thames water utilities amp6 capital delivery

Thames Water Utilities AMP6 Capital Delivery

Project Code: C725

Project Name: AMP 6 Basingstoke STW – Advanced Sludge Treatment Plant

Document Name: Flood Risk Assessment

Document Number: C725-PRO-REP-10118

DOCUMENT HISTORY

Rev. Reason for Change Date Author Checked By Approved By

01 Final Draft 24/08/2015 MC/SP K Limbrick D Watt

C725 Amp 6 Basingstoke STW – Advanced Sludge Treatment Plant

Flood Risk Assessment

Rev. No. Date:21/08/2015

UNCONTROLLED WHEN PRINTED



Rev. No. Date:21/08/2015


Contents

EXECUTIVE SUMMARY 3

1. INTRODUCTION 5

1.1. Purpose 5

1.2. Proposed Development Site 5

1.3. Report Objectives 5

1.4. Available Information 5

2. DESCRIPTION OF THE EXISTING SITE 6

2.1. Basingstoke STW 6

3. DESCRIPTION OF THE PROPOSED DEVELOPMENT 7

3.1. Proposed Extension to Basingstoke STW 7

4. POTENTIAL SOURCES OF FLOOD RISK 9

4.1. Sources of Flooding 9

5. FLOOD ZONES 10

5.1. Planning Practice Guidance Flood Zones 10

6. FLOOD RISKS TO THE SITE 11

6.1. Sources of Flooding 11

6.2. Flooding from Rivers and the Sea 11

6.3. Flooding from Land 12

6.4. Flooding from Groundwater 12

6.5. Flooding from Sewers 13

6.6. Flooding from Reservoirs, Canals and Other Artificial Sources 13

7. PLANNING PRACTICE GUIDANCE 14

7.1. Proposed Land Use Classification 14

7.2. PPG Requirements 14

7.3. Sequential Test 14

8. CONSULTATION 15

8.1. Environment Agency Consultation 15

9. FLOOD RISK CONSIDERATIONS 15

9.1. Key Considerations 15

9.2. Remain Operational in Times of Flood 15



Rev. No. Date:21/08/2015


9.3. No Net Loss of Floodplain Storage 16

9.4. No Impediment to Flood Water Flows 16

9.5. Surface Water Runoff 16

9.6. Risks to People 18

9.7. Access to flood defence and management facilities 18

9.8. Cumulative Impact 19

10. CONCLUSIONS 19

Appendices 21

Appendix A – Site location and existing and proposed ground levels 23

Appendix B – The Proposed Development Layout 25

Appendix C ‐ Flood risk figures and Environment Agency correspondence 27

Appendix D – Channel and Hydraulic Structure Survey of the Petty’s Brook and River Loddon 29

Appendix E – Technical Note on Flood Estimation and Flood Modelling 31

Appendix F – DVD of InfoWorks ICM run Files 43

Appendix G – Surface Water Drainage Philosophy 45

Appendix H – Safe storage of excess surface water on site under a 1 in 200 year + climate change event 47



Rev. No. Date:21/08/2015 Page 3 of 35


EXECUTIVE SUMMARY

This document presents the findings of a Flood Risk Assessment (FRA) undertaken for a proposed extension to the Basingstoke Sewage Treatment Works (STW). The STW is a 117,000 Population Equivalent (PE) Activated Sludge Plant (ASP) located northeast of Basingstoke, approximately 1.7 km east of Chineham (Grid Ref. SU 67419 55225). Basingstoke STW is owned and operated by Thames Water Utilities Limited (TWUL).

The STW is shown to be partially located within the floodplains of the Petty’s Brook and the River Loddon according to the Environment Agency’s Flood Map. These watercourses are designated as ‘Main Rivers’ by the Environment Agency. As such, current planning regulations require that the planning application for the proposed extension is accompanied by an appropriate FRA.

This FRA has been prepared in accordance with the National Planning Policy Framework (NPPF) and supporting Planning Practice Guidance (PPG) and is intended to accompany a planning application for the development of:

A new advanced sludge treatment plant utilising thermal hydrolysis.

The proposed development is required in order to increase the capacity at Basingstoke STW to treat indigenous sludge and sludge arising from four satellite STWs. The new plant will allow the unsustainable practise of liming sludge at the satellite sites, prior to disposal, to cease and will generate sufficient sustainable electricity to export to the grid as well as satisfy the power demand of Basingstoke STW.

According to the Environment Agency’s Flood Map, Flood Zone 2 (Medium Probability) is shown to encroach onto the northern, eastern and southern boundaries of the proposed development site, with a small area of land along the northern and north‐eastern boundary also being affected by Flood Zone 3 (High Probability). The proposed development is classified as ‘Less Vulnerable’ under the PPG, which is considered an appropriate land use type for these two Flood Zones.

The Environment Agency does not currently hold any flood level information for the two watercourses that are shown to represent a potential flood risk to the peripheral areas of the site. During the preparation of this FRA, therefore, a detailed 1D/2D river model was constructed of the Petty’s Brook and the River Loddon with which to estimate flood level data for the areas surrounding the proposed development.

Using the output from this detailed river model, this FRA is able to confirm that the proposed development will meet the requirements of the NPPF/PPG. Specifically, this FRA has demonstrated that:

The location of the proposed advanced digestion plant is considered to have a Low Probability of flooding;

The proposed land use is appropriate for its associated risk of flooding and the development proposals will not involve an increase in the number of staff working within a high flood risk area;





The proposed development will remain operational in times of flooding over the duration of its intended lifetime;

The development proposals will not cause an adverse impact on floodplain storage or conveyance; and

The new build will not result in an increase in the rate of surface runoff leaving the site.

In summary, this FRA has demonstrated that the proposed development is appropriate for its location, will remain operational in times of flooding, and will not result in an increase in off‐site flood risk.





1. INTRODUCTION

1.1. Purpose

This document presents the findings of a Flood Risk Assessment (FRA) undertaken for a proposed extension to the Basingstoke Sewerage Treatment Works (STW).

