15 flooding, hydrology and water resources · 15 flooding, hydrology and water resources 15.1...

Volume 1: Environmental Statement – Text and Figures

Northamptonshire County Council

15-1 Chapter 15 – Flooding, Hydrology and Water Resources

15 Flooding, Hydrology and Water Resources

15.1 Introduction 15.1.1 This Chapter assesses the likely significant environmental effects of the Proposed Scheme on

flooding, hydrology and water resources in the context of the Site and surrounding area. In particular it considers the potential effects on surface water drainage systems, water bodies and flood risk on-site, in the vicinity of and downstream of the Site during the construction and operational phases of the Proposed Scheme. The potential effects are identified, along with the proposed mitigation measures and the subsequent anticipated residual effects.

15.1.2 This Chapter (and its associated figures and appendices) should be read together with the Introductory Chapters of this ES, Chapters 1 – 6, as well as Chapter 19 - Cumulative Effects.

15.1.3 Potentially significant effects to groundwater and surface water quality due to chemical contamination are reported in Chapter 16 - Ground Conditions, Hydrogeology and Contamination. Potentially significant effects to biodiversity that relate to water habitats are reported in Chapter 12 – Biodiversity.

15.2 Legislation, Policy and Guidance

Legislative Framework 15.2.1 The applicable legislative framework is summarised as follows:

Land Drainage Act 1991 (Ref. 15.1) consolidates enactments relating to internal drainage boards and the functions of these boards and of local authorities in relation to land drainage. Internal Drainage Boards (IDB) exercise a general supervision over all matters relating to the drainage of land within their district and have powers to perform duties imposed on them within the Act.

The Water Resources Act 1991 (Ref. 15.2) sets outs regulatory controls and restrictions that provide protection to the water environment through controls on abstraction, impounding and discharges as well as identifying water quality and drought provisions;

Land Drainage Act 1994 (Ref. 15.3) adds new environmental duties to the Land Drainage Act 1991. It places on the Internal Drainage Board (IDB) and Local Authorities to further the conservation and enhancement of natural beauty and the conservation of flora, fauna and geological or physiographical features of special interest and to take into account any effects which the proposals would have on the beauty or amenity of any rural or urban area or on any such flora, fauna or features;

The Environment Act 1995 (Ref. 15.4) sets out the responsibilities of the Environment Agency (EA) in relation to water pollution, resource management, flood defence and fisheries;

Water Act 2003 (Ref. 15.5) amends the Water Resources Act 1991 and the Water Industry Act 1991 to formalise the Government’s commitment to the sustainable management and use of water resources;

The Water Environment (Water Framework Directive) (England and Wales) Regulations, 2003 (WFD) (2000/60/EC) (Ref. 15.6) establishes a framework for protecting the water environment and is designed to improve and integrate the way water bodies are managed throughout Europe. The WFD requires all inland and coastal waters to achieve ‘good’ chemical and ecological water quality targets by 2015; and,

The Groundwater (England and Wales) Regulations (2009) (Ref, 15.7) transposed certain elements of the Water Framework Directive, as they relate to groundwater and Article 6 of the




2006 Groundwater Daughter Directive (2006/118/EC). The regulations are an environmental protection measure which provides enhanced protection for groundwater by preventing the input of ‘hazardous’ substances into groundwater and limiting the input of ‘non-hazardous’ pollutants into groundwater.

EU Flood Directive & Flood Risk Regulations (2009) (Ref. 15.8) these regulations require the preparation of Preliminary Flood Risk Assessments, flood risk maps, hazard maps and flood risk management plans for flood risk from all sources of flooding. Areas of nationally significant risk must also be identified, and flood risk maps, hazard maps and management plans must be produced for those areas.

Flood and Water Management Act 2010 (Ref. 15.9) revises and consolidates legislation relating to flooding, land drainage, coastal erosion and reservoir safety. The Act implements new statutory responsibilities for unitary and county councils to manage local flood risk. The Act also includes provisions regarding flood risk management assets, sustainable drainage, powers to undertake environmental works, reservoir safety and a number of alterations to water and sewerage provision.

Planning Policy 15.2.2 Planning policy at the national and local level and its relevance to environmental design and

assessment is discussed in (Chapter 6 – Planning Policy and Context). A summary of the Proposed Scheme’s compliance with legislation and planning policy has been included in Table 15.6 Policies of particular relevance to flooding, hydrology and water resources are discussed below.

National Planning Policy Framework

15.2.3 The National Planning Policy Framework (NPPF) (Ref. 15.10) was published on 27 March 2012 and is a key part of the reforms to make the planning system less complex and more accessible, to protect the environment and to promote sustainable growth. There is an overarching presumption in favour of sustainable development that should be the basis of every plan and every decision.

15.2.4 The NPPF consolidates all of the previous Planning Policy Statements (PPSs) and Planning Policy Guidance Notes (PPGs) into one document. The NPPF replaces Planning Policy Statement 25 (PPS25) Development and Flood Risk Practice Guide. Following launch of the National Planning Policy Guidance (NPPG) (Ref. 15.11) on 6 March 2014 the Technical Guidance to the NPPF has been revoked.

15.2.5 The NPPF sets out the Government's national policies for flood risk management in a land use planning context within England. The NPPF (Ref. 15.10) contains the following policies relating to flood risk and is supported by the NPPG; these documents also retain key elements of PPS 25:

"Inappropriate development in areas at risk of flooding should be avoided by directing development away from areas at highest risk, but where development is necessary, making it safe without increasing flood risk elsewhere" [Para 100 Planning and Flood Risk];

"… a site-specific flood risk assessment must demonstrate that the development will be safe for its lifetime taking account of the vulnerability of its users, without increasing flood risk elsewhere, and, where possible, will reduce flood risk overall" [Para 102 Planning and Flood Risk]; and,

"When determining planning applications, local planning authorities should ensure flood risk is not increased elsewhere and only consider development appropriate in areas at risk of flooding where, informed by a site-specific flood risk assessment following the Sequential Test, and if required, the Exception Test, it can be demonstrated that:

Within the Site, the most vulnerable development is located in areas of lowest flood risk unless there are overriding reasons to prefer a different location; and

Development is appropriately flood resilient and resistant, including safe access and escape routes where required, and that any residual risk can be safely managed, including by




emergency planning; and it gives priority to the use of sustainable drainage systems" [Para 98 Planning and Flood Risk].

To prevent unacceptable risks from pollution and land instability, planning policies and decisions should ensure that new development is appropriate for its location. The effects (including cumulative effects) of pollution on health, the natural environment or general amenity, and the potential sensitivity of the area or proposed development to adverse effects from pollution, should be taken into account. Where a site is affected by contamination or land instability issues, responsibility for securing a safe development rests with the developer and/or landowner. [Para 120]

Local Plan

15.2.6 The saved policies from the Daventry District Council Local Plan were published in November 2010 (Ref 15.12). The relevant policy is summarised below:

Policy ENV35 – “Planning permission will not be granted for development that would adversely affect the following ecologically important sites, Daventry Reservoir, Drayton Reservoir, Disused Daventry / Braunston Railway Line, Staveton Valley, Grand Union and Oxford Canals. In recognition of their ecological value, the District Council will seek to ensure that proposals for new development have no detrimental environmental effects and that the sites are managed in such a way as to ensure public access where practical.”

The saved policies from South Northamptonshire Local Plan were published in September 2007 (Ref. 15.13). The relevant policy is summarised below:

Policy EV25 – “The council will not permit development that would adversely affect the nature conservation, landscape or wildlife value of dismantles railways or waterways and watercourses.”

15.2.7 Policies EV26 and EV27 (Flood Protection and Sewerage and Sewage Treatment Infrastructure) have been deleted from the South Northamptonshire Local Plan.

Guidance

15.2.8 The NPPG (Ref. 15.11) contains guidance on water supply, waste water and water quality and contains the following guidance and policies:

Adequate water and wastewater infrastructure is needed to support sustainable development. A healthy water environment will also deliver multiple benefits, such as helping to enhance the natural environment generally and adapting to climate change. [Para 001 Water supply, wastewater and water quality];

The EU Water Framework Directive applies to surface waters (including some coastal waters) and groundwater (water in underground rock). It requires member states, among other things, to prevent deterioration of aquatic ecosystems and protect, enhance and restore water bodies to ‘good’ status. Local planning authorities must, in exercising their functions, have regard to the river basin management plans on the EA website that implement the Water Framework Directive. These plans contain the main issues for the water environment and the actions needed to tackle them. [Para 001 Water supply, wastewater and water quality];

Local planning authorities should adopt proactive strategies to mitigate and adapt to climate change, taking full account of flood risk, coastal change and water supply and demand considerations. [Para 94 Meeting the challenge of climate change, flooding and coastal change]; and

Local planning authorities should set out the strategic priorities for the area in the Local Plan. This should include strategic policies to deliver…the provision of infrastructure for transport, telecommunications, waste management, water supply, wastewater, flood risk and coastal change management, and the provision of minerals and energy (including heat). [Para 165 Plan Making].




15.2.9 The following Pollution Prevention Guidelines (PPGs) have been issued by the EA and are considered relevant to this assessment:

PPG 1 ‘Understanding Your Environmental Responsibilities - Good Environmental Practices’’ (Ref. 15.14);

PPG 5 ‘Works in, Near or Liable to Affect Watercourses’ (Ref.15.15);

PPG 6 ‘Working at Construction and Demolition Sites’ (Ref. 15.16); and,

PPG 21 ‘Pollution Incident Response Planning’ (Ref. 15.17).

Other best practice guidance documents relevant to this assessment include the following:

CIRIA C532 (2001) ‘Control of Pollution from Construction Sites’ (Ref. 15.18); and

CIRIA 697 (2007) ‘The SuDS Manual’ (Ref. 15.19). This document provides best practice guidance on the planning, design, construction, operation and maintenance of Sustainable Drainage Systems (SuDS) to facilitate their effective implementation within developments.

15.2.10 The EA’s Policy Statement for the Upper Nene Catchment (Ref 15.20) states that:

For Upper Nene catchment (the area through Northampton and within the Nene catchment upstream of Northampton) surface water attenuation should be provided up to the 1 in 200 year flood event plus climate change for all new development. The general standard of 1 in 100 year plus climate change protection is increased to 1 in 200 year plus climate change due to the history of flooding in the catchment;

Sustainable Drainage System (SuDS) features must be located outside the 1 in 100 year floodplain in the River Great Ouse catchment and SuDS features (including balance ponds) outside the 1 in 200 year (plus climate change) floodplain in the River Nene catchment;

If the outfall of a proposed attenuation system is likely to be submerged in a 0.5% rainfall event, then within 24 hours of top water level being attained in a 0.5% probability flood event the regulation facility must be capable of storing 80% of the additional run-off arising from a 10% probability flood; and,

The volume of runoff must not be greater than the greenfield runoff for the 1 in 200 year, 6 hour rainfall event, and the peak flow rate discharged must not be greater than greenfield runoff rate for each of the following:

1:1 year rainfall event;



1:200 year rainfall event; and

1:200 year plus climate change.

15.3 Assessment Methodology and Significance Criteria

Scope of the Assessment 15.3.1 A scoping report was submitted to Northamptonshire County Council (NCC) in April 2014. This

section provides an update on the scope of the assessment and re-iterates the evidence base for insignificant effects.

Insignificant Effects

15.3.2 The following insignificant effects have been identified in the site preparation, earthworks and construction phase:




Increase in potable water demand associated with the construction works.

15.3.3 The construction works are not considered likely to generate a notable demand for potable water, and as such it is not considered likely to be a significant effect.

15.3.4 The following insignificant effects have been identified during the operational phase:

Increase in potable water demand and/ or foul drainage capacity associated with the Proposed Scheme.

