Flood Risk Assessment for Residential and Industrial Developement at Aston Timber Yard, Sedge Fen, Lakenheath

FLOOD RISK ASSESSMENT FOR RESIDENTIAL AND INDUSTRIAL DEVELOPEMENT AT ASTON TIMBER YARD, SEDGE FEN, LAKENHEATH

FINAL REPORT

ECL0407/ACORUS RURAL PROPERTY SERVICES

DATE JANUARY 2021

ELLINGHAM CONSULTING LTD Email: [email protected]

DISCLAIMER This document has been prepared solely as a Flood Risk Assessment in support of a planning application for a proposed residential and commercial development at Sedge Fen, Lakenheath. “Ellingham Consulting Ltd” accepts no responsibility or liability whatsoever for any use made of this document other than by the client “Mr B Rutterford” for the purposes it was originally commissioned and prepared. All comments and opinions made are based upon information available to “Ellingham Consulting Ltd” during the necessary investigative process, and the conclusions and recommendations could, therefore, differ in the event of material subsequently being found erroneous, incomplete, or misleading. “Ellingham Consulting Ltd” therefore, accepts no liability should this prove to be the case. CONTENTS

1.0 INTRODUCTION

2.0 SITE LOCATION AND DESCRIPTION 2.1 Site Location 2.2 Existing Site 2.3 Proposed Development 2.4 Local Development Documents 2.5 Available Flood Risk Information

3.0 FLOOD RISK VULNERABILITY 3.1 The Sequential and Exception Test 3.2 Vulnerability Classification 3.3 Application of the Sequential Test

4.0 SITE SPECIFIC FLOOD RISK 4.1 Local Flood Assets 4.2 Sources of Flooding 4.3 Probability of Flooding 4.4 Historic Flooding 4.5 Climate Change and Residual Risk

5.0 FLOOD RISK MITIGATION 5.1 Summary of Risks 5.2 Mitigation Measures 5.3 Surface Water Management

6.0 CONCLUSIONS AND RECOMMENDATIONS

ATTACHMENT 1 – Site Location and Block Plan (Dwg RUTTERFORD S1 20-01)

ATTACHMENT 2 – Environment Agency Flood Risk Information

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1.0 INTRODUCTION

This Flood Risk Assessment has been prepared in accordance with National Planning Policy Framework (NPPF) and supporting planning practice guidance (PPG) on Flood Risk and Coastal Change.

In areas at risk of flooding or for sites of 1 hectare or more, developers are required to undertake a site-specific Flood Risk Assessment to accompany an application for planning permission. This Flood Risk Assessment has been produced on behalf of Mr B Rutterford in respect of a development that consists of two residential dwellings and two industrial units at Aston Timber Yard, Sedge Fen, Lakenheath.

A planning application for the proposed development is to be submitted by Acorus Rural Property Services.

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2.0 SITE LOCATION AND DESCRIPTION

2.1 Site Location

The site is situated at Aston Timber Yard, Sedgefen Road, Sedge Fen, Lakenheath, IP27 9LQ. The National Grid Reference of the site is 56637/28446.

The location of the site is shown on Figure 1.

Site Location

Figure 1 – Location Plan (© OpenStreetMap contributors)

2.2 Existing Site

The site is a timber yard on the northern side of Sedgefen Road in the village of Sedge Fen. The building within the site was destroyed by fire in 2019. There are agricultural buildings to the east and west of the site and agricultural land to the north. The area of development is approximately 0.70 hectares.

Environment Agency LIDAR information shows that ground levels within the site are typically between +0.8m OD and +0.4m OD with the highest levels in the southern part of the site and the lowest levels along the northern boundary. The carriageway level of Sedgefen Road alongside the site is +0.9m OD. The agricultural land to the north of the site is below datum.

The site is in the Burnt Fen Internal Drainage Board’s (IDB) area. Surface water at the site drains naturally through soakaway and hence to the IDB drain system. There is a riparian drain 200m north of the site and an IDB main drain approximately 400m east of the site.

The online British Geological Survey maps indicate that the site is likely to be underlain by Gault Formation mudstone. The bedrock is shown to be overlain with superficial deposits of Alluvium (silt and clay).

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2.3 Proposed Development

The proposed development consists of two residential dwellings and two industrial units to accompany the dwellings. The dwellings will have two storeys. The existing building at the site will be demolished. A Site Plan is provided in Attachment 1.

2.4 Local Development Documents

The Forest Heath Core Strategy Development Plan 2001-2026 adopted May 2010 provides the overall vision for the former Forest Heath District Council area. West Suffolk have commenced a review of the local plan which will set out the long term planning and land use policies for the area.

