SURREY HEATH STRATEGIC FLOOD RISK ASSESSMENT

SURREY HEATH STRATEGIC FLOOD RISK ASSESSMENT

For SURREY HEATH BOROUGH COUNCIL

SUMMARY REPORT

June 2008

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Surrey Heath SFRA

Surrey Heath Borough Council i June 2008

Surrey Heath Strategic Flood Risk Assessment

Summary Document

Report REV 1.0 / June 2008

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DOCUMENT REGISTER

It is accepted that the content of the Surrey Heath SFRA will need to be reviewed and amended as new information becomes available. This document provides a summary of the Woking and Surrey Heath SFRA and the Blackwater Valley SFRA. When either these documents are updated, this summary report will also need to be reviewed.

It is the responsibility of the reader to be satisfied that they are using the most up to date information and that this has been included within the Surrey Heath SFRA.

The Surrey Heath SFRA Summary Report (this document) is a live document requiring review in the event of an improvement or change in the fundamental principles or best available data underpinning the strategy. This is likely to include, but should not be limited to:

• An improvement in the best available information or a reduction in uncertainty.

• Revision to relevant policy, plans or guidance.

• Outcomes of neighbouring strategies.

Revisions to this document should be recorded below in Table 1.0 to maintain clarity for those making decisions involving flood risk issues.

Table 1.0 Document Register

Version Date Issued by Issued to

Rev 1.0 18th June 2008

CSL SHBC

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CONTENTS

DOCUMENT REGISTER .............................................................................................................. II

FOREWORD ................................................................................................................................ IV

1. INTRODUCTION ................................................................................................... 1-1

Introduction .......................................................................................................... 1-1

Study Area ............................................................................................................ 1-1

SFRA Reports....................................................................................................... 1-1

2. HOW FLOOD RISK IS ASSESSED...................................................................... 2-1

Defining Flood Risk ............................................................................................. 2-1

Assessing Flood Risk.......................................................................................... 2-1

3. SUMMARY OF FLOOD RISK IN SURREY HEATH............................................. 3-1

Flooding from Rivers........................................................................................... 3-1

Flooding from Land ............................................................................................. 3-4

Flooding from Sewers ......................................................................................... 3-5

Flooding from Groundwater ............................................................................... 3-5

Flooding from Artificial Sources ........................................................................ 3-6

4. SFRA DECISION SUPPORT GUIDANCE............................................................ 4-1

5. SFRA MAINTENANCE AND MANAGEMENT ..................................................... 5-1

Data Collection..................................................................................................... 5-1

Data Ownership.................................................................................................... 5-1

Monitoring the SFRA ........................................................................................... 5-4

Incorporating new datasets ................................................................................ 5-4

Updating SFRA reports and figures................................................................... 5-5

6. CONCLUSIONS AND RECOMMENDATIONS..................................................... 6-1

7. GLOSSARY........................................................................................................... 7-1

LIST OF FIGURES

Figure A – SFRAs covering Surrey Heath Borough Figure B - Historic Flooding (all sources) Figure C - SFRA Flood Zones (2, 3a, and 3b) Figure D - Climate Change Figure E - Artificial Sources of Flooding (Canal)

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FOREWORD

Surrey Heath Borough Council is required to prepare a Strategic Flood Risk Assessment (SFRA) to support their Local Development Frameworks (LDF).

The borough is within two river catchments, the River Blackwater catchment and the Hale and Addlestone Bourne catchment. To ensure the flooding mechanisms and therefore flood risks in the different catchments were appropriately assessed two SFRAs have been completed, one for each catchment. The Hale and Addlestone Bourne (Woking and Surrey Heath) SFRA has been prepared with Woking Borough Council. The Blackwater Valley SFRA has been prepared in conjunction with Hart Borough Council. The findings of these two SFRAs in relation to Surrey Heath are summarised in this report.

The SFRAs create a strategic framework for the consideration of flood risk when making planning decisions. They have been developed with reference to Planning Policy Statement 25 (PPS25): development and flood risk and additional guidance provided by the Environment Agency.

The fundamental concepts that underpin the SFRA are outlined in PPS25. The guidance provided in PPS25 requires local authorities and those responsible for development decisions to demonstrate that they have applied a risk based, sequential approach in preparing development plans and consideration of flooding through the application of a sequential test. Failure to demonstrate that such a test has been undertaken potentially leaves planning decisions and land allocations open to challenge during the planning process.

