appendix j.1 flood risk assessment › council...1.12 the site is shown on environment agency (ea)...

Environmental Statement - Appendices Great Billing Sand and Gravel Extraction and Restoration

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Appendix J.1 Flood Risk Assessment

Sand and Gravel Extraction, Great Billing, Northamptonshire Flood Risk Assessment

Sand and Gravel Extraction and Restoration, Great Billing, Northamptonshire Ref: CCE/F191/FRA-03 June 2017 For Anglian Water Services Ltd.


Document Review Sheet This document has been prepared for the use of Anglian Water Services Ltd. Its content should not be relied upon by others without the written authority of Cannon Consulting Engineers. If any unauthorised third party makes use of this report they do so at their own risk and Cannon Consulting Engineers owes them no duty of care or skill.

Reference Date Author Checked

CCE/F191/FRA-01 July 2012 JOH RBT

CCE/F191/FRA-02 November 2016 JOH/RTW RBT

CCE/F191/FRA-03 June 2017 JOH/RTW RBT


Contents 1. Introduction 2. Forms of Flooding 3. Flood Management 4. Water Management 5. Conclusions

Figures 1. Site Location Plan 2. Flood Zone & Reservoir Inundation Map 3. Groundwater Source Protection Zone Map 4. Surface Water Flooding Map

Appendices

Appendix A – Existing Site Topographical Survey Environment Agency Data Flood Outline Overlay Ground Investigation Data Appendix B – Proposed Site Development Phase Plans Indicative Groundwater Mitigation Measures Restoration Strategy Plan Typical Details – Proposed Temporary Crossing of Ecton Brook


Summary Table Site location Land to the east of Great Billing Water

Recycling Centre, Great Billing, Northampton, NN3 9BX

Grid reference 483224, 262177

Planning application Full

Existing site Site area

Greenfield – agricultural with a small area of woodland 130 hectares (approximately)

Proposed development Extraction of sand and gravel comprising excavations and associated temporary infrastructure and subsequent restoration

Flood Zone Surface water flooding Reservoir inundation zone

Zones 1, 2 and 3 Extensive areas of ‘Low’ – ‘High’ Yes

Other sources of flooding None assessed as significant Flood Management Water Management

Plant and Operations Area located within Flood Zone 1. No loss of flood storage during extraction. Restoration levels no higher than existing levels within areas susceptible to flooding. Restoration levels in the southernmost part of the site reduced by approximately 0.50m to 0.75m. Flood Response Plan for people and mobile plant working within the areas susceptible to flooding. Cut-off ditch and underdrain to manage any potential upstream groundwater mounding post-restoration. Runoff generated from the limited areas of proposed hardstanding to drain to the large Water Management ponds.

Local groundwater abstraction and application to recharge trenches / basins and stilling ponds.


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1.0 Introduction Context 1.1 This Flood Risk Assessment (FRA) has been prepared on behalf of Anglian Water Services Ltd

to support an application for the proposed extraction of sand and gravel and subsequent restoration on land to the east of the Great Billing Water Recycling Centre (WRC), Northamptonshire, NN3 9BX (refer to Figure 1 for a site location plan).

1.2 The site makes up the majority of the allocated 153 ha site referred to as “MA5: Earls Barton West Extension” in the Northamptonshire Minerals and Waste Development Framework (MWDF) Development Plan Document “Locations for Minerals Development” (adopted March 2011).

1.3 The site is also identified in Policy 4 of the Northamptonshire Minerals and Waste Local Plan (MWLP) Update: Submission Plan as a site for the provision of sand and gravel (site reference M4).

1.4 The development proposal is for a sand and gravel quarry that provides a supply of construction aggregates. The main use for the processed sand and gravel is in the manufacture of concrete and hence a concrete batching plant also forms part of the development proposal.