The STW is a 117,000 Population Equivalent (PE) works located northeast of Basingstoke, approximately 1.7km east of Chineham (Grid Ref. SU 67419 55225). This FRA has been produced in accordance with the National Planning Policy Framework (NPPF) and supporting Planning Practice Guidance (PPG). The NPPF and PPG require that flood risk is taken into account at all stages of the planning process to ensure that new development is appropriate for its location, will remain safe, and will not increase flood risk elsewhere.

1.2. Proposed Development Site

The Basingstoke STW is located on Whitmarsh Lane, which itself is located perpendicular to the A33, approximately 1.7km east of Chineham, Hampshire. The proposed development will be located on a parcel of land to the east of the existing STW, within the existing STW boundary, as shown on Figure1, Appendix B.

1.3. Report Objectives

The objectives of this report are to a) quantify the flood risk to the site; b) demonstrate that the proposed development will remain operational during flood conditions over its intended lifetime taking climate change into consideration; and c) demonstrate that the proposed development will not increase flood risk elsewhere.

This FRA concentrates on the flood risk issues over the operational lifetime of the proposed development.

1.4. Available Information

This assessment is based on the following available information:

Site location and existing and proposed ground levels (Appendix A);

The proposed development layout (Appendix B);

Flood risk figures and Environment Agency correspondence (Appendix C);

DVD of channel and hydraulic structure survey of the Petty’s Brook and River Loddon (Appendix D);

Technical Note on flood estimation and flood modelling (Appendix E); and

DVD of InfoWorks ICM run files – (Appendix F);

Surface Water Drainage Philosophy (Appendix G); and





Safe storage of excess surface water on site under a 1 in 200 year + climate change event (Appendix H).

At the time of writing, MWH UK Ltd. is unaware of any previous FRA having been undertaken for the Basingstoke STW.

2. DESCRIPTION OF THE EXISTING SITE

2.1. Basingstoke STW

The existing sludge treatment at the Basingstoke STW consists of primary and secondary digestion and digested sludge dewatering, storage and disposal.

The STW site boundary is located approximately 10m to the south of Petty’s Brook and 250m north‐west of the River Loddon as shown in Appendix A (Figure 1).

The grid reference for the site is:

SU 67419 55225 (Easting 467419, Northing 155225)

The postal address for the site is:

Basingstoke STW,

Whitmarsh Lane,

Sherfield on Loddon,

Basingstoke,

Hampshire,

RG24 8LL.

The existing site layout is shown on dwg B466‐A1‐24001 Appendix B and a topographical survey showing the existing ground levels is included in Appendix A.

The site consists of areas of impermeable hard standing associated with the existing STW; storm water from these areas is drained back to the head of the works for treatment. The proposed extension will be located on a plot formerly used as the land treatment area for excess storm water leaving the STW. Excess storm water from the STW was once directed to this plot for land treatment before being discharged to the Petty’s Brook and the River Loddon.

As such, although this parcel of land appears to be currently undeveloped, it would have once received storm flows from the STW and consequently would have formed part of the waste water treatment process. As a result of investment in new technology by TWUL over time, this area of land is no longer required to form part of the treatment process in this way.





Existing ground levels throughout the parcel of land intended for the proposed development are shown in the topographical survey included in Appendix A. Existing ground levels in the centre‐western part of the site are approximately 64m AOD, and gradually grade down to a level of approximately 63.4m AOD in the centre‐eastern area of the site.

Immediately beyond the northern boundary of the proposed development plot, ground levels rise to between 65.2 to 64.5m AOD before grading down towards the channel of the Petty’s Brook. Immediately beyond the eastern boundary, existing levels are approximately 64.5 to 64m AOD. On the southern boundary, ground levels are between 64 and 63.6m AOD and gradually fall away to the River Loddon to the south. Ground levels vary throughout the existing STW to the west of the proposed development site although are generally between 65.5 and 64.8m AOD.

The responsibility for protecting the site infrastructure and ensuring its ongoing operation lies with TWUL.

3. DESCRIPTION OF THE PROPOSED DEVELOPMENT

3.1. Proposed Extension to Basingstoke STW

The proposed extension to Basingstoke STW involves the construction of a new advanced sludge treatment plant utilizing thermal hydrolysis. The new assets will be constructed on the undeveloped plot of land to the east of the existing STW, within the existing site boundary, that was formerly used to receive excess storm flows from the works.

The proposed development comprises the following details:

Imported sludge cake facility;

Imported liquid sludge facility;

Indigenous sludge collection and transfer;

Liquid sludge blending, buffering and screening;

Pre‐THP dewatering, polyelectrolyte storage, preparation and dosing for pre‐THP dewatering;

Dilution;

THP feed silo;

Thermal hydrolysis plant;

Sludge cooling and transfer to digesters;

Digesters, chemical dosing (antifoam and pH control);

Digested sludge collection and transfer;





Digested sludge dewatering and storage (existing);

Biogas storage and utilization;

Gas engines and composite boilers;

Ventilation and odour control plant;

Liquor collection and return;

Final effluent supply;

Potable water supply; and

Auxiliary fuel storage.

Further details of the above can be found in the Process Design Statement appended to the Basis of Design Report (C725 PRO REP 10048 Rev 03) and in the layout drawing of the proposed development included in Appendix B. Details of the proposed ground levels for the new advanced sludge treatment plant are shown in Appendix A.





4. POTENTIAL SOURCES OF FLOOD RISK

4.1. Sources of Flooding

Flooding can occur from a number of sources as shown in Table 4.1.

Table 4.1 Possible Sources of Flooding Identified in the PPG

Source Description

Flooding from rivers River flows which exceed the flow capacity of the river channel (or culverts) can cause flooding from rivers.

Flooding from the sea High tides and/or storm surges can cause flooding from the sea.

Flooding from land Intense rainfall that cannot soak into the ground or enter drainage systems can quickly run off the land and result in local flooding.

Flooding from groundwater Groundwater flooding occurs when water levels in the ground rise above surface elevations.

