northampton north-west relief road€¦ · prepared by sam willis rory clements signature checked...

MAY 2020 PUBLIC

Northamptonshire County Council

NORTHAMPTON NORTH-WEST

RELIEF ROAD

Flood Risk Assessment

Northamptonshire County Council

NORTHAMPTON NORTH-WEST RELIEF ROAD

Flood Risk Assessment

PUBLIC

FINAL (VERSION 1.0) PUBLIC

PROJECT NO. 70021598

DATE: MAY 2020

WSP

The Mailbox Level 2 100 Wharfside Street, Birmingham B1 1RT

Phone: +44 121 352 4700

WSP.com

Northampton North-West Relief Road PUBLIC | WSP Project No.: 70021598 May 2020 Northamptonshire County Council

QUALITY CONTROL

Issue/revision First issue Revision 1 Revision 2 Revision 3

Remarks Changes to flood management strategy

Date June 2019 June 2020

Prepared by Sam Willis Rory Clements

Signature

Checked by Louise Markose Sam Willis / Joanne Goodwin

Signature

Authorised by Sam Willis Sam Willis

Signature

Project number 70021598 70021598

Report number Version 1.0 Version 2.0

File reference


CONTENTS

EXECUTIVE SUMMARY

1 INTRODUCTION 1

1.1 PROJECT OVERVIEW 1

1.2 SITE DETAILS 1

1.3 ASSESSMENT METHODOLOGY 3

2 LEGISLATIVE FRAMEWORK AND GUIDANCE 5

2.2 EUROPEAN POLICY 5

WATER FRAMEWORK DIRECTIVE (2000/60/EC) 5

GROUNDWATER DIRECTIVE (2006/118/EC) 5

2.3 NATIONAL POLICY 5

NATIONAL PLANNING POLICY FRAMEWORK 2018 5

FLOOD AND WATER MANAGEMENT ACT 2010 8

ENVIRONMENTAL PERMITTING (ENGLAND AND WALES) REGULATIONS 2010 9

LAND DRAINAGE ACT 1991 9

2.4 LOCAL POLICY 10

WEST NORTHAMPTONSHIRE JOINT CORE STRATEGY LOCAL PLAN (PART 1) (2014 - 2029) 10

NORTHAMPTONSHIRE LOCAL FLOOD RISK MANAGEMENT STRATEGY (2016) 10

LOCAL STANDARDS AND GUIDANCE FOR SURFACE WATER DRAINAGE IN NORTHAMPTONSHIRE (2017) 10

2.5 CONSULTATION 10

2.6 INTERPRETATION OF PLANNING POLICY FOR THE PROPOSED DEVELOPMENT 14

3 DESK BASED REVIEW OF FLOOD RISK 16

3.2 FLUVIAL FLOOD RISK 16

INDICATIVE FLOOD MAPPING 16


3.3 SURFACE WATER FLOOD RISK 17

3.4 GROUNDWATER 18

3.5 FLOOD RISK FROM RESERVOIRS 19

4 HYDRAULIC ASSESSMENT 21

4.1 OVERVIEW 21

4.2 HYDROLOGY 21

4.3 HYDRAULIC MODELLING 22

MODEL SCHEMATISATION 22

KEY HYDRAULIC CONTROLS 24

4.4 EXISTING FLOOD RISK EXTENTS 25

5 DEVELOPMENT RISKS 28

5.1 DEVELOPMENT PROPOSALS 28

5.2 FLUVIAL FLOOD RISK 31

FLOOD RISK TO THE SCHEME 31

FLOOD RISK TO THE SURROUNDING AREA 32

REVIEW OF COMPENSATORY STORAGE VOLUMES 36

BRAMPTON BROOK 38

FLOOD RISK DURING CONSTRUCTION PHASE 38

RESIDUAL RISKS 38

5.3 SURFACE WATER FLOOD RISK 39

5.4 GROUNDWATER FLOOD RISK 40

6 DRAINAGE STRATEGY 43

6.1 EXISTING SITE AND DRAINAGE REQUIREMENTS 43

OBJECTIVE OF THE STUDY AND METHODOLOGY 43

EXISTING DRAINAGE 43

DESIGN MANUAL FOR ROADS AND BRIDGES (DMRB) 43

6.2 SUDS FOR HIGHWAYS 44

APPLICATION OF SUDS FOR ROAD DRAINAGE 44


SUDS ASSESSMENT 44

6.3 DRAINAGE DESIGN 44

ATTENUATION FEATURES 45

6.4 FURTHER WORK 45

7 CONCLUSIONS 47

TABLES

Table 2-1 - Flood Zone Categorisations 6

Table 2-2 - Flood risk vulnerability and Flood Zone compatibility 6

Table 2-3 - Recommended peak river flow allowances for the Anglian River Basin District 7

Table 2-4 - Peak rainfall intensity allowance in small and urban catchments 8

Table 2-5 - Record of communications with the Environment Agency 11

Table 4-1 - Final design flows for the Flood Risk Assessment 22

Table 4-2 - Key hydraulic structures on the River Nene 24

Table 5-1 - Summary of objective and effect of the various flood mitigation structures 29

Table 5-2 – Comparison of the 200yr+65% event peak water levels and design levels at key pinch points along the Proposed Scheme 32

Table 5-3 - Increases in flood depths upstream of A5199 35

Table 5-3 - Comparison of the 1000yr+65% event peak water levels and design levels at key pinch points along the Proposed Scheme 39

Table 5-4 - Design flows from local drainage catchments 40

FIGURES

Figure 1-1 - Overview of the site location 2

Figure 1-2 - Overview of the Scheme 3

Figure 3-1 – Extract from the Environment Agency’s Flood Map for Planning 16

Figure 3-2 - Extract from the Environment Agency’s Surface Water Flood Risk map 18

Figure 3-3 - Location of groundwater monitoring logs 19


Figure 3-4 – Extract from the Environment Agency's Flood Risk from Reservoirs map 20

Figure 4-1 - Model schematisation of the two hydraulic models constructed for the study 23

Figure 4-2 - Flood Risk Maps in the Existing Situation for the River Nene 26

Figure 4-3 - Flood Risk Maps for the Existing Situation for Brampton Brook 27

Figure 5-1 - Location of elements of flood mitigation measures referenced in Table 5-1 29

Figure 5-2 - Comparison of 200yr+65% flood extent on the River Nene before and after the Proposed Scheme 33

Figure 5-3 - Comparison of 200yr+65% flood extent on Brampton Brook before and after the Proposed Scheme 34

Figure 5-4 - Changes to the downstream hydrograph resulting from the proposals 36

Figure 5-5 - Comparison of floodplain volume for NWRR Mainline for Baseline and Proposed Scheme layouts 37

APPENDICES

MAIN RIVER CROSSING DRAWINGS

HYDROLOGY

FEH CALCULATION RECORD

SURFACE WATER HYDROLOGY

HYDRAULIC MODELLING LOG

FLOOD COMPENSATION FEATURE CROSS-SECTIONS

DRAINAGE STRATEGY DRAWINGS


EXECUTIVE SUMMARY

WSP was commissioned by Northamptonshire Country Council (NCC) to undertake a Flood Risk Assessment (FRA) to support the Environmental Impact Assessment (EIA) and planning application for a proposed relief road located to the north west of Northampton, hereafter referred to as the Proposed Scheme.

The Proposed Scheme crosses the River Nene and Brampton Brook watercourses and is situated within their associated floodplains. The main element of the Proposed Scheme runs along a north-south alignment to the west of the River Nene and is referred to as the NWRR Mainline. A second element of the Proposed Scheme, hereafter referred to as the Causeway, bisects the River Nene floodplain.

In determining the location and alignment of the Proposed Scheme NCC have undertaken an options appraisal process, set out in the Design and Access Statement, to identify a preferred option to deliver on seven key objectives. Through this process it is considered NCC have appropriately addressed the Sequential Test and the Part 1 of the Exception Test. Part 1 of the Exception Test requires the Proposed Scheme provides wider sustainability benefits to the community that outweigh flood risk. These key sustainability benefits include:

Unlocking housing development at Dallington Grange. Supporting the delivery and purchase of planned housing development sites in North West

Northampton to support economic growth. Improving access to employment sites for residents in North West Northampton. Reducing the number and severity of accidents in North West Northampton, particularly in hot

spot areas including the A508. Supporting the reduction of emissions and better air quality in the vicinity of AQMAs in

Northampton.

This FRA appraises Parts 2 and 3 of the Exception Test; that the Proposed Scheme remains operational and safe for users in times of flood and that the Proposed Scheme results in no net loss of floodplain storage and does not impede water flows or increase flood risk elsewhere.

