose sand). Fine- to coarse-grained rounded marine quartz sand, glauconitic in part. Commonly silty with few clay wisps or seams. Locally clean silver sands. Locally cemented to iron pan or gritty carstone. Cross-bedded. Some pebbles and phosphatic nodules to base. Typically grey or greenish grey weathering to ochreous yellow-brown. Minor lignite and pyrite.

Uncertain

West Watton & Ampthill Clay

Undifferentiated complex succession of medium to coarse grained sand, with an intervening silt unit. Thick Mudstone and Limestone unit towards the base with silty Mudstone and Cementstone nodules.

Uncertain

Oxford Clay Formation

Silicate-mudstone, grey, generally smooth to slightly silty, with sporadic beds of argillaceous limestone nodules. Bituminous in lower part.

NO


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Potton

Drift – Alluvium deposits adjacent to Potton Brook, with local bands of Head deposits. Solid – Woburn Sands Formation overlying West Watton & Ampthill Clay Formation. Within the eastern extreme of the settlement the Gault Formation is exposed. Settlement’s soakage/infiltration potential – Uncertain – requires soakaway test pits to confirm within Woburn Sands and West Watton & Ampthill Clay formations.

Biggleswade

Drift – Glacial deposits located within the east / south east of the settlement (Glaciofluvial deposits – south east, and Glacial Till – east and south of the settlement). River Terrace Deposits located within the west of the settlement, and Alluvium adjacent to the River Ivel to the west. Solid – Woburn Sands Formation overlying the West Watton & Ampthill Clay Formation. The Woburn Sands are exposed within the centre of the settlement. Settlement’s soakage/infiltration potential – Uncertain / YES – requires soakaway test pits to confirm within Glaciofluvial deposits, Woburn Sands and West Watton & Ampthill Clay Formations. Positive drainage potential within River Terrace deposits.

Sandy

Drift – Alluvium and River Terrace Deposits adjacent to the River Ivel, through the centre of the site. Solid – Oxford Clay Formation. Within the eastern and western extremes of the settlement, the Oxford Clay is exposed. Settlement’s drainage potential – YES – within the River Terrace Deposits.

Shefford

Drift – Alluvium and River Terrace Deposits adjacent to the River Ivel, through the centre of the settlement. Towards the outskirts of the settlement there are localised areas of Head and Glacial deposits. Solid – Woburn Sands Formation, with the Gault Formation present to the north and south. The Woburn Sands are exposed in the north west and south west, and the Gault is exposed in the extreme north and south of the settlement.


Settlement’s soakage/infiltration potential – YES / Uncertain – requires soakaway test pits to confirm with Woburn Sands. There is a positive potential within River Terrace Deposits, although this is limited in extent.

Arlesey

Drift – Limited to none. Made Ground is noted to the west of the settlement. Solid – Lower Chalk formation over the Gault Formation. The Lower Chalk is exposed across the majority of the settlement. Settlement’s soakage / infiltration potential – NO

Stotfold

Drift – Alluvium adjacent to the River Ivel, with Glaciofluvial deposits present across the majority of the settlement. Solid – Lower Chalk Formation. Settlement’s soakage / infiltration potential – NO / Uncertain – requires soakaway test pits to confirm within the Glaciofluvial deposits.

Flitwick

Drift – Alluvium adjacent to the River Flit, Glacial deposits located within west and east (Glacial Sands & Gravels – east and northwest, and Glacial Till – west). Head deposits are also present in the north, south east and extreme east of the settlement. Solid – Woburn Sands Formation. The Woburn Sands are exposed in the centre of the settlement. Settlement’s soakage / infiltration potential – Uncertain – requires soakaway test pits to confirm within Glaciofluvial Sands & Gravels, Head deposits and Woburn Sands Formation.

Ampthill

Drift – Alluvium and Head deposits located in the south of the settlement. Solid – Woburn Sands Formation. The Woburn Sands are exposed in the majority of the settlement, with localised areas of exposed West Watton & Ampthill Clay formation. Settlement’s soakage / infiltration potential – Uncertain – requires soakaway test pits to confirm within the Head deposits and the Woburn Sands and West Watton & Ampthill Clay Formations.


Marston Moretaine

Drift – Alluvium through the centre of the site, with Valley Gravels located within the extreme east of the settlement. Solid – Oxford Clay Formation. The Oxford Clay is exposed across the majority of the settlement. Settlement’s soakage / infiltration potential – YES– within the Valley Gravels, although this is a localised deposit.

Cranfield

Drift – Glacial Till is present across the majority of the settlement. Solid – Oxford Clay Formation. The Oxford Clay is exposed in the south east of the settlement.

Settlement’s soakage / infiltration potential – NO

Wixams New Town

Drift - Limited to no significant drift deposits. Solid – Oxford Clay Formation. The Oxford Clay is exposed across the majority of the settlement. Settlement’s soakage / infiltration potential – NO

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8.3.1 The underlying ground conditions of a development site will often determine the type of SUDS approach to be used at development sites for the disposal of surface water runoff. This will need to be determined through ground investigations carried out on-site, however an initial assessment of a sites suitability to the use of SUDS can be obtained from the review of the available soils / geological survey of the area presented in Tables 8.1 and 8.2.

8.3.2 The SUDS viability plans presented in Appendix H of the Strategic Flood Risk Assessment gives an indication of the ground conditions found in the Mid Bedfordshire district area in terms of their permeability and appropriateness for the use of SUDS infiltration techniques. These definitions are based on a desk study review of available information and our experience and should not supersede site-specific data and ground investigations carried out.

8.3.3 In the design of any drainage system and SUDS approach, consideration should be given to site-specific characteristics and where possible be based on primary data from site investigations. The information presented in the Tables 8.1 and 8.2 is provided as a guide and should not be used to accept or refuse SUDS techniques.


8.3.4 Further advice must also be sought in terms of water resources, contaminated land, archaeology, ecology etc.

8.3.5 The initial appraisal of SUDS Viability will, however, help to inform the allocation of land for development in a sustainable manner as preference should be given to sites underlain by ground conditions that are suitable for SUDS infiltration techniques over other sites having less viable ground conditions. This may be used as a criteria for ranking sites in terms of suitability where a number of alternative sites are situated within the same Flood Risk Zone.

8.3.6 Potential SUDS techniques are discussed in Section 8.4 below.

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8.4.1 Traditionally, built developments have utilised piped drainage systems to manage storm water and convey surface water run-off away from developed areas as quickly as possible. Typically these systems connect to the public sewer system for treatment and/or disposal to local watercourses. Whilst this approach rapidly transfers storm water from developed areas, the alteration of natural drainage processes can potentially impact on downstream areas by increasing flood risk and reducing water quality. Receiving watercourses are therefore much more sensitive to rainfall intensity, volume and catchment land uses after a catchment or areas of a catchment have been developed.

8.4.2 Due to the difficulties associated with upgrading sewer systems it is uncommon for sewer and drainage systems to keep pace with the rate of development / re-development and the increasingly stringent controls placed on discharges to watercourses. As development progresses and / or urban areas expand these systems become inadequate for the volumes and rates of storm water they receive, resulting in increased flood risk and / or pollution of watercourses. Allied to this are the implications of climate change on rainfall intensities, leading to flashier catchment responses and surcharging of piped systems.