Flooding from sewers Sewer flooding can occur when piped systems are overwhelmed by heavy rainfall, when sewers become blocked or when sewers are of inadequate capacity.

Flooding from reservoirs, canals and other artificial sources

Non‐natural or artificial sources of flooding can include reservoirs, canals and lakes where water is retained above natural ground level.





5. FLOOD ZONES

5.1. Planning Practice Guidance Flood Zones

The PPG defines three flood zones as shown in Table 5.1.

Table 5.1 PPG Flood Zones

Flood Zone Return Period (Annual Exceedance Probability)

1 Low probability ‐ less than 1 in 1,000 year (<0.1%) for river or sea flooding.

2 Medium probability ‐ between 1 in 1,000 year (0.1%) and 1 in 100 year (1%) for river flooding or between 1 in 1,000 year (0.1%) and 1 in 200 year (0.5%) for sea flooding.

3a High probability ‐ 1 in 100 year (1%) or greater for river flooding or 1 in 200 year (0.5%) or greater for sea flooding.

3b The Functional Floodplain ‐ land where water has to flow or be stored in times of flood. There is not a strict definition of the annual probability of flooding in this zone, but the 1 in 20 year (5%) or greater return period should provide a starting point for consideration.

According to the Environment Agency’s Flood Map, Flood Zone 2 (Medium Probability) is shown to encroach onto the northern, eastern and southern boundaries of the proposed development site, with a small area of land along the northern and north‐eastern boundary also being affected by Flood Zone 3 (High Probability). The Environment Agency’s Flood Map for the Basingstoke STW is shown in Appendix C (Figure 2).





6. FLOOD RISKS TO THE SITE

6.1. Sources of Flooding

The Flood Maps, Appendix C Figures 2 to 5, provide a high level overview of the likely flood risk to a specific area or region, although the flood extents shown do not account for the presence of existing flood defences or the likely impacts of climate change. Each of these sources of flood risk is considered in more detail below.

6.2. Flooding from Rivers and the Sea

The Environment Agency’s Flood Map (Appendix C ‐ Figure 2) indicates that the most likely source of flooding to the proposed development is fluvial from the Petty’s Brook to the north and north‐east and the River Loddon to the east and southeast of the site.

The Environment Agency was contacted in March 2014 to determine whether flood risk data were available for these watercourses. At the time of this enquiry, the Environment Agency confirmed that it was unable to provide flood levels for these watercourses given the absence of detailed flood modelling for the area. This Environment Agency correspondence is provided in Appendix C.

Consequently, topographical survey data were obtained for the channels and hydraulic structures of the Petty’s Brook and the River Loddon. The results of this survey are included on a DVD in Appendix D. In addition, LiDAR Digital Terrain Model (DTM) data were also obtained for the floodplain areas of both watercourses.

The topographical survey and DTM data were combined to create a linked 1D‐2D hydraulic model of the Petty’s Brook and River Loddon in the area of the Basingstoke STW. The hydraulic assessment was undertaken using InfoWorks Integrated Catchment Modelling (ICM) software. Further information regarding the flood modelling element of this FRA is provided as a Technical Addendum in Appendix E; the model run files are included on a DVD in Appendix F.

The results from the detailed flood modelling study are presented in Appendix C (Figure 6a and Figure 6b) for the site and entire model domain respectively for the 1 in 1000 year return period event. This was the most extreme flood event included in the modelling study and can be considered equivalent to PPG Flood Zone 2 (Medium Probability). All land falling outside this detailed 1 in 1000 year flood outline can be considered to lie within PPG Flood Zone 1 (Low Probability).

Appendix C (Figure 6b) compares favourably with the Environment Agency’s Flood Map shown in Appendix C (Figure 2) in terms of the general shape and location of the areas thought to be at the highest risk of flooding. Appendix C (Figure 6b), however, shows less area at risk under the 1 in 1000 year event than shown in the Environment Agency mapping (i.e. Flood Zone 2 shown in Appendix C ‐ Figure 2). This is most likely because the Environment Agency mapping for this area is based on flood estimates generated using the FEH Rainfall Runoff method, which has now been replaced by other hydrological modelling techniques. The FEH Rainfall Runoff method is known to overestimate peak flows and the 1 in 1000 year peak flow estimates generated using this method are significantly higher than





those generated using the more accurate techniques used in this FRA (this is discussed further in Appendix E). As such, it is most likely for this reason that the 1 in 1000 year extent shown in Appendix C (Figure 6b) covers less area than the outline for Flood Zone 2 shown in Appendix C (Figure 2).

Appendix C (Figure 6a) indicates that the 1 in 1000 year flood outline does not encroach upon the site of the proposed development. Appendix C (Figure 6a) also displays the maximum flood levels under the 1 in 1000 year and 1 in 100 year (plus climate change) events at various locations surrounding the proposed development site.

Under the 1 in 1000 year event, peak flood levels along the northern boundary of the site range from 63.69 to 63.35m AOD. These levels then drop to 62.81 and 62.73m AOD within the floodplain of the Petty’s Brook and River Loddon to the east and southeast of the site respectively (Appendix C ‐ Figure 6a).

Under the 1 in 100 year (plus climate change) event, peak flood levels along the northern boundary of the site range from 63.34 to 62.82m AOD. These levels then drop to 62.43m to the southeast of the site.

As outlined in Section 2.1 and Appendix A, existing ground levels throughout the site of the proposed development range from 64 to 63.4m AOD, and ground levels rise beyond these levels to both the north and east before dropping towards the channels of the Petty’s Brook and the River Loddon. As such, it is not expected that the site of the proposed development would experience inundation under the 1 in 1000 year or the 1 in 100 year (plus climate change) return period flood events. Accordingly, this source of flood risk is not considered further.

6.3. Flooding from Land

Thames Water does not hold any records of the site being flooded by surface runoff. The Environment Agency Flood Map for Surface Water (Appendix C ‐ Figure 3) confirms that an area of Medium to High Surface Water flood risk is present in the north eastern and eastern area of the proposed development. This represents the potential for surface water ponding in the northeast and eastern areas of the site behind the higher ground that separates the site from the Petty’s Brook. The accumulation of surface water shown in Appendix C (Figure 3) will be accommodated as part of the surface water drainage strategy for the proposed advanced sludge treatment plant. This is explained further in Section 9.5 below.