The greatest concerns for the Proposed Scheme are associated with fluvial flood risk. A series of mitigation measures have been derived to offset the impact of the Proposed Scheme. The findings of this FRA (following the implementation of these mitigation measures) are as follows:

The Proposed Scheme is not inundated in the 200yr+65% event. The location of the Proposed Scheme near the junction with Brampton Lane, where the lowest

standard of protection is achieved, is partially inundated during the 1000yr+65% event, however depths on the southern carriageway at this location are generally between 5mm and 70mm. Emergency service vehicles could pass through the shallow depths during the 1000yr+65% event. On this basis it is considered the Proposed Scheme passes Part 2 of the Exception Test.


The Proposed Scheme will result in an increase in water levels upstream of the Causeway. A single residential property, currently at risk of flooding, is impacted by this increase in water levels and there will be an associated marginal increase in flood frequency. The Boughton Mill Equestrian Centre riding track and Boughton Mill may be marginally impacted by an increase in 200yr+65% flood extent. Additional information is needed to confirm if the various buildings of Boughton Mill are put at increased risk of flooding as this seems unlikely based on the proposed model extents. For these properties the Proposed Scheme does not pass Part 3 of the Exception Test and agreement will be needed with the landowner to confirm the change in flood risk is acceptable. If this agreement is not forthcoming further measures will be required to manage the change in flood risk to this property.

In principle it has been confirmed that floodplain compensation can be appropriately achieved through the construction of flood compensation basins and a flood mitigation bund with associated ordinary watercourse diversion. Further iterations of the design through the detailed design phase to accommodate additional data will be required.

The floodplain compensation basins and flood mitigation bund should be constructed and operational prior to construction works on the NWRR Mainline for the reach that cuts across the Brampton Brook floodplain to the north of Brampton Brook to prevent increases in downstream flood risk.

The ordinary watercourse diversion associated with the flood mitigation bund should be completed and operational prior to construction of the flood mitigation bund to ensure there is no impoundment of normal ordinary watercourse flows behind the feature.

The largest proposed flood compensation basin, north of the existing railway line, is situated within an area of known high groundwater. There is likely to be continual ingress of groundwater into the basin; the proposed outfall should be situated at the lowest point to ensure the basin is empty prior to a flood event.

There are no concerns associated with flood risk from surface water. Further work is required through the detailed design stage to understand the implications of the

Proposed Scheme on groundwater flooding and flow regime. This will need to confirm the floodplain compensation basin can be safely constructed, remain empty prior to a significant fluvial flood event and there will be no negative impact on the railway embankment structure following the construction of the Proposed Scheme.

Contact name Sam Willis

Contact details +44 121 352 4700 | [email protected]

Northampton North-West Relief Road Flood Risk Assessment WSP Project No.: 70021598 | Our Ref No.: May 2020 Northamptonshire County Council Page 1 of 48

1 INTRODUCTION

1.1 PROJECT OVERVIEW

1.1.1. WSP was commissioned by Northamptonshire Country Council (NCC) to undertake a Flood Risk Assessment (FRA) to support the Environmental Impact Assessment (EIA) and planning application for a proposed relief road located to the north west of Northampton, hereafter referred to as the Proposed Scheme. The assessment has been conducted in accordance with:

the National Planning Policy Framework (NPPF) and associated Planning Practice Guidance (PPG),

the Design Manual for Roads and Bridges (DMRB) LA113, the CIRIA Sustainable Drainage Systems (SuDS) Manual guidance and local planning policy as well as other relevant standards as agreed through consultation with

NCC and the Environment Agency (EA).

1.1.2. The Proposed Scheme is situated crosses the Brampton Branch of the River Nene (hereafter referred to as the River Nene) and Brampton Brook watercourses and their associated floodplains. Review of the EA’s Flood Map for Planning (Rivers and Sea) confirms that a large proportion of the Proposed Scheme is located within the high-risk Flood Zone 3. Flood Zone 3 is defined as land with a 1% (1 in 100) or greater annual probability of flooding from fluvial (river) sources. On this basis the EA standing advice on flood risk states that a FRA will be required to support the planning application for this development.

The assessment includes the following:

A review of national and local policy applicable to the FRA; Confirmation of the sources of flooding which may affect the Scheme; A quantitative assessment of the risk of fluvial and surface water flooding to the Proposed

Scheme and to adjacent sites as a result of the Proposed Scheme; Identification of possible measures which could reduce flood risk to acceptable levels and a

summary of residual risks; and A summary of the proposed surface water drainage strategy.

1.2 SITE DETAILS

1.2.1. The Proposed Scheme is located to the north west of the town of Northampton; an overview of the Scheme location is provided in Figure 1-1. The main element of the Proposed Scheme runs along a north-south alignment to the west of the River Nene, and is referred to as the NWRR Mainline. To the north, it will connect into Sandy Lane, via a new roundabout, before connecting into the A5199 Welford Road. To the south the Proposed Scheme crosses Brampton Brook and the railway line before connecting into a new roundabout, east of Grange Farm, which will provide access to the proposed Dallington Grange residential development.

1.2.2. From the new roundabout to the north a second element of the Proposed Scheme, hereafter referred to as the Causeway, will cross the River Nene parallel to the A5119, re-joining the A5119 via a second roundabout at Brampton Lane.

1.2.3. The site is generally rural and flood risk to properties is limited. Four properties are however identified that are of relevance to the FRA; these are a residential property located on the left bank


immediately upstream of the A5199 and the Brampton View Care Home, Boughton Mill Equestrian Centre and Boughton Mill itself all of which are located on the left bank of the River Nene further upstream of the A5199.

Figure 1-1 - Overview of the site location

1.2.4. With specific regard to aspects relating to flooding, the Proposed Scheme includes:

Earth embankments within the floodplain to support the road alignment. A new concrete pre-cast span bridge crossing the River Nene (details included in Appendix A). A new culvert to convey Brampton Brook through the road alignment (embankment details

included in Appendix A). Diversion of surface water flows and two new surface water culverts beneath the NWRR

Mainline. One access track culvert on the ditch downstream from the new southern surface water culvert.

This structure has no significance from a flood risk perspective and has not been discussed further within this report.

Three highway drainage attenuation ponds within the River Nene floodplain.

1.2.5. Full details of the proposed flood mitigation measures are discussed in Section 5, but broadly these included:

Two large flood relief culverts beneath the NWRR Mainline. Two large flood relief culverts beneath the Causeway.


Excavation of the existing floodplain to the west of the NWRR Mainline. One large flood compensation basin north of the railway line to the east of the Proposed Scheme.

The basin uses existing ground levels on the north-eastern boundary to inform top height; levels in this location are generally between 64.1-64.2m AOD. Plan area is approximately 26500m2 and the volume available below 64.1m AOD is approximately 30,600m3. The basin has a proposed base level excavated to 62.51m AOD.

Earth flood mitigation bund within the floodplain downstream of the Proposed Scheme to attenuate peak hydrograph flows.

Diversion of an ordinary watercourse around the new flood mitigation bund.

The location of these works is illustrated in Figure 1-2

Figure 1-2 - Overview of the Scheme

1.3 ASSESSMENT METHODOLOGY

In brief the methodology used for this FRA comprises:

Review of available relevant flood risk information to identify existing risks from all sources. The information reviewed includes: EA’s online maps for flood risk and groundwater, and data provided by EA on historical flooding during consultation.


Consultation with the EA and NCC to confirm local flood risk concerns and agree principles for the mitigation of flood risk to the Proposed Scheme and third-party land arising from the Proposed Scheme;

A detailed assessment of how the Proposed Scheme may affect fluvial flood risk through the construction of a 1D 2D Flood Modeller TUFLOW hydraulic model;

Development of mitigation measures, as necessary, to reduce flood risk to the Proposed Scheme and third-party land to an acceptable level as informed by the hydraulic modelling;

Discussion of the implication of the Proposed Scheme on groundwater; and A summary of the strategy for the management of site generated surface water runoff.


2 LEGISLATIVE FRAMEWORK AND GUIDANCE

2.1.1. The coordination of policies for the water environment is managed by the UK Government. Many flood risk and water quality requirements are set at European level, which are then transposed into UK law. The EA has a strategic overview regarding the management of all of sources of flooding and an operational responsibility for managing the risk of flooding from main rivers, reservoirs, estuaries and tidal sources. Lead Local Flood Authorities (LLFAs), in this case NCC, are responsible for managing the risk of flooding from local sources, including surface water, groundwater and ordinary watercourses.