8.4.3 In addition, as flood risk has increased in importance within planning policy, a disparity has emerged between the design standard of conventional sewer systems (1 in 30 year) and the typical design event (1 in 100 year). This often results in drainage inadequacies for the flood return period developments need to consider, often resulting in potential flood risk from surface water sewers and combined sewer systems.

8.4.4 A sustainable solution to these issues is to reduce the volume and / or rate of water entering the sewer system and watercourses.

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8.4.5 Sustainable Drainage Systems (SUDS) are the preferred method (as set by the Environment Agency, Government, and Building Regulations) for managing the surface water runoff generated by developed sites. SUDS seek to manage surface water runoff as close to source as possible. Typically this approach involves a move away from piped systems to softer engineering solutions inspired by natural drainage processes, where adoptable drainage criteria allow.


8.4.6 In addition, they should be designed to take into account the surface water run-off quantity, rates and also water quality ensuring their effective operation up to and including the 1 in 100 year design event.

8.4.7 Where possible, a SUDS technique should seek to contribute to each of the three goals identified below with the favoured system contributing significantly to each objective:

� Reduce flood risk (to the site and neighbouring areas),

� Reduce pollution, and,

� Provide landscape and biodiversity benefit.

8.4.8 In keeping with the guidance of Planning Policy Statement 25, Local Planning Authorities should encourage the application of SUDS techniques. This chapter presents a summary of the SUDS techniques currently available and a review of the soils and geology of the study area, enabling Mid Bedfordshire District Council to identify where SUDS techniques could be employed.

8.4.9 Typically, the application of SUDS techniques is not limited to one technique per site. Often a successful SUDS solution will utilise a number of techniques in combination, providing flood risk, pollution and biodiversity benefits. In addition, SUDS can be employed on a strategic scale, for example with a number of sites contributing to large scale jointly funded and managed SUDS.

8.4.10 Appropriate policy relating to Sustainable Drainage Systems (SUDS) will be adopted in line with Development Control Policy documents; section DC 4 of the draft Core Strategy Preferred Options Paper (September 2007), provides additional information relating to SUDS.

8.4.11 In the draft Core Strategy Preferred Options paper (DC 4), the council stipulates that SUDS are required for all major residential and commercial schemes.

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8.4.12 SUDS techniques can be used to reduce the rate and volume and improve the water quality of surface water discharges from sites to the receiving environment (i.e. natural watercourse or public sewer etc). Various SUDS techniques are available, however the techniques operate on two main principles:

� Infiltration

� Attenuation

All systems generally fall into one of two categories, or a combination of the two.

8.4.13 The design of SUDS measures should be undertaken as part of the drainage strategy and design for a development site. A ground investigation will be required to access the suitability of using infiltration measures, with this information being used to assess the required volume on on-site storage. Hydrological analysis should be undertaken using industry approved procedures such as the Flood Estimation Handbook to ensure a robust design storage volume is obtained.


8.4.14 The viability of alternative SUDS techniques would need to be robustly assessed on an individual site basis as part of any site specific Flood Risk Assessment, and must take full account of a number of criteria, which include the following:

� Underlying Geology and results of on-site Geo-Environmental Investigations;

� Proximity of Groundwater Table;

� Long term maintenance and management of the SUDS asset;

� Sewerage Undertaker’s criteria relating to any public sewer systems that would rely upon the function of the SUDS asset; Environment Agency criteria relating to protection of underlying groundwater and water resources; Contaminated Land issues (refer to Mid Bedfordshire District Council Contaminated Land Officer).

8.4.15 During the design process, liaison should take place with the Local Planning Authority, the Environment Agency, and Anglian Water in order to establish that the design methodology is satisfactory and to also agree on a permitted rate of discharge from the site.

8.4.16 Building Regulations advise that a SUDS hierarchy be followed in order to select the appropriate solution for a development site, with preference given to infiltration techniques over attenuation systems, provided that underlying conditions allow.

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8.4.17 This type of Sustainable Drainage System relies on discharges to ground, where suitable ground conditions allow. Therefore, infiltration SUDS are reliant on the local ground conditions (i.e. permeability of soils and geology, the groundwater table depth and the importance of underlying aquifers as water resources etc) for their successful operation.

8.4.18 Various infiltration SUDS techniques are available from directing the surface water run-off to ground. However, development pressures and a desire to maximise development potential often results in typically small land areas available for infiltration systems. This small area, allied to the rapid rates of runoff generation often require some form of supplementary attenuation as part of the infiltration system. The storage may be provided in the sub-base of a permeable surface, within the chamber of a soakaway, as a pond / water feature, or within underground storage pipes / tanks.

8.4.19 Infiltration measures include the use of permeable surfaces and other systems that are generally located below ground.

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8.4.20 Permeable surfaces are designed to allow water to drain through to a sub-base at a rate greater than the rain that falls onto the surface. Permeable surfaces act by directly intercepting the rain where it falls and are therefore true source controls. In theory this system would prevent any surface water running off the site, however in reality it is impractical to design permeable surfaces to directly infiltrate intense rainfall events. The permeable sub-base can be used to temporarily store infiltrated run-off underground allowing it to percolate into the ground below. Alternatively stored water within the sub-base can be discharged from the site.


8.4.21 Maintenance programmes will need to ensure that the surface is kept clear of silt and voids are clear. The use of grit and salt during the winter months will adversely affect the drainage potential of paved surfaces, however this should not be required often as ice is less likely to form on these types of surfaces.

8.4.22 Types of permeable surfaces include:

� 4��G�1��H Grassed or landscaped areas provide a permeable surface that allows for the infiltration of rain falling onto these areas, and potentially also run-off from adjacent impermeable areas. Grassed or landscaped areas are a relatively inexpensive SUDS measure however they are likely to be restricted to areas where vehicles are not present.

� ��4�H Techniques are available that allow grass to be incorporated into a pavement type surface. These provide varying ratios of hard-pavement to grass dependant on the site requirements. These range from concrete block arrangements to plastic meshes and can be utilised in those areas where the hard permanence of a typical pavement might be undesirable, such as in conservation areas, roadside verges, emergency services access, canal towpaths, farm tracks, rural settings etc. In the past these systems have been typically adopted for situations where a load bearing surface is required to fit into the surrounding environment, however these systems are often now installed for surface water management purposes. The grass: hard pavement ratio will be one of the dominant factors that determine the rate of infiltration through to the sub surface.

� 4�,��H A bed of gravel with a high void space on a permeable sub-base offers a cost effective solution for trafficked areas. Rain falling directly onto the area is able to infiltrate through to the sub-base.

� ��0�,��6 � ��"��H Solid paving can be installed in such a way that voids are present that can be in-filled with a permeable material such as grass or gravel etc. If this is to be used as a SUDS measure, a permeable sub-base is required to allow infiltration into the underlying ground and/or temporary storage to attenuate discharges.