6.4. Flooding from Groundwater

There is no historical record of flooding of the site by groundwater. This is not expected to change as a result of the proposed development. Groundwater flood risk information was obtained for the Basingstoke STW as part of this FRA. This information is shown in Appendix C (Figure 4). The groundwater flood risk data indicates that the site of the proposed development is not expected to be at significant risk. Accordingly, this source of flood risk is not considered further.





6.5. Flooding from Sewers

The site of the proposed development is undeveloped and was once used as the land treatment area for excess storm flows from the STW. As such, the existing site is not at risk of flooding from overwhelmed sewers. The storm water drainage strategy for the proposed development is described in Section 9.5 below. Accordingly, this source of flood risk is not considered further.

6.6. Flooding from Reservoirs, Canals and Other Artificial Sources

There are no artificial water bodies in the area surrounding the site and the Environment Agency Risk from Reservoirs Map (Appendix C ‐ Figure 5) indicates that there is no risk to the Basingstoke STW from reservoir failure. Accordingly, this source of flood risk is not considered further.





7. PLANNING PRACTICE GUIDANCE

7.1. Proposed Land Use Classification

The proposed extension to the existing STW is classified as ‘Less Vulnerable’ development under the PPG as the land use type falls under the definition of ‘Sewage treatment works, if adequate measures to control pollution and manage sewage during flooding events are in place’.

As per the PPG, this means that the proposed advanced sludge treatment plant is appropriate for location ‘in principle’ within Flood Zones 2 and 3 despite the fact that the results of the detailed flood modelling described above confirm that the site would not be at risk under these events.

7.2. PPG Requirements

Given the scale and nature of the proposed development, the key considerations of this FRA are to ensure that the new advanced sludge treatment plant will:

Remain operational in times of flooding;

Result in no net loss of floodplain storage;

Not impede flood water flows;

Not increase the volume and rate of surface water runoff leaving the site;

Not have an adverse effect on a watercourse, floodplain or its flood defences;

Not impede access to flood defence and management facilities; and

Ensure that the cumulative impact of such developments would not have a significant effect on local flood storage capacity or flood flows.

7.3. Sequential Test

The Sequential Test aims to steer new development to areas that have the lowest probability of flooding. Given that the proposed development would constitute an extension to an existing business premises, the PPG suggests a pragmatic approach to the Sequential Test in such circumstances: “in considering planning applications for extensions to existing business premises it might be impractical to suggest that there are more suitable alternative locations for that development elsewhere.” As such, the requirement to pass the Sequential Test is not considered further.





8. CONSULTATION

8.1. Environment Agency Consultation

The Environment Agency was consulted in March 2014 to determine the flood level information available for the area of interest. In response to this request, the Environment Agency confirmed that it does not hold detailed modelling for the Basingstoke STW area and as such is unable to provide modelled flood levels for use within this FRA (Environment Agency reference: WT15091). This Environment Agency correspondence is provided in Appendix C.

9. FLOOD RISK CONSIDERATIONS

9.1. Key Considerations

As described in Section 7.2, to meet (or exceed) the PPG requirements, ‘Less Vulnerable’ land use such as that proposed at Basingstoke STW will be considered suitable provided it can:

Remain operational in times of flooding;

Result in no net loss of floodplain storage;

Not impede flood water flows;

Not increase the volume and rate of surface water runoff leaving the site;

Not have an adverse effect on a watercourse, floodplain or its flood defences;

Not impede access to flood defence and management facilities; and

Ensure that the cumulative impact of such developments would not have a significant effect on local flood storage capacity or flood flows.

Each of these requirements is discussed in relation to the proposed development in Sections 9.2 to 9.8 below.

9.2. Remain Operational in Times of Flood

Fluvial Flooding

The proposed advanced sludge treatment plant consists of a number of smaller developments as shown in Appendix B. As confirmed by the results of the detailed flood modelling described in Section 6.2 above, the proposed development site would not be expected to experience flooding under the 1 in 1000 year event (Appendix C ‐ Figure 6a).

As such, the proposed advanced sludge treatment plant would be able to remain operational in times of fluvial flooding throughout the duration of its intended lifetime without the need for further measures to be put in place.





Pluvial Flooding

The surface water drainage strategy for the proposed advanced sludge treatment plant is summarised in Section 9.5 below and detailed in Appendix G. TWUL has a commitment to ensuring that the new advanced sludge treatment plant would remain operational under fluvial and pluvial flood events up to and including the 1 in 200 year (plus climate change) standard. Under this event, excess storm water would be safely contained on the surface of the site (Appendix H). All operational elements of the proposed advanced sludge treatment plant would be set above the level that would result from this accumulation of excess storm water.

As such, the proposed advanced sludge treatment plant would be able to remain operational in times of pluvial flooding throughout the duration of its intended lifetime without the need for further measures to be put in place.

9.3. No Net Loss of Floodplain Storage

The proposed development will be located on land that is not at risk under the 1 in 1000 year fluvial event and will not, therefore, result in any loss of floodplain storage.

9.4. No Impediment to Flood Water Flows

The hydraulic modelling carried out for this FRA demonstrates that the site falls outside the 1 in 1000 year fluvial flood extent.

The proposed advanced sludge treatment plant would not, therefore, result in any impediment to flood water flows.

9.5. Surface Water Runoff

The surface water drainage strategy for the proposed development is detailed in the Drainage Philosophy Report in Appendix G. The surface water generated across the new impermeable area associated with the proposed advanced sludge treatment plant will be considered to be ‘dirty’ and thus will need to be returned to the head of the STW for treatment before being discharged to the River Loddon.

The existing STW has a finite capacity for accepting storm flows and as such the additional storm runoff generated across the advanced sludge treatment plant site would be directed to the disused sludge drying beds shown in Appendix A (Figure 1). These drying beds will be made ready to act solely as storm water attenuation features over the lifetime of the proposed development.

