WASTE SERVICES

Proposed Household Waste Recycling Centre (HWRC) Flood Risk Assessment

Project Reference: CA105369/FRA

March 2021

Designer Name: Mike Reed Delivery & Transformation Manager Waste Services County Council

Design Office Address: County Offices, Newland, Lincoln, LN1 1YL

Client: Lincolnshire County Council

Proposed Development: Construction of a Household Waste Recycling Centre (HWRC)

Site Address: Kirkby Lane, Tattershall Thorpe, LN4 4PD

OS Site Co-Ordinates: E 523151 N 360222

Date: March 2021

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DOCUMENT CONTROL

Tattershall Household Waste Recycling Centre CA105369/FRA

Client: Lincolnshire County Council (Waste Services) Document Title: Flood Risk Assessment

ORIGINAL Originated By: Name Signature

Dan Brook

Approved By: Name Signature

Mike Reed

REVISION 1 Originated By: Name Signature

Revised By: Name Signature

Approved By: Name Signature

REVISION 2 Originated By: Name Signature

Revised By: Name Signature

Approved By: Name Signature

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CONTENTS

Item Page

1.0 INTRODUCTION 7

1.1 Context 7 1.2 Structure of report 7

2.0 SITE DESCRIPTION 8

2.1 Location 8 2.2 Topography 8 2.3 Previous and existing land use 8 2.4 Hydrology 8 2.5 Hydrogeology 8 2.6 Proposed development 9

3.0 FLOOD RISK ANALYSIS 10

3.1 Surface water runoff 10 3.2 Existing sewers 11 3.3 Fluvial sources 11 3.4 Groundwater 11 3.5 Overland flow 11 3.6 Residual risk 12

4.0 PROPOSED DRAINAGE STRATEGY 13

4.1 Surface Water 13 4.1.1 Attenuation 13 4.1.2 Source Control 14 4.1.3 Climate Change 14 4.1.4 Pollution Control 14 4.1.5 Infiltration Technique 14 4.2 Existing sewers 15 4.3 Fluvial sources 15 4.4 Maintenance of drainage systems 15

5.0 CONCLUSION 16

6.0 REFERENCES 17

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APPENDICES

Appendix A 18

A1 Site Location Plan 18 A2 Indicative flood map for the site 19 A3 Proposed site layout 20 A4 Drainage authority sewer records 21 A5 Correspondence with Witham Third District IDB 22

Appendix B 23

B1 Greenfield runoff rate calculation 23 B2 Attenuation storage estimate 24

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1.0 Introduction

1.1 Context

Lincolnshire County Council (LCC) has a network of Household Waste Recycling Centres (HWRCs) located across the county of Lincolnshire. Out of the 11 sites which are open to the public 10 are owned by the authority. LCC are looking to replace the existing privately owned and operated site at when its service contract expires with a view at looking at medium to long term cost savings.

This Flood Risk Assessment (FRA) has been commissioned to accompany the Planning Application for a HWRC at Tattershall. The assessment has been carried out in accordance with the requirements of Planning Practice Guidance (PPG): Flood Risk and Coastal Change, and on the outcomes of discussions with the local water authority.

1.2 Structure of Report

Section 2 of this report provides a description of the site and the proposed development. Analysis of the potential sources of flooding on the site is presented in section 3 and section 4 discusses proposals for mitigating and managing flooding on the site. Proposals for reducing the risk of pollution to surface and groundwater sources are also discussed in section 4. Section 5 presents the conclusions and recommendations of the report which is followed by the appendices which contain supporting information.

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2.0 Site Description

2.1 Location

The site is closed landfill situated approximately 3km north-east of the village of Tattershall, Lincolnshire. The overall site is rectangular in shape and is bound by Kirkby Lane to the west, the Old River Bain to the east, farmland to the south and a former gravel pit to the north. Access to the site is via a gate off Kirkby Lane.

There are several lagoons across the site, each separated from one another by approx. 1m high bunds. A tarmac road runs around the site. The site is currently overgrown with vegetation.