� 0��*��0�,�� H Permeable pavements allow the rain falling directly onto the area to infiltrate through into a sub-base and where suitable, through into the underlying strata. Permeable pavements are constructed using porous concrete blocks allowing the infiltration of rainwater. Small projects of less than 100m2 (depending on sub-grade permeability) can often be managed using 100% infiltration, whereas larger schemes will often require a combined system, with some form of attenuation provided as back-up for periods of exceptional flows. The use of geo-membranes can trap pollutants and prevent them being carried into the receiving environment. The use of an impermeable membrane beneath the sub-base will work to contain any pollutants within the sub-base. One major advantage of tanked permeable paving systems is that any significant contamination, for example, a diesel spillage, will be restricted to the immediate area and not transported into local sewers or watercourses. The performance of permeable pavements will dramatically decrease over a period of time with the clogging of voids and this should be taken into consideration during the design process and maintenance.

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8.4.23 Where permeable surfaces are not a practical option more defined infiltration systems are available. In order to infiltrate the generated run-off to ground, a storage system is provided that allows the infiltration of the stored water into the surrounding ground through both the sides and bottom of the storage. These systems are constructed below ground and therefore may be advantageous with regards to the developable area of the site, however consideration needs to be given of construction methods and maintenance access to any development that takes place over an underground infiltration system. Consideration is also needed of the depth to the groundwater table. The provision of large volumes of infiltration / storage underground has potential cost implications and infiltration devices should not generally be built within 5m of a building, under a road or on soil that may dissolve or be washed away, in line with Bulding Regulations advice.

8.4.24 Various methods for providing sub-surface infiltration include:

� 4��' ��H Modular block systems can be used to provide an underground infiltration facility. The modular structures are usually made of plastic and can be staked side by side or on top of each other to construct an infiltration/storage unit of the required size. The modular blocks are usually sited upon a highly permeable sub-base through which the surface water run-off is discharged (usually through the perforated pipes). The outlet from the pipes are restricted which causes the run-off to rise up through the sub-base into the geocellular storage system. The storage systems are usually tanked with a geomembrane. These types of systems are quick and easy to install, flexible in their configuration, and have minimal maintenance requirements (providing the inflow of silt is limited). While many manufacturers claim that their products are suitable for installation beneath roadways or car parks, their use in these areas should be taken with caution. Geocellular systems can also be utilised for providing storage without infiltration in order to attenuate discharge rates. In these situations the system is tanked with a geomembrane or similar.

� �� H A filter drain is a trench that contains a perforated or porous pipe that runs along its bottom. The trench is filled with a suitable filter material, granular material or lightweight aggregate fill, all with a high void space. The fill may be exposed at the ground surface or capped with turf, topsoil etc that allow the trench to flood (i.e. not an impermeable surface that could pressurise the trench). Surface water run-off generated by the site is directed through the perforated pipe which then flows into the trench and infiltrates into the surrounding ground. Filter drains have been used extensively for highways and car parking areas, where they have generally been constructed in the verge and median strip.


� ��6 �'�>��*�?H Surface water run-off is directed to a chamber set in the ground with holes in the sides and base. This allows the stored water to soak into the ground. The storage capacity of soakaway chambers are limited and therefore they are more suited to serve dwellings rather than large developed areas. The chambers are prone to silting up over time and, therefore, need regular maintenance.

� ��6 �'�> ��?H Where the linear space is available, soakaways that use a trench rather than a chamber may be used to manage the runoff from larger areas.

� ��6 �'�>4��6 �'?H similar to a filter drain, a soakaway (either chamber or trench) filled with a high percentage void, granular material can be used to store surface water runoff as it is infiltrated into the surrounding ground. The granular fill will offer structural strength to the soakaway although the storage volume will be substantially reduced.

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8.4.25 Where site ground conditions are deemed unsuitable for the widespread implementation of infiltration techniques, the surface water runoff discharged from a site will need to be attenuated utilising on-site attenuation storage. While this is a SUDS technique that will reduce the rate of discharge from the site, the overall volume will not be minimised using on-site storage alone. An important factor that needs to be taken into consideration when assessing the suitability of on-site storage as part of a proposed development is the volume required and the associated impacts the storage will impose on development proposals and risks to neighbouring properties.

8.4.26 Should the use of infiltration to either partially or fully dispose of the surface water runoff from a developed site be found to be unfeasible, on-site storage will be required in order to attenuate the discharge from the site. An allowable rate of discharge from the site will need to be agreed with the Environment Agency, Anglian Water, and the Local Planning Authority. This can have significant implications to the proposed development with regards to the large volume of storage that may be required. On-site storage can be constructed above ground or below ground; the above ground systems usually being the cheaper option on a cost per m3 of storage basis. It should be noted, however, that the below ground systems may pose less constraints on the developable area of the site.

On-site ‘above ground’ storage measures include basins and ponds, with ‘below ground’ facilities generally following the more engineered forms of underground storage.

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8.4.27 Basins are areas that have been re-profiled (or alternatively embanked) to allow for the temporary storage of runoff from a developed site. Basins are drained in such a way that ensures that they are free from water in dry weather. Therefore between periods of rainfall they can be used for other purposes such as open public space, recreation etc. Basins treat run-off in a variety of ways i.e. settlement of solids in still water, absorption by aquatic vegetation or biological activity etc. The construction of basins uses relatively simple techniques. Local varieties of vegetation should be used wherever possible and should be fully established before the basins are used. Access to the basin should be provided such that a maintenance programme can be implemented. This would include inspections, regular cutting of grass, annual clearance of aquatic vegetation and silt removal as required.


8.4.28 Various types of basins are available for utilising as SUDS measures. These include:

� �� H A detention basin is designed and constructed to store surface water run-off temporarily in order to attenuate flows over a minimal period of time. Detention basins provide better flow attenuation than floodplains as they store water until the flood has passed. The stored water is then released at a controlled rate after the storm to avoid flooding downstream. If the runoff is held back for long enough, solids start to settle out of the water, which improves water quality.

� �: �� H If the period of detention increases to approximately 24 hours, the basin is referred to as an extended detention basin. This results in the surface water runoff being stored beyond the time normally required for attenuation which provides extra time for natural processes to remove some of the pollutants in the water.

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8.4.29 Ponds are similar to basins except that the outflow is configured such that a volume of water is contained during dry weather, usually for amenity, recreational, or agricultural purposes. Ponds are designed to hold the additional surface water runoff generated by the site during rainfall events. Like basins, ponds are designed to control discharge rates by storing the collected runoff and releasing it slowly once the risk of flooding has passed. Ponds can provide wildlife habitats, water features to enhance the urban landscape and, where water quality and flooding risks are acceptable, they can be used for recreation. It may be possible to integrate ponds and wetlands into public areas to create new community ponds. Ponds and wetlands trap silt which may need to be removed periodically. Ideally, the contaminants should be removed at source to prevent silt from reaching the pond or wetland in the first place. In situations where this is not possible, consideration should be given to a small detention basin placed at the inlet to the pond in order to trap and subsequently remove the silt. Depending on the setting of a pond, health and safety issues may be important issues that need to be taken into consideration. The design of the pond can help to minimise any health and safety issues (i.e. shallower margins to the pond to reduce the danger of falling in). A fence may also be required for keeping children and wildlife out.