2.1.2. The applicable legislative framework is summarised below.

2.2 EUROPEAN POLICY

WATER FRAMEWORK DIRECTIVE (2000/60/EC)

2.2.1. The overall objective of the Water Framework Directive (WFD) is to bring about the effective co-ordination of water environment policy and regulation across Europe. The main aims of the legislation are to ensure that all surface water and groundwater reaches ‘good’ status (in terms of ecological and chemical quality and water quantity, as appropriate), promote sustainable water use, reduce pollution and contribute to the mitigation of flood and droughts.

2.2.2. The WFD also contains provisions for controlling discharges of dangerous substances to surface waters and groundwater and includes a ‘List of Priority Substances’. Various substances are listed as either List I or List II substances, with List I substances considered the most harmful to human health and the aquatic environment. The purpose of the directive is to eliminate pollution from List I substances and reduce pollution from List II substances.

GROUNDWATER DIRECTIVE (2006/118/EC)

2.2.3. This Groundwater Directive aims to set groundwater quality standards and introduce measures to prevent or limit pollution of groundwater, including those listed within the ‘List of Priority Substances’. The directive has been developed in response to the requirements of Article 17 of the WFD, specifically the assessment of chemical status of groundwater and objectives to achieve ‘good’ status.

2.3 NATIONAL POLICY

NATIONAL PLANNING POLICY FRAMEWORK 2018

2.3.1. The National Planning Policy Framework (NPPF) sets out the Government’s planning policies for England, providing a framework within which local councils can produce their own plans that better reflect the specific needs of their communities. Planning Practice Guidance (PPG) has been published alongside the NPPF to set out how certain policies, including those relating to flood risk, should be implemented. The PPG for Flood Risk and Coastal Change is updated regularly to respond to changes in guidance and best practice.

PPG for flood risk and coastal change

2.3.2. The PPG for flood risk and coastal change identifies how new developments must take flood risks into account and steer development to those areas at lowest risk.


2.3.3. The PPG identifies Flood Zones in relation to flood frequency. The zones refer to the probability of river (fluvial) and sea (tidal) flooding, whilst ignoring the presence of defences. Table 2-1 summarises the relationship between Flood Zone category and the identified flood risk.

Table 2-1 - Flood Zone Categorisations

Flood Risk Area Identification Annual probability of fluvial flooding

Annual probability of tidal flooding

Zone 1 Low probability < 0.1% < 0.1% Zone 2 Medium probability 1% - 0.1% 0.5% - 0.1% Zone 3a High probability >1% >0.5% Zone 3b* Functional Floodplain >5% >5%

*The definition of the functional floodplain should take account of local circumstances. The annual flood probability is stated as a starting point for consideration.

2.3.4. The consequence of a flood event describes the potential damage, danger and disruption caused by flooding. This is dependent on the mechanism and characteristics of the flood event and the vulnerability of the affected land and land use.

2.3.5. The PPG identifies five classifications of flood risk vulnerability and provides recommendations on the compatibility of each vulnerability classification with the Flood Zones, as shown in Table 2-2. Full details of the Flood Zones and flood risk vulnerability classifications can be found in the PPG for flood risk and coastal change.

Table 2-2 - Flood risk vulnerability and Flood Zone compatibility

EA Flood Zone

Essential Infrastructure

Water Compatible

Highly Vulnerable

More Vulnerable

Less Vulnerable

Zone 1 ✓ ✓ ✓ ✓ ✓ Zone 2 ✓ ✓ Exception test

required ✓ ✓

Zone 3a Exception test required

✓ Exception test required

✓

Zone 3b* Exception test required

✓

✓ Development considered acceptable Development considered unacceptable

2.3.6. The PPG requires the following sources of flooding, relevant to this study, to be considered:

Fluvial flood risk from nearby watercourses; Surface water flooding from within the site and adjacent land; Groundwater flooding; and Flood risk from other artificial sources such as canals and impounded reservoirs.


2.3.7. The PPG sets out the requirement to consider Sustainable Drainage Systems (SUDS) within all new development where appropriate. It states that developments should aim to discharge surface run off as high up the following hierarchy of drainage options as reasonably practicable:

1. Into the ground (infiltration);

2. To a surface water body;

3. To a surface water sewer, highway drain, or another drainage system;

4. To a combined sewer.

2.3.8. Information regarding expected minimum standards is provided within the Non-Statutory Technical Standards for Sustainable Drainage Systems (SUDS) published by DEFRA in March 2015 (discussed below).

PPG for climate change

2.3.9. Scientific consensus is that the global climate is changing as a result of human activity. While there remain uncertainties as to how a changing climate will affect areas already vulnerable to flooding, it is expected to increase risk significantly over time. For the UK, projections of future climate change indicate that more frequent short-duration high-intensity rainfall events and more frequent periods of long-duration rainfall could be expected.

2.3.10. Updated climate change recommendations were published by the EA in August 2017, which superseded the previous recommendations that were included within the NPPF. The impacts of climate change are expected to increase over time and the EA guidance provides a range of estimates for increases in peak river flow, peak rainfall intensity and sea level rise over the next 100 years. This is reflected by larger allowances recommended for developments with a longer design life.

2.3.11. The precise extent of the impacts of climate change is unknown. This is reflected in the EA’s guidance which provides ‘Central’, ‘Higher Central’ and ‘Upper End’ estimates that are based on the 50th, 70th and 90th percentile predictions for climate change.

2.3.12. The increases in peak fluvial flows are also expected to vary depending on geographical location. To account for this the EA guidance divides England into eleven river basin districts. The Scheme that is the focus of this FRA is located within the Anglian River Basin District. Table 2-3 shows the recommended increase in peak river flow in this district.

Table 2-3 - Recommended peak river flow allowances for the Anglian River Basin District

Allowance category

Total potential change anticipated

2017 - 2039


2040 - 2069


2070 - 2115

Peak river flow allowances for Anglian

Upper End 25% 35% 65% Higher Central 15% 20% 35% Central 10% 15% 25%

2.3.13. Table 2-4 summarises the EA’s guidance for increases to peak rainfall intensity throughout England. This information is typically applied to the assessment of surface water runoff but can also be


applied to small watercourses that have a catchment of less than c.3km2 which respond much more quickly to intense rainfall events.

Table 2-4 - Peak rainfall intensity allowance in small and urban catchments

Allowance category


2017 - 2039


2040 - 2069


2070 - 2115

Peak rainfall intensity allowance

Upper End 10% 20% 40% Central 5% 10% 20%

FLOOD AND WATER MANAGEMENT ACT 2010

2.3.14. The Flood and Water Management Act 2010 created the role of the LLFA (in this case NCC) to take responsibility for leading the co-ordination of local flood risk management in their areas. In accordance with the Act the EA is responsible for the management of risks associated with main rivers, the sea and reservoirs. LLFAs are responsible for the management of risks associated with local sources of flooding such as ordinary watercourses, surface water and groundwater.

2.3.15. The Act is also guiding the role of the LLFA in the review and approval of surface water management systems. This has led to a recent change that requires the LLFA to review and comment on significant development in regard to SUDS.

2.3.16. Schedule 3 of the Flood and Water Management Act introduces National Standards for SUDS against which proposed drainage systems should comply. These are discussed below.

Non-Statutory Technical Standards for Sustainable Drainage Systems 2015

2.3.17. The Non-Statutory Technical Standards for SUDS, published by DEFRA in March 2015, set out the core technical standards for SUDS proposed within England. These standards should be used in accordance with the NPPF and PPG for flood risk and climate change. The standards include guidance on controlling flood risk within a development boundary and elsewhere, peak flow and runoff volume control, and the structural integrity of SUDS.

LA 113 Road Drainage and the Water Environment

2.3.18. The Design Manual for Roads and Bridges (DMRB) (LA 113) sets out the requirements for the assessment and management of the impacts that road projects can have on the water environment, including those related to flood risk.

2.3.19. Of key importance is that LA 113 states that road projects must be compliant with relevant national policy and meet the requirements of the relevant overseeing organisation (in this case the EA and NCC).

2.3.20. It states that, if required, a FRA shall be carried out in accordance with the NPPF and associated PPG on Flood Risk and Coastal Change.

2.3.21. It also states that all projects on motorways and all-purpose trunk roads shall be designed to:

• remain operational and safe for users in times of flood; • result in no net loss of floodplain storage; • not impede water flows; and • not increase flood risk elsewhere.


2.3.22. The assessment of flood risk must apply the latest climate change allowances published by the EA for the river basin district in which the scheme is located.