The underlying objective of the risk based sequential allocation of land is to reduce the exposure of new development to flooding and reduce the reliance on long-term maintenance of built flood defences. Within areas at risk from flooding, it is expected that development proposals will contribute to a reduction of flood risk.

SFRAs are essential to enable a strategic and proactive approach to be applied to flood risk management. The assessment allows us to understand current flood risk on a wide-spatial scale and how this is likely to change in the future.

The main objective of the Surrey Heath SFRAs is to provide a summary of flood information:

• so that an evidence based and risk based sequential approach can be adopted when making planning decisions, in line with PPS25;

• that is strategic in that it covers a wide spatial area and looks at flood risk today and in the future;

• that supports sustainability appraisals of the local development frameworks; and

• that identifies what further investigations may be required in flood risk assessments for specific development proposals.

The SFRA is a live document that is intended to be updated as new information and guidance become available. The outcomes and conclusions of the SFRA may not be valid in the event of future changes to the data or the baseline flooding situation. It is the responsibility of the user to ensure they are using the best available information.

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1. INTRODUCTION

Introduction

This document provides a summary of the Flood Risk from various sources in the Borough of Surrey Heath compiled from two Strategic Flood Risk Assessments (SFRAs) carried out in the Woking and Surrey Heath, and the Blackwater Valley area.

Flood Risk in Surrey Heath has been assessed through two SFRAs due to its administrative boundaries encompassing sections of two river catchments:

• the River Blackwater catchment; and

• the Addlestone and Hale Bourne catchments

The two SFRAs are summarised here and with this document form the Surrey Heath SFRA.

The Woking and Surrey Heath SFRA concentrates mainly on the Addlestone Bourne and Hale Bourne catchment, whereas the Blackwater Valley SFRA concentrates on the River Blackwater. The areas assessed in each SFRA are outlined in the section below and the relevant SFRA to consult for information on Flood Risk is illustrated in Figure A of this report.

Study Area

The study areas of each SFRA can be seen in Figure 1.1 (Blackwater Valley SFRA, Volume 3) and in Appendix B1 (Woking and Surrey Heath SFRA, Volume 3).

The Blackwater Valley SFRA extends east to west from Hook to Camberley, including the entire District of Hart and the eastern area of Surrey Heath. The main settlements included in this area are Camberley, Frimley, Yateley, Fleet, Hartley Wintney and North Warnborough. There are a number of rivers included within this study area, however the most relevant to Surrey Heath is the River Blackwater. The Blackwater flows from Rushmoor, along the boundary with Guildford before entering Surrey Heath. The river flows adjacent to the A331 before leaving the borough near the A30 and flowing along the northern boundary of Hart District Council. There are a number of smaller watercourse, within the eastern sector of Surrey Heath, which flow into the Blackwater, including the Balmoral Stream.

Woking and Surrey Heath SFRA extends west to east from Camberley to Byfleet, covering the administrative boundaries of Surrey Heath Borough Council and Woking Borough Council. The main settlements within Surrey Heath covered by this SFRA include Bagshot, Lightwater, Windleham, West End, Bisley, and Chobham. The main catchment within this SFRA relevant to Surrey Heath is the Addlestone and Hale Bourne catchment. However the SFRA also considers flood risk from the River Wey and Hoe Stream, which affect Woking and Byfleet.

Flood Risk from other sources such as surface water, sewers, groundwater, and artificial water bodies are also relevant to Surrey Heath.

SFRA Reports

The SFRAs are set out in three volumes. The first volume contains summaries of the catchments within the Study Area. It also contains supporting guidance for planners, emergency planners, engineers, management and developers.

Volume 2 is the Technical Report. This contains details of the approach and methodology, analysis and assessment for all flooding sources within the SFRA.

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All Figures including flood extent maps, tables, and flow charts can be found in Volume 3. The maps in Volume 3 of each SFRA are referred to in the Volume 1 and Volume 2 reports. The SFRA reports and maps should be used in conjunction with this document. The SFRA reports and maps should be referred to for:

• information on the sources of flooding within the borough;

• details of the level of flood risk from the various sources;

• description of the flooding mechanisms within the borough;

• understanding the methodologies employed to assess flood risk;

• description of the limitations of the data used in the analysis; and

• guidance on using the SFRAs for planning policy, development control, and emergency planning.