8.4.30 Various types of ponds are available for utilising as SUDS measures. These include:

� ��G�� 0��H A balancing or attenuation pond is designed only to store surface water run-off and attenuate discharge until the flood / storm peak has passed. Therefore, storage periods may not be long enough to significantly improve water quality capacity compared with ponds with longer retention times. They contain some water at all times with the water level fluctuating as the runoff passes through the device.

� �� ,��H Flood storage reservoirs are very similar to balancing / attenuation ponds except that they are usually much larger. They are generally used to attenuate fluvial flood flows rather than surface water runoff from developed area and are therefore likely to be utilised as a SUDS measure. Should an existing flood storage reservoir in the vicinity of the proposed development be identified to offer spare capacity, then depending on ownership and agreement, it


may be possible to utilise the spare capacity for storm water management of development areas.

� 1��H Should the surface water run-off have a high suspended solids content, a lagoon could be a suitable method for attenuating its discharge to the receiving environment. Lagoons are similar to balancing / attenuation ponds except that they are also designed for the settlement of suspended soils. Usually they are long and narrow in shape to ensure the longest retention time and therefore an efficient removal of suspended solids. However lagoons are usually free of vegetation and therefore do not provide any biological treatment.

� �� 0��H Retention ponds are designed to detain the surface water run-off for periods between several days and two weeks. This allows for a higher level of settlement, biological treatment and limited attenuation of flows. Retention ponds provide a greater degree of treatment than extended detention basins.

� 9 � ��H A wetland is similar to a pond except that it has a high proportion of emergent vegetation in relation to open water. Wetlands use plants to make the treatment of surface water run-off more efficient and can allow the detention times required to fully treat the run-off to be reduced to a couple of weeks. Constructed wetlands are ponds with irregular perimeters and undulating bottom contours into which wetland vegetation is purposely placed to enhance pollutant removal from surface water runoff. Surface water runoff enters a constructed wetland through a forebay where the larger solids and course organic material settle out. The runoff discharged from the forebay passes through emergent vegetation which acts to filter organic materials and soluble nutrients. The use of constructed wetlands can be looked at from two perspectives. The first is that the wetland is used primarily to maximise pollutant removal from storm water runoff and also help to attenuate storm water flows. Alternatively, it may be used primarily to control storm water flows, with increased pollutant removal capabilities.

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8.4.31 In many situations the development of a site may involve proposals that would inhibit the use of basins or ponds as a means of managing the surface water runoff discharged from the site. This may be due to land take, economic feasibility, or other issues such as health and safety etc. In these situations it may be appropriate to use a storage option that is viewed as being more ‘engineered’ than an open basin or pond. Most of these methods involve the provision of storage beneath the ground surface, which may be advantageous with regards to the developable area of the site, however, consideration needs to be given to construction methods, maintenance access and to any development that takes place over an underground storage facility. The provision of large volumes of storage underground also has potential cost implications.


8.4.32 Methods for providing alternative attenuation include:

� �� : Deep shafts can be utilised in areas with low groundwater tables. Shafts have small footprints and are therefore useful on sites with low land availability, however pumping from the shaft into the receiving drainage network will be required. Therefore substantial mechanical and electrical works will be required as potentially would major civil engineering requirements. With a relatively small cross-sectional area, a shaft would be prone to siltation at a rapid rate depending on the quality of the run-off. Therefore a maintenance programme would be required as would regular servicing of any pump works.

� 4��' ��H See previous discussion.

� ),��I��H Oversizing the pipes that make up the on-site drainage network is a cost effective method that is often used for providing attenuation storage within the network. The main draw back with using this method is that it can be very difficult to obtain the required level of storage as the pipe diameters are often restricted by the depth of cover (vertical distance between top of pipework and ground surface) available and the need to gravity drain into an existing network. A solution to this is to lay multiple pipes side by side however this increases the excavation areas and may also place restrictions on the development footprint. The use of oversized pipes is not an effective method of providing on-site storage if the network is at a relatively steep gradient. This is because the storage at the upstream end of the pipe is unlikely to be utilised.

� ��6 � ��8�,� ��H Rainwater harvesting is the process of collecting rain that falls directly onto roofs (and in some cases hardstanding areas) such that it can be re-used for non-potable uses around the home or business (typically flushing toilets, car washing and irrigation etc). The simplest form of rainwater harvesting involves the collection of runoff from a roof via a water butt situated at the bottom of a down pipe. This water can then be used for irrigation. For systems where the collected runoff is to be used for toilet flushing, washing machines etc, it is likely that the water would be pumped from a storage tank installed on the grounds of the property. Packaged systems are available although the costs (including ongoing maintenance) may outweigh the payback in terms of reduced water supply charges. The use of rainwater harvesting fits well with the overall water cycle, as it may assist with a minor reduction in volumes of off-site runoff ‘post development’ together with reducing overall demand for potable water.

� ��H The use of tanks for the provision of storage to attenuate surface water runoff is varied in the number of options available. While most storage tanks are installed beneath the ground surface, above ground storage tanks may be feasible a part of industrial or commercial type developments where amenity issues or space are not significant restrictions. Tanks are likely to be prefabricated, but could also be constructed in-situ for below ground concrete tanks. Plastic and Glass Reinforced Plastic tanks are also often utilised with sizes of up to 25m3 being available off the shelf (although larger tanks used above ground may have additional reinforcing requirements). Consideration must be given to below ground tanks with regards to cover depths, load bearings, and also invert depths should the tank be drained by gravity. The storage requirements need to be based on a sound hydrological assessment as undersizing the tank would be costly to remediate.


� 4��H A roof area that is used for growing appropriate types of vegetation, which provides a degree of retention and attenuation, it also promotes evapotranspiration. In addition, vegetation and substrate can absorb a range of pollutants. Green roofs are more suitable for public and institutional buildings that have good maintenance programmes and support. Green roofs are available in both prefabricated and in-situ construction however they are heavy systems and can have major structural implications for buildings and are usually impractical to retro-fit.

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8.4.33 In most situations, SUDS systems include both infiltration and storage. Most of the techniques identified above can be used in combination, however dedicated infiltration and attenuation systems include swales and filter strips.

� �6 ��H A swale is a grass-lined channel designed to control both the flow rate and quality of surface water run-off that is generated by the adjacent site. Not only does the water run down the sides of the swale at a reduced speed, but it can also be slowed further as it flows along the channel before being discharged from the site. This detention of the runoff also increases the infiltration from the swale.

� �� H A filter strip runs along the edge of a permeable area and is sloped to allow the sheet flow across the vegetated strip. Unlike a swale however, no storage is offered other than what is held back by the vegetation. The overland flow across the filter strip is likely to run into a water course at some point rather than being formally collected and discharged at a controlled rate from the site.


9 Strategic Infrastructure & Residual Risk Management

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9.1.1 Locations of key sewerage infrastructure have been provided in Appendix F.

9.1.2 A meeting was held with key personnel within Anglian Water in order to gather information relating to capacity issues at local Sewage Treatment Works, and to assess the likely impact of future growth upon the existing infrastructure.

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9.2.1 Where Anglian Water have indicated that a Sewage Treatment Works within the study area is running at or near capacity, it can be taken that further investment would be required to accommodate any growth in excess of a 1% increase in population. This would have to be staged within the works development.