The grid reference for the site is 523151, 306222. The site is 1.87 acres (0.758 hectares). A site location plan (see A1) is included in Appendix A.

2.2 Topography

The existing site is undulating, it consists of a circulatory road and several bunded dry lagoons. It is proposed to develop approx. a quarter of the site. Typical ground levels are:  Road: 9.400 - 9.950m  Bund: 10.000 - 10.500m  Lagoon: 9.300 - 9.500m  Perimeter: 8.500 - 9.500m

2.3 Previous and existing land use

The site is brownfield and was historically used as an LCC waste site, records show that it was operational between 1963 and 1992. This is backed up by the 1972 Ordnance Survey map which shows the waste site. It has since been left untouched for a number of years. The types of waste listed include household, commercial and liquids/sludge.

2.4 Hydrology

The site has an Internal Drainage Board (IDB) maintained watercourse running along its boundary to the north, which connects into the Old River Bain approx. 120m to the east. Reference to the Environment Agency’s (EA) web-based flood maps shows that the site lies predominantly in flood 1one 1, with a small area in flood zone 2 on the northern edge. The flood zone map (see A2) for the site is included in Appendix A.

2.5 Hydrogeology

The British Geological Survey 1:50,000 geology sheet for the area indicates the site to be situated on superficial Lower River Terrace Deposits. Areas surrounding the site have been or are currently worked for sand and gravel extraction. It is anticipated that the majority of the ground across the site will compose of made ground/fill material.

A ground investigation was undertaken by Lincs Lab in March 2018 (see accompanying report G40805). Several boreholes were undertaken to a depth of between 1.0m and 3.0m. The top 1m BGL was found to be made ground consisting of mainly loose sandy topsoil with some fragments of glass, plastic and metal. Below

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this level River Terrace Deposits were found consisting mainly of loose coarse grained sand. No ground water was found within the boreholes.

The samples were sent for chemical analysis and the results are included in the report, this shows the ground within the lagoons to be contaminated.

2.6 Proposed development

The proposed development comprises of an office/welfare building, a service road and a HGV manoeuvring area. The development, covering a total area of 7,577m2, will have an impermeable area of 4,170m2 comprising:

Roof area 57m2 On-site road, footways, and car parks 4113m2

A layout of the proposed development (see A3) is included in Appendix A.

The PPG seeks to direct development to the areas of lowest flood risk in preference to the areas of higher risk, which is known as the sequential test. As mentioned in section 2.4 the proposed development is predominantly within flood zone 1, with a small area in flood zone 2 on the northern edge. This is therefore defined as predominantly low risk to medium risk. The small area in flood zone 2 falls within the 9m easement of the watercourse to the north, which will be left undeveloped. Therefore for the purpose of this report it will continue to be deemed that the development is located in flood zone 1.

From 'Table 1: Flood Zones' of the PPG the proposed development is deemed to have a low probability of river and sea flooding i.e. less than 1 in 1000 annual probability. From 'Table 2: Flood Risk Vulnerability Classification' of the PPG the proposed development falls within the "More Vulnerable" category. As per 'Table 3: Flood Risk Vulnerability and Flood Zone Compatibility' of the PPG, the development is deemed appropriate.

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3.0 Flood Risk Analysis

This section presents an analysis of the risk posed by potential sources of flooding. The analysis assesses the likelihood of flooding and the severity of flooding from each of the potential flood sources. The following potential sources of flooding are considered:

 Surface water runoff,  Existing sewers,  Fluvial sources,  Groundwater, and  Overland flow.

3.1 Surface water runoff

The proposed development is brownfield and has a 676m2 area of existing road. The proposed development will remove/replace this area, with the total impervious area becoming 4,170m2. This will, inevitably, generate relatively large volumes of runoff compared to the existing conditions. The additional runoff generated as a result of the proposed development, if not properly managed, has the potential to cause flooding to the developed site, as well as to adjacent land. The probability of flooding from surface runoff, if not properly managed, is deemed to be high. Also, such flooding will pose a risk to adjacent land; hence the impact of such flooding is also classified as ‘high’.