15.3.5 Given the Proposed Scheme comprises a highway and no buildings; it is considered that there will not be an increase in potable water demand or a requirement for foul drainage.

Potentially Significant Effects

13.4.4 The likely significant effects that have been identified for inclusion in the assessment are as follows:

Site Preparation, Earthworks and Construction Phase

Potential increase in risk of flooding (fluvial, pluvial, drainage and groundwater) to site preparation, earthworks and construction workers;

Potential increase in physical contamination (i.e. sediments) entering surface water bodies and drainage features due to increased sediment loading; and,

Potential changes to pattern and quantity of groundwater and surface water flows and subsequent effects on water receptors (e.g. Aquifers and surface water courses).

Operational phase

Potential increase in risk of flooding (fluvial, pluvial, drainage and groundwater) to future Site users;

Potential increase in physical contamination (i.e. sediments) entering surface water bodies and drainage features due to increased sediment loading; and,

Potential changes to pattern and quantity of groundwater and surface water flows and subsequent effects on water receptors (e.g. Aquifer and surface water courses).

Extent of the Study Area 15.3.6 The study area for Flooding, Hydrology and Water Resources encompasses the Site and sensitive

water receptors within the potential area of influence. The environmental setting (hydrology and hydrogeology) is based on a 1km search area from the Site boundary. The extent of the Site boundary is shown on Figure 1.2 and a description of the Site and surrounding area is presented within Chapter 3 – The Existing Site and Surroundings.

Consultation Undertaken to Date 15.3.7 Table 15.1 provides a summary of the consultation activities undertaken in support of the preparation

of this Chapter.

Table 15.1: Summary of Consultation Undertaken to Date

Body / organisation Individual / s at body/organisation

Meeting dates and other forms of consultation Summary of outcome of discussions

Environment Agency

Rachael Storey (Partnership and Strategic Overview Officer)

Meeting (29th September 2013) and various email correspondence dated between 14th October 2013 and 11th December 2013)

Provision of details relating to hydraulic model and hydraulic model




Body / organisation Individual / s at body/organisation

Meeting dates and other forms of consultation Summary of outcome of discussions

Environment Agency Vicky Ellison (Customers & Engagement Officer)

Email correspondence dated 7th February 2014

The EA has confirmed that it does not hold any records of flooding affecting the Site itself or within the vicinity of the Site.

Anglian Water Gary Fulton (Growth Planning & Equivalence Team)

Email correspondence dated 25th February 2014

Anglian Water has confirmed that it does not hold any records of flooding affecting the Site itself or within the vicinity of the Site.


Ruth Burnham (Senior Flood and Water Officer)

Email correspondence dated 28th November 2013

Northamptonshire County Council confirmed that they have a number of records of historic flooding within the vicinity of the proposed development. This records are listed in section 15.6.45.

Daventry District Council

Joy Smith (Planning Admin Officer)

Email correspondence dated 18th March 2014

Daventry District Council confirmed that it does not hold any records of flooding affecting the Site itself or within the vicinity of the Site.

Method of Baseline Data Collation 15.3.8 The following reports comprise the main references for baseline information in this Chapter:

WSP, ‘NPPF Flood Risk Assessment’ (FRA), Draft, Reference 70000131, October 2014 (The FRA is submitted as a separate report in support of the planning application);

WSP, ‘A45 Flore – Weedon Bypass, Preliminary Sources Study Report’ Reference 12171165-001, March 2010 (Appendix 16.1) This report assesses the historical land uses, environmental setting and potential ground conditions for 3 proposed bypass route options;

WSP, ‘A45 Daventry Link Road, Preliminary Sources Study Report’ Ref: 00039880-001-R01, September 2013 (Appendix 16.2). This report assesses the historical land uses, environmental setting and potential ground conditions for the Scheme;

WSP (June 2014) ‘A45 Daventry Link Road, Ground Investigation Report’ draft issue Ref: 00039880-001-R02 (Appendix 16.3);

WSP (July 2014) ‘Interpretative Geotechnical Report’ draft Issue Ref. 00039880/10135 (Appendix 16.4);

WSP, ‘Water Framework Directive Assessment (WFDA)’, draft issue, Reference 6200160538889, November 2014 (Appendix 16.5);

Geotechnics Limited (November 2014) ‘Ground Investigation at A45 Daventry Link Road’, Ref. PC145708 (Appendix 16.7); and

WSP (November 2014) ‘Contamination Assessment’, Ref. 70007479-R01 (Appendix 16.8).

15.3.9 Other sources of desk study information are referred to throughout this Chapter where relevant and a full list of references is presented at the end of the Chapter.

Assessment Modelling Flood Risk

15.3.10 The NPPF (Ref. 15.10) follows a sequential risk-based approach in determining the suitability of land for development in flood risk areas, with the intention of steering all new development to the lowest flood risk areas.

15.3.11 The NPPG (Ref: 15.11) Table 3 confirms the ‘Flood Risk Vulnerability Classification’ of a site depending upon the proposed usage. This classification is subsequently applied to determine whether:




The Proposed Scheme is suitable for the flood zone in which it is located; and,

Whether an exception test is required for the Proposed Scheme.

15.3.12 The FRA has been prepared in accordance with the requirements of the NPPF covering effects of construction and operation. The findings of the FRA are summarised in this Chapter.

15.3.13 The FRA is a holistic risk based assessment of potential flooding from possible sources, including fluvial, groundwater and surface water run-off. The assessment also identifies and examines the residual flood risk to the Proposed Scheme and third party land.

15.3.14 Preliminary assessment of drainage infrastructure was undertaken to investigate the potential effects of the Proposed Scheme on the drainage system, and to assist the development of the drainage design.

15.3.15 Mitigation for potential flood risk effects identified has been ‘designed-in’ to the Proposed Scheme and is therefore considered in the pre-mitigation stage of the assessment.

15.3.16 The FRA focuses on assessing the practical flood risk issues at the Site as follows:

Identification of potential sources of flooding at the Site (i.e. groundwater, fluvial, pluvial and drainage);

Assessment of the existing flood risk at the Site and the potential effect of the Proposed Scheme on flood risk; and

Consideration of the flood risk implications, taking into account the potential allowance for climate change over the lifetime of the Proposed Scheme; the identification of measures to design out flood risk for the Proposed Scheme; and, the effect of flood risk to other land due to the Proposed Scheme.

Significance Criteria 15.3.17 The assessment of potential effects as a result of the Proposed Scheme, has taken into account both

the site preparation, earthworks and construction phase and operational phase. The significance level attributed to each effect has been assessed based on the magnitude of change due to the Proposed Scheme and the sensitivity of the affected receptor/receiving environment to change, as well as a number of other factors that are outlined in more detail in Chapter 2 - Approach to the EIA. Magnitude of change and the sensitivity of the affected receptor/receiving environment are both assessed on a scale of high, medium, low and negligible (as shown in Chapter 2 - Approach to the EIA).

Effect Significance

15.3.18 The following terms have been used to define the significance of the effects identified:

Major effect: where the Proposed Scheme could be expected to have a very significant effect (either positive or negative) on flooding, hydrology and water resources and associated sensitive receptors;

Moderate effect: where the Proposed Scheme could be expected to have a noticeable effect (either positive or negative) on flooding, hydrology and Water Resources and associated sensitive receptors;

Minor effect: where the Proposed Scheme could be expected to result in a small, barely noticeable effect (either positive or negative) on flooding, hydrology and Water Resources and associated sensitive receptors; and

Negligible: where no discernible effect is expected as a result of the Proposed Scheme on flooding, hydrology and Water Resources and associated sensitive receptors.




15.4 Sensitive Receptors 15.4.1 The following are the sensitive receptors which will be assessed in the following assessment:

On-site surface water courses (Whilton Branch of the River Nene, Grand Union Canal, Hollandstone Brook and ponds);

Off-site surface water courses (the River Nene and ponds);

Bugbrooke Meadow Site of Special Scientific Interest (SSSI) is located approximately 475m to the south of the eastern extent of the Site. The SSSI is located adjacent to the River Nene and could therefore potentially be impacted by changes in surface water flow;

Secondary A Aquifer and Secondary Undifferentiated Aquifer underlying the Site;

Future drainage systems on-site;

Earthworks and construction workers;

Third parties; and

Future site users.

15.4.2 The location of surface water features is shown on Figure 15.1.

15.5 Baseline Conditions

Site Description and Current Site Use 15.5.1 The Site is located on predominantly Greenfield agricultural land to the north of the villages of

Weedon Bec and Flore in Northamptonshire. A motocross track is present just off the A5 in the west.

15.5.2 The Proposed Scheme crosses the Grand Union Canal, the West Coast Main Line railway and the Whilton Branch, a tributary of the River Nene in the west.

15.5.3 The eastern and central western parts of the Proposed Scheme cross two former landfill sites (OS Field No. 0756 Landfill in the east, and Dodmoor Farm Landfill in the west).

Topography 15.5.4 The topography predominantly comprises gently undulating rural land dominated by two hills on

either side of the valley. These hills are Weedon Hill (162m AOD), located to the south west of Weedon, and Glassthorpehill (142m AOD) to the east of Flore.

15.5.5 The western end of the Proposed Scheme is at an elevation of approximately 115m AOD. The elevation immediately decreases as the Proposed Scheme descends into the Nene Valley, the base of which lies at approximately 80m AOD. From here the Proposed Scheme then begins to ascend from the valley towards Hobhill which has a maximum elevation of approximately 105m AOD at Hobhill Spinney. The Proposed Scheme then turns to the southeast trending roughly parallel to the M1. This section of the Proposed Scheme undulates but overall it descends towards a small, flat bottomed valley at approximately 70m AOD, Hollandstone Farm Brook is present at this level. The Proposed Scheme then ascends and crosses gently undulating ground until its intersection with the A45 east of Upper Heyford at an elevation of approximately 80m AOD.

Surrounding Land Use 15.5.6 The Site is predominantly surrounded by agricultural fields. The M1 motorway is present to the east

of the Site. The residential settlements of Flore and Weedon are present to the south of the Site.




Hydrology 15.5.7 Pertinent surface water features on-site identified within 1km are described below in Table 15.2 and

shown on Figure 15.1:

Table 15.2: Summary of Local Surface Water Features

Surface Water Feature Approximate Distance from the Site RBMP* Chemical Quality Classification RBMP Ecological Quality

Classification

River Nene Variable from south west to south east

Does not require assessment (north west), Good and predicted to remain so by 2015 (south east)

Moderate and predicted to remain so by 2015 (northwest and south east)

River Nene – Whilton Branch (incorporating Whilton Brook and Old Mill Race)

On-site (west) Good and predicted to remain so by 2015 Moderate and predicted to remain so by 2015

Grand Union Canal On-site (west) Good and predicted to remain so by 2015 Moderate and predicted to remain so by 2015

Hollandstone Farm Brook On-site (east) NA NA

* RBMP = River Basin Management Plan (EA). NA = Not Assessed

15.5.8 A total of 15 ponds are present within 250m of the Site, with a further 15 ponds within 250m and 500m of the Site. The location of the ponds is shown on Figure 15.1. Six ponds are located within the red line boundary. Pond 5 is a fully enclosed water body and is therefore not considered further in this assessment.

15.5.9 The hydrology in the area is dominated by the River Nene, which is located to the south of the Site in an off-site location and flows approximately west to east passing through Weedon Bec and to the south of Flore.

15.5.10 The western third of the Proposed Scheme is set within the Nene Valley. The Whilton Branch of the River Nene flows from the north of Daventry and along the general route of the M1 before crossing the Proposed Scheme between Flore and Weedon Bec (all off-site). The main channel of the River Nene flows from the west and through Weedon Bec. The two channels converge to the east of Weedon Bec before flowing generally east. Hollandstone Farm Brook flowing north to south joins the River Nene to the south east of Flore and crosses the Site prior to this.