2.3.23. Environmental permits/ licences (or registered exemptions) must be obtained prior to carrying out any activity that has the potential to affect flood risk, in accordance with the relevant legislative requirements.

ENVIRONMENTAL PERMITTING (ENGLAND AND WALES) REGULATIONS 2010

2.3.24. The Environmental Permitting (England and Wales) Regulations 2010 replaced the Water Resources Act 1991 as the key legislation for water pollution in the UK. Under the Environmental Permitting Regulations, it is an offence to cause or knowingly permit a water discharge activity, including the discharge of polluting materials to freshwater, coastal waters, relevant territorial waters or groundwater, unless complying with an exemption or an environmental permit.

2.3.25. An environmental permit is obtained from the EA. The EA sets conditions which may control volumes and concentrations of particular substances or impose broader controls on the nature of the effluent, taking into account any relevant water quality standards from EC Directives.

2.3.26. With regards to the water environment any works in, under or near a main river requires permission from the EA to ensure no detrimental impacts on the watercourse. Previously, this was a Flood Defence Consent; however, in April 2016 consent for flood risk activities was included under the Environmental Permitting Regulations, and is now provided as a Flood Risk Activities Permit (FRAP).

LAND DRAINAGE ACT 1991

2.3.27. The Land Drainage Act sets out the duty for the Environment Agency to maintain a map of the Main River network. Main Rivers are the responsibly of the Environment Agency and are defined as a watercourses from which extensive flooding occurs or from which flows contribute to extensive flooding. All other watercourses and channels that convey water that are not Main River are classified as ordinary watercourses.

2.3.28. Local Authorities and Internal Drainage Boards have additional duties and powers associated with the management of flood risk under the Land Drainage Act 1991. As Land Drainage Authorities, consent must be given for any permanent or temporary works that could affect the flow within an ordinary watercourse under their jurisdiction in order to ensure that local flood risk is not increased.

2.3.29. The Land Drainage Act specifies that the following works will require formal consent from the appropriate authority:

Construction, raising or alteration of any mill dam, weir or other like obstructions to the flow of a watercourse;

Construction of a new culvert; Any alterations to an existing culvert that would affect the flow of water within a watercourse.

2.3.30. The Land Drainage Act also sets out the maintenance responsibilities riparian owners have in order to reduce local flood risks. Riparian owners, who are land owners with a watercourse either running through their land or adjacent to, have the responsibility to ensure that the free flow of water is not impeded by any obstruction or build-up of material within the watercourse.


2.4 LOCAL POLICY

WEST NORTHAMPTONSHIRE JOINT CORE STRATEGY LOCAL PLAN (PART 1) (2014

- 2029)

2.4.1. The West Northamptonshire Joint Core Strategy (2014-2029) provides guidance for new development within the council’s administrative area up to 2029. In order to achieve the vision set out in the plan, a number of policies have been adopted including Policy BN7.

2.4.2. Policy BN7 (Dealing with flood risk) sets out to ensure that development will be directed to areas of the lowest probability of flooding. All proposals for development will need to demonstrate that the proposed development is safe from flooding, there is no increased risk of flooding to existing properties and that proposals seek to improve existing flood risk management.

2.4.3. Where flood risk management requires the use of sustainable drainage systems to manage surface water runoff, these should:

1. Separate surface water from foul and combined sewers;

2. Be accompanied by a long-term management and maintenance plan;

3. Protect and enhance water quality

2.4.4. The policy is also consistent with correspondence received from the EA at the start of the project. It states that the design standard for the Upper Nene catchment is the 200yr event plus an appropriate allowance for climate change. The design of surface water attenuation should also be to this standard.

NORTHAMPTONSHIRE LOCAL FLOOD RISK MANAGEMENT STRATEGY (2016)

2.4.5. Northamptonshire County Council’s Local Flood Risk Management Strategy (LFRMS) provides information and technical guidance on how flood risk will be managed within Northamptonshire. The overarching aim of the strategy is to provide a robust local framework that employs a full range of complementary approaches towards managing and communicating the risks and consequences of flooding arising from all sources of flood risk for Northampton.

LOCAL STANDARDS AND GUIDANCE FOR SURFACE WATER DRAINAGE IN

NORTHAMPTONSHIRE (2017)

2.4.6. These guidelines are intended to assist developers in the design of surface water drainage systems, alongside supporting Local Planning Authorities, in considering drainage proposals for new developments within Northamptonshire. The guidance is clear and extremely comprehensive highlighting clearly where there is divergence from national guidelines.

2.5 CONSULTATION

2.5.1. The EA has been consulted throughout this process. Table 2-5 details the dates of consultation and the key information relating to the parameters of the study provided at the time.


Table 2-5 - Record of communications with the Environment Agency

Details of Correspondence

Communication

Letter received August 2017

Proposed routes cross land located within Flood Zone 3 as defined by the EA Flood Map as having a high and medium probability of flooding. Paragraph 103, footnote 20 of the NPPF requires applicants for planning permission to submit a FRA when development is proposed in such locations. … all new development in the Upper Nene catchment must be designed for a flood with a 0.5% probability (1 in 200 chance) occurring in any year, including an appropriate allowance for climate change. This includes design of mitigation for main river flooding and any surface water attenuation. This applies across the whole of the Upper Nene catchment including all branches and arms of the Nene …. It’s recommended that any main river crossings are clear span (no piers in the river). If the road is classified as ‘essential infrastructure’ it should not flood in the 0.1% (1 in 1000) plus climate event. The soffit of the structures should be a minimum of 600mm above the 0.5% (1 in 200) plus climate change flood level and no lower than 300mm above either of the upstream bank tops. There should be no loss of floodplain as a result of the proposed road and the FRA should provide further details on any raising or lowering of land within the floodplain. Any loss of floodplain should be compensated for on a level for level, volume for volume basis (i.e. re-grade the land at the same level as that taken up by the development) therefore providing a direct replacement for the lost storage volume. Within the FRA, detailed information must be provided to demonstrate how this can be achieved.

Meeting held January 2018

The EA consider the road to be classified as essential infrastructure although ultimately it is for the LPA (Local Planning Authority) to determine vulnerability. The terms of reference if the road is classified as essential infrastructure are:

• The FRA is required to demonstrate the scheme does not flood to the 1000yr plus 65% climate change (the upper end allowance).

• Floodplain compensation will be required to offset the impacts of the scheme. Floodplain compensation will be required to demonstrate no adverse impact to the 200yr plus 65% climate change to maintain the standard of defences within Northampton.

Meeting held December 2018

WSP asked if the Causeway will be classified as essential infrastructure. EA confirmed they will treat the whole scheme as essential infrastructure, but it will be the LPA who determines the classification of the infrastructure. The Causeway has the potential to act as a flood barrier, increasing water levels upstream but reducing flows downstream. WSP queried if using this structure in this way would be worth exploring. EA advice to get 3rd party agreement from properties upstream that could be affected by a perceived increase in water levels. WSP asked if the floodplain compensation needs to be for the 65% CC as detailed at the previous meeting. There appears to be a contradiction in requiring essential infrastructure to provide protection from flooding to downstream properties compared to any other development in a similar position. The EA noted the point and agreed to check and confirm.

Letter received January 2019 in response to meeting in December 2018

65% CC allowance should be used for all aspects of the road including floodplain compensation if in Flood Zone 3a or 3b as stated on gov.uk: https://www.gov.uk/guidance/flood-risk-assessments-climate-change-allowances “Using peak river flow allowances for flood risk assessments” … the minimum design level of the road should be the 0.5% (1 in 200) plus climate change as this is the design standard for Northampton and upstream. … all new development in the Upper Nene catchment will be designed for a flood with a 0.5% probability (1 in 200 chance) occurring in any year, including an appropriate allowance for climate change. This includes design of mitigation for main river flooding and any



Communication

surface water attenuation. This applies across the whole of the Upper Nene catchment including all branches.

Letter received July 2019, WSP response April 2020 and EA response May 2020

WSP raised queries in relation to the bulleted objections detailed in EA Letter AN/2019/129224. Five main points were discussed via email chain, pivotal points are summarised below, with the EA 2nd responses being received in May 2020.

ID EA Comment WSP Response EA 2nd Response

1. EA requested model files and model report for review. Noted that model scope previously requested.

Modelling files have been issued and now reviewed as fit for purpose.

Modelling scope was issued on 22nd March 2018 and reissued on 17th April 2018 following requests for response. No response on this was received.

Accepted. WSP agreed (email 27 March 2020) to update the modelling log (Appendix C of the FRA). This will consist updating the simulation run list page to reflect the latest model and including an additional tab summarising model run parameters and the outstanding modelling warnings with associated justification as flagged in the model review.