To manage the flood risk posed by surface water runoff, the drainage system will aim to mimic the pre-development drainage regime as much as possible. Runoff from the developed site will have to be attenuated to ensure that discharge of runoff from the site does not exceed the predevelopment runoff rate, wherever possible. The pre- development runoff rates for the site have been calculated using the UKSuDS 'Greenfield runoff rate estimation for sites' tool based on the IH124 methodology. The results of the calculations are shown in Table 1 below and were based on the following parameters:

Catchment area: 0.758ha Percentage paved: 10% Average annual rainfall (SAAR): 587mm Soil: 0.300m Hydrological Region: Region 5

Table 1 Calculated Greenfield Runoff Rates Return Period (Years) 1 30 100

Greenfield Runoff Rate (l/s) 0.98 2.75 4.00

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The calculated Mean Annual Flood Flow (QBAR) for the site was 1.12l/s, which translates to a Greenfield runoff rate of 1.48l/sec/ha.

The output sheet for the Greenfield runoff rate estimation (see B1) is included in Appendix B.

3.2 Existing sewers

A search of Anglian Water's sewer records shows that there isn't a surface water or foul sewer in the vicinity of the proposed development. The nearest being approx. 2km to the south in /Tattershall. Therefore there is no risk of flooding from this source. Also it will not be feasible to connect into either of these, as they are both so far from the site.

Anglian Water sewer utility records are enclosed in A4 in Appendix A

3.3 Fluvial sources

The nearest major watercourse is the Old River Bain approx. 120m east of the proposed development. EA flood maps indicate that the site lies within flood zone 1, which has a less than 1 in 1000 annual probability of river or sea flooding (<0.01%).

In order to inform the process of risk assessment and site selection District Council produced a Strategic Flood Risk Assessment (SFRA) (March 2017). It found that 38% of the District is at risk from coastal flood risk, with additional risk over the whole District coming from surface water flooding i.e. from rivers, drains and localised flooding.

The site itself lies within the Witham Third District IDB area. To the north of the proposed development is the Tattershall Thorpe/Kirkby IDB maintained drain. This in turn discharges into the Old River Bain which is to the east of the site and is a Main River maintained by the EA.

3.4 Groundwater

Groundwater flooding is the emergence of groundwater at the ground surface. It can occur in locations where there are permeable strata on the ground surface or close to the ground surface. The potential for groundwater flooding is enhanced if the underlying strata are also permeable.

There was no groundwater encountered in the exploratory bore holes (1-3m deep). Upon checking the EA's Groundwater Vulnerability Map the likelihood of a pollutant reaching the groundwater is Medium-Low. Upon checking the EA's Groundwater Source Protection Zones the site is not within a groundwater protection zone. Therefore the risk of groundwater flooding is deemed low.

3.5 Overland flow

The proposed development borders a ditch/road to the west and IDB maintained drain to the north. To the east and south its immediate border will be the existing closed land fill site. Beyond this the surrounding land is gravel pits and farm land. As there are no historical records of overland flooding at the site, the risk of flooding posed by overflow is deemed minimal. The proposed development levels will be built up.

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3.6 Residual Risk

The main residual risk that remains for the proposed development is for rainfall events greater than the drainage design criteria affecting the proposed drainage system. On-site drainage elements will be designed in accordance with the relevant regulations and design guides to take account of overland flood flow routes and to divert any excess floodwater around and away from the proposed building thresholds and pedestrian routes.

To cater for the potential effects of climate change, a 20% increase in peak rainfall intensity will be considered and managed as part of the drainage strategy for the site.

A summary of the flood risk assessment is shown in Table 2.