15.5.11 The Grand Union Canal follows a similar path to the Whilton Branch of the River Nene, crossing the main channel of the River Nene before its confluence with the Whilton Branch, and then flowing generally to the south east.

15.5.12 From east to west, the Proposed Scheme will cross Hollandstone Farm Brook, the Whilton Branch of the River Nene and the Grand Union Canal. The Grand Union Canal is expected to be lined with a low permeability barrier.

15.5.13 Licensed surface water abstractions within 1km of the Site are listed in Table 15.3 below.

Table 15.3: Licensed Surface Water Abstractions

Operator Distance and Direction Stated Use

R S Smith & Son 281m north Private Water Supply (Amenity etc.), and Impounding (Vicarage Brook, upstream of the Site)

R W Amos Esq 691m west Private Water Supply (Amenity etc.), and Impounding

Heygate & Sons 928m south east Food And Drink: Process Water (from Nene)




15.5.14 The two nearest surface water abstractions are located up gradient of the Site and will therefore not be impacted by the Proposed Scheme.

15.5.15 Active or potentially active discharge consents direct to surface water within 1km of the Site listed in the Envirocheck in (Appendix 16.6), are presented in Table 15.4 below.

Table 15.4: Licensed Surface Water Discharge Consents

Operator Reference Distance and Direction Stated Use and Water Body

Mr D F Pennell Pr5nf2165 135m west Not supplied

Orbit Housing Association Prnnf12098 376m south east Sewage Discharges - Final/Treated Effluent - Not Water Company. Brockhall Road Roadside Ditch.

Botterill Builders Ltd. Prnnf00126 504m south east Discharge of Other Matter-Surface Water

John Arthur Banner Pr5nf2156 532m west Not supplied, but from single domestic property.

Mr Trevor Spokes & Mrs Rachel Spokes Npswqd008549 566m east Sewage Discharges - Final/Treated Effluent - Not Water

Company. Un-named tributary of River Nene

Anglian Water Services Ltd Aw5nf292 622m west Discharge of Other Matter-Surface Water. Whilton Branch River Nene

Anglian Water Services Ltd Awnnf13423 671m south west Sewage Discharges - Pumping Station - Water Company. Tributary of River Nene.

The Occupier, 2 Willow End Dodford, Weedon. Pr5nf2002 724m north west Not supplied. To Wilton Branch.

Mrs Elaine Hamer Prnnf01914 805m west Sewage Discharges - Final/Treated Effluent - Not Water Company. To tributary of Dodford Brook.

Anglian Water Services Ltd AW5NF/65 710m west and 833m south east

Sewerage and Other Matter Discharge - Storm Sewerage. River Nene.

Anglian Water Services Ltd Aw5nf1766 929m south west Sewage Discharges - Pumping Station - Water Company. River Nene.

Tasty Blackboard Co Ltd Pr5nf777 978m east Sewage Discharges - Final/Treated Effluent - Not Water Company. River Nene.

Anglian Water Services Ltd Aw5nf163 991m south Sewage Discharges - Pumping Station - Water Company. River Nene.

15.5.16 A further six consents are listed for discharges directly to land or soakaway within 1km of the Site.

Geology and Ground Conditions 15.5.17 Published 1:50,000 geological mapping (Ref 15.21) shows that within the valleys the geology is

predominantly influenced by the River Nene with Alluvium indicated adjacent to the River Nene and its larger tributaries. Glaciofluvial Deposits are also shown within the valleys at the eastern section of the Site, whilst in the western section of the Site superficial deposits of Glacial Till (Oadby Member) are indicated.

15.5.18 The geological mapping reviewed indicates that beneath the superficial deposits the Site is underlain by Middle and Upper Lias Clay. These have been re-named in the east as the Whitby Mudstone Group (mudstone with interbeds of sandstone, siltstone and limestone) and at the western extent of the Site as Dyrham Formation (interbedded siltstone and mudstone). In the central and northern




section of the Site the bedrock geology is indicated to be Marlstone Rock Formation (limestone interbedded with sandstone and mudstone).

15.5.19 The ground investigation (Appendix 16.3) confirmed the anticipated geological sequence, which is summarised in Table 15.5.

Table 15.5: Summary of Ground Conditions

Stratum Typical Description Occurrence*

Made Ground – Historical Landfills

Soft and firm sandy / gravelly clay.

Fine to coarse sand and fine to coarse gravel of brick, concrete and road planings.

With cobbles and boulders (600-1200mm) of concrete.

Associated with Dodmoor Farm Landfill and landfill OS Field No. 0756 Landfill.

Topsoil Soft, brown, sandy / gravelly clay. Fine to coarse, angular to rounded gravels. Occasionally with a low cobble content.

Almost all exploratory boreholes and trial pits.

Alluvium

Very soft to firm, sandy, peaty in places, clay with some shells and decomposed organic remains.

Clayey fine to coarse sand with subangular to subrounded gravel.

Angular to rounded gravel of flint, shale, sandstone, mudstone and limestone.

Encountered in recent exploratory holes adjacent to the Whilton Branch of the River Nene..

Localised deposits encountered in historical exploratory holes between the West Coast Main Line and the Whilton Branch of the River Nene

Granular Glacial Deposits

Loose to very dense, silty/clayey sand/gravel predominantly comprising angular to rounded flint, chalk, sandstone, ironstone and limestone.

With cobbles and boulders of limestone and flint.

Localised bands/lenses of clay.

Encountered as localised bands in exploratory holes between in the west.

Encountered in exploratory holes north of Upper Heyford.

Cohesive Glacial Deposits

Soft to stiff, sandy/gravelly clay with fine to coarse subangular to rounded gravel of flint, limestone, chalk, sandstone and ironstone. Some cobbles and boulders of flint.

Soft to firm silt.

Some partings/lenses of sand.

Encountered in exploratory holes in the west.

Encountered in exploratory holes north of Upper Heyford.

Whitby Mudstone Group (Upper Lias)

Firm and stiff, grey, orangish brown, yellowish brown and light brown mottled, sandy / gravelly in places, micaceous, calcareous, silt / clay occasionally grading to siltstone / mudstone and occasionally containing bands of limestone.

Occasionally described as friable, containing trace fossils and fissured.

Shear surfaces described in five exploratory holes over the recent and historical ground investigations.

At surface in the west.

Thickness is variable from <1.0m to 10m.

Shear surfaces are described in historical exploratory holes BH51, BH53, BH57, BH62 and TP84.

Marlstone Rock Beds Medium strong and strong, fossiliferous or bioclastic, ferruginous limestone interbedded with extremely weak to weak mudstone, siltstone and sandstone.

Encountered at surface or underlying the Upper Lias in the north and east.

Dyram Formation (Middle Lias)

Soft to very stiff, bluish grey, dark grey, orangish brown, sandy / gravelly in places, calcareous, silt / clay grading to siltstone / mudstone and containing bands of limestone.

Frequently described as laminated and fissured.

Shear surfaces described in seven exploratory holes over the recent and historical ground investigations.

Underlying the entire route.

At surface or underlying drift soils.

Shear surfaces are described in SATP03, SATP06A and historical exploratory holes BH32, BH39, BH43, BH129 and TP95.

* - Chainages given are approximate and based on available ground investigation data.




Hydrogeology 15.5.20 The EA has designated the Whitby Mudstone Formation as unproductive strata, the Marlstone Rock

Formation as a ‘Secondary ‘A’ Aquifer’, and the Dyrham Formation as a Secondary ’Undifferentiated Aquifer’.

15.5.21 The overlying superficial deposits of Glaciofluvial Deposits and Alluvium are classified as ‘Secondary A Aquifers’, whilst the Oadby Member is classified as ‘Unproductive Strata’.

15.5.22 Groundwater is likely to be present within the more permeable strata (e.g. sandstone / limestone) and is potentially confined by less permeable horizons including clay and mudstone. Groundwater flow direction in the superficial deposits is likely to be influenced by the local surface water features that cross the Site in the west and east, and the Site topography. Deeper groundwater flow is likely to be influenced by the general dip of the strata and regional groundwater flow direction, which has not been determined as part of this assessment..

15.5.23 The Site is not located within an EA designated groundwater source protection zone (SPZ). The underlying groundwater quantitative quality and chemical quality is classified ‘Good’ under the EA’s Water Framework Directive River Basin Management Plan, and is predicted to remain as such by 2015 (Ref. 15.22).

15.5.24 Licensed groundwater abstractions within 1km of the Site are presented in Table 15.6.

Table 15.6: Groundwater Abstractions

Operator Distance and Direction Stated Use

R S Smith 225m north east General Farming and Domestic

G & F Chaplin 686m east General Farming and Domestic

15.5.25 During consultations as part of the Water Framework Directive Assessment (WFDA) (Appendix 16.5), the EA has confirmed that “There are no current surface water or groundwater abstractions in the area”. It is noted that the extent of the search area was not stated.

15.5.26 Goundwater monitoring wells were installed and soakaway testing completed as part of ground investigations undertaken on the Site (Appendices 16.3, 16.4 and 16.7). Details of the monitoring programme and soakaway test results are presented in Chapter 16 - Ground Conditions, Hydrogeology and Contamination.

15.5.27 Surface water elevations within the Whilton Branch of the River Nene and Hollandstone Farm Brook were recorded between 76.11m and 79.0m AOD. The surface water elevation of the Great Union Canal was recorded at 89.84m AOD. On the whole, groundwater levels are higher than their surface water counterparts indicating that surface water bodies are gaining systems and receive a portion of their flow from groundwater.

15.5.28 A groundwater flow direction could not be determined based on the ground investigation completed. However, shallow groundwater flow is considered likely to be towards the surface watercourses or coinciding with the general fall in surface topography. Shallow groundwater recorded on-site potentially represents a continuous lateral groundwater body within the underlying aquifers (on-site and off-site). Groundwater at the Site has the potential to be in continuity (locally or otherwise) with the on-site surface watercourses and off-site surface water bodies such as the River Nene. It is not known if the ponds identified in the vicinity of the Site are lined or in continuity with groundwater. As such, in order to provide a conservative assessment, it is assumed that they are potentially in continuity with groundwater.




Hydraulic Regimes 15.5.29 On the whole, groundwater levels are higher than their surface water counterparts indicating that

surface water bodies (Table 15.2) are likely to be gaining systems. The hydraulic regime for each surface water feature is described below.

Grand Union Canal Hydraulic Regime

15.5.30 The Grand Union Canal is expected to be lined with a low permeable barrier, creating a hydraulic barrier between surface water and groundwater. Boreholes BH4, 5, 10, and 7 are located within close proximity to the canal. The levels indicate if the surface water feature was not lined with a hydraulic barrier, it would receive a proportion of flow from groundwater and would be classified as a gaining system.

Whilton Branch of the River Nene Hydraulic Regime

15.5.31 Comparison of surface water elevations with the groundwater elevations from the closest boreholes (boreholes BH7 and 11) indicate that the Whilton Branch should receive a proportion of flow from groundwater and is therefore classified as a gaining system.

Hollandstone Brook

15.5.32 Borehole BH30 is located 15m to the east of the brook. The borehole log suggests that there is no hydraulic barrier between the Alluvium and the water bearing Dyrham Formation, however, the Dyrham Formation at this location is predominantly clay which is likely to have a poor vertical permeability limiting the hydraulic link between groundwater in the two units. The monitoring results indicate the tributary will receive a proportion of flow from groundwater and is therefore classified as a gaining system.

Ponds

15.5.33 Fifteen ponds are located within 250m of the Site. Six of which are located within the red line boundary. No referenced surface water elevations were available. It has not been possible to assess the groundwater to surface water interface, as a conservative approach it has been assumed that there is connectivity.