1.5 The phasing of the mineral extraction will allow for a continuous, rolling program of restoration. The restoration proposals involve returning areas of the site to agricultural use, grassland, and the creation of a riverside wetland corridor in the south of the site (refer to Appendix B). The concept design for the restoration is to increase the area of wetland habitat and to increase the flood storage capacity of the site.

1.6 It is envisaged that final restoration of the site will be completed after approximately 20 years (depending on detailed design, mineral demand, etc). It is anticipated that restoration of each phase will begin within a few months of its completion. An illustrative phasing plan showing how the extraction and restoration will progress is included in Appendix B.

1.7 This assessment takes account of the National Planning Policy Framework (NPPF) and associated Planning Practice Guidance (PPG) and the definitions of sources of flooding within the Flood and Water Management Act (FWMA) 2010.

Baseline Information

1.8 The site is approximately centred on Ordnance Survey grid reference 483317, 261910. The site lies to the east of the Great Billing WRC, between the A45 and the River Nene.


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1.9 The site extends to approximately 130 hectares, the majority of which is currently laid to agricultural land.

1.10 Topographical information (refer to Appendix A) shows that ground levels fall across the site

from approximately 55.5 m AOD in the north-west to approximately 48.0 m AOD in the south-east.

1.11 The site is located approximately 200 m to the north of the River Nene. The River Nene is

designated as a Main River and flows eastwards. Two tributaries converge with the River Nene in the vicinity of the site; the Ecton Brook (designated Main River) serving a catchment of approximately 5.75 km2 and, the Barton Brook (designated Main River) serving a catchment of approximately 12.2 km2.

1.12 The site is shown on Environment Agency (EA) mapping to be affected by Flood Zone 3 and 2 (refer to Figure 2 and EA data in Appendix A) and by surface water flooding (refer to Figure 4). Individuals and plant working within the flood prone areas of the site would be at risk in the event of a flood and therefore this impact will need to be managed. However, the nature of the proposals means that the impact of flooding on the majority of the proposed development site will be low.

1.13 The Ecton Brook flows south through the Ecton Brook Linear Park along the western edge of Great Billing, towards the Great Billing WRC. It passes beneath the A45, turns east and flows towards Commander’s Spinney where it turns south, flowing to the Nene. The banks of the channel are raised (embanked) as it passes through the site. The Ecton Brook also receives flows from two smaller tributaries which rise just to the east and south of Ecton village.

1.14 The Barton Brook flows south from Sywell Wood some 7.5 km to the north of the site. Its route takes it beneath the A45 from where it flows along the eastern boundary of the site and into the River Nene. Barton Brook is joined at Sywell Reservoir (approximately 2.7 km north of the site) by a tributary originating in the north-east of Sywell village.

1.15 The most notable surface water body in the area (aside from the various riverside bodies

associated with the River Nene located primarily to the south of the site) is the Sywell Reservoir to the north (approximately 2.7km). Other more distant bodies are identified on EA reservoir inundation mapping and these, and their potential impact on the site, are discussed in Section 2 of this report.

1.16 British Geological Survey (BGS) records indicate that the site in underlain with bedrock geology of Whitby Mudstone Formation and superficial deposits of Alluvium (Clay and Silt). Intrusive site investigations confirm this geology and also indicate a band of river terrace sands and gravels positioned between the Alluvium and Mudstone. The sand and gravel deposits in the Nene Valley are recognised as a major source of mineral supply; much of the adjacent valley has been, or will be, worked for minerals.


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1.17 The application site has been the subject of two intrusive surveys in 1991 and 2004, the latter included the installation of fourteen water monitoring boreholes (piezometers). Groundwater levels have been recorded nearly every month since November 2005 and are still being recorded. A total of 98 readings within each piezometer were made available at the time of writing this report. The groundwater levels at the site range from 1.0 to 4.0 m below ground level; the monitoring indicates a shallow gradient from north-west to south-east, suggesting that groundwater within the sands and gravels contribute to flows in the Nene.