The storm runoff would be pumped to these drying beds and temporarily stored within these features until capacity within the STW is restored following the storm event. The storm water contained within these drying beds would then discharge via gravity and an existing pumping station to the head of the STW for treatment before being discharged to the River Loddon.





Runoff from the advanced sludge treatment plant site up to the 1 in 30 year return period critical duration event will be pumped directly to these drying beds. The drying beds have a storage volume, which is more than adequate to accept the 1 in 30 year return period critical duration event storm volume from the advanced sludge treatment plant site.

TWUL has a commitment to ensuring that the new advanced sludge treatment plant would remain operational under fluvial and pluvial flood events up to and including the 1 in 200 year (plus climate change) standard.

The additional volume of water generated for storms between the 1 in 30 and 1 in 200 year (plus climate change) events will be safely stored on the surface of the advanced sludge treatment plant site as demonstrated in Appendix H. This water would then be subsequently pumped back to the drying beds once capacity within the pumping system is restored.

The operational components of the advanced sludge treatment plant site would be set above the level that could be reached by the accumulation of this storm runoff and the resultant surface water depths would not be expected to pose a risk to site operators

The attenuation process for the additional volume and rate of surface water generated by the proposed development can, therefore, be summarised as follows:

Storms up to the 1 in 30 year return period (plus climate change):

1. Direct pumping to the disused sludge drying beds;

2. Storage within the sludge drying beds until capacity within the STW is restored;

3. Discharge via gravity and an existing pumping station from the drying beds to the head of the STW;

4. Conveyance and storage within the treatment process of the STW; and

5. Discharge to the River Loddon.

Storms between the 1 in 30 year and the 1 in 200 year return period (plus climate change):

1. Direct pumping to the disused sludge drying beds for flows up to the 1 in 30 year rate;

2. Storage of the 1 in 30 year equivalent volume within the sludge drying beds until capacity within the STW is restored;

3. Storage of the difference between the 1 in 30 year and 1 in 200 year volumes safely on the surface of the advanced sludge treatment plant site (the proposed ground levels and layout will ensure that this volume is contained and would not be able to leave the site);

4. Discharge via gravity and an existing pumping station of 1 in 30 year equivalent volume from the drying beds to the head of the STW;





5. Direct pumping of the difference between the 1 in 30 year and 1 in 200 year volumes from the advanced sludge treatment plant site to the disused sludge drying beds;

6. Discharge via gravity and an existing pumping station of the difference between the 1 in 30 year and 1 in 200 year volumes from the disused sludge drying beds to the head of the STW once capacity is restored;

7. Conveyance and storage within the treatment process of the STW; and

8. Discharge to the River Loddon.

The proposed drainage strategy demonstrates that the additional storm water volume generated from the proposed advanced sludge treatment plant site will be stored on site for a sufficient length of time to ensure that an adverse impact on flood risk on the receiving watercourse is avoided.

For example, the water stored in the disused sludge drying beds will only be able to discharge via gravity to the head of the STW once capacity within the works has been restored. Given the residence time within the sewage catchment draining to the STW, this is only likely to occur a considerable length of time after the initial storm has ended.

Consequently, the drainage strategy will ensure that the proposed development will not lead to an increase in the rate of surface water leaving and site and will not, therefore, result in an increase in surface water flood risk elsewhere.

9.6. Risks to People

The proposed development will inevitably result in an increase in the number of TWUL staff present on site in order to operate and manage the new advanced sludge treatment plant.

As described in Section 6.2 above, however, the new advanced sludge treatment plant will be located on land that will not be affected under the 1 in 1000 year return period flood event. Storm water from the new advanced sludge treatment plant’s drainage system will be stored on the surface of the site under the 1 in 200 year (plus climate change) event although will not reach depths that would be unsafe for operational staff (Appendix H).

Consequently, the proposed development will not result in an increase in the number of people having to work in an area of high flood risk.

9.7. Access to flood defence and management facilities

The proposed development will take place within the confines of TWUL’s Basingstoke STW. There are no Environment Agency or Local Authority flood defence or management facilities within the Basingstoke STW. The proposed development will not, therefore, inhibit access to flood defence and management facilities.





9.8. Cumulative Impact

The proposed advanced sludge treatment plant site will be located outside the 1 in 1000 year floodplain and also the 1 in 100 year (plus climate change) ‘design’ floodplain. As such, the proposed works will not contribute to a cumulative impact on the ‘design’ floodplain.

Although the proposed advanced sludge treatment plant site will inevitably involve the creation of additional impermeable area, the drainage strategy described above will ensure that the rate of surface water leaving the site will not increase as a result. As such, the proposed development will not contribute to a cumulative impact on surface water flood risk.

10. CONCLUSIONS

This FRA has used detailed flood modelling to quantify the flood risk to the site of the proposed new advanced sludge treatment plant at Basingstoke STW. The detailed flood data have been used to confirm that the proposed advanced sludge treatment plant site will remain operational during times of flooding over the duration of its intended lifetime, whilst taking climate change into consideration.

This FRA has also demonstrated that the proposed advanced sludge treatment plant site will not have an adverse impact on floodplain storage or conveyance and will not result in an increase in surface water flood risk elsewhere.

The proposed land use type is considered suitable for its location and the development proposals will not increase the number of people working in or visiting an area of high flood risk.

The conclusions of this FRA are, therefore, that the proposed development:

Is appropriate for its location from a flood risk perspective;

Will remain operational in times of flooding;

Will not represent an increase in risk to people;

Will not increase flood risk elsewhere;

Will not restrict access to flood defence and management facilities; and

Will not contribute to a cumulative adverse flood risk impact.





Appendices





•





Appendix A – Site location and existing and proposed ground levels





Appendix B – The Proposed Development Layout





Appendix C ‐ Flood risk figures and Environment Agency correspondence





Appendix D – Channel and Hydraulic Structure Survey of the Petty’s Brook and River Loddon – (DVD available on request)





Appendix E – Technical Note on Flood Estimation and Flood Modelling





1.1 INTRODUCTION

A detailed hydraulic flood model was constructed to help inform this FRA. The model was a fully linked 1D‐2D model created using InfoWorks Integrated Catchment Modelling (ICM). The steps in the physical model build, flood flow estimation and model simulation are described in following sections.