Ecological Designations 15.5.34 Bugbrooke Meadow Site of Special Scientific Interest (SSSI) is located approximately 475m to the

south of the eastern extent of the Site. The SSSI is located adjacent to the River Nene and could therefore potentially be impacted by changes in surface water quality. Further ecological designations within the study area are discussed in Chapter 12 - Biodiversity.

Existing Surface Water Infrastructure 15.5.35 The Site is classified as a Greenfield development site. It is assumed that the Site currently drains via

infiltration and overland flows, towards the existing watercourses on-site (the River Nene, Whilton Branch of the River Nene, Hollandstone Farm and the Grand Union Canal).

Flood Risk 15.5.36 Baseline information on flood risk summarised below is taken from the FRA unless otherwise stated.

15.5.37 West Northamptonshire Level 1 Strategic Flood Risk Assessment (Level 1 SFRA) (Ref 15.23) provides evidence on flood risk to support assessment of potential development sites.




15.5.38 The SFRA notes that a FRA is required ‘to assess the flood risk posed to proposed developments and to ensure that, where necessary, appropriate mitigation measures are included in the development’.

15.5.39 The Level 1 SFRA also notes that a FRA is required if:

The development site is located in Flood Zone 2 or 3;

The proposed development is classed as a major development and located in Flood Zone 1. These are residential developments consisting of sites greater than 0.5ha or greater than 10 dwellings and commercial developments that are greater than 1ha or have a floor area greater than 1000m2;

The development site is located in an area known to have experienced flooding problems from any flood source; and

The development is located within 20m (water environment) of any watercourse regardless of Flood Zone classification.

15.5.40 West Northamptonshire (Daventry & South Northamptonshire) Level 2 Strategic Flood Risk Assessment (Ref 15.24) suggests that suitable surface water mitigation measures are incorporated into any development plans in order to reduce and manage surface water flood risk to, and posed by the proposed development. This should ideally be achieved by incorporating SuDS.

Historical Flood Events

15.5.41 The EA has confirmed that it holds no records of historical fluvial or pluvial flooding at the Site.

15.5.42 Northamptonshire County Council holds a number of records of historic flooding within the vicinity of the Proposed Highway, these are listed below:

Nov 2009: Highways Agency flood incident in Weedon Bec;

April 2012: Highways Agency flood incident in Weedon Bec;

Sept 2012: Highways Agency flood incident on M1;

Nov 2012: Surcharging manhole in Flore;

Nov 2012: Highways Agency flood incident on M1;

Dec 2012: Surcharging manhole in Flore;

Dec 2012: Flooding in Upper Heyford;

Dec 2012: Flooding in Weedon Bec; and

Unknown: Canal breach in Weedon Bec.

15.5.43 Daventry District Council has confirmed that it holds no records of historic flooding at the Site.

15.5.44 The Bedford Group of Internal Drainage Boards has confirmed that it holds no records of historic flooding at this time.

15.5.45 Anglian Water has confirmed that it holds no records of historic sewer related flooding at the Site.

15.5.46 The Highways Agency has confirmed that the area to the north of Junction 16 of the M1 is ‘A Very High’ flooding hotspot and there have been several flood events of varying severity.

Flooding From Fluvial Sources

15.5.47 The flood risk areas are shown on Appendix H in the FRA.




15.5.48 The EA Flood Map identifies the areas alongside the on-site watercourses (Whilton Branch of the River Nene and Hollandstone Farm Brook) to be in Flood Zone 2 and 3. Approximately 300m of the Proposed Scheme is located within Flood Zone 3.

15.5.49 Land in Flood Zone 2 has been assessed as having an annual probability of flooding from rivers or the sea between 1 in 100 years and 1 in 1000 years (1% - <0.1%). Land in Flood Zone 3 has an annual probability of river flooding of 1 in 100 years or greater (>1%).

15.5.50 The remainder of the Proposed Scheme is on land within Flood Zone 1. This zone comprises land assessed as having a less than 0.1% 1 in 1,000 annual probability of river or sea flooding in any given year.

15.5.51 The Environment Agency confirmed that they do not hold suitable flood levels for the Hollandstone Farm Brook and their hydraulic model for the Upper Nene does not have sufficient coverage for the Whilton Branch of the River Nene. It was therefore necessary to carry out hydraulic modelling for the Hollandstone Farm Brook and extend the Environment Agency’s Upper Nene model to determine flood levels to inform the design of the Proposed Highway. The results of the hydraulic modelling are appended to the FRA and have received approval from the EA.

15.5.52 Hollandstone Farm Brook: The baseline hydraulic model results show that the M1 culvert immediately upstream of the proposed crossing is the critical hydraulic structure as it throttles the flow in the watercourse resulting in flood waters backing upstream. Flood flows downstream at the location of the proposed crossing are therefore restricted to only the channel without spilling into the floodplain during the 1 in 200 year plus climate change event.

15.5.53 Whilton Branch: The predicted peak water levels for the Whilton Branch of the River Nene indicate that flood flows are generally out of bank at the proposed crossing for all modelled return periods. The floodplain is significantly wider than that of Hollandstone Farm Brook.

15.5.54 The EA has confirmed within their correspondence (dated 23 May 2013, and confirmed to still be relevant on 7 February 2014) that they have no record of fluvial flooding at the Site and no formal flood defences are present. The natural channel of the Whilton Branch of the River Nene is maintained by the EA, and provides a nominal protection against a flood event with a 20% chance of occurring in any year (1 in 5). The EA inspects the channel regularly to ensure that any potential defects are identified early.

15.5.55 The risk of flooding from fluvial sources to the Site is considered to be moderate by the FRA.

Flooding From Pluvial Sources

15.5.56 In extreme storms the ground may become saturated and the drains and sewers which carry away surface water may not be able to accommodate these flows, or may even be blocked with debris, leading to surface water flooding.

15.5.57 The EA updated Flood Map for Surface Water indicates that there are a number of areas shown to be at risk from surface water flooding for the 1 in 30, 1 in 100 and 1 in 1,000 year return periods. The majority of areas shown to be at risk of surface water flooding within the Site are alongside the Whilton Branch of the River Nene and Hollandstone Farm Brook (1 in 30 chance of flooding). The high risk areas associated with surface water therefore double as fluvial flood plains. Approximately 250m of the Proposed Highway is shown to be at risk of surface water flooding for the 1 in 1,000 year event to the north of Flore.

15.5.58 A few smaller areas along and adjacent to the alignment are shown to be at risk of surface water flooding for the 1 in 30 and 1 in 100 year events, including a stretch of land alongside the M1.




15.5.59 Northamptonshire County Council provided a number of records of historic flooding in the vicinity of the Proposed Scheme as detailed in the previous historical flood records above.

15.5.60 The risk of pluvial/overland flow to the Site is considered to be moderate by the FRA.

Drainage and Infrastructure

15.5.61 Anglian Water has confirmed that they have no record of flooding occurring in the vicinity of the Site due to the surcharging of public sewer system. Flooding could occur as a result of infrastructure failure, for example burst water mains or failed pumping stations.

15.5.62 No records have been made available of water mains and water infrastructure flooding. Given the topography and general depths of construction, the risk of flooding from sewers and drainage infrastructure at the Site is considered to be negligible by the FRA.

15.5.63 Flooding could occur as a result of infrastructure failure, for example breach or overtopping. The Grand Union Canal is not modelled as part of the EA’s Upper Nene Model. The Grand Union Canal is at an elevation lower than that of the Proposed Highway. Therefore, in the unlikely event of a breach, the breach waters will discharge into the Whilton Branch of the River Nene without increasing the risk to the surrounding properties or flooding the highway. There are no dams within the vicinity of the Site and no such infrastructure in the wider catchment is known to have an influence on the Site. The Site does not lie within the inundation area of any reservoirs. Therefore, the risk of flooding from dam and canal infrastructure at the Site is considered to be negligible by the FRA.

Flooding From Groundwater Sources

15.5.64 Groundwater flooding occurs when the water table rises above the level of the ground. The ground investigation (Appendix 16.3) recorded shallow groundwater beneath the Site. During the monitoring period, groundwater levels at the base of the River Nene valley were generally located within close proximity to ground level and artesian conditions have been recorded locally associated with shallow beds of rock. Groundwater levels across the rest of the Proposed Scheme were below ground level during the monitoring period.

15.5.65 The general groundwater flow direction and water bearing strata at the Site is not confirmed by the ground investigation. However, groundwater flow direction is likely to be influenced by the local surface water features (i.e. towards the River Nene or tributaries). Deeper groundwater flow is likely to be influenced by the general dip of the strata and regional groundwater flow direction.

15.5.66 The EA has confirmed that it holds no records of groundwater flooding at the Site.

15.5.67 Therefore the risk of groundwater flooding at the Site is considered to be Low to Moderate by the FRA especially at the locations where the Proposed Scheme will be in cutting.

Baseline Flood Risk Summary

15.5.68 A summary of the baseline flood risk as presented within the FRA, is presented in Table 15.7.

Table 15.7: Summary of Baseline Flood Risk

Potential Source of Flooding Baseline Flood Risk

Fluvial Moderate

Pluvial Moderate

Drainage and Infrastructure Negligible / Low

Groundwater Low / Moderate




Future Baseline 15.5.69 Should the Proposed Scheme not proceed, it is considered that the future baseline conditions in

relation to flooding, hydrology and water resources at the Site would remain relatively unchanged. The volume and intensity of precipitation falling on the Site could increase due to climate change. This could have a corresponding effect on flood risk associated with the drains and water courses on-site and in the vicinity of the Site.

15.6 Assessment of Effects, Mitigation and Residual Effects


Design Solutions and Assumptions

15.6.1 As detailed in Chapter 4 - Description of The Proposed Scheme, the site preparation, earthworks and construction phase of the Proposed Scheme will be undertaken in accordance with industry best-practice including the Considerate Constructors Scheme.

15.6.2 A temporary drainage solution will be implemented during the site preparation, earthworks and construction phase as part of the design solution to collect, attenuate and subsequently discharge surface water. Further information is provided in Chapter 4 - Description of The Proposed Scheme.

15.6.3 Prior to the commencement of the construction phase, Site run-off will be controlled to mitigate both flood risks and sediment loading. It is assumed that a phased temporary drainage network will be implemented to prevent sediment laden surface run-off from leaving the Site or entering surface water. The proposed temporary drainage strategy for this phase of the Proposed Scheme will be developed during the detailed design stage.

15.6.4 Any increase in surface water run-off above the pre-development (or Greenfield) volumes will be attenuated on site. It is proposed that onsite attenuation will be provided by a series of source control, filtration, swales/conveyance channel, and retention/detention methods before entering surface water courses.

15.6.5 The assessment completed as part of the FRA is summarised below:

The post development model results (see the FRA) indicate that the inclusion of the Proposed Scheme crossing over Hollandstone Farm Brook will have a minimal impact on flood levels in all areas upstream and downstream of the Proposed Scheme. The Proposed Scheme will still be operational during the 1 in 1000 year plus climate change event.

The post development hydraulic model results indicate that the inclusion of the proposed crossing over the Whilton Branch of the River Nene will result in increases in the flood levels of up to 2.7m and floodplain extent area of 11.7 ha on the land immediately upstream during the 1 in 200 year plus climate change event. However, Northamptonshire Highways have held discussions with all the landowners who have confirmed that the above impacts are acceptable on their parcels of land. It should be noted that there are no notable increases in flood levels in all areas outside these parcels.

Modelling reported in the FRA indicates that the Proposed Scheme does not flood in the vicinity of the Whilton Branch of the River Nene in the 1 in 1000 year event plus climate change.

The risk of flooding to and arising from the Proposed Highway, regarding pluvial/overland flow, is moderate in the FRA.