Table 2 Risk assessment for proposed development Source of Likelihood Severity Risk* Action to reduce risk flooding

 Adopt site control. Surface runoff High High High  Provide on-site attenuation.  Incorporate flood paths and temporary storage for extreme rainfall events. Existing sewers Low Low Low -  Old River Bain Fluvial sources Low High High maintained by the Environment Agency.  Watercourse to the north maintained by IDB.  Site levels to be built up.  Sign up for Flood Warnings. Groundwater Low Low Low -

Overland flow Low Low Low -

*Risk rating if risk control measures are not put in place.

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4.0 Proposed Drainage Strategy

This section outlines the proposed drainage strategy for the site and measures for managing and mitigating the flood risk identified in section 3 of this report. The overall principle of the proposed drainage strategy is to minimise risk of flooding to the proposed development, whist at the same time to not exacerbate any existing flood risk associated with properties situated upstream or downstream of the site. The site will have segregated drainage systems for foul and surface water.

It is felt that the development provides wider sustainability benefits to the local community and surrounding area that outweigh the flood risk. As mentioned in the introduction this proposed HWRC is to replace the existing privately owned site at Kirkby on Bain when its service contract expires. The proposed development will provide continuity/guarantee of service for members of the public. The purpose of the facility is to temporarily store waste and recyclable material delivered from residents of Lincolnshire prior to the onward transportation in bulk carriers for recovery of materials, energy recovery and composting to reduce the amount of waste in the county of Lincolnshire for final disposal.

4.1 Surface Water

The PPG and Part H of the Building Regulations 2000, recommends the use of Sustainable Drainage Systems (SuDS) within new development sites wherever technically feasible. Careful design of SuDS features can ensure that the site surface water drainage closely reflects the natural hydrology and hydrogeology of the site.

The choice of SuDS techniques for the proposed development will follow the management train principles of:

- Prevention: to prevent or reduce pollution and runoff quantities. This may include good housekeeping, to prevent spills and leaks, storage in water butts, rainwater harvesting systems, and alternative roofs (i.e. green roofs).

- Source Control: control of runoff at or very near its source (such as the use of rainwater harvesting, pervious pavements or green roofs).

- Site Control: management of water from several sub-catchments (including routeing water from roofs and car parks to one large basin or pond for the whole site).

- Regional Control: management of runoff from several sites, typically in a detention pond or wetland.

Surface water runoff generated on the site will be collected in underground pipes, attenuated on site and control discharged into the existing Witham Third District IDB maintained watercourse to the north of the site.

4.1.1 Attenuation

Runoff generated by the proposed development will be attenuated to ensure the flood risk to downstream properties is not exacerbated. Approximately 0.417ha of impermeable area will require attenuation storage. This comprises all impermeable areas such as the site road and roof. The calculations are based on a rate of 1.48l/sec/ha and an area of approximately 0.758ha.

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However, a minimum discharge rate of 2l/s would accord with best practice and mitigate against future risk of blockages. Witham Third District IDB have stated that a Greenfield runoff rate of 1.4l/sec/ha would be the best achievable, however for any discharge above that a proportionate fee would be required.

The restrictive discharge from the site will include rainfall events up to and including the 100 year return period event plus a 20% allowance for climate change. Necessary consent/s shall be sort from Witham Third District IDB for discharge rate/structure and working within 9m of the top of bank.

Correspondence with the IDB is enclosed in A5 Appendix A.

To achieve the level of attenuation described above, maximum storage volumes of approx. 363m3 will be required at the site as per the UKSuDS 'Surface water storage requirements for sites' tool, output sheet (see B2) included in Appendix B. The required storage at the site will be provided by an above ground attenuation pond, fitted with a flow control device, exact details of this will be determined at the detailed design stage.

The proposed development will fall away from the highway boundary, so as to contain the surface water run off generated by the site. The area of the upgraded access to the development located within the highway will fall towards the existing road. Due to superelevation the runoff will be directed to the inside of the bend where there are existing kerb offlets.

4.1.2 Source control

No source control methods form part of the Flood Risk Assessment.

4.1.3 Climate change

It is recommended that a climate factor should be incorporated into the design of surface water drainage systems to cater for the effects of climate change. In line with this requirement, the design rainfall intensities will be increased by 20%.