9.2.2 A brief appraisal has been undertaken of each Sewage Treatment Works (STW) serving the key settlements; in turn:

Bedford STW (serves Wixams)

� Maintenance programme and process appraisal underway to enhance capacity.

� Capacity extensions proposed to provide capacity for growth to 2015. Anglian Water will be seeking funding within their draft 2009 business plan.

� Potential for expansion of the Works at its current location.

� Likely increases in effluent discharge to River Great Ouse would need to be assessed at a more detailed level.

Biggleswade STW (serves Biggleswade)

� Inlet works recently upgraded; appraisal underway to enhance capacity.

� Capacity available to AMP5 period. Capacity extensions required to accommodate growth to 2021.

� Land potentially available for expansion of the Works at its current location, but the site is situated in Flood Zone 3 and constraints may be imposed.

� Likely increases in effluent discharge to River Ivel would need to be assessed at a more detailed level.

.


Clifton STW (serves Shefford)

� Flows at or exceeding peak dry weather flows are currently pumped to Poppy Hill STW at Henlow.

� A new treatment plant is currently being commissioned with increased consented dry weather flow which will cater for additional growth.

� Pumping to Poppy Hill STW will shortly discontinue.

� Land potentially available for further expansion of the Works at its current location.

� Likely minor increases in effluent discharge to Henlow Brook (River Ivel) would need to be assessed at a more detailed level.

Flitwick STW (serves Flitwick and Ampthill)

� Capital scheme in asset plan programme (2007 – 2008).

� Additional capacity currently available.

� Land potentially available for expansion of the Works at its current location, but a portion of the site is situated within Flood Zone 3 and constraints may be imposed.

� Likely increases in effluent discharge to River Flit would need to be assessed at a more detailed level.

Marston Moretaine STW (serves Marston Moretaine)

� Potential water quality issues at this location; additional capacity available.


� Likely minor increases in effluent discharge to Marston Brook would need to be assessed at a more detailed level.

Poppy Hill STW (serves Stotfold and Arlesey)

� Additional capacity currently available which will increase following discontinuation of pumped flows from Clifton STW.

� May require minor upgrading works and is included within the 2007 Asset Plan.

� Land available for expansion of Works at its current location, but the site is situated in Flood Zone 3 and constraints may be imposed.



Potton STW (serves Potton)

� Additional capacity available; capacity extensions may be required to accommodate growth to 2021.


� Likely minor increases in effluent discharge to Sutton Brook (River Ivel) would need to be assessed at a more detailed level.

Sandy STW (serves Sandy)

� Capital scheme underway to upgrade works capacity. Revised flow consents are currently being discussed with the EA.

� Limited capacity available to accommodate considerable future growth. Capacity extensions will be required to accommodate growth to 2021.



Stewartby STW (serves Stewartby and Marston Moretaine)

� Limited capacity available.

� Water quality issues at this location.

� No scope for expansion of the Works at its current location.

� Current strategic option is to close the Works and replace it with a pumping station, lifting flows to Marston Moretaine STW, or Bedford STW.

9.2.3 Any future flood mapping must include an allowance for the future increases in flow from each Sewage Treatment Works in order to assess the impact.

9.2.4 A ranking of the main sewage treatment works in the district based on the information given in section 9.2 and their available capacity in relation to future urban growth in the district is shown below;

STW Area Served Current Capacity

Available Capacity

EA Consent Overall Group Ranking

Bedford Wixams 128,386 domestic population equivalent

4,728 dwellings

Yes 3

Biggleswade Biggleswade

17,773 population equivalent

1,463 dwellings

Yes 7


Clifton Shefford 12,690 population equivalent

1,771 dwellings

Increase in consent is currently being negotiated with the EA

5

Flitwick Flitwick and Ampthill


9,388 dwellings

Yes 1

Marston Moretaine

Marston Moretaine

9,068

population equivalent

3,222 dwellings

Yes 4

Poppy Hill Stotfold and Arlesey


4,762 dwellings

Yes 2

Potton Potton 5,028 population equivalent

1,587 dwellings

Yes 6

Sandy Sandy 10,989 population equivalent

184 dwellings

Yes 9

Stewartby Stewartby and Marston Moretaine


220 dwellings

Yes 8

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9.3.1 Significant growth and urban expansion is predicted within the adjoining districts of North Hertfordshire and Luton.

9.3.2 As a result, future increases in the volume and rates of treated foul effluent are expected from both Hitchin and Letchworth Sewage Treatment Works (North Hertfordshire) into the River Hiz and Pix Brook respectively. The potential also exists for increased volumes and frequency of storm discharges into these watercourses. Increased flows will eventually impact upon the downstream receiving watercourse, the River Ivel.

9.3.3 Future increases in the volume and rates of treated foul effluent are expected from Chalton Sewage Treatment Works (Luton) into the River Flit. The potential also exists for increased volumes and frequency of storm discharges into the watercourses. Increased flows will eventually impact upon the downstream receiving watercourse, the River Ivel.

9.3.4 Any future flood mapping must include an allowance for the future increases in flow from each Sewage Treatment Works in order to assess the impact.


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9.4.1 Locations of key strategic infrastructure have been indicated in Appendix F. Facilities highlighted include principal highway routes, hospitals, fire stations, and police stations and sewage treatment works.

9.4.2 Key observations from a comparison of flood mapping to strategic sites have been set out below:

� Potton Fire Station lies within the floodplain of Potton Brook and thus safe access and egress for vehicles, fire appliances and pedestrians may be significantly disrupted during significant flood conditions;

� The A1 at Beeston has been affected historically by flooding. Safe access along a principal highway route may be significantly disrupted during significant flood conditions.

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9.5.1 Assessment of residual flood risks in terms of hazard and consequence should be considered on a more site specific basis and are generally not appropriate to be covered by way of a strategic assessment where principal residual risks such as flood defence breach (failure) are not applicable.

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9.6.1 Due to the hydrological characteristics of the majority of watercourse catchments within the district, the likely duration of extreme flooding is likely to be relatively short. This, in turn, may mean that the length of advance warning of the inset of flooding may often be short.

9.6.2 Environment Agency Flood Warning System coverage extends along a reach of the River Ivel, incorporating only the following key settlements within the study area:

� Biggleswade

� Sandy

� Shefford

9.6.3 Bedfordshire and Luton Local Resilience Forum currently have a ‘Flood Response Plan’ which sets out what the Council do in terms of setting up centres for displaced persons during an emergency. Specific references to flood warnings and evacuation procedures are given.

9.6.4 Mid Bedfordshire District Council has also issued a ‘Flooding Guide’ which gives advice on what to do in the event of extensive flooding, detailing the emergency services that can assist.

9.6.5 Emergency services and facilities are generally well placed geographically to deal with flooding situations and should remain operational during extreme flood conditions.


10 Planning And Development Issues

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10.1.1 Potential future growth areas have been presented within Appendix G.

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10.2.1 A sequential risk-based approach to determining the suitability of land for development in flood risk areas is central to Planning Policy Statement 25 and should be applied at all levels of the planning process.

10.2.2 Mid Bedfordshire District Council, as part of the Local Development Framework process of allocating land for development, should apply the Sequential Test to demonstrate that there are no reasonably available sites in areas of lower probability of flooding that would be appropriate to the type of development or land use proposed.