2. 1 in 20yr event to be run

This has been run and supplied.

Accepted.

3. Drawings of the main river crossings to be provided to confirm access

Drawings of the River Nene bridge were submitted originally in March 2019 and EA response in a letter dated 28th March 2019 confirmed that access was suitable. The drawings were also reissued in response to EA letter AN/2019/129224 on 9th August 2019.

Requested that drawings be included in the FRA.



Communication

4. Details of floodplain compensation basins including dimensions to be provided. Details of how the basins will provide level for level, volume for volume compensation should be provided.

Dimensions of the floodplain compensation basins were provided. The FRA highlighted that these are concept designs at this stage and will be refined in detailed design.

The purpose of building the 2D model was to confirm that there would be no increase in downstream flows. It was noted in the minutes of the first meeting in January 2018 that due to the length of the scheme and the changing water level over that length that manual level for level compensation calculations would not be suitable. At least one element of the scheme is being used to attenuate flows which again does not lend itself to a level for level compensation analysis.

The FRA demonstrates that the proposals will reduce downstream flows for all points on the hydrograph. This is equivalent to the level for level compensation assessment.

EA requested that the area and volume of the storage areas be added to figures. Also state the volume of floodplain lost and the volume gained through compensation. EA clarified they would need to put a ‘scheme to be agreed’ condition on for this if the final design is not yet available.

5. Increase of flood risk to residential owner is not acceptable without agreement

Discussions are ongoing on this and it is not possible at this stage to include a statement in the FRA that agreement from the landowner has been provided. It is expected this will be resolved through the detailed design stage.

EA are not able to remove objection on this element until either the flood risk is accepted by the property or flood risk mitigation is proposed.


2.6 INTERPRETATION OF PLANNING POLICY FOR THE PROPOSED

DEVELOPMENT

2.6.1. In determining the alignment of the NWRR, NCC undertook an options appraisal process to identify a preferred solution that could deliver on 7 key objectives for Northampton:

O1 - To support free flow traffic movements on key local routes in North West Northampton during off peak periods, and minimise delay during peak periods.

O2 - To increase journey time reliability and minimise travel costs on key strategic routes in North West Northampton.

O3 – To unlock housing development at Dallington Grange. O4 - To support the delivery and purchase of planned housing development sites in North West

Northampton to support economic growth. O5 - To improve access to employment sites for residents in North West Northampton. O6 - To reduce the number and severity of accidents in North West Northampton, particularly in

hot spot areas including the A508. O7 - To support the reduction of emissions and better air quality in the vicinity of AQMAs in

Northampton.

2.6.2. Of the options assessed only the NWRR fully addressed the identified problems, with the intervention supporting the development of housing and wider support to the growth of employment sites. The NWRR will also provide greater journey time reliability and reduce traffic flows in conservation areas such as Chapel Brampton and Church Brampton. The intervention is likely to provide a positive air quality due to additional capacity reducing stop/start movements. It was recognised as part of this assessment that the NWRR would have a negative impact on the local environment as a result of land requirements. The NWRR supports economic growth with a high VfM and the potential for outputs to support job creation and the delivery of 6,600 dwellings across three SUEs. The NWRR is a priority project for NCC. It has been strongly supported in the West Northamptonshire Joint Core Strategy by the Joint Planning Unit and its constituent local authority members.

2.6.3. The options appraisal process is set out in detail in the Design and Access Statement and on this basis it is considered that the Sequential Test has been appropriately addressed by NCC.

2.6.4. The Proposed Scheme is considered to be ‘Essential Infrastructure’ and therefore according to Table 3 of the Flood Risk and Coastal Change PPG, this development can be located in all Flood Zones, provided an Exception Test is satisfied. In order to meet Part 1 of the Exception Test it must be demonstrated that the Proposed Scheme provides wider sustainability benefits to the community that outweigh flood risk. The Proposed Scheme would deliver a number of sustainability benefits to local community, as summarised in the Proposed Scheme objectives above and expanded on in more detail in the Design and Access Statement.

2.6.5. To satisfy Parts 2 and 3 of the Exception Test a FRA is required to demonstrate the Proposed Scheme remains operational and safe for users in times of flood, results in no net loss of floodplain storage and does not impede water flows or increase flood risk elsewhere.

2.6.6. The West Northamptonshire Joint Core Strategy Local Plan requires that all new development in the Upper Nene catchment must be designed for a flood with a 0.5% probability occurring in any given year, including an appropriate allowance for climate change.


2.6.7. The PPG for Climate Change requires the upper end allowance be used for Essential Infrastructure within Flood Zone 3b. The design life of the Scheme is 100 years. In accordance with the guidance above taking into account the development’s vulnerability and the discussions with the EA detailed in Table 2-5, the parameters of the study are as follows:

Scheme to be designed to the 0.5% (200yr) event plus 65% climate change; Floodplain compensation to be provided to the 0.5% (200yr) event plus 65% climate change; Design of proposed surface water drainage systems to attenuate to the 0.5% (200yr) event plus

65% climate change.


3 DESK BASED REVIEW OF FLOOD RISK

3.1.1. The NPPF requires the assessment of flood risk from the following sources as part of the FRA:

Fluvial flood risk from nearby watercourses; Surface water flooding from within the site and adjacent land; Groundwater flooding; and Flood risk from other artificial sources such as canals and impounded reservoirs.

3.1.2. To support this assessment the EA has produced a series of national flood risk maps that cover the risk of flooding from fluvial, tidal, reservoir and surface water sources across England. These and other readily available datasets have been used as appropriate to complete a desk based assessment of the key flood risk considerations appropriate to the Proposed Scheme.

3.2 FLUVIAL FLOOD RISK

INDICATIVE FLOOD MAPPING

3.2.1. The EA’s Flood Map for Planning available from the EA website (shown in Figure 3-1) shows Flood Zones 2 and 3 and confirms the Proposed Scheme overlaps significant portions of Flood Zone 3 along both the River Nene and Brampton Brook.

Figure 3-1 – Extract from the Environment Agency’s Flood Map for Planning


3.3 SURFACE WATER FLOOD RISK

3.3.1. The risk of flooding from surface water is defined as high, medium, low and very low by the EA’s flood risk maps using the following criteria:

High: Land assessed as having a greater than 1 in 30 (3.3%) annual probability of flood risk from surface water.

Medium: Land assessed as having between a 1 in 30 (3.3%) and 1 in 100 (1%) annual probability of flood risk from surface water;

Low: Land assessed as having between a 1 in 100 (1%) and 1 in 1000 (0.1%) annual probability of flood risk from surface water;

Very low: Land assessed as having less than 1 in 1000 (0.1%) annual probability of flood risk from surface water.

3.3.2. The EA’s surface water flood risk map indicates that the site is at high risk of surface water flooding, as illustrated in Figure 3-2. These maps have been developed by allowing ‘water’ to follow the existing topography. As such it is expected that the River Nene and Brampton Brook and their associated floodplains would be picked up in these maps as the watercourse represents the natural low point in the area to which runoff will drain. The management of flood risk from these channels falls under the fluvial element of the FRA.

3.3.3. The map also shows three surface water flow routes draining the catchments to the west that pass beneath the alignment of the Proposed Scheme before joining the River Nene floodplain. The changes to these surface water flow paths has been considered as part of this FRA.


Figure 3-2 - Extract from the Environment Agency’s Surface Water Flood Risk map

3.4 GROUNDWATER

3.4.1. Site investigations into groundwater levels across the site are ongoing. At the time of writing, boreholes have been drilled at three locations to the south of the railway line, four locations north of the railway, two locations north of Brampton Brook at the southern end of the site and two locations at the location of the proposed Causeway south of Sandy Lane. Monitoring logs were installed in these boreholes and groundwater level data collection is ongoing. Figure 3-3 shows the location of the boreholes with monitoring logs installed.


Figure 3-3 - Location of groundwater monitoring logs

3.4.2. The results from BH335 (which is available now) confirm that groundwater levels are around 0.4m below ground level. The ground levels to the south of the railway line are significantly higher than the River Nene floodplain. The ground level at BH335 is approximately 7.5m above the River Nene floodplain.

3.4.3. The railway line is a significant raised structure cutting across the site and there is currently no understanding as to how groundwater flows past this structure. Notwithstanding the railway line, it would appear that there may be some groundwater emergence on the slopes down to the floodplain north of the railway line.

3.4.4. Elsewhere across the site the Proposed Scheme is generally situated within the River Nene floodplain. Groundwater elevations in the floodplain will be heavily influenced by the water levels in the River Nene itself and would be expected to be close to existing ground levels.