4.1.4 Pollution control

To comply with pollution prevention good practice guidance, the surface water drainage system will be passed through an appropriate oil separator before entering the watercourse. Runoff from the road ways and vehicle turning areas will be passed through a Class I Bypass Separator.

In addition to the infrastructure proposed, should any potentially polluting liquid be accidentally spilled during site operations, it will be contained and soaked up using an appropriate proprietary material. This will be part of the sites operating procedure to ensure that there is no risk of pollution to the surface water drainage system.

4.1.5 Infiltration technique

Due to ground conditions discovered in the ground Investigation, infiltration techniques such as soakaways and filter drains have been discounted for this site.

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4.2 Existing sewers

Domestic waste water generated on site will be collected in a below ground pipe and discharged into an underground sealed cesspool. This will be emptied by a tanker and disposed of at a permitted site. Although not being adopted the foul drainage system will be designed to comply with Sewerage Sector Guidance recommendations and will be designed to achieve self-cleansing velocities.

The nature of trade effluent needing to be disposed of from the site is green waste leachate (waste water from grass cuttings and leaves), which drops from the green waste containers. It will be captured separately from other surface water sources, the volume of this will be minimal and will also be collected in the cesspool.

4.3 Fluvial sources

Due to the nature of the existing site the proposed development will generally be built up. The finished floor level of the welfare building will be raised as much as possible (taking into account the need for disabled access) so that it is the highest point of the proposed development. This design will protect the proposed development from fluvial flooding as far as reasonably possible.

4.4 Maintenance of Drainage Systems

On completion of the proposed development, Lincolnshire County Council (Waste Services) will be responsible for the maintenance of the HWRC. The maintenance regime will follow standard drainage maintenance procedures and will form part of the site operating manual.

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5.0 Conclusion

This report presents the results obtained from the Flood Risk Assessment carried out in accordance with the requirements and recommendations of Planning Practice Guidance: Flood Risk and Coastal Change and UKSuDS online tools. The risk of flooding from water courses, foul sewers, overland flow and groundwater have been assessed and deemed not to be significant.

This report concludes that the site for the proposed development is located in flood zone 1 on the EA flood maps. To minimise risk of flooding to the area all rivers maintained by the EA, and local watercourses maintained by Witham Third District IDB are in a good maintained condition. Therefore there is not a significant risk from fluvial flooding for 1 in 100 year flood event. Although the proposed land use is classified as "More Vulnerable" within the PPG it is still appropriate to develop in the area.

It is proposed that surface water runoff generated at the development site shall be attenuated with a controlled discharge into an existing IDB maintained drain to the north of the site. The runoff from the HWRC will be calculated with a restriction of 1.4l/s/ha for all rainfall events up to and including the 1 in 100 year storm plus a 20% allowance for climate change. All runoff from the site will pass through a Class I bypass separator before being discharged into the attenuation area.

Domestic wastewater and leachate shall be contained in a separate sealed underground storage tank and collected periodically by tanker.

From the above summary, it is concluded that:

i) there is a low risk of flooding to the proposed development that will be managed accordingly, and

ii) the proposed development does not increase the risk of flooding elsewhere.

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6.0 References

British Geological website. http://bgs.ac.uk

Communities and Local Government (2014) National Planning Policy Framework Planning Practice Guidance: Flood Risk and Coastal Change

Environment Agency website, www.environment-agency.gov.uk

Institute of Hydrology (1994). Flood Estimation for Small Catchments. Report No. 124.

National SUDS Working Group (2004). Interim Code of Practice for Sustainable Drainage Systems (ICoP SUDS).

Water UK (2019). Sewerage Sector Guidance.

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APPENDICES

Appendix A

A1 Site location plan

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A2 Indicative flood map for the site

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A3 Proposed Site Layout

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A4 Drainage authority sewer record

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A5 Correspondence with Witham Third District IDB

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Appendix B

B1 Greenfield runoff rate calculation

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B2 Attenuation storage estimate

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