10.2.3 Table D.1, Annex D of Planning Policy Statement 25 (below) provides definitions for the flood zones, referring to the probability of fluvial and tidal flooding, ignoring the presence of defences.

PPS25 Table D.1 : Flood Zones & Appropriate Land Uses

Zone 1 Low Probability

Definition This zone comprises land assessed as having a less than 1 in 1000 (< 0.1%) annual probability of river or sea flooding in any year.

Appropriate uses All uses of land are appropriate in this zone.

FRA requirements For development proposals on sites comprising one hectare or above the vulnerability to flooding from other sources as well as from river and sea flooding, and the potential to increase flood risk elsewhere through the addition of hard surfaces and the effect of the new development on surface water run-off, should be incorporated in a Flood Risk Assessment. This need only be brief unless the factors above or other local considerations require particular attention. See Annex E of PPS25 for minimum requirements.

Policy aims In this zone, developers and local authorities should seek opportunities to reduce the overall level of flood risk in the area and beyond through the layout and form of the development, and the appropriate application of sustainable drainage techniques.


Zone 2 Medium Probability

Definition This zone comprises land assessed as having between a 1 in 100 and 1 in 1000 annual probability of river flooding (1% – 0.1%) or between a 1 in 200 and 1 in 1000 annual probability of sea flooding (0.5% – 0.1%) in any year.

Appropriate uses The water-compatible, less vulnerable and more vulnerable uses of land and essential infrastructure in Table D.2 are appropriate in this zone.

Subject to the Sequential Test being applied, the highly vulnerable uses in Table D.2 are only appropriate in this zone if the Exception Test (see para. D.9.) is passed.

FRA requirements All development proposals in this zone should be accompanied by a Flood Risk Assessment. See Annex E of PPS25 for minimum requirements.

Policy aims In this zone, developers and local authorities should seek opportunities to reduce the overall level of flood risk in the area through the layout and form of the development, and the appropriate application of sustainable drainage techniques.


Zone 3a High Probability

Definition This zone comprises land assessed as having a 1 in 100 or greater annual probability of river flooding (>1%) or a 1 in 200 or greater annual probability of flooding from the sea (>0.5%) in any year.

Appropriate uses The water-compatible and less vulnerable uses of land in Table D.2 of PPS25 are appropriate in this zone.

The highly vulnerable uses in Table D.2 should not be permitted in this zone.

The more vulnerable and essential infrastructure uses in Table D.2 should only be permitted in this zone if the Exception Test (see para. D.9) is passed. Essential infrastructure permitted in this zone should be designed and constructed to remain operational and safe for users in times of flood.


Policy aims In this zone, developers and local authorities should seek opportunities to:

i. reduce the overall level of flood risk in the area through the layout and form of the development and the appropriate application of sustainable drainage techniques;

ii. relocate existing development to land in zones with a lower probability of flooding; and

iii. create space for flooding to occur by restoring functional floodplain and flood flow pathways and by identifying, allocating and safeguarding open space for flood storage.


Zone 3b The Functional Floodplain Definition This zone comprises land where water has to flow or be stored in times of flood. SFRAs should identify this Flood Zone (land which would flood with an annual probability of 1 in 20 (5%) or greater in any year or is designed to flood in an extreme (0.1%) flood, or at another probability to be agreed between the LPA and the Environment Agency, including water conveyance routes).

Appropriate uses Only the water-compatible uses and the essential infrastructure listed in Table D.2 should be permitted in this zone. It should be designed and constructed to:

– remain operational and safe for users in times of flood;

– result in no net loss of floodplain storage;

– not impede water flows; and

– not increase flood risk elsewhere.

Essential infrastructure in this zone should pass the Exception Test.


Policy aims In this zone, developers and local authorities should seek opportunities to:

i. reduce the overall level of flood risk in the area through the layout and form of the development and the appropriate application of sustainable drainage techniques; and

ii. relocate existing development to land with a lower probability of flooding.

10.2.4 A sequential approach should be used in areas known to be at risk from other forms of flooding.

10.2.5 The Environment Agency’s Flood Zone Maps form the basis for the sequential testing of Planning Policy Statement 25 whereby land is categorised as being in one of a range of zones, Flood Zone 1 to Flood Zone 3, according to the probability of flooding to the land. Planning Policy Statement 25 advises on the appropriate planning response for different types of development in relation to the flood risk as categorised by the various Flood Zones.

10.2.6 Flood Zone Mapping for the study area has been based primarily upon the Environment Agency’s Flood Zone Maps. The maps have been refined and enhanced at this stage where appropriate in light of more detailed hydraulic modelling data. Maps have been prepared across the study area with the refined Flood Zone Maps overlain (set out in Appendix E) and are the starting point for the sequential approach.


10.2.7 As set out within Annex D of Planning Policy Statement 25, the aim of the Sequential Test is to steer new development to areas at the lowest probability of flooding, preference should be given to locating development in Flood Zone 1. If there is no reasonably available site in Flood Zone 1, the flood vulnerability of the proposed development (see Table D.2, Annex D Planning Policy Statement 25 below) can be taken into account in locating development in Flood Zone 2 and then Flood Zone 3.

PPS25 Table D.2 : Flood Risk Vulnerability Classification Essential Infrastructure

Essential transport infrastructure (including mass evacuation routes) which has to cross the area at risk, and strategic utility infrastructure, including electricity generating power stations and grid and primary substations.

Highly Vulnerable

• Police stations, Ambulance stations and Fire stations and Command Centres and telecommunications installations required to be operational during flooding.

• Emergency dispersal points.

• Basement dwellings.

• Caravans, mobile homes and park homes intended for permanent residential use.

• Installations requiring hazardous substances consent.

More Vulnerable

• Hospitals.

• Residential institutions such as residential care homes, children’s homes, social services homes, prisons and hostels.

• Buildings used for: dwelling houses; student halls of residence; drinking establishments; nightclubs; and hotels.

• Non–residential uses for health services, nurseries and educational establishments.

• Landfill and sites used for waste management facilities for hazardous waste.

• Sites used for holiday or short-let caravans and camping, subject to a specific warning and evacuation plan.


Less Vulnerable

• Buildings used for: shops; financial, professional and other services; restaurants and cafes; hot food takeaways; offices; general industry; storage and distribution; non–residential institutions not included in ‘more vulnerable’; and assembly and leisure.

• Land and buildings used for agriculture and forestry.

• Waste treatment (except landfill and hazardous waste facilities).

• Minerals working and processing (except for sand and gravel working).

• Water treatment plants.

• Sewage treatment plants (if adequate pollution control measures are in place).

Water-compatible Development

• Flood control infrastructure.

• Water transmission infrastructure and pumping stations.

• Sewage transmission infrastructure and pumping stations.

• Sand and gravel workings.

• Docks, marinas and wharves.

• Navigation facilities.

• MOD defence installations.

• Ship building, repairing and dismantling, dockside fish processing and refrigeration and compatible activities requiring a waterside location.

• Water-based recreation (excluding sleeping accommodation).

• Lifeguard and coastguard stations.

• Amenity open space, nature conservation and biodiversity, outdoor sports and recreation and essential facilities such as changing rooms.