1.18 EA mapping indicates the site lies outside of a Groundwater source protection zone (refer to Figure 3). EA Groundwater Vulnerability mapping identifies that the northern region of the site is located within a zone underlain by a Minor Aquifer with Intermediate vulnerability and the southern region is located within a zone underlain by a Minor Aquifer with High vulnerability (refer to Figure 3).


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2.0 Forms of Flooding Watercourses 2.1 The EA Flood Map for Planning shows that parts of the site are affected by the high probability

flood area (Flood Zone 3) and the medium probability flood area (Flood Zone 2).

2.2 The EA has provided modelled flood level data and flood mapping for the River Nene and Ecton Brook. The fluvial flood data is taken from the Middle Nene Model (dated 2012). The EA flood data shows that the floodplain on the site is attributable to the River Nene and Barton Brook as flood levels for Ecton Brook are retained ‘in-bank’ for all modelled flood events. A plan showing the EA flood outlines overlain onto a larger scale site plan is included in Appendix A.

2.3 The flood levels provided by the EA include a 20% increase in peak river flow to account for climate change. Current climate change guidance for water-compatible development with a proposed development lifetime of circa 20 years recommends the flows are increased by 10% to account for climate change. The levels provided by the EA are therefore appropriate.

Surface Water Flooding / Overland Flow

2.4 The EA Surface Water Flood Map (refer to Figure 4) highlights areas where runoff is likely to

flow and gather. The mapping shows four surface water flood risk categories from ‘Very Low’ to ‘High’. Commonly, the map indicates existing watercourses, ponds, topographical depressions, tracks and roads as surface water flood areas.

2.5 EA mapping indicates an extensive coverage of varying probabilities of surface water flood risk from ‘Low’ to ‘High’ across the site.

2.6 Closer inspection of the EA surface water flood depth mapping indicates the following: High risk The area of ‘High’ risk flood extent (i.e. greater than 3.3% annual exceedance probability of flooding in any given year) is confined primarily to the surrounding Main Rivers, within only isolated areas indicated across the site. The corresponding depth for the majority of the surface water flooding is indicated as below 300mm, with relatively minor areas indicated as between 300 to 900mm (refer to Figure 4). Medium risk The majority of the site lies outside of the area of ‘Medium’ probability (i.e. between 1% AEP and 3.3% annual exceedance probability chance of flooding in any given year) save for a number of isolated areas within the north-western region and south-eastern region of the


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site. The corresponding depth of surface water flooding is indicated as below 300mm, with deeper 300 to 900mm indicated in the south-eastern region of the site (refer to Figure 4). Low Risk EA mapping indicates an extensive coverage of the ‘Low’ probability flood zone (i.e. less than 1% annual exceedance probability but greater than or equal to 0.1% annual exceedance probability chance of flooding in any given year). The corresponding depth of surface water flooding is largely indicated as below 300mm, with deeper 300 to 900mm indicated in the south-eastern and central regions (adjacent the Ecton Brook) of the site (refer to Figure 4).

2.7 The pattern of surface water flooding on the site suggests that the floodwater is the result of one of two mechanisms: • Surface water being conveyed to the site via the Barton Brook and has therefore been

assessed in paragraphs 2.1 to 2.3; or • Local runoff from the site and land between the site and the A45 (road) pooling in the

low points.

Surface Water Sewers 2.8 There are no Anglian Water (AW) public surface water sewers in the vicinity of the site and

therefore this is not a source of flooding that is considered further.

Groundwater 2.9 Extensive groundwater monitoring shows levels at the application site to range from 1.0 to

4.0 m below ground level (bgl). The groundwater levels mean that groundwater will need to be managed during the extraction phase of the works. However, the nature of the proposals mean that the development is not considered to be adversely affected by any groundwater flooding.

Reservoirs/Lakes

2.10 The site is shown as lying wholly within a reservoir inundation area. The likelihood of reservoir failure is low so as to not be considered as a notable source of flooding for the proposed development.