The FRA states that the Proposed Scheme is embanked and in cutting along its alignment but generally follows the existing topography and is therefore unlikely to act as a barrier to overland flows.




The Proposed Highway will cross the Grand Union Canal via a bridge. The integrity of the Grand Union Canal will not be impacted by the construction of the bridge. In the event of a breach in the canal, it is likely that water discharge into the Whilton Brook based on the topographical survey, and will not impact the Proposed Scheme.

Potential increase in risk of flooding (fluvial, pluvial, drainage and groundwater) to site preparation, earthworks and construction workers

Flooding From Fluvial Sources

15.6.6 The Site lies partly within Flood Zone 2 (between a 1 in 100 and 1 in 1000 annual probability of flooding / medium probability) and Flood Zone 3 (less than 1 in 100 annual probability of flooding / high probability) associated with the Whilton Branch of the River Nene and Hollandstone Farm Brook. The remainder of the Proposed Scheme is on land within Flood Zone 1 (less than 1 in 1,000 annual probability of river or sea flooding in any given year).

15.6.7 Flood modelling has been undertaken for Hollandstone Farm Brook and the Whilton Branch of the River Nene, as reported in the FRA. Predicted peak water levels for the Whilton Branch of the River Nene indicate that flood flows are generally out of bank at the proposed crossing for all modelled return periods. The baseline hydraulic model results for Hollandstone Brook show that the M1 culvert immediately upstream of the proposed crossing is the critical hydraulic structure as it throttles the flow in the watercourse resulting in flood waters backing upstream. Flood flows downstream at the location of the proposed crossing are therefore restricted to only the channel without spilling into the floodplain during the 1 in 200 year plus climate change event.

15.6.8 Correspondence with the EA (dated 23rd May 2013, and confirmed to still be relevant on 7th February 2014) states that there are no formal flood defences protecting the Site. The natural channel of the Whilton Branch of the River Nene is maintained by the EA and provides a nominal protection against a flood event with a 20% chance of occurring in any year (1 in 5). The EA inspects the channel regularly to ensure that any potential defects are identified early.

15.6.9 There are no raised flood defences on the Whilton Branch of the River Nene, Hollandstone Farm Brook or the Grand Union Canal.

15.6.10 In the event of fluvial flooding, excavations within areas classified as Flood Zone 2 and 3 may fill with water, increasing instability of the excavations and risk to workers within or near excavations.

15.6.11 The risk of fluvial flooding to earthworks and construction worker during construction is considered to be moderate.

Flooding From Pluvial Sources / Overland Flow

15.6.12 The EA indicates that there are a number of areas shown to be at risk from surface water flooding for the 1 in 30, 1 in 100 and 1 in 1,000 year return periods. The majority of areas shown to be at risk of surface water flooding within the Site are alongside the Whilton Branch of the River Nene and Hollandstone Farm Brook (1 in 30 chance of flooding). Approximately 250m of the Proposed Scheme is shown to be at risk of surface water flooding for the 1 in 1,000 year event to the north of Flore. A few smaller areas along and adjacent to the alignment are shown to be at risk of surface water flooding for the 1 in 30 and 1 in 100 year events, including a stretch of land alongside the M1.

15.6.13 Northamptonshire County Council has provided a number of records of historic flooding in the vicinity of the Proposed Scheme as detailed in the previous historical flood records above.

15.6.14 Existing surface cover predominantly comprises permeable surfacing (vegetation). The area of hard-standing is likely to increase during the site preparation and construction phase, with associated hard drainage systems installed. The overall surface water run-off volume at the Site is likely to increase.




15.6.15 In extreme storms, the ground may become saturated and the drains and sewers which carry away surface water may not be able to accommodate these flows, or may even be blocked with debris, leading to surface water flooding.

15.6.16 Overland flow across, into or out of the Site would currently follow the line of least resistance and follow the natural topography which is likely to change during construction. Surface water flooding, especially after extreme rainfall events, has the potential to harm workers on-site if it is received in large volumes, particularly if they are working in excavations which have the potential to fill with water.

15.6.17 The baseline risk of pluvial flooding / overland flow to earthworks and construction worker during construction is considered to be moderate.


15.6.18 Anglian Water has confirmed that they have no records of water mains and water infrastructure flooding. No records are available of flooding occurring in the vicinity of the Site due to the surcharging of public sewer system. The Grand Union Canal is not modelled as part of the EA’s Upper Nene Model. There are no dams within the vicinity of the Site and no such infrastructure in the wider catchment is known to have an influence on the Site. The Site does not lie within the inundation area of any reservoirs.

15.6.19 Flooding could occur as a result of infrastructure failure, for example for example breach or overtopping, burst water mains or failed pumping stations. It is likely that flows resulting from the risk of surcharging of off-site drainage and sewer system during extreme rainfall events would be short-term, relatively shallow depth and would pass through the Site following the natural topography.

15.6.20 Drainage flooding, especially after extreme rainfall events, has the potential to harm earthworks and construction workers on-site if it is received in large volumes, particularly if they are working in excavations which have the potential to fill with water.

15.6.21 The risk of sewers /drainage infrastructure flooding to earthworks and construction worker at the Site during construction is considered to be low.

Flooding From Groundwater Sources

15.6.22 Groundwater flooding occurs when the water table rises above the level of the ground. The ground investigation (Appendix 16.3) recorded groundwater beneath the Site at shallow depth or within influence of the proposed cuttings. Locally, artesian conditions were noted in the base of the Nene Valley.

15.6.23 The earthworks for the scheme involve areas of ‘cut’ which are likely to intercept groundwater and may result in localised flooding, increased instability of the excavations and risk to workers within or near excavations. Within the Earthworks Section 6 Cutting area (chainage 2000-3700m), groundwater levels are recorded above the proposed cutting elevation which may result with ingress of groundwater into the cutting during excavation.

15.6.24 Embankment construction can create obstructions to both surface water and groundwater movement and effect natural surface water percolation to the substrata.

15.6.25 During the construction phase groundwater may fill excavations causing instability and risk to ground workers and construction workers within excavations.

15.6.26 The risk of groundwater flooding earthworks and construction worker at the Site during construction is considered to be moderate.

15.6.27 Prior to the commencement of works, the proposed temporary drainage strategy will be implemented. Implementation of a SuDS management train for surface water drainage. The surface




water will discharge into the existing watercourses on-site at the existing Greenfield runoff rates and any increase in surface water run-off above the pre-development (or Greenfield) volumes will be attenuated on-site. Therefore, the construction works will result with no additional increase in flows.

15.6.28 The sensitivity of ground workers and construction workers is high, and the magnitude of the change prior to mitigation, is medium, based on the worst case flood risk identified above. Therefore there is likely to be a direct, temporary, medium-term effect on-site preparation, earthworks and construction workers of moderate to major negative significance prior to the implementation of mitigation measures.

Mitigation

15.6.29 Measures will be put in place to manage the safety of earthworks and construction workers in the event of a flood event. A flood warning and evacuation plan will be prepared in consultation with the emergency services prior to construction commencing. The construction site office should sign up to the EA’s Floodline Warnings Direct free service that provides flood warnings by phone, text or email. EA flood warnings will be reviewed on a daily basis and construction will be temporarily halted in the event of an EA Severe Flood Warning across the Site.

15.6.30 Maintenance and implementation of the proposed temporary drainage strategy.

15.6.31 Further groundwater and surface water monitoring is being undertaken as part of a 12 month monitoring programme, due to be reported in early 2015. Groundwater levels should be monitored during all excavations. Dewatering measures will be employed if necessary.

15.6.32 Workers on-site will be made aware of the risks and how to mitigate them through working practices prior to commencement of the construction works.

15.6.33 Movement of materials around the Site will be managed under an appropriate Materials Management Plan and in line with the CIRIA Code of Practice (Ref. 15.16) to ensure the placement of materials does not alter the flood risk within the Site.

15.6.34 Where groundwater levels are in close proximity to the current ground elevation in areas of proposed embankments. A thick layer of high permeability and high porosity fill may be required to prevent the creation of heavily confining and unstable ground conditions occurring in the proposed fill locations. Conceptual and quantitative modelling may be required to determine if this could be a risk and to assess the required storage to prevent artesian conditions occurring.

15.6.35 Groundwater levels will be monitored during excavations. In particular within Earthworks Section 6. Dewatering calculations will be carried out and dewatering measures will be employed if necessary. If dewatering is required, abstraction and discharge licences will be sought from the EA under the terms of the Water Resources Act 1991 (Ref. 15.2).

15.6.36 Workers on-site will be made aware of the risks and how to mitigate them through working practices prior to commencement of the construction works.

Residual Effect

15.6.37 The sensitivity of construction workers and activities to fluvial, pluvial, drainage and groundwater flooding at the Site is high and the magnitude of change should this occur, following mitigation, is negligible. Therefore, there is likely to be a direct, temporary, medium-term effect of negligible significance to earthworks and construction workers following the implementation of mitigation measures.

Potential increase in physical contamination (i.e. sediments) entering surface water bodies and drainage features due to increased sediment loading

15.6.38 The key potential surface water receptors are:




The Grand Union Canal crosses the Site in the western extent and runs north-west to south-east through Weedon Bec to the south of Flore towards Nether Heyford;

The Whilton Branch of the River Nene crosses the site in the west;

Hollandstone Farm Brook to the east of Flore;

Ponds on-site and in the vicinity of the Site; and

The River Nene, which is located to the south of the Site in an off-site location and flows approximately west to east passing through Weedon Bec and to the south of Flore.

15.6.39 The current EA WFD RBMP (Ref. 15.26) classifies The River Nene, The Grand Union Canal and Whilton Branch as ‘good’ chemically and the ecological quality as ‘moderate’.

15.6.40 The Site is classified as a Greenfield development and it is assumed that the Site drains via infiltration and overland flows, towards the existing watercourses on-site.

15.6.41 Surface water abstractions from the River Nene and tributaries have been identified within 1km of the Site and are up gradient of the Proposed Scheme therefore will not be impacted.

15.6.42 Both recorded groundwater abstractions (if currently active) are up hydraulic gradient of the Site. As such, the abstractions are unlikely to be affected by physical contamination as a result of increased sediment loading.

15.6.43 During the site preparation and construction phase of the Proposed Scheme there will be a number of activities which could reduce surface water quality with respect to physical contaminants. These include:

Emplacement of embankments constructed to a maximum height of 13m (Appendix 16.3); and

Excavation of cuttings extending to maximum depth of 10m (Appendix 16.3);

15.6.44 Eight structures have been proposed where the Proposed Scheme encounters existing infrastructure. For ease of assessment, the geotechnical report divides the Proposed Scheme into ten Earthworks Sections (Earthworks 1-10). In addition, there are eight structures comprising bridges and one culvert which carries the link road over Hollandstone Farm Brook and a public footpath. The locations of the earthworks and structures are shown on the Earthworks and Structures Plan in Appendix 16.8 Where the Proposed Scheme crosses surface water features bridges will be provided (i.e. STR7001 over the Grand Union Canal (combined with the railway bridge), STR7003 bridge over Whilton Branch, STR7007 bridge over Hollandstone Farm Brook) as shown on Earthworks and Structures Plan in Appendix 16.8 and on Figures 4.2, 4.3 and 4.7:

Movement and use of static and mobile plant/construction vehicles such as excavators, dumper trucks, hydraulic breakers and, haulage trucks;

Breaking out of existing hardstanding (e.g. roads/tracks, former foundations associated with the former farm buildings and access tracks). This is likely to be minimal in extent;

Materials handling, storage, stockpiling, spillage and disposal;

Earthworks to ground formation levels;

Construction of drainage features; and

Installation of infrastructure and roads.