3.5 FLOOD RISK FROM RESERVOIRS

3.5.1. The EA’s mapped extents of areas deemed to be at risk of flooding from reservoirs indicates those areas that could be flooded if a large reservoir were to fail and release the water it holds. A large reservoir is one that holds over 25,000 cubic metres of water.

3.5.2. The reservoir flood risk map indicates that the site is at risk of flooding in the event of failure of Pitsford Water, Hollowell Reservoir or Ravensthorpe Reservoir. Of these Pitsford Water is by far the


largest and it is assumed the source of the greatest flood risk. In the event of failure of this reservoir, the worst-case scenario flooding would inundate the properties at the Brampton Lane junction, including the care home and a number of the properties that back onto the River Nene opposite the Proposed Scheme.

Figure 3-4 – Extract from the Environment Agency's Flood Risk from Reservoirs map

3.5.3. The flow from this reservoir would have to make its way down the River Nene before impacting the site. This would indicate that flood risk from the reservoir will follow the same flooding mechanism as a fluvial flood event. The risk of a failure of the reservoir is low and in this case, it is considered that this risk will be adequately considered in the detailed appraisal of the fluvial flood risk, and in particular the residual risk scenarios presented in Section 5.2.


4 HYDRAULIC ASSESSMENT

4.1 OVERVIEW

4.1.1. The scale and extent of the interaction of the Proposed Scheme with the River Nene floodplain and the sensitivity of the downstream flood receptors in Northampton means that a detailed understanding of the potential change in flood risk associated with the Proposed Scheme is required.

4.1.2. Floodplain compensation to offset the significant loss of floodplain resulting from the Proposed Scheme is required. The EA require level for level compensation to be provided. Given the distance over which the Proposed Scheme is impacting the River Nene floodplain, simple level for level calculations will not provide the clarity and confidence required to confirm that mitigation has been achieved. Instead the FRA will use a hydraulic model to demonstrate the impacts of the Proposed Scheme on downstream flow hydrographs.

4.1.3. The Upper Nene Flood Map Improvements Project, hereafter referred to as the Upper Nene Study, was completed by the EA in September 2013. This project updated the hydraulic model for the Upper Nene catchment to Billing Wharf downstream of Northampton, and included the Brampton Branch of the River Nene (the reach of interest for this study). The study was updated by the EA in 2016; the EA requested that WSP consider the impact of the updated modelling. This data represents the best available hydraulic model for the site. The Upper Nene Study developed a 1D 2D Flood Modeller Pro TUFLOW model of the Nene, however the 2D element of the model does not start until Mill Lane which is approximately 2km downstream of the Brampton Brook inflow.

4.1.4. The work completed as part of this FRA has built upon and developed the data available in Upper Nene Study model.

4.1.5. The following sections set out the findings of the hydraulic assessment. In the first instance a baseline model has been developed. The Proposed Scheme and associated floodplain compensation has then been incorporated into the model to demonstrate the changes in flood risk resulting from the operational scheme.

4.1.6. The 2016 EA update of the Upper Nene 2013 model did not include newer hydrology. The update did result in changes to within the location of the downstream boundary to the WSP model (details below), as such the 2016 study has been utilised to update the downstream boundary of the WSP study model by extraction of the stage discharge relationship from the 2016 model at the location of the railway for use in the WSP model as the downstream boundary.

4.2 HYDROLOGY

4.2.1. Catchment hydrology was required for the River Nene and Brampton Brook for this study. The catchments of the River Nene and Brampton Brook are markedly different in size; the catchments to the confluence of the two watercourses are 190km2 for the River Nene and 30km2 for Brampton Brook. This difference in size highlights the different responses the can be expected in the event of a flood.

4.2.2. There are two gauges on the River Nene that are of use for this study, the Geldhart’s Mill gauge and St Andrew’s Mill gauge. The Geldhart’s Mill gauge in particular is located approximately 200m


downstream of the confluence of the River Nene and Brampton Brook and so is directly applicable to this study. St Andrew’s Mill gauge is approximately 3.5km downstream of the confluence.

4.2.3. Hydrology for the River Nene was developed as part of the Upper Nene Study. A review of the applicability of this hydrology to the Proposed Scheme location was completed using the data available from the Geldhart’s Mill gauge as part of the hydrological assessment. The hydrology was found to be suitable and has been used directly for this study with no changes proposed.

4.2.4. The Upper Nene Study did not derive flows for Brampton Brook and a full Flood Estimation Handbook (FEH) analysis has been completed to determine the flows from this watercourse. Full details of this assessment are provided in the FEH calculations record in Appendix B. The findings of the assessment recommended the statistical approach be used to derive the design flows for Brampton Brook using the Geldhart’s Mill and St Andrew’s Mill gauges as donor sites for QMED.

4.2.5. The final design flows for the River Nene and Brampton Brook at the Proposed Scheme are presented in Table 4-1.

Table 4-1 - Final design flows for the Flood Risk Assessment

Site code Flood peak (m3/s) for the following return periods (in years)

2 5 10 25 50 75 100 200 1000

River Nene

24.4 36.1 43.3 53.0 60.8 68.6 73.1 84.7 109.4

Brampton Brook

3.72 5.46 6.67 8.32 9.69 10.55 11.18 13.17 18.67

4.2.6. A hydrological assessment has also been completed for the surface water flow paths from the catchments to the east of the Proposed Scheme. These flows have been used to determine appropriate culvert sizes for the surface water culverts beneath the NWRR Mainline and so details of the calculated flows are provided with that assessment in Section 5.3.

4.3 HYDRAULIC MODELLING

MODEL SCHEMATISATION

4.3.1. A 1D 2D FMP TUFLOW model has been constructed of the River Nene and Brampton Brook in the vicinity of the scheme. The 1D FMP model from the Upper Nene Study has been used as the basis for the 1D model. The upper limit of the Upper Nene Study model is approximately 550m upstream of the A5199. Additional survey was collected of the River Nene so that the model could be extended upstream a further kilometre the road bridge at Chapel Brampton.

4.3.2. The Brampton Brook is not included in the Upper Nene Study model and additional survey of this watercourse was collected covering a distance of 800m upstream of its confluence with the Nene. Additional check survey was collected at the Geldhart’s Mill gauge site to support calibration of the model in this location. Finally, a number of infill and check survey points were collected where there were gaps in the Upper Nene Study model, for example at the A5199 Road Bridge.

4.3.3. Initially the Upper Nene Study model was trimmed at Mill Lane, located approximately 2km downstream of Brampton Brook, and the downstream boundary derived from a stage flow relationship extracted from the Upper Nene Study model. The downstream boundary was moved during the study to the railway line, located approximately 850m downstream of the Brampton Brook, to facilitate the number of design iterations that were required. The downstream boundary at


the railway line was derived from the updated EA 2016 update to the 2013 Upper Nene Study model.

4.3.4. Similarly, the initial model construction incorporated both the River Nene and Brampton Brook. The small width of Brampton Brook necessitated a 4m cell size to be appropriately represented in the model. The model was split into two separate models during the study, allowing a coarser cell size to be used for the River Nene model, to facilitate the model stabilisation and the number design iterations that were required in the Proposed Scheme model.

4.3.5. The final models used to assess the existing flood risk and impacts and mitigation required as a result of the Proposed Scheme are:

• An FMP TUFLOW model of the River Nene only between Chapel Brampton and the railway bridge. This model has an 8m cell size.

• An FMP TUFLOW model of the River Nene between the A5199 and the railway bridge and Brampton Brook for 800m upstream of its confluence with the River Nene. This model has a 4m cell size.

4.3.6. Full details of the hydraulic models are provided in Appendix C. Figure 4-1 provides an overview of the model schematisation for the two hydraulic models.

Figure 4-1 - Model schematisation of the two hydraulic models constructed for the study


KEY HYDRAULIC CONTROLS

4.3.7. The River Nene and Brampton Brook are essentially rural in the vicinity of the Proposed Scheme and as such the main hydraulic control is the River Nene floodplain itself, which is in the region of 200m to 300m wide in this location. There are however a couple of structures that are important in understanding the existing flood risk at the site, these are presented in Table 4-2.