• Essential ancillary sleeping or residential accommodation for staff required by uses in this category, subject to a specific warning and evacuation plan.


10.2.8 Flood Zone ‘compatibility’ (see Table D.3, Annex D Planning Policy Statement 25 below) forms the basis for the Sequential Test to be undertaken. Note that this table does not show the application of the Sequential Test which guides development to Flood Zone 1 first, then Flood Zone 2, and then Flood Zone 3.

PPS25 Table D.3. Flood Risk Vulnerability and Flood Zone ‘Compatibility’

Key:

� Development is appropriate

x Development should not be permitted

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10.3.1 Planning Policy Statement 25 expands on the Sequential Test by incorporating an Exception Test whereby if, following application of the Sequential Test, it is not possible, consistent with wider sustainability objectives, for the development to be located in zones of lower probability of flooding, the Exception Test can be applied.

10.3.2 The Exception Test is appropriate for use when there are large areas in Flood Zones 2 and 3, where the Sequential Test alone cannot deliver acceptable sites, but where some continuing development is necessary for wider sustainable development reasons, taking into account the need to avoid social or economic blight and the need for essential civil infrastructure to remain operational during floods. It may also be appropriate to use it where restrictive national designations (e.g. Sites of Special Scientific Interest) prevent the availability of unconstrained sites in lower flood risk areas.

10.3.3 The Exception Test provides a mechanism for managing flood risk while still allowing necessary development to occur. It should not, however, be used to justify ‘highly vulnerable’ development in Flood Zone 3a, or ‘less vulnerable’, ‘more vulnerable, and ’highly vulnerable’ development in Flood Zone 3b.

Flood Risk

Vulnerability

Classification (see

Table D.2)

Essential

Infrastructure

Water

Compatible

Highly

Vulnerable

More

Vulnerable

Less

Vulnerable

Zone 1 ��

Zone 2 � �� Exception Test

required

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Zone 3a Exception Test required

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required

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d Zo

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ee T

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Zone 3b

‘Functional

Floodplain’

Exception Test required

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10.3.4 For development to be allocated or permitted, all three elements of the Exception Test criteria (set out below) will have to be passed:

� It must be demonstrated that the development provides wider sustainability benefits to the community that outweigh flood risk, informed by a Strategic Flood Risk Assessment where one has been prepared. If the Development Plan Document has reached the ‘submission’ stage (see Figure 4 of Planning Policy Statement 12 : Local Development Frameworks) the benefits of the development should contribute to the Core Strategy’s Sustainability Appraisal;

� The development should be on developable previously-developed land or, if it is not on previously developed land, that there are no reasonable alternative sites on developable previously-developed land; and

� A Flood Risk Assessment must demonstrate that the development will be safe, without increasing flood risk elsewhere, and, where possible, will reduce flood risk overall.

10.3.5 This Strategic Flood Risk Assessment study takes no account of other socio-economic or sustainability factors other than flood risk and drainage infrastructure. These wider issues are to be considered by Mid Bedfordshire District Council as part of their Sequential Test and Exception Test procedure, as required.


11 Review Of Proposed LDF Future Growth Areas

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11.1.1 A review of the future growth locations has been carried out; these areas/locations have been shown within Appendix G. Fluvial flood risk has been assessed based upon the plans provided within Appendix E; these provide a pictorial representation of the variation in fluvial flood risk across the key settlements within the study area.

11.1.2 Planning Policy Statement 25 was applied to identify the planning constraints posed as a result of flood risk. Reference should be made to Tables D.1, D.2, and D.3 (Section 10) in relation to Flood Zone Maps (Appendix E) in order to apply the Sequential Test to future development proposals.

11.1.3 A summary of the appropriateness of potential growth locations has been provided below. Clearly, the suitability of all sites in flood risk terms will be subject to ratification by the EA, a detailed site-specific Flood Risk Assessment being prepared to support any planning application, and demonstration that surface water runoff from the development will pose no detrimental impact to off-site areas.

Potential Growth Area Flood Zone Appropriate in Flood Risk Terms?

Ampthill 1,2,3 Yes, subject to steering vulnerable development away from flood zone 3

associated with wc163

Arlesey 1,2,3 Yes, subject to steering vulnerable development away from flood zone 3 associated with the River Hiz and Pix

Brook

Biggleswade 1,2,3 Yes, subject to steering vulnerable development away from flood zone 3

associated with the River Ivel and wc74

Cranfield 1 Yes

Flitwick 1,2,3 Yes, subject to steering vulnerable development away from flood zone 3

associated with the River Flit

Marston Moretaine 1,2,3 Yes, subject to steering vulnerable development away from flood zone 3

associated with wcB1(4)

Potton 1,2,3 Yes, subject to steering vulnerable development away from flood zone 3

associated with Potton Brook

Sandy 1,2,3 Yes, subject to steering vulnerable development away from flood zone 3

associated with the River Ivel


Shefford 1,2,3 Yes, subject to steering vulnerable development away from flood zone 3

associated with the River Ivel (Navigation), the River Hit and the River Flit

Stotfold 1,2,3 Yes, subject to steering vulnerable development away from flood zone 3, associated with the River Ivel and Pix

Brook

Wixams 1,2,3 Yes, subject to steering vulnerable development away from flood zone 3

associated with wcB3(1)

11.1.4 The potential growth areas/locations, presented in Appendix G are considered appropriate in flood risk terms. The sequential approach has been followed, in line with PPS25, whereby proposed new development should be steered towards areas of lowest flood risk in the first instance.

11.1.5 As previously stated, in order to apply a precautionary principle, the EA have requested that all areas shown in Flood Zone 3 should be considered as Flood Zone 3b (functional floodplain) unless otherwise proven. The hydraulic modelling undertaken along the River Ivel (Atkins) and River Flit (Hannah Reed) does not illustrate the extent of the Functional Floodplain. Based on the above, the EA have requested that these Flood Zone 3 areas are also classified as 3b (Functional Floodplain). However, this situation may change as additional hydraulic modelling or refinement of the existing models is undertaken along these watercourses.

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11.2.1 A Flood Risk Assessment ‘toolkit’ has been set out within Appendix I.

11.2.2 Guidance on appropriate treatment of climate change impacts, control of surface water runoff, implementation of appropriate SUDS techniques, and consideration of residual risks have been offered to assist Mid Bedfordshire District Council and future developers of sites in the study area.

11.2.3 Site specific Flood Risk Assessment ‘toolkits’ are offered as guidance only and will be subject to Environment Agency approval and current policy at the time of submission of a planning application.


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11.3.1 Several potential opportunities for strategic flood mitigation have been identified during the Stage 2B study. These opportunities include:

Location Potential Strategic Flood Mitigation Opportunities

Stotfold (South)

Land south of Stotfold provides the potential for the provision of a strategic flood storage facility. The scheme could benefit flood prone areas downstream along the Pix Brook at Brook Street. The scheme could have the potential benefits of alleviating adjacent existing development deemed to be at high probability of flooding, and may offer amenity benefits within a carefully designed water body, and may offer biodiversity benefits by way of wetland habitat creation.