15.6.45 The construction activities including areas of cut (Earthworks Section 4, 6 and 9) and embankment fill (Earthworks Section 1, 2, 3, 5, 7, 8, 10) may lead to the disturbance and mobilisation of physical contaminants (i.e. dust, sediments and muds). Particularly during periods of heavy rainfall, vehicle movements resulting in damage to soil structure may generate increased sedimentation within surface water run-off. In addition, during periods of dry, windy weather, wind-blown dusts generated




by the excavation of soils and vegetation clearance has the potential to directly reduce the quality of surface water features.

15.6.46 Construction of watercourse crossings and work next to water bodies is likely to generate the highest risk.

15.6.47 These activities may result in sediments indirectly (e.g. wind-blown) or directly (via surface water run-off) entering surface water features, potentially impacting on the physical quality of surface water receptors crossing the Site and in the surrounding area.

15.6.48 Prior to the commencement of works, the proposed temporary drainage strategy will be implemented. Implementation of a SuDS management train for surface water drainage. All site works and ground works should be undertaken in accordance with the Considerate Contractors Scheme to help ensure a well-managed operation which minimises environmental risks.

15.6.49 Due to the presence of surface water features on-site, the proximity and connectivity with the River Nene and the WFD RBMP classifications, the sensitivity of the surface water receptors to physical contaminants is considered to be medium, taking into account the temporary drainage system the magnitude of the change prior to mitigation, is medium. Therefore, there is likely to be a direct and/or indirect, temporary, medium term effect on surface water bodies and drainage features of moderate negative significance prior to the implementation of mitigation measures.

Mitigation 15.6.50 Movement of materials around the Site will be managed under an appropriate Materials Management

Plan and in line with the CIRIA Code of Practice (Ref. 15.18) to ensure the placement of materials does not alter the flood risk within the Site.

15.6.51 Good environmental site practices will also be implemented to avoid or minimise effects at the source. Such measures include, but are not limited to, the following:

Working areas shall be clearly defined to ensure the disturbance of soils is minimised, where possible;

The cleaning of vehicle wheels prior to leaving Site;

Controlled and covered waste storage areas;

Dust suppression (i.e. damping down);

Provision of environmental awareness training for site workers; and

Installation of systems such as silt traps and swales designed to trap silty water including adequate maintenance and monitoring of these to ensure effectiveness, particularly after adverse weather conditions.

15.6.52 The position and extent of working areas should reflect the sensitivity of surrounding areas and works being carried out. The contractor should appraise the suitability of such working areas in this respect as part of working method statements.

15.6.53 Where appropriate, all site works will be undertaken in accordance with the EA’s Pollution Prevention Guidelines, in particular:

PPG1 ‘Understanding Your Environmental Responsibilities - Good Environmental Practices’ (Ref. 15.14);

PPG5 ‘Works in, Near or Liable to Affect Watercourses’ (Ref. 15.15);

PPG6 ‘Working at Construction and Demolition Sites’ (Ref. 15.16); and

PPG21 ‘Pollution Incident Response Planning’ (Ref. 15.17).




15.6.54 Earthworks and construction activities should also be undertaken in accordance with CIRIA guidance ‘C532 – Control of Pollution from Construction Sites’ (Ref. 15.18).

15.6.55 Dewatering effluent from earthworks at below ground level should be discharged via the temporary drainage system for settlement before discharge to a watercourse.

15.6.56 Best practice recommendations for the prevention of contamination will be outlined in more detail in a Construction Environment Management Plan (CEMP), which will be agreed with the appropriate regulators, prior to site preparation or earthworks commencing. The CEMP will include the following:

Programme and phasing details of works;

A broad plan of demolition and construction works, highlighting the various stages and their context within the proposed scheme, including a full schedule of materials and manpower resources and plant and equipment schedules;

Detailed site layout arrangements (including requirements for temporary works), plans for storage, accommodation, vehicular movements, delivery and access;

Prohibited or restricted operations (locations, hours, etc.);

Details of plant used; and

Details of operations likely to result in ground disturbance, with an indication of the expected duration of each phase with key dates.

Residual Effect 15.6.57 Following implementation of the measures described above the risk of increased sediment

mobilisation to controlled water receptors will be mitigated and there should not be a reduction in status of the WFD RBMP chemical and ecological classification of the on-site and nearby surface watercourses.

15.6.58 The sensitivity of surface water receptors in the vicinity to physical contaminants is medium and the magnitude of change, following mitigation, is negligible. Therefore, there is likely to be a direct and/or indirect, temporary, medium-term effect of negligible significance on surface water bodies and drainage features following the implementation of mitigation measures.

Potential changes to pattern and quantity of groundwater and surface water flows and subsequent effects on water receptors (e.g. Aquifers and surface water courses)

15.6.59 Controlled water receptors being considered on-site and / or off-site as part of this assessment include the underlying Aquifers (Secondary A and Secondary Undifferentiated) and surface water features including the Grand Union Canal, Whilton Branch of the River Nene, Hollandstone Farm Brook, ponds (on-site and off-site), and the River Nene (off-site). These features are shown on Figure 15.1.

15.6.60 No groundwater abstractions are recorded by the EA within the vicinity of the Site (it is noted that two are listed within 1km of the Site in the Envirocheck report, but are not considered to be at risk from the Site).

15.6.61 A SSSI is located 475m to the south of the eastern extent of the Site adjacent to the River Nene and could be impacted by any changed in groundwater or surface water flows.

15.6.62 The Site is currently predominantly agricultural land which drains via infiltration and overland flow towards the existing on-site surface water courses and ultimately into the off-site surface watercourses (River Nene).

15.6.63 The ground investigation (Appendix 16.3) recorded groundwater beneath the Site at shallow depth or within influence of the proposed cuttings. Locally, artesian conditions were noted in the base of the Nene Valley. The general groundwater flow direction at the Site is considered likely to be towards the




surface watercourses, with local variations in proximity to the on-site surface water courses / features.

15.6.64 The current EA WFD RBMP (Ref. 15.22) classifies The River Nene, The Grand Union Canal and Whilton Branch as ‘good’ chemically and the ecological quality as ‘moderate’. Hollandstone Farm Brook is not classified.

15.6.65 The underlying groundwater quantitative quality and chemical quality is classified ‘Good’ under the EA WFD RBMP, and is predicted to remain as such by 2015 (Ref. 15.22).

15.6.66 A temporary Drainage Strategy will be implemented which will aim to mimic the current conditions, where possible. Surface water run-off will be collected and discharged at Greenfield run-off rates to the surface water courses on-site.

15.6.67 There is likely to be to an increase in impermeable surface during the construction phase (compared to the existing agricultural conditions) and a potential increase in surface water run-off. However, due to the generally cohesive nature of the underlying geology, it is anticipated that surface water run-off currently comprises a relatively large portion of total rainfall collected by the Site. The run-off will be directed into surface watercourses. Therefore, the change in relative proportion of rainfall infiltration and run-off is not anticipated to be notable during the construction stage.

15.6.68 The earthworks include areas of cut which extend below the groundwater table (e.g. Earthworks Section 6 proposed to 10m bgl). Groundwater levels throughout Earthworks Section 6 (see Appendix 16.3) cutting are located above the proposed cutting elevation, indicating that groundwater ingress may occur from the Marlstone Rock Formation. The preliminary information provided in the WFDA indicates that there is potential for there to be a hydraulic link between the cutting and pond locations to the north of the Site. Therefore, these ponds (Pond 13, 15 and 16 shown on Figure 15.1) could be in the zone of influence from dewatering during the earthworks / construction stage.

15.6.69 Dewatering will be completed as necessary within excavations. An increase in run-off (less permeable surface), a possible decrease in infiltration, coupled with increased dewatering (in areas of cut and excavations) may create a reduction overall in groundwater base flow to the surface watercourses. However, the overall quantity of water recharge to the watercourses will remain approximately the same, as surface water run-off will continue to be discharged directly to the watercourses and groundwater will be returned to the aquifer (where possible).

15.6.70 The sensitivity of surface water and groundwater flows is medium. The magnitude of change, prior to mitigation, is medium. Therefore, there is likely to be a direct, temporary, medium term effect on groundwater flow and groundwater in the underlying aquifer of moderate negative significance, prior to the implementation of mitigation measures.

Mitigation

15.6.71 Implementation and maintenance of temporary drainage system.

15.6.72 Piping though embankments, removal of temporary construction infrastructure (e.g. sheet piled walls) and water reinjection should be considered to mitigate the loss of inflow to groundwater.

15.6.73 On-going groundwater and surface water monitoring will be carried out and a baseline factual report will be issued in early 2015 which will include all of the surface water and groundwater monitoring data. Further ground investigation and / or monitoring will be completed as appropriate during detailed design to assess whether the ponds could be impacted by the scheme construction and mitigation measures will be developed to protect them. These may include limiting dewatering rates, or discharge of water directly to the ponds.




Residual Effects

15.6.74 Following implementation of the measures described above, the potential effects of changing groundwater and/or surface water flows will be mitigated and there should not be a reduction in status of the WFD RBMP classification for groundwater and surface water.

15.6.75 The sensitivity of surface water and groundwater flow is medium and the magnitude of change is negligible. Therefore, there is likely to be a direct, temporary, medium-term effect on surface water and groundwater flow of negligible significance.

Operational phase Design Solutions and Assumptions

15.6.76 The following design solutions and assumptions have been taken into account during the pre-mitigation assessment.

15.6.77 The drainage strategy at the Site is described in the FRA and Chapter 4 - Description of the Proposed Development and a summary is provided below. It is assumed that the FRA and associated drainage strategy will be fully implemented prior to operation.

15.6.78 The drainage strategy at the Site will discharge surface waters to the existing surface watercourses. It will utilise a Sustainable Drainage Systems (SuDS) management train for surface water drainage in accordance with the SuDS Manual (Ref. 15.19). Attenuation storage will be provided through a series of source control, filtration, swales/conveyance channel, and retention/detention method before discharging into the existing watercourses on-site at the existing Greenfield run-off rates.

15.6.79 To provide protection to groundwater and watercourses it is proposed that a minimum of two stages of SuDS treatment are adopted as part of this strategy, with three provided wherever possible.

15.6.80 As a further measure of protection against the risk of fuel spills or fire water entering local watercourses or infiltrating to ground, it is proposed that SuDS features are constructed with an impermeable liner. Particularly where SuDS features are in close proximity to existing watercourses and water bodies.

15.6.81 At locations where infiltration is possible, then it is proposed that the detention basins are unlined to allow for soakage. All SuDS features upstream of these unlined features will still require lining to provide the necessary protection to groundwater. In addition penstocks or other control valves should be considered at outfall locations.

15.6.82 Gullies will collect surface water from the proposed carriageways and associated footways and will route surface water towards swales or to fin drains adjacent to the highway. It is intended that the gullies will be trapped, therefore providing the first line of defence against pollution.

15.6.83 Where kerbs and gullies are not present, surface water will flow over grassed filter strips and into the swales or fin drains. The swales and fin drains will provide attenuation volume, slow the rate of discharge of surface water and provide additional protection against the migration of pollutants suspended in the runoff. If additional storage is required check dams will be considered to further reduce the rate of discharge towards the proposed final stage of treatment, the detention basins. At the location of the proposed bridges the surface water will be routed through surface water sewers beneath the bridge deck.

15.6.84 Detention basins are located at or near to the low points of each sub catchment with consideration given, as far as practicable, to the gradient of the existing ground, the extent of Flood Zones 2 and 3 (as identified by the EA Flood Maps) and the presence of environmentally sensitive areas.




15.6.85 A control device will limit the flow of surface water from the detention basins to the receiving watercourses at the existing Greenfield run-off rates and it is intended that released water will have passed through sufficient stages of treatment to provide adequate, low maintenance and sustainable pollution control.