Table 4-2 - Key hydraulic structures on the River Nene

Structure Description

A5199 Road Bridge and flood relief culverts

The A5199 is a significant control that crosses the River Nene perpendicular to the flow path. It has four passing locations for the River Nene: the River Nene bridge itself, shown opposite, and three flood relief culverts. The bridge consists of two openings both of which are rectangular on the upstream face and arch on the downstream face. The arch provides the smaller opening capacity of the two and both are approximately 4.2m wide and 2.8m high. The River Nene comes to the bridge at an angle of around 45 degrees from the east and the right bank is encroaching across the upstream face as a result (shown in the figure opposite). The two arches themselves are clear beneath the bridge. The three flood relief culverts are located in the left bank floodplain, two in the centre of the channel and one on the left-hand side of the floodplain. The two central culverts are sprung arch approximately 2.5m wide and 1.7m high. The left-hand culvert is a small arch 3m wide and 0.9m high. The A5199 generally sits between 1m and 2m above the floodplain. The lowest section of the road on the eastern side adjacent to the access road to the Care Home and, during periods of high flows, significant overtopping of the A5199 in this location is predicted.

Geldhart’s Mill Gauge

Whilst not a key driver of flood risk the gauge is important for the validation of the model given the limited flood history at the site. The control is a v crump weir. The weir is bypassed at high flows on the right bank in particular. Out of bank flows from Brampton Brook can also bypass the gauge, but in this instance form a separate flow path and so do not contribute the level changes at the gauge. These flows cannot therefore be assessed by this gauge.


4.4 EXISTING FLOOD RISK EXTENTS

4.4.1. The baseline hydraulic models have been run for the 5yr, 10yr, 20yr, 25yr, 50yr, 100yr and 200yr+65% event to provide an understanding of the existing flood risk and flood frequency and the design flood extent (the 200yr+65% flows) against which the effectiveness of the proposed flood mitigation measures have been assessed.

4.4.2. Figure 4-2 provides the extents for the River Nene and Figure 4-3 provides the extents for Brampton Brook. The flood extents shown for the River Nene have been derived assuming all flows are along the River Nene. The flood extents for Brampton Brook assume a fixed flow on the River Nene equivalent to a 2yr event. A review of past events completed as part of the hydrology calculations for Brampton Brook suggested an average lag between peak flows from Brampton Brook and the River Nene of around 11.5 hours. It is reasonable on this basis to conclude that higher flows on the River Nene would not be appropriate for the assessment.

4.4.3. There is little in the way of calibration data to verify these flood extents. Of note is the frequency with which Sandy Lane is predicted to overtop; this is at the 100yr return period event. No data has been received to validate or reject this finding.


Figure 4-2 - Flood Risk Maps in the Existing Situation for the River Nene


Figure 4-3 - Flood Risk Maps for the Existing Situation for Brampton Brook


5 DEVELOPMENT RISKS

5.1 DEVELOPMENT PROPOSALS

5.1.1. The Proposed Scheme consists of a significant structure that both cuts across and constrains the width of the floodplain of both the River Nene and Brampton Brook. Different elements of this structure will have different hydraulic effects on the watercourses as follows:

• The Causeway bisects the floodplain immediately downstream of the A5199. This structure will increase upstream water levels and so reduce the flows passing through the existing A5199 flood relief culverts. The result is increased water levels upstream of the A5199 and some marginal attenuation of flows.

• The NWRR Mainline cuts across the western limit of the River Nene floodplain. The NWRR Mainline will restrict the width of the floodplain compared to the existing situation and so reduce the attenuating effect of this area of the floodplain. Currently flows overtop the right bank of the River Nene downstream of the Causeway, head in a south-westerly direction towards Brampton Brook and fill the floodplain to the north of Brampton Brook. The NWRR Mainline will cut off this flow path and divert flows southwards. Following the implementation of the NWRR Mainline, flows will return to the channel more quickly and no attenuation in the floodplain within the NWRR Mainline footprint or to the west of the structure will occur.

• The NWRR Mainline cuts across the Brampton Brook floodplain. The NWRR Mainline will reduce downstream flows from Brampton Brook. Peak runoff from this watercourse will be attenuated and potentially the peak will be delayed. The difference in runoff response times between the two watercourses, thought to be on average around 11.5 hours, means there is no risk of this exacerbating peak flows on the River Nene.

5.1.2. A flood management scheme has been developed iteratively, balancing the contradictory impacts of the Proposed Scheme (namely increases in attenuation provided by the Causeway and decreases in attenuation resulting from the NWRR Mainline) on downstream flows against the available capacity for floodplain compensation. The complexity of the hydraulic interactions across the whole scheme means that it is only feasible to demonstrate the impacts of the proposal and confirm changes in associated flood risk through the use of the hydraulic model.

5.1.3. The proposals detailed here represent the concept with which the various hydraulic effects of the Proposed Scheme will be mitigated against in the final design. This FRA confirms there is a viable hydraulic solution, however further iterations will be required through the detailed design, particularly associated with the floodplain compensation basin, flood mitigation bund and channel diversion as more information on the environmental constraints and ground conditions becomes available.

5.1.4. The flood mitigation proposals are shown in Figure 5-1 and summarised in Table 5-1. Cross sections through the floodplain compensation basin and flood mitigation bund are provided in Appendix D.


Figure 5-1 - Location of elements of flood mitigation measures referenced in Table 5-1

Table 5-1 - Summary of objective and effect of the various flood mitigation structures

ID Structure Location Objective and effect

1 The Causeway Across the River Nene floodplain to the south of the A5199

The objective of this structure is not to manage flood risk, it will however attenuate downstream flows during flood events. There is a corresponding increase in upstream water levels associated with this impact.

2 A single span bridge over the River Nene for the Causeway. The bridge will be a concrete span structure 19m wide with a crest level approximately 3m above bank top.

Over the River Nene beneath the Causeway.

To allow the in-channel River Nene flows to pass downstream. The structure will reduce the impact of the Proposed Scheme on upstream water levels. The capacity of this structure is greater than the existing A5199 bridge.

3 Flood relief culvert beneath the Causeway. The structure will have an opening equivalent to 8m wide and 0.9m high.

Eastern side of the drainage attenuation basin that sits to the north of the Causeway

To allow floodplain River Nene flows to pass downstream. The structures will reduce the impact of the Proposed Scheme on upstream water levels.



4 Flood relief culvert beneath the Causeway. The structure will have an opening equivalent to 6m wide and 0.8m high.

Western side of the drainage attenuation basin that sits to the north of the Causeway

The capacity of these structures is greater than the existing A5199 flood relief culverts.

5 Flood relief culvert beneath the NWRR Mainline. The structure will have an opening equivalent to 4m wide and 1.6m high.

Situated between the two drainage attenuation basins. This culvert is beside a drainage culvert on the NWRR Mainline

To maintain the connection of the River Nene with the floodplain on the west of the NWRR Mainline. The structures will allow the floodplain to the east of the Proposed Scheme to attenuate floodplain flows, albeit not as effectively as is currently the case.

6 Flood relief culvert beneath the NWRR Mainline. The structure will have an opening equivalent to 4m wide and 1.8m high.

Situated to the south of the drainage attenuation basins.

7 A drainage culvert beneath the NWRR Mainline. The culvert will be a 1.2m diameter pipe culvert.

Situated in the drainage ditch beneath the NWRR Mainline between the two drainage attenuation basins (and beside the flood relief culvert).

To convey runoff from the catchment to the west of the NWRR Mainline into the River Nene. The structure will convey the minor flows from the drainage channels to the west.

8 A drainage culvert beneath the NWRR Mainline. The culvert will be 1.2m wide by 0.8m high.

Situated on the existing overland flow route from the west at the northern end of the NWRR Mainline.

To convey runoff from the catchment to the west of the NWRR Mainline into the River Nene. The structure will attenuate the minor flows from the overland flow route to the west.

9 A culvert structure on Brampton Brook. The structure will sit on either bank with a soffit level in excess of the of the 200yr plus 65% flood event.

Over the Brampton Brook beneath the NWRR Mainline.

To maintain the connection of the River Nene with the Brampton Brook floodplain on the west of the NWRR Mainline. The structure will allow the Brampton Brook floodplain to attenuate floodplain flows. It will also reduce the impact of the Proposed Scheme on upstream water levels on Brampton Brook.

10 Flood compensation basin to the west of the NWRR Mainline to the north of Brampton Brook.

The basin will sit between the golf course and the NWRR Mainline. It will connect to the northern NWRR Mainline flood relief culvert on its eastern side.

The basin will increase the capacity of the floodplain to the west of the NWRR Mainline to offset the floodplain lost beneath the Scheme. The structure will allow the floodplain to the east of the NWRR to attenuate floodplain flows.

11 Large floodplain compensation basin and flood flow channel to the east of the NWRR Mainline and to the north of the railway line. Brampton Brook banks and area slightly north lowered to 64.1m AOD.