A Level 2 Strategic Flood Risk Assessment (SFRA) for the former Forest Heath District Council area was prepared in 2011.

The Suffolk Lead Local Flood Authority (LLFA) is a statutory consultee for surface drainage proposals for major developments. The Suffolk Flood Risk Management Partnership, which is made up of all the relevant Risk Management Authorities relevant to flood risk in Suffolk, have produced the Suffolk Flood Risk Management Strategy which is a tool to understand and manage flood risk.

2.5 Available Flood Risk Information

The site is located within Flood Zone 3, an area with a high probability of flooding benefitting from defences, of the Environment Agency Flood Maps for Planning as shown in Figure 2.

Figure 2 – Environment Agency Flood Map for Planning 3

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The Environment Agency Long Term Flood Risk Maps show that:

• the site has a low risk of flooding from rivers (annual probability between 0.1% and 1.0%); • the site has a very low risk of surface water flooding (annual probability less than 0.1%); and • the site is not within an area at risk of reservoir flooding.

Flood risk information provided by the Environment Agency for a nearby site is provided within Attachment 2.

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3.0 FLOOD RISK VULNERABILITY

3.1 The Sequential and Exception Test

The NPPF requires the application of a Sequential Test to ensure that new development is in areas with the lowest probability of flooding.

The Exception Test is a method to demonstrate and help ensure that flood risk to people and property will be managed, while allowing necessary development to go ahead in situations where suitable sites at lower risk of flooding are not available.

3.2 Vulnerability Classification

Table 2 of the PPG Flood Risk and Coastal Change categorises different types of uses and development according to their vulnerability to flood risk. The proposed develop includes residential dwellings and is therefore classified as ‘More Vulnerable’.

Table 3 of the PPG Flood Risk and Coastal Change sets out Flood Risk Vulnerability and flood zone ‘compatibility’. The site is in Flood Zone 3 and the development is ‘More Vulnerable’ therefore it is necessary to complete the Exception Test.

PPG Flood Risk and Coastal Change defines that the lifetime of the development in terms of flood risk and coastal change is 100 years.

3.3 Application of the Sequential and Exception Test

It is for the Local Planning Authority, taking advice from the Environment Agency as appropriate, to consider the Sequential Test.

Large parts of West Suffolk to the west of Lakenheath and Mildenhall are within Flood Zone 3. The defences along the Little Ouse River were not considered during the preparation of the Environment Agency Flood Maps. When the level of protection provided by the defences alongside the Little Ouse River is considered the site has a low risk of flooding. Consequently, alternative sites in Flood Zone 1 could not be considered to have a lower level of risk. As such the site is considered to pass the Sequential Test.

The Exception Test requires consideration of the wider sustainability benefits of a development and that the development would be safe and residual risks managed.

The Core Strategy Policy CS7 for Housing defines the housing need and distribution for new dwellings across the district. It estimates that 340 new homes a year are required over the period 2011 and 2031. The proposed development will contribute to this target and the provision of rural housing is considered to provide a sustainability benefit. In addition, the industrial units will support the local economy.

Section 5 of this Flood Risk Assessment describes the flood mitigation measures and the management of the residual risks, demonstrating that this development will be 5

ECL0407 Ellingham Consulting safe and not increase flood risk elsewhere. The development is considered to pass the Exception Test.

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4.0 SITE SPECIFIC FLOOD RISK

4.1 Local Flood Assets

The site is approximately 1.3km south west of the Little Ouse River. The Little Ouse River is an embanked main river. The Little Ouse has its confluence with the River Ely Ouse approximately 8km downstream of the site.

Both the River Ely Ouse and the Little Ouse River are the responsibility of the Environment Agency. There is a long-term strategy for the maintenance of the Environment Agency defences which is reviewed and updated every 5 years.

There is an extensive local drainage network managed by the Burnt Fen Internal Drainage Board. There is an IDB main drain approximately 400m east of the site. The site, and surrounding land, drains by gravity to Lark Pumping Station to discharge into the River Lark. The River Lark is also a tributary of the River Ely Ouse.

During the operation and maintenance of its pumping stations, associated structures, and channel systems, the IDB seeks to maintain a general standard capable of providing flood protection to its district. A routine maintenance programme is in place to ensure that the Board’s assets are commensurate with the standard of protection that is sought.

Current maintenance standards of the King’s Lynn IDB and the Environment Agency’s defences are generally good.