Impact of Flooding on the Proposals

2.11 As discussed in Section 1, the nature of the development proposals are such that they would

be unlikely to be significantly affected by any floodwater from any source. Any overland flows will inherently be managed and adapt to the constant presence of water and its transportation around the site.


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3.0 Flood Management

3.1 Proposed temporary built development associated with the proposals is limited to the Plant and Operations Area. This will be sited in the west of the site in Flood Zone 1.

3.2 No formal compensatory flood storage is considered necessary as the raised structures (the temporary bunds) will not have a measurable or notable impact on flood levels in the area because of their minimal footprint when compared to the floodplain in the area. The bunds will also allow for the passage of floodwater between them to avoid having a notable impact on flood plain flows.

3.3 Notwithstanding the limited potential impact on flood levels associated with the bunds, the excavation works will provide ongoing indirect compensatory storage of a greater magnitude (refer to the phasing plans in Appendix B).

3.4 The proposed restoration involves lowering ground levels in the south of the site in order to

help create a wetland area running parallel to the River Nene. The levels in this location will be reduced by approximately 0.50m to 0.75m so this area becomes susceptible to flooding under a 1 in 2 year storm. The proposed lowering of ground levels will provide an increase in potential floodplain storage at the site.

3.5 Flood risk to people and mobile plant (excavation and haulage machinery) will be managed via a flood response plan which will enable plant to be stowed securely and any active areas of excavation / reclamation within the flood plain to be evacuated in advance of a predicted flood.

3.6 The flood response plan will be compiled by the site operator (to be established at a later stage, post-planning). The response plan will include (but will not be limited to): • Contact details of all staff and management to enable any off-site staff to be informed of

the situation ,e.g. a flood warning has been received, the site has been closed / part-closed and made safe, etc;

• The EA Flood Warning service and levels of flood ‘warnings’; • An evacuation procedure; • Details of where mobile plant should be placed/parked and secured, i.e. the Plant and

Operations Area; • Details of any necessary shut down procedures for in-situ processing equipment; • The location and contents of the on-site flood kit; and • Contact details of the relevant emergency services.

3.7 As the reclaimed material is likely to have a lower permeability that the extracted material, a

precautionary approach to groundwater flood management has been adopted. The mitigation measures propose a cut-off ditch and underdrain to be constructed close to the


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northern edge of the reclaimed material to manage any potential upstream groundwater mounding. This feature will serve to intercept groundwater should it reach 2m bgl and divert the water to the retained mineral “veins” that run north to south. There is also a proposed route to Wind Spinney and the reed bed area in the south (refer to the Indicative Groundwater Management Measure plan in Appendix B). Observations

3.8 The proposed temporary crossing of the Ecton Brook will be bridged with a clear span structure, or a suitably sized box culvert will be installed; all will be subject to an Environmental Permit with full details agreed with the EA. Typical details of a proposed temporary crossing are included in Appendix B.


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4.0 Water Management

4.1 Both runoff and an element of groundwater are proposed to be managed within the three stilling ponds to be provided in the Water Management Area in the west of the site. However, it should be noted that this proposal is only illustrative at this stage as the number and size of ponds is an operational matter that needs to be determined at the later planning /design stages so that sufficiently clean water can be returned to the processing plant.

4.2 Surface water runoff from the Plant and Operations Area will drain to the stilling ponds. 4.3 During the excavation and restoration process, groundwater will primarily be managed

through pumping to temporary recharge trenches / basins from where the water is allowed to disperse (return to groundwater) via infiltration. Some losses to evaporation will also occur.

4.4 The process of dewatering the working phase is widely carried out at sand and gravel quarries.

The general approach is to excavate a trench along the edge of the working phase to intercept the flow of groundwater. The inflow is then collected in channels which are connected to a sump from where the water is pumped to the above-mentioned recharge features. In some instances the abstracted water may need to be pumped to the three stilling ponds in the Water Management Area, or may need to be discharged to the local watercourse network.