The basin sits within the 200yr+65% floodplain and will be drowned out during events of this scale. To maximise capacity, it is proposed to construct the base below the local floodplain level.

The basin will increase the capacity of the floodplain on the rising limb of the hydrograph to offset the floodplain lost beneath and to the west of the NWRR Mainline. The structure will collect floodplain flows that come out of bank to the north of Brampton Brook. These will be diverted along the eastern face of the NWRR Mainline, via a flood flow channel into the basin, this is assisted by lowering



the Brampton Brook banks and area immediately north to 64.1m AOD.

12 Flood mitigation bund to the east of the railway line within the River Nene floodplain.

The flood mitigation bund will be constructed within the floodplain of the River Nene and is within the current 200yr+65% extent. The structure crosses an existing section of ordinary watercourse.

The flood mitigation bund will restrict peak flow during the 200yr+65% hydrograph reducing peak flows and pass forward flow volume to offset the floodplain lost beneath and to the west of the NWRR Mainline. The structure will restrict floodplain flows from the River Nene immediately upstream from the structure location, this will result in an increase in maximum flood level upstream and a reduction in peak flow downstream.

13 Diversion of ordinary watercourse within the River Nene floodplain.

The channel diversion will be constructed within the floodplain of the River Nene and is within the current 200yr+65% extent.

The objective of the channel diversion is to ensure that normal flows within the ordinary watercourse can continue to flow towards the River Nene. The watercourse diversion avoids the need for a culvert structure through the flood mitigation bund. The watercourse diversion will need to be heavily planted, recreating the habitat observed in the upstream channel, following construction to ensure that flows are not increased towards the River Nene when compared with the baseline situation.

5.2 FLUVIAL FLOOD RISK

FLOOD RISK TO THE SCHEME

5.2.1. There are two elements to consider with respect to the fluvial flood risk: the risk of flooding to the Proposed Scheme itself and the implications of the Proposed Scheme on the surrounding flood risk.

5.2.2. The Proposed Scheme has been classified as essential infrastructure and the EA have confirmed that they require it to be flood free to the 200yr+65% event. The critical pinch points of Proposed Scheme have been extracted to confirm the minimum freeboard levels of the highway above the 200yr+65% event. The results are shown in Table 5-2. The review confirms that the Proposed Scheme will not flood in the 200yr+65% event. The critical location is at the eastern end of the Causeway where the available freeboard is 0.4m only. It is noted in this location that a design constraint is to tie into the existing A5199, and the A5199 does flood in the 200yr+65% event immediately to the north of the Causeway. It is for this reason that the freeboard on the Causeway is minimal in this location.

5.2.3. A review of the freeboard in the 1000yr+65% event is provided in the discussion on residual risks below. This confirms that the NWRR Mainline does not flood in this event either but that the Causeway is partially inundated but will remain passable for emergency services as flooding on southern carriageway is of shallow depth.


Table 5-2 – Comparison of the 200yr+65% event peak water levels and design levels at key

pinch points along the Proposed Scheme

Pinch Point Coordinates Scheme Level (m

AOD)

200yr+65% Flood Level

(m AOD)

Freeboard (m)

Lowest level on the NWRR Mainline. Located immediately downstream of the southern drainage attenuation basin.

473292, 264357

67.40 65.16 2.2

Upstream limit of the NWRR Mainline before the change in gradient up to the Causeway roundabout. Located 100m downstream of the Causeway

473362, 265142

68.07 66.15 1.9

Lowest level of the Causeway and highest water levels upstream of the Causeway. Located at the eastern limit of the Causeway.

473610, 265237

67.85 67.43 0.4

FLOOD RISK TO THE SURROUNDING AREA

5.2.4. The mitigation measures outlined in Section 5.1 have been developed to manage changes in fluvial flood risk resulting from the Proposed Scheme. Figure 5-2 and Figure 5-3 compare the pre and post 200yr +65% flood extents on the River Nene and Brampton Brook respectively.


Figure 5-2 - Comparison of 200yr+65% flood extent on the River Nene before and after the Proposed Scheme


Figure 5-3 - Comparison of 200yr+65% flood extent on Brampton Brook before and after the Proposed Scheme

Northampton North-West Relief Road PUBLIC | WSP Project No.: 70021598 May 2020 Northamptonshire County Council Page 35 of 48

5.2.5. Table 5-3 summarises the increases in water levels associated with the proposed Scheme at various locations upstream of the A5199.

Table 5-3 - Increases in flood depths upstream of A5199

Site 5yr 10yr 25yr 50yr 100yr 200yr65%CC

Floodplain upstream A5199 50mm 80mm 90mm 170mm

A5199 Not inundated 10mm 70mm 170mm

Residential Property Not inundated

Inundation around property 60mm

80mm 90mm 150mm

Brampton View Care Home Not inundated Boughton Mill Equestrian Centre

Not inundated

Boughton Mill Not inundated 150mm. Extent of inundation of property unclear.

5.2.6. Increases to water levels upstream of the A5199 occur at all events with a predicted increase in water levels of approximately 0.15m in the 200yr+65% event. There will therefore be an increase in flood risk to the properties on the left bank of the River Nene upstream of the A5199. At present it is thought that only the single residential property on the left bank immediately upstream of the A5199 is affected by this increase in water levels. The existing standard of protection to this property is estimated to be around the 50yr event at which point flood depths are in the region of 400mm at the property. For this event the predicted increase in water levels resulting from the Proposed Scheme is less than 0.1m.

5.2.7. Other properties that are shown as being marginally brought into the flood extent are those associated with Boughton Mill and the riding track for Boughton Mill Equestrian Centre. The model does not explicitly represent the various buildings associated with Boughton Mill and it appears unlikely that this marginal increase in modelled flood extent will result in the buildings flooding. Further information is required to confirm this assumption. Boughton Mill Equestrian Centre and the Care Home on the left bank are not affected.

5.2.8. Given the increase in water levels it is recognised that this will also equate to a marginal increase in flood frequency. Agreement will be needed from the residents and property owners located upstream of the A5199 for the Proposed Scheme to progress. Where agreement cannot be reached further consideration of mitigation measures for these properties will be required.

5.2.9. To assess the impacts of the proposals downstream of the site, flows have been extracted from the hydraulic model at the railway line. All flows return to the channel at this location making comparison between scenarios relatively simple. Figure 5-4 details the change in flow hydrograph between the existing and proposed scenarios. To facilitate an understanding of the role the floodplain compensation basin and flood mitigation bund will play in managing the changes to the floodplain hydraulics in this area, a hydrograph has also been included showing the impacts of the Proposed Scheme excluding the floodplain compensation basin and flood mitigation bund (collectively referred to as ‘attenuation’ in the figure).


Figure 5-4 - Changes to the downstream hydrograph resulting from the proposals

5.2.10. Figure 5-4 highlights the two periods during the design event where floodplain compensation is required. The first period is between 15 and 20 hrs where flows that would be attenuated within the Brampton Brook floodplain are diverted back towards the River Nene. The second period is towards the peak of the event where the attenuating effect of the Causeway is offset by the floodplain lost to the Proposed Scheme.

5.2.11. Figure 5-4 confirms that the use a floodplain compensation basin and flood mitigation bund in the area to the north of the railway line and to the west of the NWRR Mainline is sufficient to offset the impacts of the Proposed Scheme on downstream flood risk. It is however recognised that the proposals set out within this FRA are subject to further adjustments through the detailed design stage pending further data collection of ground conditions and feedback from environmental surveys. Further iterations of the design through the detailed design phase and final confirmation of the effectiveness of the proposed mitigation will be required on completion of the detailed design stage.

REVIEW OF COMPENSATORY STORAGE VOLUMES

5.2.12. As discussed and presented above, level for level compensation is not suitable for the Proposed Scheme due to the length and linear morphology of the NWRR Mainline embankment and the change to floodplain hydraulics that result from the overall Proposed Scheme proposals. However, to assist with understanding the volume of floodplain storage lost and gained following construction of the NWRR Mainline and flood compensation basin, a comparison exercise has been undertaken.

0

20

40

60

80

100

120

140

160

10 15 20 25 30 35

Flo

w (

m3

/s)

Duration (hrs)

Baseline Results Scheme excluding attenuation Scheme including attenuation

Operation of attenuation basin

Operation of embankment


5.2.13. The volume of floodplain storage available within the NWRR Mainline footprint has been estimated by comparing the Baseline and Proposed Digital Elevation Models (DEM) produced by the 2D hydraulic model with the Baseline 200yr+65% maximum water level grid. The baseline maximum water level has been chosen to provide a consist

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