Arlesey (North East)

Land north east of Arlesey provides the potential for the provision of a strategic flood storage facility. The scheme could benefit flood prone areas downstream along the Pix Brook off Stotfold Road. The scheme could have the potential benefits of alleviating adjacent existing development deemed to be at high probability of flooding, and may offer amenity benefits within a carefully designed water body, and may offer biodiversity benefits by way of wetland habitat creation.


Shefford (South and North West)

Land south and north west of Shefford provides the potential for the provision of a strategic flood storage facility. The scheme could benefit flood prone areas downstream within the town centre by attenuating flows within Watercourse 151 and the Flit respectively. The scheme may offer amenity benefits within a carefully designed water body, and may offer biodiversity benefits by way of wetland habitat creation.

Shefford (South)

Future urban expansion of Shefford to the south provides an opportunity for the impoundment of floodwater upstream of the A507 road embankment, thus providing potential benefit to flood risk areas downstream. The schemes may offer amenity benefits within a carefully designed water body, and may offer biodiversity benefits by way of wetland habitat creation.

Marston Moretaine (South and West)

Land to the south and west of Marston Moretaine provides the potential for rationalising local watercourse arrangements, or for the provision of a strategic flood storage facility. The scheme could facilitate committed development downstream along Watercourse B40. The scheme may offer amenity benefits within a carefully designed water body, and may offer biodiversity benefits by way of wetland and / or stream habitat creation.

11.3.2 It is recommended that Mid Bedfordshire District Council adopt the appropriate policies in line with their Core Strategy, relating to the promotion of strategic flood mitigation facilities within the district and how this can aim to reduce the level of flood risk to certain areas.


12 Recommendations

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12.1.1 Mid Bedfordshire District Council are guided to carry out the Sequential Test for allocating land for future development based upon supporting evidence and information set out in Appendix I of this report and the Local Development Framework Constraints Plans in Appendix E in relation to the test criteria set out within Section 10.

12.1.2 Mid Bedfordshire District Council are guided to support the implementation of SUDS by way of robust planning conditions and / or Section 106 agreements (see Section 8.4.10). Appropriate policies relating to SUDS should be adopted in line with Development Control policy documents; Section DC4 of the draft Core Strategy Preferred Options paper (Sept 07), provides additional info relating to SUDS.

12.1.3 Mid Bedfordshire District Council are guided to seek the implementation of strategic flood mitigation opportunities (some examples were highlighted within Section 11.3 (i.e. flood storage facilities in Arlesey, Stotfold, Shefford and Marston Moretaine) by way of developer negotiation, planning condition, or Section 106 agreements.

12.1.4 To safeguard the future operation and function of flood defence and attenuation storage infrastructure, responsibility and management for such facilities should be steered towards the Bedfordshire and River Ivel Internal Drainage Board or Mid Bedfordshire District Council maintenance body rather than individual management companies set up by developers. The Marston Vale Surface Waters Plan would be a good template to follow whereby the Internal Drainage Board control watercourses, implement and maintain and control strategic flood defence infrastructure by way of a developer contribution (applied per m² of impermeable development) in return for a more favourable discharge rate to the watercourse. This contribution could equally be applied per property by way of a ‘roof tax’ or similar.

12.1.5 Mid Bedfordshire District Council are guided to investigate the application of a ‘roof tax’ or similar mechanism to supplement flood defence and strategic flood alleviation scheme development to safeguard the future of existing settlements that are deemed to be at risk of flooding currently and in the future taking into account climate change impacts.

12.1.6 Mid Bedfordshire District Council should ensure developers and their consultants make reference to this Strategic Flood Risk Assessment study prior to the formulation of development proposals and planning applications in order that opportunities are maximised, and the key requirements of Planning Policy Statement 25 (supplemented by recommendations within the Strategic Flood Risk Assessment) are met.

12.1.7 Mid Bedfordshire District Council should ensure developers carry out Flood Risk Assessments for their proposals in line with the Environment Agency’s latest standing advice on flood risk and the requirements of a Flood Risk Assessment.

12.1.8 Mid Bedfordshire District Council are guided to develop an Emergency and Flood Evacuation Plan for all settlements within the district.

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12.2.1 Future growth directions within the Mid Bedfordshire District Council study area may alter from the direction currently evolving from the Local Development Framework process. It is recommended that the Strategic Flood Risk Assessment be re-visited


periodically and updated during the Plan period to ensure that it remains a ‘live’ document.

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12.3.1 Areas for further investigation following the completion of the Stage 2B Strategic Flood Risk Assessment study principally focus upon refinements to the existing hydraulic modelling data and included the following key elements:

� Address deficiencies within the River Ivel hydraulic modelling where flood extents may be over-cautious due to crude modelling techniques being used to assimilate the presence of raised embankments;

� Assess any deficiencies within the River Flit model, which covers two areas along the River Flit upstream of Shefford;

� Extend the River Ivel model to incorporate Stotfold and the lower reach of the River Hiz at Arlesey;

� Run hydraulic models available within the study catchment for a 1 in 20 year return period event in order to better define the functional floodplain (Flood Zone 3b), as required by Planning Policy Statement 25. Currently, all Flood Zone 3 areas within the district have been classified as (3b), at the request of the Environment Agency;

� Run hydraulic models available within the study catchment for a 1 in 100 year return period event, incorporating an allowance for climate change (typically a 20% increase in fluvial flow), in order to better define the predicted flood extents for the lifetime of the proposed developments, as required by Planning Policy Statement 25;

� Run hydraulic models available within the study catchment for a 1 in 1000 year return period event, in order to better define Flood Zone 2, as required by Planning Policy Statement 25.


13 Conclusions

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13.1.1 A strategic assessment of flood risk issues has been carried out across 11 key settlements within the Mid Bedfordshire District, to assist Mid Bedfordshire District Council with their risk-based approach to the allocation of land for development as part of their ongoing Local Development Framework process.

13.1.2 Land allocations must be made with reference to the Sequential Test and, where appropriate, the Exception Test, as set out within Planning Policy Statement 25.

13.1.3 Particular reference should be made to the Local Development Framework Flood Risk Constraints Plans provided in Appendix E, and to the appraisal of strategic sewerage infrastructure set out in Section 9 of this report.

13.1.4 Reference should also be made to Section 11.0 of this report which summarises the appropriateness of the proposed LDF growth locations.

13.1.5 Recommendations have been set out within Section 11.3 of this report that seek to allow the implementation of strategic flood mitigation opportunities and enhanced flood protection of existing properties by way of developer contribution.

13.1.6 A site specific Flood Risk Assessment ‘toolkit’ has been provided to assist Mid Bedfordshire District Council, the Environment Agency, and future developers in identifying the key flood risk issues within the study area and to assist with the formulation of solutions to the management of flood risk and surface water runoff that are of benefit strategically rather than locally.

13.1.7 This Strategic Flood Risk Assessment has been based upon planning policies and information available at the time of report issue (September 2008). Flood risk classifications may be subject to change in line with future planning policy. It should also be noted that flood zoning may be subject to change following consideration of detailed topographical information, and following investigation of site specific flood risk issues in further detail as part of any site specific Flood Risk Assessments required to be undertaken by developers to accompany future planning applications for sites across the District.

Appendices, Figures & Tables

11500767 Mid Bedfordshire District Council

Appendix A Study Area

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