4.2 Sources of Flooding

The following potential sources of flooding have been identified during this assessment:

• blockages to the local drainage network in the vicinity of the site; • an event in the local drainage network that exceeds the standard of protection; • failure of Lark Pumping Station; and • overtopping and/or breaching of the Little Ouse River defences.

There are no artificial waterbodies in the immediate vicinity of the site. The Environment Agency Long Term flood maps show that the site is not at risk from a breach of a reservoir. The risk of flooding from Anglian Water assets including the foul sewerage system and water supply infrastructure is considered to be a residual risk.

4.3 Probability of Flooding

The probability of flooding associated with blockages in the Burnt Fen IDB drainage system is low due to the maintenance standards already achieved and managed by the IDB.

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The standard of drainage provided by Burnt Fen IDB is assessed at 2% annual probability (1 in 50 chance each year), compatible with the Department of the Environment, Food and Rural Affairs (DEFRA) target level of service for rural drainage and flood defence works. The risk associated with flooding due to events greater than 2% annual probability (1 in 50 chance each year) is lowered due to the Burnt Fen IDB main drains incorporating freeboard. This provides storage during events greater than 2% annual probability (1 in 50 chance each year).

The site benefits from defences on the Little Ouse River that provide protection during a fluvial event with a 1% annual probability (1 in 100 chance each year). The information provided by the Environment Agency show that the defence levels on the Little Ouse River typically exceed +3.7m OD. The flood level does not exceed +3.0m OD and therefore the risk of overtopping is very low.

The defended model flood outline provided by the Environment Agency, which is in Attachment 2, confirms that the site is not at risk during the 0.1% annual probability (1 in 1000 chance each year) event.

4.4 Historic Flooding

During the preparation of this assessment, no evidence was discovered of the site being flooded.

4.5 Climate Change and Residual Risk

Climate change is likely to impact the site through increased rainfall intensity and duration affecting the local drainage network and flood levels in the Little Ouse River.

A comparison of the defence level and flood level during a 1% annual probability (1 in 100 chance in any year) fluvial event with climate change in the Little Ouse shows that the risk of overtopping is very low. This is supported by the Environment Agency defended climate change model flood outlines which show that the site is not at risk during the 1% annual probability (1 in 100 chance each year) fluvial event with a 20% allowance for climate change.

In summary the existing systems and defence levels are appropriate for the design life of the development (i.e. 100 years).

The risk of a breach in the Little Ouse River is considered to be very low. A breach normally occurs because of water initially overtopping the defences and as can be seen from the levels above this is unlikely.

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5.0 FLOOD RISK MITIGATION

5.1 Summary of Risks

The probability of this development flooding from localised drainage systems is low due to the standards of the Burnt Fen IDB. Failure of Lark Pumping Station could lead to an increased level of risk at the site.

The probability of the site flooding from any Environment Agency system is less than 1% annual probability (1 in 100 chance each year) because of the standards of the existing flood defences. Over time there will be a gradual increase in risk to the site due to climate change. During the design life of the development it is not anticipated that the site would flood.

The development reduces the impermeable area at the site so there is no potential that flood risk will be increased elsewhere due to surface water.

5.2 Mitigation Measures

Based upon the information available during the preparation of this flood risk assessment, to mitigate against the remote risk of a flooding it is recommended that:

• the floor level of the dwellings is 0.3m above surrounding ground levels and there is 0.3m of flood resilient construction above finished floor level; and • it is recommended that the floor levels of the industrial units are above surrounding ground levels.

The developer should ensure that the eventual occupier of the dwelling is sufficiently aware of the risk of flooding, and the standard of the existing defences. The Environment Agency provides a Flood Warning Service which includes Flood Warning Codes and uses direct warning methods where the risks and impacts of flooding are high.

In addition to direct and indirect flood warnings, the Environment Agency operates a 24 hour a day Floodline Service providing advice and information on flooding. The occupier of the dwelling should register with the Floodline Direct Warnings Service to receive any future flood warnings.

Should there be a failure of Lark Pumping Station and conditions were such to put properties and land at risk of flooding, the Internal Drainage Board would take emergency action to maintain the drainage level of service by utilising temporary pumping equipment.

5.3 Surface Water Management

At present surface water runoff from the impermeable areas within the site drain to the edge of the surfaced areas. In general, surface water is discharged to the land to the north of the site where it infiltrates. 9

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The proposed development will reduce the impermeable area at the site. It is not proposed that there will be any significant changes to the drainage regime or approach to the disposal of surface water because of the development.

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6.0 CONCLUSIONS AND RECOMMENDATIONS

As a result of the assessment, the following conclusions have been reached.

• The proposed development consists of two residential dwellings and two industrial units to accompany the dwellings at Sedgefen Road, Sedge Fen, Lakenheath.