1.2 RIVER CROSS ‐SECTION AND STRUCTURE DATA

A topographic river cross‐section survey was commissioned for the Petty’s Brook, the River Loddon and an unnamed ditch. The cross‐sections were surveyed at 60m intervals along the Petty’s Brook and at 120m intervals on the River Loddon, based on measured bank widths as per Environment Agency guidance for flood modelling. The locations of the surveyed river cross‐sections are indicated in red on Figure E.1.

Five bridges on the Petty’s Brook and one bridge on the River Loddon were also fully surveyed as these represent key hydraulic structures within the model domain.

Figure E.1: Surveyed Cross‐sections on Petty’s Brook, River Loddon and the Ditch





1.3 FLOOD FLOW ESTIMATION

A hydrological study was undertaken to calculate the design flows to use as input to the flood model. Firstly, the contributing FEH catchments for the Petty’s Brook and the River Loddon were identified. The River Loddon catchment also contained the ditch which merges with the Petty’s Brook. This ditch is represented explicitly in the model, and hence the River Loddon catchment was subdivided following the contours of the region to provide individual subcatchment boundaries. The catchment boundaries are shown in Figure E.2.

Figure E.2: Catchment boundaries





Catchment descriptors for the Petty’s Brook and the River Loddon were extracted using the FEH CD‐ROM Version 3. These are shown in Table E1 below.

Table E1: FEH Catchment Characteristics

Catchment Characteristics

Petty's Brook River Loddon

VERSION FEH CD‐ROM Version 3 FEH CD‐ROM Version 3

CATCHMENT SU 67950 55250 SU 67950 55150

CENTROID SU 64492 54623 SU 62758 50732

AREA 11.3 42.44

ALTBAR 89 114

ASPBAR 52 31

ASPVAR 0.36 0.32

BFIHOST 0.475 0.915

DPLBAR 4.87 8.66

DPSBAR 25.3 37.1

FARL 1 0.955

FPEXT 0.1059 0.0613

FPDBAR 0.585 0.351

FPLOC 0.827 0.849

LDP 10.92 14.04

PROPWET 0.35 0.35

RMED‐1H 11 10.9

RMED‐1D 31.7 34

RMED‐2D 40 42.3

SAAR 725 768

SAAR4170 736 803

SPRHOST 32.53 8.11

URBCONC1990 0.665 0.649

URBEXT1990 0.1243 0.1212

URBLOC1990 1.064 0.922

URBCONC2000 0.848 0.863

URBEXT2000 0.2405 0.2315

URBLOC2000 1.1 0.94

C ‐0.02877 ‐0.02714

D1 0.33557 0.36405

D2 0.27021 0.25934

D3 0.35308 0.37108

E 0.30388 0.29891

F 2.50818 2.50695

C(1 km) ‐0.029 ‐0.029

D1(1 km) 0.328 0.328

D2(1 km) 0.271 0.271

D3(1 km) 0.335 0.335

E(1 km) 0.306 0.306

F(1 km) 2.502 2.502





The catchment characteristics were reviewed to identify the most appropriate flood estimation procedure to use to calculate the design flows to be used in the model. Three methods were compared:

- FEH Rainfall‐Runoff;

- ReFH (Revitalised Flood Hydrograph); and

- FEH Statistical (i.e. WINFAP v3).

The FEH rainfall‐runoff method was discarded because it is considered redundant in most situations except for reservoir safety work and in pumped catchments.

The ReFH method has superseded the FEH rainfall‐runoff method and tends to give results which are more consistent with the FEH statistical method. But as with other rainfall‐runoff methods, ReFH struggles with permeable catchments (BFIHOST > 0.65) where it gives unrealistic estimates of losses. Furthermore, as recommended in the Environment Agency’s Flood Estimation Guidelines (2012), ReFH is not deemed appropriate for heavily urbanised catchments (URBEXT > 0.125). The URBEXT2000 values for both the Petty’s Brook and River Loddon catchments are greater than 0.125 and BFIHOST for the River Loddon is also in excess of 0.65. As such, the ReFH method was also discounted.

WINFAP is a statistical method which uses observed data from gauging stations in catchments similar to the study catchment. Pooling groups are created to provide the required 500 year of peak flow data. These data are used to calculate QMED (median annual flood flow), and based on QMED the peak flow for required return periods is generated.

In this study, the WINFAP statistical method is preferred over the ReFH method because:

1) The Loddon catchment is highly permeable (BFIHOST > 0.65)

2) Both the catchments are heavily urbanised (URBEXT2000 > 0.125)

To calculate the flood peaks the WINFAP‐FEH 3 software (version 3.3.4) was used. The catchment descriptor files (*.cd3) extracted from the FEH CD‐ROM Version 3 were imported into WINFAP separately for the Petty’s Brook and the River Loddon. Pooling groups were then created for the Petty’s Brook and the River Loddon.

1.3.1 Petty’sBrook

The pooling group for the Petty’s Brook is presented in Table E2.