4.5 If formal discharge to the Ecton Brook, Barton Brook, or River Nene of any water arising from the proposed development is required then an Environmental Permit will be sought from the Environment Agency. Any discharge will meet the relevant quantity and quality criteria.


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5.0 Conclusions 5.1 All built development (generally limited to the Plant and Operations Area) and raised levels

(bunds primarily) will be temporary in nature. It is envisaged that the final restoration of the site will be completed within 20 years of the commencement of activity at the site.

5.2 The proposed Plant and Operations Area will be sited in Flood Zone 1 (the low probability area). The working element of the proposals are not considered to be particularly vulnerable to flooding.

5.3 The temporary bunds will not have a measurable or notable impact on flood levels in the area

because of their minimal footprint when compared to the floodplain in the area. The bunds will allow for the passage of floodwater between them to avoid having a notable impact on flood plain flows. Notwithstanding the limited potential impact on flood levels associated with the bunds, the excavation works will provide ongoing indirect compensatory storage of a greater magnitude.

5.4 A flood response plan will be prepared at a later stage once the site operator is confirmed.

The response plan will detail measures to be taken to evacuate working areas of the site that are susceptible to flooding and to secure plant and machinery.

5.5 The proposed restoration involves lowering ground levels in the south of the site in order to

help create a wetland area running parallel to the River Nene. The proposed lowering of ground levels will provide an increase in potential floodplain storage at the site.

5.6 Surface water runoff from the Plant and Operations Area will be managed within the main

stilling ponds. 5.7 During the excavation and restoration process, groundwater will primarily be managed

through pumping to temporary recharge trenches / basins from where local infiltration will occur. In some instances the abstracted water may need to be pumped to the three stilling ponds in the Water Management Area, or may need to be discharged to the local watercourse network.

5.8 All water management facilities will be designed to manage both quality and quantity.


Figures

Scale:

Date:

Job Title: Project No:Drawing Title:

Client: Figure No:

Drawn By:

Cambridge House, Lanwades Business Park, Kentford, Newmarket, CB8 7PN

Tel: 01638 555107 Fax: 01638 555106

Email: [email protected] www.cannonce.co.uk

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13/07/2017

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GREAT BILLING, NORTHANTS F191

FIGURE 1ANGLIAN WATER SERVICES LIMITED

SITE LOCATION PLAN

Reproduced from Promap by permission of Ordnance Survey® on behalf of The Controller of Her Majesty’s Stationery Office. ©Crown copyright 2016. All rights reserved. Licence number 100020449.

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N

Scale:

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Report: Figure No:

Drawn By:

Cambridge House, Lanwades Business Park,Kentford, Newmarket, CB8 7PN

Tel: 01638 555107


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GREAT BILLING,NORTHAMPTONSHIRE

F191

FIGURE 2

FLOOD ZONE AND RESERVOIR INUNDATION MAPS

FLOOD RISK ASSESSMENT

SITE

FLOOD ZONE 3

FLOOD ZONE 2

KEY

MAIN RIVERS

EXTENT OF RESERVOIR FLOODING

AREAS BENEFITING FROM FLOOD DEFENCES

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N

© Crown copyright. All rights reserved. Environment Agency, 100026380, 2016.

RESERVOIR INUNDATION MAP

FLOOD ZONE MAP

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Report: Figure No:

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Tel: 01638 555107


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F191

FIGURE 3FLOOD RISK ASSESSMENT

GROUNDWATER SOURCE PROTECTION ZONE PLAN

SITE

INNER ZONE

OUTER ZONE

KEY

TOTAL CATCHMENT

SPECIAL INTEREST

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FIGURE 4FLOOD RISK ASSESSMENT

SURFACE WATER FLOODING PLAN

HIGH

MEDIUM

KEY

LOW

VERY LOW

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Appendix A – Existing Site Topographical Survey

Environment Agency Data Flood Outline Overlay

Ground Investigation Data

appendix j.1 flood risk assessment › council...1.12 the site is shown on environment agency (ea)...

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