15.6.86 Drainage strategies will allow for an increase in rainfall intensities of 30%. The EA aspires to reduce the rate of surface water discharge from new developments for any given storm event and specifies that the drainage system should be designed to control run-off for up to the 1 in 200 year rainfall event (increased from the general standard of 1 in 100 due to the history of flooding in the catchment).

15.6.87 The on-site SUDS will be designed to accommodate the 1 in 200 year event plus climate change (30%) rainfall event and discharge into the existing watercourses on-site at the existing Greenfield run-off rates.

15.6.88 Floodplain compensation will be included where required to mitigate the loss of floodplain storage and any corresponding increase in flood risk caused by the bypass and / or its crossing. Where floodplain is displaced by a development proposal, compensatory flood storage will be provided within the Site (or upstream) on a ‘level for level’ basis up to and including the 1 in 200 year flood level incorporating climate change. This compensation will be achieved on land that does not currently flood and is attached to the existing floodplain.

15.6.89 The potential effects of dewatering earthworks cuttings during the operational phase on groundwater and surface water recharge will have been considered as part of the final drainage strategy.

Potential increase in risk of flooding (fluvial, pluvial, drainage and groundwater) to future Site users

15.6.90 The baseline conditions for flooding are provided in the construction phase section above and the baseline conditions section, Section 15.6.

15.6.91 Reference should be made to the FRA for detailed information on baseline flood risk, the potential effects of the Proposed Scheme on flood risk and development of the mitigation measures to protect on-site and off-site third parties from the effects of flood events.

Fluvial

15.6.92 During times of peak flow, surface water levels within the on-site surface water courses could rise and overflow into the Site. The hydraulic modelling undertaken for Hollandstone Farm Brook and the Whilton Branch of the River Nene (reported in the FRA) indicates that the Proposed Scheme does not flood in the vicinity of the two watercourses in the 1 in 1000 year event plus climate change. Where on embankments, the Proposed Scheme will sit above existing ground level decreasing he risk of future flooding.

15.6.93 The FRA states that no formal flood defences protect the Site currently. The Whilton Branch of the River Nene is maintained by the EA and provides a nominal protection against a flood event with a 20% chance of occurring in any year (1 in 5).

15.6.94 The Proposed Scheme lies partly within Flood Zone 2 (between a 1 in 100 and 1 in 1000 annual probability of flooding / medium probability) and Flood Zone 3 (less than 1 in 100 annual probability of flooding / high probability) associated with Whilton Branch of the River Nene and Hollandstone Farm Brook. The FRA states that changes in landscape (i.e. cuttings and embankments) have been incorporated into the hydraulic model for Hollandstone Farm Brook and the proposed crossing. The model concludes that the crossing will have a minimal impact on flood levels.

15.6.95 The post development hydraulic model for Whilton Branch of the River Nene indicates that the inclusion of the Proposed Scheme crossing will result in notable increases in the flood levels and floodplain extent area of 11.7 ha on the upstream during the 1 in 200 year plus climate change event.




Northamptonshire Highways has held discussions with the landowners who have confirmed that the above impacts are acceptable on the land. Flood risk is also increased on the A5 immediately to the north of the crossing. Flood mitigation will be employed and will include a bund along the eastern edge of the A5 to protect the highway.

15.6.96 The Proposed Scheme will cross the Grand Union Canal via a bridge. The integrity of the Grand Union Canal will not be impacted by the construction of the bridge. In the event of a breach in the canal, it is likely that water discharge will be into the Whilton Branch of the River Nene (after passing over the existing A5) based on the topographical survey. The Grand Union Canal and existing A5 are at an elevation lower than that of the Proposed Scheme, which is located on embankments in that area.

Flooding from Pluvial Sources

15.6.97 The increased proportion of hard standing across the Site during the operational phase could result in localised ponding of water on-site if the drainage system fails.

15.6.98 The Proposed Scheme is embanked and in cut along its alignment but generally follows the existing topography and is therefore unlikely to act as a barrier to overland flows. Alteration of/or development within the existing areas at risk of surface water flooding could result in increased risk of flooding from pluvial sources.

15.6.99 The M1 motorway is located to the north of the site acts a hydrological boundary by holding back any overland flows from the north.

15.6.100 To ensure the effective operation of the site drainage network, penstocks or other control valves will be considered at outfall locations to restrict the backing up of the site drainage system when the surface watercourses are in flood. Control devices will be installed on all outfalls to restrict flows from each sub catchment to their target discharge rates.

15.6.101 In summary, the run-off will be managed by the collection of surface water and discharge from the Site at Greenfield rate, particularly during peak flows. This will provide a reduction in flood risk on-site and downstream of the Site during times of peak flow as the surface water run-off discharge rate will be controlled at a suitable rate.


15.6.102 The Proposed Scheme will not connect to any part of existing public sewer and drainage infrastructure.

15.6.103 Extreme rainfall events may lead to over topping of drainage features and subsequent surface water flows. However, as part of the drainage strategy for the Proposed Scheme total storage volumes have been calculated for the 1 in 200 year event including climate change (30%). Therefore, overtopping is considered unlikely and would only occur if the drainage system failed.

15.6.104 Should flooding from drainage and infrastructure sources occur at the Site, the impact would be greatest on the health and safety of future site users.

Groundwater

15.6.105 During periods of high rainfall, groundwater levels could rise and flooding and over topping of the drainage system may result during the operational phase should the drainage system fail.

Overall flood risk during operation

15.6.106 The proposed surface water drainage strategy will seek to replicate the existing drainage regime of the Site as far as possible. On-site SUDS will be designed to accommodate the 1 in 200 year event plus climate change (30%) rainfall event with no additional increase in flows, thus providing additional betterment in the future. These measures are in line with EA requirements.




15.6.107 Surface water drainage arrangements will ensure that volumes and peak discharge rates, leaving the Site, are no greater than the rates pre development and where possible returned to those associated with the Greenfield conditions of the Site. Any increase in surface water run-off above the pre-development (or Greenfield) volumes will be attenuated on-site. Therefore, reducing the flood risk.

15.6.108 Provided the drainage strategy and flood risk measures are implemented, the sensitivity of future populations and third parties to flooding at the Site is high and the magnitude of change prior to mitigation is negligible. Therefore, there is likely to be a direct, permanent, long-term effect on future populations and third parties of negligible significance prior to the implementation of mitigation measures.

Mitigation

15.6.109 No further mitigation measures are considered necessary other than those set out in paragraphs 15.6.76 to 15.6.89.

Residual Effects

15.6.110 The sensitivity of future populations and third parties to flooding at the Site is high and the magnitude of change following mitigation is negligible. Therefore, there is likely to be a direct, permanent, long-term effect on future populations and third parties of negligible following the implementation of mitigation measures.


15.6.111 As discussed in the construction phase section above, the key surface water receptors are on-site surface water courses (Grand Union Canal, Whilton Branch of the River Nene and Hollandstone Farm Brook) and off-site surface watercourses (River Nene).

15.6.112 Surface water run-off currently drains to on-site surface watercourse and ultimately drains into the River Nene. A SSSI is located 475m to the south of the eastern extent of the Site adjacent to the River Nene and could be impacted by any increases in sediments.

15.6.113 The current EA WFD RBMP (Ref. 15.22) classifies The River Nene, The Grand Union Canal and Whilton Branch of the River Nene as ‘good’ chemically and the ecological quality as ‘moderate’.

15.6.114 The impermeable area on-site during the operational phase will increase compared to the current agricultural conditions. The impermeable area will serve to hold any looser topsoil materials present in arable fields in-situ and therefore potentially reduce the sediment loading to surface watercourses. However, this is likely to be balanced in part by an increase in sediments from the increase of traffic during the operational stage.

15.6.115 Bearing the above in mind, overall, during the operational phase the potential for sediment entrainment in surface water run-off is anticipated to remain at similar levels or lower due to the nature of Proposed Scheme, which will involve minimal or localised disturbance of soil such as during maintenance works and potential sediments resulting from the increase of traffic.

15.6.116 Gullies will collect surface water from the proposed carriageways and associated footways and will route surface water towards swales or to fin drains adjacent to the highway. Where kerbs and gullies are not present, surface water will flow over grassed filter strips and into the swales or fin drains. If additional storage is required check dams will be considered to further reduce the rate of discharge towards the proposed final stage of treatment, the detention basins. At the location of the proposed bridges the surface water will be routed through surface water sewers beneath the bridge deck.

15.6.117 Provided the drainage strategy and flood risk measures are implemented, the sensitivity of surface water receptors in the Site and local area to physical contaminants is medium and the magnitude of




change, prior to mitigation, is negligible. Therefore, there is likely to be a direct, permanent, long-term effect on surface water receptors of negligible significance prior to the implementation of mitigation measures.

Mitigation

15.6.118 As the drainage strategy will be implemented and maintained in accordance with the SuDS Manual (Ref. 15.19), no further mitigation is considered necessary.

Residual Effects

15.6.119 Following implementation of the measures described above the risk of increased sediment mobilisation to controlled water receptors will be mitigated and there should not be a reduction in status of the WFD RBMP ecological classifications.

15.6.120 The sensitivity of surface water receptors in the vicinity to physical contaminants is medium and the magnitude of change, following mitigation, is negligible. Therefore, there is likely to be a direct, permanent, long-term effect of negligible significance to surface water receptors following the implementation of mitigation measures.

Potential changes to pattern and quantity of surface water and groundwater flows and subsequent effects on water receptors (e.g. Aquifer and surface watercourses)

15.6.121 The Site is currently predominantly agricultural land with drains which ultimately flow directly into the on-site surface water courses.

15.6.122 The current EA WFD RBMP (Ref. 15.22) classifies The River Nene, The Grand Union Canal and Whilton Branch of the River Nene as ‘good’ chemically and the ecological quality as ‘moderate’.

15.6.123 The underlying groundwater quantitative quality and chemical quality is classified ‘Good’ under the EA’s Water Framework Directive River Basin Management Plan, and is predicted to remain as such by 2015 (Ref. 15.22).

15.6.124 Shallow wells in boreholes BH13, 33, 34 and 35 (screened within Glacial Deposits) have consistently remained dry during all monitoring rounds. In the remaining wells, groundwater levels were generally between 2m and 3m bgl in shallow wells, and 6m to 8m bgl in deeper wells.

15.6.125 Shallow groundwater (artesian in locally in the base of the Nene valley BH06 / BH07) was recorded during the Ground Investigation (Appendix 16.3) at depths between 71.85m AOD and 103.09m AOD and based on the environmental setting of the Site the groundwater is likely to be in continuity with the on-site and off-site surface watercourses. The underlying bedrock is classified as Secondary A and Secondary Undifferentiated Aquifers and no groundwater abstractions are recorded within influence of the Site.

15.6.126 Groundwater flow is likely to be towards the surface water features. During the operational stage, the presence of below ground structures and areas of cut up to 10m bgl (which intersect the groundwater table) may alter the existing groundwater flows. The groundwater may collect in the drainage system and lead to overtopping, however, the system will be designed to collect to groundwater flows and discharge to ground (subject to permeability testing) and surface water courses at Greenfield rates. The preliminary information provided in the WFDA indicates that there is a potential hydraulic link between Earthwork 6 (cutting) and ponds. Based on the preliminary information, ponds 13, 15 and 16 could be in the zone of influence from any change in groundwater flow /dewatering during the operational stage.

15.6.127 If there is a change in aquifer recharge volume on-site post development caused by an increase in run-off and a possible decrease in infiltration, this may create a reduction overall in groundwater base flow to the surface watercourses. However, it is considered that the overall quantity of water recharge




to the watercourses will remain approximately the same, as surface water run-off will continue to be discharged directly to the watercourses.