Table E2 ‐ Petty’s Brook Pooling Group

Station Distance(km)

Years of data

QMED (m3s‐1) L‐CV L‐SKEW Discordancy

26802 (Gypsey Race @ Kirby Grindalythe) 0.97 13 0.109 0.261 0.199 0.535

25019 (Leven @ Easby) 1.079 34 5.538 0.347 0.394 0.796

27051 (Crimple @ Burn Bridge) 1.175 40 4.539 0.222 0.149 0.941

203046 (Rathmore Burn @ Rathmore Bridge) 1.245 30 10.934 0.136 0.091 1.126

20002 (West Peffer Burn @ Luffness) 1.246 41 3.299 0.292 0.015 1.937

27010 (Hodge Beck @ Bransdale Weir) 1.422 41 9.42 0.224 0.293 0.537

44008 (South Winterbourne @ Winterbourne Steepleton) 1.448 33 0.42 0.395 0.332 1.043

36010 (Bumpstead Brook @ Broad Green) 1.478 45 6.759 0.418 0.228 1.919

27073 (Brompton Beck @ Snainton Ings) 1.543 32 0.813 0.197 ‐0.022 0.771

45816 (Haddeo @ Upton) 1.605 19 3.456 0.324 0.434 0.863

22003 (Usway Burn @ Shillmoor) 1.617 26 19.22 0.303 0.303 1.284

47022 (Tory Brook @ Newnham Park) 1.624 19 7.331 0.257 0.071 0.579

72014 (Conder @ Galgate) 1.645 45 17.703 0.193 0.059 0.922

41020 (Bevern Stream @ Clappers Bridge) 1.677 43 13.49 0.214 0.208 0.871

73015 (Keer @ High Keer Weir) 1.678 21 12.239 0.156 0.001 0.797

28033 (Dove @ Hollinsclough) 1.689 33 4.666 0.266 0.415 1.079

Total 515

Weighted means 515 0.263 0.198

Gauging station 49006 Camel at Camelford was removed from the pool as it only had 6 years of data, which is considered to be a short record for statistical analysis. The total number of years of data for analysis is 515. The Goodness‐of‐fit test showed that the Generalised Logistic (GL) Distribution method gave the best fit.

The estimation of QMED can be carried out using catchment descriptors or using a donor station (the Data Transfer method). As per the guidance in WINFAP‐FEH 3 (2009) the use of the Data Transfer method to improve the estimate of QMED is not recommended in heavily urbanised catchments such as Petty’s Brook and the River Loddon (URBEXT2000 > 0.03). Hence QMED was estimated using catchment descriptors and an Urban Adjustment Factor (UAF) using the following equation.

QMED = UAF x QMEDrural ‐ Equation 1

It should also be noted that the Environment Agency Flood Estimation Guidelines (2012) indicate that the Statistical method (QMEDrural) with urban adjustment can be used to estimate extreme event flows for urbanised catchments in which sewer flow can be neglected.

The QMEDrural based on catchment descriptors is 2.464 m3/s. The UAF for Petty’s Brook is 1.411 and based on that the estimated QMED is 3.476 m3/s.

Even though the Donor Station method is not recommended for the study catchments, QMED for Petty’s Brook was also calculated using a range of donor stations for comparison,





which all returned lower estimates, confirming that QMED based on catchment descriptors with urban adjustment is conservative for flood peak estimation.

The peaks for various return periods in Petty’s Brook are shown in Table E3.

Table E3 – Petty’s Brook ‐ Peak flows using WINFAP

Return Period(Years)

GL ‐ Peak flow (m3/s)

2 3.476

5 4.777

10 5.764

25 7.245

50 8.564

100 10.104

200 11.912

500 14.804

1000 17.451

1.3.2 RiverLoddon

Following the same procedure as for Petty’s Brook, a default pooling group was created for the River Loddon catchment (Table E4). In this case, all the gauging stations have a sufficient period of data for statistical analysis. The Goodness‐of‐fit test showed that the Generalised Extreme Value (GEV) distribution method gives the best fit for the River Loddon catchment.





Table E4 ‐ River Loddon Pooling group

Station Distance(km)

Years of data

QMED (m3s‐1) L‐CV L‐SKEW Discordancy

53017 (Boyd @ Bitton) 0.353 39 13.073 0.243 0.112 0.016

41022 (Lod @ Halfway Bridge) 0.363 39 16.044 0.287 0.214 0.737

39033 (Winterbourne Stream @ Bagnor) 0.458 50 0.393 0.336 0.369 2.066

41020 (Bevern Stream @ Clappers Bridge) 0.471 43 13.49 0.214 0.208 1.134

28058 (Henmore Brook @ Ashbourne) 0.495 14 9.006 0.168 ‐0.102 1.391

42011 (Hamble @ Frogmill) 0.503 40 8.028 0.159 0.013 0.996

30004 (Lymn @ Partney Mill) 0.556 50 6.778 0.236 0.059 0.14

24007 (Browney @ Lanchester) 0.602 15 10.981 0.222 0.212 1.948

44003 (Asker @ Bridport) 0.621 30 14.636 0.253 0.221 0.427

36004 (Chad Brook @ Long Melford) 0.63 45 4.938 0.306 0.199 0.617

43806 (Wylye @ Brixton Deverill) 0.637 21 1.914 0.383 0.222 1.408

20006 (Biel Water @ Belton House) 0.663 28 11.748 0.375 0.128 1.839

33054 (Babingley @ Castle Rising) 0.7 36 1.129 0.214 0.069 0.407

26803 (Water Forlornes @ Driffield) 0.703 13 0.684 0.215 0.069 0.918

26003 (Foston Beck @ Foston Mill) 0.736 52 1.739 0.243 ‐0.015 0.955

Total 515

Weighted means 0.258 0.138

As with Petty’s Brook, the QMED was estimated using the Equation‐1 above. The QMEDrural for the River Loddon based on catchment descriptors is 1.14 m3/s. The UAF for River Loddon is 3.986 and based on that the estimated QMED is 4.544 m3/s. As with Petty’s Brook, a comparison of QMED calculated using a range of donor stations shows that the catchment descriptor‐based QMED estimate with urban adjustment is more conservative for flood peak estimation.

Peak flows for various return periods in the River Loddon catchment are presented in Table E5.

Table E5 – River Loddon ‐ Peak flows using WINFAP

Return Period(Years)

GEV ‐ Peak flow (m3/s)

2 4.543

5 6.339

10 7.54

25 9.07

50 10.214

100 11.358

200 12.506

500 14.032

1000 15.194





1.3.3 100yrPeakFlowAdjustments

As noted previously, part of the River Loddon FEH catchment drains to a ditch. A small part of the total flow from this catchment was assigned to the ditch based on its catchment area. The catchment area of the River Loddon at this location is 42.44km2. The catchment area of the ditch was measured to be 0.4km2. As such the catchment area of the River Loddon was reduced by this amount to allow the ditch to represent a stand‐alone catchment area in the flood model. The flood flow estimates for the River Loddon were pro‐rated accordingly so that the 1 in 100 year (plus climate change) flow on the ditch was 0.128m3s‐1.