• The proposed development is in Flood Zone 3. Defences on the Little Ouse River provide protection during the 1% annual probability (1 in 100 chance each year) event with climate change.

• The site is located within an Internal Drainage Board catchment with a minimum standard of drainage of 2% annual probability (1 in 50 chance each year) which accords with DEFRA guidelines for rural development. The risk of flooding is lowered further due to the Burnt Fen IDB watercourses incorporating a significant freeboard. This provides storage during events greater than 2% annual probability (1 in 50 chance each year).

• It is recommended that the finished floor levels of the dwellings are 0.3m above the surrounding ground with 0.3m of flood resilient construction above finished floor level. The floor level within the industrial units should be above the surrounding ground levels.

• The development passes the Sequential Test and Exception Test and is therefore suitable for the proposed location.

ATTACHMENT 1

SITE LOCATION AND BLOCK PLAN (Dwg RUTTERFORD S1 20-01)

ATTACHMENT 2

ENVIRONMENT AGENCY FLOOD RISK INFORMATION

(The Product Data was provided following a request for a nearby site. The breach maps cover the proposed development but are centred on the nearby site.)

Ean/2020/188479 Product 4 Request Standard of Overall Statutory Upstream Downstream Protection Condition Defence Crest Crest Unique ID (Label) Easting Northing (Return Period) Grade Level Level Level Little Ouse d/s Hockwold 114968 567848 285072 1 in 100 (1%) 3 Not known 3.7 3.85 125198 567426 285136 1 in 100 (1%) 3 Not known 4.157 3.205 86619 567217 285281 Not known 4 Not known 3.3 3.6 6023 567081 285455 1 in 100 (1%) 3 Not known 4.157 4.005 114965 566809 285714 1 in 100 (1%) 4 Not known 3.9 3.8 113011 571797 280709 1 in 100 (1%) 3 Not known 5.158 4.894 98443 571457 282438 1 in 100 (1%) 3 Not known 5.515 5.245 113009 571579 284268 1 in 100 (1%) 3 Not known 4.888 5.425 113010 571046 283169 1 in 100 (1%) 3 Not known 5.132 4.727 117092 562309 279945 1 in 100 (1%) 3 Not known 3.69 3.929 117091 561854 281131 1 in 100 (1%) 3 Not known 3.929 3.833 117090 561457 282019 1 in 100 (1%) 3 Not known 3.833 3.672 Defended Model Flood Outlines NGR TL6604782588 Ref 188479 centred on Harris Farm Burnt Fen Turnpike Bury St Edmunds Suffolk IP28 8EB Created 27/10/2020

Environment Agency Bromholme Lane, / Brampton, Cambridgeshire PE28 4NE

Legend ^_ Site 5% AEP 1% AEP 0.1% AEP Main river

0 0.5 1 2 km

Information Model Tolerance - Any data included in this product is subject to a standard modelling tollerance of +/- 150mm.The fluvial models used ^_ to produce these results are intended for strategic scale use only.

Flood Risk Assessments - The Environment Agency recommends any Flood Risk Assessment should only consider these results in the context of a site specific assessment.

AEP - Annual Exceedance Probability - The probability of a given event occurring in any one year. Please note this is not a return period.

Strategic Scale Model - This model has been designed for catchment wide flood risk mapping. It should be noted that it was not created to produce flood levels for specific development sites within the catchment. Modelled outlines take into account catchment wide defences if present.

Copyright © Environment Agency copyright and / or database rights 2020. All rights reserved. © Crown Copyright and database right. All rights reserved. Environment Agency, 100026380, 2020. Contact Us: National Customer Contact Centre, PO Box 544, Rotherham, S60 1BY Tel: 03708 506 506 (Mon-Fri 8-6). Email: [email protected] NGR TL6604782588 Defended Climate Change Model Flood Outlines Ref 188479 centred on Harris Farm Burnt Fen Turnpike Bury St Edmunds Suffolk IP28 8EB Created 27/10/2020

Environment Agency Bromholme Lane, / Brampton, Cambridgeshire PE28 4NE

Legend ^_ Site 1% AEP + 20% climate change Main river

0 0.5 1 2 km

Information Model Tolerance - Any data included in this product is subject to a standard modelling ^_ tollerance of +/- 150mm.The fluvial models used to produce these results are intended for strategic scale use only.

Flood Risk Assessments - The Environment Agency recommends any Flood Risk Assessment should only consider these results in the context of a site specific assessment.

AEP - Annual Exceedance Probability - The probability of a given event occurring in any one year. Please note this is not a return period.