15.6.128 Although there is likely to be to an increase in impermeable surface during the operational phase (compared to the existing agricultural Site conditions) and a potential increase in surface water run-off, it is anticipated that surface water run-off currently comprises a relatively large portion of rainfall due to the cohesive geology. Therefore, the change in relative proportion of rainfall infiltration and run-off is not anticipated to be notable following development.

15.6.129 Therefore it is anticipated that the net effect on the water balance to the surface water courses will remain relatively intact and the WFD RBMP classifications will not be degraded.

15.6.130 Following implementation of the drainage strategy, the sensitivity of surface water and groundwater flows is medium. The magnitude of change, prior to mitigation, is negligible. Therefore, there is likely to be a direct, permanent, long-term effect on groundwater flow and groundwater in the underlying aquifer of negligible significance, prior to the implementation of mitigation measures.

Mitigation

15.6.131 Implementation of the drainage strategy which will aim to mimic the current conditions, where possible. As such, no further mitigation measures are considered necessary.

Residual Effects

15.6.132 Following implementation of the measures described above, the potential effects of changing groundwater and/or surface water flows will be mitigated and there should not be a reduction in status of the WFD RBMP classification for the surface water features.

15.6.133 The sensitivity of surface water and groundwater flow is medium and the magnitude of change is negligible. Therefore, there is likely to be a direct, permanent, long-term effect on surface water and groundwater flow of negligible significance.

Limitations and Assumptions 15.6.134 The principles for the surface water drainage strategy during the construction and operational phases

have been established. Should the design details vary in a manner that compromises the existing design, further assessment will be required.

15.6.135 It is assumed that risk assessments will be updated following receipt of further information, including the results of the on-going surface water and groundwater monitoring programme, and the third phase of ground investigation that has not yet been reported.

15.6.136 It is assumed that the mitigation measures presented in this Chapter will be undertaken by the designers (completion of a CEMP and additional monitoring etc.), and appointed construction contractors (e.g. adherence to the CEMP).

15.7 Cumulative Effects Effect Interactions

15.7.1 The effects on sensitive on-site surface water receptors from physical contaminants will be allied with effects from chemical contaminants which are discussed in Chapter 16 ‘Ground Conditions, Hydrogeology and Contamination’. Although these two types of contaminants will cumulatively reduce the water quality of any receiving water body, the impacts will be mitigated by implementing similar measures including good environmental site practices.




In-combination Effects

15.7.2 Up to seven planning applications have been identified in close proximity to the site, three of which have full planning permission.

15.7.3 These committed developments are predominantly residential and if earthworks are required, they are considered likely to experience similar drainage, flooding and water resources impacts which will be managed in a similar manner to the Site. Drainage systems, flooding assessment and management will be similar for all sites in the vicinity of Flood Risk Zones.

15.7.4 All committed development will need to take account of the SFRA, requirements of NPPF for flood risk management and pollution prevention measures, and / or other associated predecessor policies, for example PPS23 and PPS25. The requirements for these are delivered within the boundary of each development as part of the planning permission.

15.7.5 This development does not contribute to strategic water issues such as water supply and waste water treatment that would need to be assessed for residential development.

15.7.6 Therefore, it is anticipated that the cumulative impact will be negligible

15.8 Summary 15.8.1 The Site is currently used mainly for agricultural purposes. Three on-site surface watercourses are

present, Hollandstone Farm Brook, the Whilton Branch of the River Nene and the Grand Union Canal. In addition, ponds are located on-site and in the near vicinity of the Site. With the exception of the Grand Union Canal, surface watercourses flow into the River Nene which flows roughly west to east to the south of the Site.

15.8.2 The Wilton Branch of the River Nene and Grand Union Canal ecological potential is classified by the EA as ‘Moderate’. The River Nene ecological potential is classified by the EA as ‘Moderate’. Hollandstone Farm Brook has not been assessed.

15.8.3 Groundwater levels at the Site are typically shallow or within influence of proposed cuttings. Locally, artesian conditions have been recorded in the base of the Nene valley.

15.8.4 The Site is located partly within a Flood Risk Zone 2 and 3 (medium to high probability) adjacent to the on-site surface watercourses with the remainder within Flood Zone 1.

15.8.5 Based on the available baseline data, it is considered that the following likely significant effects may be associated with the site preparation, earthworks and construction phase and operational phases of the Proposed Scheme:

Potential increase in risk of flooding (fluvial, pluvial, drainage and groundwater) to future Site users;

Potential increase in physical contamination (i.e. sediments) entering surface water bodies and drainage features due to increased sediment loading; and

Potential changes to pattern and quantity of groundwater and surface water flows (i.e. reduction in surface cover permeability) and subsequent effects on water receptors (e.g. Aquifer and surface water courses).

15.8.6 During the site preparation, earthworks and construction phase, a temporary drainage system will be in place. Earthworks are proposed along the route involving cuttings, embankments, bridges and a culvert. Dewatering of excavations will be employed if required. Mitigation measures including surface water and groundwater monitoring and supplementary ground investigation are to be completed in advance of the earthworks. An assessment of potential risks to off-site ponds to the north due to dewatering will also be completed. Additionally, a Construction Environmental




Management Plan (CEMP) will be implemented and EA Pollution Prevention Guidance (PPGs) will be adopted to ensure that construction activities will not have a significant negative effect on water quality and that areas of flood risk will be minimised.

15.8.7 Impermeable surface areas will increase in the operational phase compared to the existing agricultural Site conditions, and surface water drainage will discharge sustainably into the on-site surface watercourses via SuDS. Floodplain compensation will be included where required to mitigate the loss of floodplain storage and any increase in flood risk caused by the bypass and / or its crossing. Where floodplain is displaced by a development proposal, compensatory flood storage will be provided within the site (or upstream) on a ‘level for level’ basis up to and including the 1 in 200 year flood level incorporating climate change. This compensation will be achieved on land that does not currently flood and is attached to the existing floodplain.

15.8.8 These design measures provide protection of the water environment and water resources in line with national, regional and local policies (including the NPPF) and in accordance with legislation including the Water Environment Regulations (2003) and the Water Act (2003).

15.8.9 Following incorporation of the mitigation measures in accordance with current best practice and the relevant guidance, the residual impacts in relation to flooding, hydrology and water resources for the site preparation, earthworks and construction phase and the operational phase are all likely to be negligible.

15.8.10 A summary of the effects on flooding, hydrology and water resources is provided in Table 15.8 below.


15-33


Chapter 15 Flooding, Hydrology and Water Resources

Table 15.8: Summary of Effects Table for Flooding, Hydrology and Water Resources

Description of Significant Effects

Receptor Significance of Effects Summary of Mitigation / Enhancement Measures

Significance of Effects Relevant Policy

Relevant Legislation

(Major, Moderate, Minor, Negligible)

Positive / Negative

(P / T) D/I) ST / MT / LT


Positive / Negative



Potential increase in risk of flooding ( fluvial, pluvial, drainage and groundwater) to site preparation, earthworks and construction workers;

Ground and construction workers

Moderate to Major

Negative T D MT Flood warning plan

Maintain / implement tem drainage strategy

Monitoring and dewatering if required

MMP in line with CIRIA

Negligible NA T D MT NPPF and NPPG EV26 and EV27

Water Act (2003) Environment Act (1995) Land Drainage Act (1991)


On-site and Off-site surface water courses

Moderate Negative T D MT Temporary drainage system.

Good environmental site practices.

Implementation of a Construction Environment Management Plan (CEMP).

Adoption of Relevant guidance

Negligible NA T D MT NPPF

Water Resources Act (1991) Part IIA of the EPA(1990) Water Act (2003) Water Environment Regulations (2003) Environment Act (1995)


15-34








Positive / Negative



Positive / Negative


Potential changes to pattern and quantity of groundwater and surface water flows and subsequent effects on water receptors (e.g. Aquifers and surface water courses)

On-site and off-site surface water courses and drainage

Moderate Negative T D MT Monitoring and dewatering if required

Temporary drainage system

Ongoing monitoring

Negligible NA T D MT EV25 Water Environment Regulations (WFD) (2003) Water Act (2003) Environment Act (1995) Land Drainage Act (1994)

Operation Phase

Potential increase in risk of flooding (fluvial, pluvial, drainage and groundwater) to future Site users

Future populations and third parties

Minor Negative P D LT Implement / maintain drainage system

Safe access and egress

Negligible NA P D LT NPPF Water Act (2003) Environment Act (1995) Land Drainage Act (1991)

Potential increase in physical contamination (i.e. sediments) entering surface water bodies and drainage features due

On-site and off-site surface water courses and drainage

Negligible NA P D LT Implement / maintain drainage strategy

Negligible NA P D LT NPPF Water Resources Act (1991) Part IIA of the EPA(1990) Water Act (2003) Water Environment Regulations (2003) Environment Act


15-35








Positive / Negative



Positive / Negative


to increased sediment loading

(1995)

Potential changes to pattern and quantity of groundwater and surface water flows and subsequent effects on water receptors (e.g. Aquifer and surface water courses).

Aquifer and surface watercourses

Minor Negative P D LT Implement / maintain drainage strategy

Monitoring Dewatering

calculations Discharge at

existing greenfield rates into existing surface watercourses

Negligible NA P D LT ENV35 EV25 NPPF

Water Environment Regulations (WFD) (2003) Water Act (2003) Environment Act (1995) Land Drainage Act (1994)

Key to table:

P / T = Permanent or Temporary, D / I = Direct or Indirect, ST / MT / LT = Short Term, Medium Term or Long Term

N/A = Not Applicable


15-36



15.9 References Ref 15.1 The Water Environment (Water Framework Directive) (England and Wales) Regulations 2003

(2000/60/EC)

Ref 15.2 Water Industry Act 1991

Ref 15.3 Land Drainage Act 1994

Ref 15.4 The Environment Act 1995

Ref 15.5 Water Act 2003

Ref 15.6 The Water Environment Water Framework Directive Regulations 2003

Ref 15.7 The Groundwater (England and Wales) Regulations 2009

Ref 15.8 EU Flood Directive & Flood Risk Regulations 2009

Ref 15.9 Flood and Water Management Act 2010

Ref 15.10 Department for Communities and Local Government (2012) National Planning Policy Framework

Ref 15.11 Department for Communities and Local Government (2014) National Planning Policy Guidance

Ref 15.12 Daventry District Council (2010). Daventry Local Plan (Adopted 2010) – Saved Policies

Ref 15.13 South Northamptonshire Council (2007). South Northamptonshire Local Plan (2007) – Saved Policies

Ref 15.14 Environment Agency (2001) Pollution Prevention Guidelines 1: Understanding Your Environmental Responsibilities - Good Environmental Practices

Ref 15.15 Environment Agency (2007) Pollution Prevention Guidelines 5: Works in, Near or Liable to Affect Watercourses

Ref 15.16 Environment Agency (2007) Pollution Prevention Guidelines 6: Working at Construction and Demolition sites

Ref 15.17 Environment Agency (2009) 21: Pollution Incident Response Planning

Ref 15.18 CIRIA (2001) C532: Control of Pollution from Construction Sites

Ref 15.19 CIRIA (2007) C697: The SuDS Manual

Ref 15.20 Environment Agency (2009), River Nene Catchment Flood Management Plan

Ref 15.21 British Geological Survey (BGS) (1980) 1:50,000 scale map Sheet 185 Northampton

Ref 15.22 Environment Agency (2014) Water Framework Directive River Basin Management Plan

Ref 15.23 West Northamptonshire Council (2009) Level 1 Strategic Flood Risk Assessment

Ref 15.24 West Northamptonshire (Daventry & South Northamptonshire) (2009) Level 2 Strategic Flood Risk Assessment

15 flooding, hydrology and water resources · 15 flooding, hydrology and water resources 15.1...

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