In addition to that, 3 no. 600 mm culverts drain into this ditch. The total pipe capacity was calculated for these inflows (1.668 m3/s) and this flow was added to proportional catchment flow for the ditch to create a peak flow of 1.796m3s‐1. It was deemed conservative to include this as a constant flow input throughout the model simulation period.

As a final step the 100yr WINFAP peak flows were increased by 20% in Table E7 to allow for climate change.

Table E7 – WINFAP Peak flows – 100 year plus climate change

Method Petty's Brook 100yr plus

climate change River Loddon 100yr plus

climate change Ditch 100yr plus climate change

Peak flow (m3/s) Peak flow (m3/s) Peak flow (m3/s)

WINFAP Statistical 12.125 13.501 1.796

1.3.4 FlowHydrographs

Hydrographs for the flow boundaries were generated using the ReFH method and these were then scaled to the peak flows derived from the statistical method described above. These hydrographs were for as inputs to the hydrodynamic flood model with the peak flow for the ditch remaining as a constant throughout the simulation period.

The flow hydrographs for 1 in 100 year plus climate change scenario are shown in Figure E3.





Figure E3 – Flow hydrographs for Petty’s Brook, River Loddon and the Ditch (1 in 100yr+CC)

1.4 FLOOD MODELLING USING INFOWORKS ICM

InfoWorks ICM is a fully integrated one‐dimensional (1D) and two‐dimensional (2D) hydrodynamic software package used to carry out hydraulic and water quality simulations. ICM models provide a considerably more realistic representation of the hydraulic capacity of river channels and floodplains when compared with the high‐level flood risk mapping data used to create the Environment Agency’s Flood Map which – by necessity – does not use surveyed river channel information or provide any representation of hydraulic structures.

Since there is no river model available for the watercourses around the Basingstoke STW, a new ICM 1D‐2D linked model was constructed using surveyed information of the river channel and associated hydraulic structures. A 1D‐2D linked hydraulic model is capable of accurately representing floodplain processes. 2D hydraulic modelling works by routing water over a grid that represents the topography of the floodplain and any physical structures contained therein.

The surveyed cross‐sections were imported into the model to create a 1D river network. The modelled lengths are: 2300m of Petty’s Brook above the confluence with the River Loddon; 1430m of the River Loddon; and 878m of the unnamed ditch above the confluence with Petty’s Brook. The river cross‐sections were joined using river reaches and the associated structures. The model contains six hydraulic structures in the form of bridges.

LiDAR Digital Terrain Model (DTM) data was acquired with which to build the 2D grid used to represent the floodplain around the Basingstoke STW. The DTM data itself was provided at 1m by 1m grid resolution which allowed the ICM model to represent the floodplain by

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00

Flow (m

3/s)

Time (hrs)

Flow Hydrograph ‐ 1 in 100yr plus climate change

Petty's Brook River Loddon Ditch





assigning an elevation value to each approximately 1m2 triangular element of the grid at the area of interest.

The 1D and 2D domains were linked with the help of bank lines. These bank lines act as irregular weirs and once the water level in the 1D domain exceeds the cross‐section bank level, it spills onto the 2D grid using the irregular weir equation.

The hydraulic properties of the floodplain were assigned in the ICM model using standard Manning’s n roughness values. A global Manning’s n value of 0.03 was applied throughout the floodplain. For the river cross‐sections a value of 0.03 was applied to the centre of the channel and 0.05 to the sides. A Manning’s n sensitivity test of +/‐ 20% was undertaken although the resultant water levels were not found to be particularly sensitive to these changes.

The input hydrographs used in the ICM model were the 1 in 100 year hydrographs including 20% uplift to allow for the impact of climate change in addition to the 1 in 1000 year hydrographs (not shown above). These were assigned as the upstream boundaries to the Petty’s Brook, the River Loddon and the ditch. At the downstream end of the model, an outfall node is applied where water leaves the 1D system without any restriction and enters the 2D system. The model extends approximately 790m downstream of the STW to allow for any backwater effect on the results.

The simulation outline for the 1 in 1000 year event and flood level data for both this event and the 1 in 100 year (plus climate change event) are presented in Appendix C (Figures 6a and 6b).

1.4.1 1in1000yearestimatesusingtheFEHRainfallRunoffmethod

Appendix C (Figure 6b) compares favourably with the Environment Agency’s Flood Map shown in Appendix C (Figure 2) in terms of the general shape and location of the areas thought to be at the highest risk of flooding. Appendix C (Figure 6b), however, shows less area at risk under the 1 in 1000 year event than shown in the Environment Agency mapping (i.e. Flood Zone 2 shown in Appendix C ‐ Figure 2). This is most likely because the Environment Agency mapping for this area is based on flood estimates generated using the FEH Rainfall Runoff method, which has now been replaced by other hydrological modelling techniques. The 1 in 1000 year peak flow estimates for the Petty’s Brook and the River Loddon generated using the FEH Rainfall Runoff method are shown in Table E8 below. These 1 in 1000 year flow estimates are considerably higher than those for the Petty’s Brook and River Loddon shown in Tables E3 and E5 respectively. As such, it is most likely for this reason that the 1 in 1000 year extent shown in Appendix C (Figure 6b) covers less area than the outline for Flood Zone 2 shown in (Appendix C) Figure 2.





Table E8 – 1 in 1000 year flow estimates generated using the FEH Rainfall Runoff method.

FEH Rainfall Runoff method: Peak flow (m3/s)

100 yr 100 yr plus climate

change 1000 yr

Petty's Brook 19.422 23.306 41.626

River Loddon 27.956 33.547 65.153





Appendix F – DVD of InfoWorks ICM run Files (Available on request)





Appendix G – Surface Water Drainage Philosophy





Appendix H – Safe storage of excess surface water on site

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