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2. HOW FLOOD RISK IS ASSESSED

Defining Flood Risk

The Blackwater Valley SFRA and the Woking and Surrey Heath SFRA define flood risk as a combination of two components, the:

"chance (or probability) of a particular flood event and the impact (or consequence) that the event would cause if it occurred."

The probability of flooding can be defined using data and statistical analysis. The consequences of flooding are assessed through considering the flood hazard and the vulnerability of the receptor. The hazard from flooding can be evaluated by considering the depth of floodwater, the velocity of flow, the speed of onset of flooding and the rate of rise of floodwater. The vulnerability of flooding can be assessed through analysis of the land use, property or people that would be affected by flooding. By reducing the hazard or vulnerability of flooding, it is possible to reduce the risk.

Information on flood risk is used to inform the Sequential Test. By including consideration of climate change the procedure is precautionary, in accordance with PPS25.

The SFRAs provide high level information for decisions on land use planning within Surrey Heath. The strategic approach defined in the SFRAs will require that information supporting all planning applications in the study area make reference to the SFRA and clearly demonstrate adoption of a risk-based sequential approach.

Assessing Flood Risk

The SFRAs provide a range of information so that the hazard of flooding, not just the probability of flooding, can be examined. In keeping with PPS25, there are four types of flood risk to be considered.

Flood Zones are defined in Table D1 of PPS25, Flood Zones show areas at risk of river and sea flooding, ignoring the presence of flood defences. It is important to recognise that because the Flood Zones ignore the presence of flood defences, they do not describe an actual level of flood risk. Thus, large areas of development behind flood defences can be shown as at risk. Flood Zones are used to define areas at low, medium and high risk of flooding. Within areas at high risk of flooding PPS25 also defines the functional floodplain. The functional floodplain is the area where water has to flow or be stored at times of flood.

Actual (defended) risk provides information on flooding, when the impact of existing flood defences is considered (assuming that they operate as they are supposed to). The actual risk of river flooding is usually assessed using the 1 per cent AEP flood event, with defences in place. For the purpose of the Blackwater Valley SFRA, the defended flood risk is considered comparable to the undefended flood risk, due to the lack of raised defences along the watercourse. This is discussed further in Volume 2, Chapter 4 of the Blackwater SFRA.

Residual risk (overtopping or exceedance) recognises that engineered flood reduction measures cannot completely eliminate flood risk, there is a need to be aware of the residual risk generated by an event more severe than that for which the defences have been designed to provide protection. Accordingly, this risk assessment usually considers the flooding associated with an extreme event (such as a 0.1 per cent AEP). For the purpose of the Blackwater Valley SFRA, the residual risk of river flooding has not been assessed due to the lack of raised defences along the watercourse.

Residual risk (breach and/or failure) involves the assessment of breach or failure of flood defences or other features, which may act as a defence. Such scenarios may include

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collapse of a flood defence wall, blockage of a culvert or structural failure of a canal or reservoir embankment. Whilst the probability of a breach or failure is generally low, the consequences of an event are often very high. Following the precautionary principle, such high hazards should be considered when making land use planning decisions. Breach and failure hazards are site specific and should be assessed in individual flood risk assessments. However probable locations for key breach and failure scenarios can be considered during a SFRA and locations have been identified where there is a risk of breach along the Basingstoke Canal.

PPS25 requires that all sources of flooding be examined. Flood risk from rivers is a good starting point for this assessment; however other sources of flooding must be examined, even if a proposed development lies within a low probability Flood Zone.

Risk of flooding from other sources (land, groundwater, sewers and artificial sources) can be assessed using a range of analyses. However, for the level of assessment required in an SFRA, these sources are usually assessed via a review of historic flood incidents records and a qualitative analysis of catchment characteristics.

A range of data sets have been used to provide a broadscale assessment of Flood Risk across the Borough, these include:

• Environment Agency Flood Zones

• Environment Agency Detailed Flood Risk Mapping

• Historic Flooding Records

• Information on the elevation of the land, the soils, geology and land use

The best available information (at time of production) was used in each SFRA to assess flood risk. Details of the data collected and the source of information is documented in Volume 2 of the Blackwater Valley SFRA, and Volume 2, Chapter 6 of the Woking and Surrey Heath SFRA. There is however a need to update the SFRAs with new data, to ensure it remains a relevant document. A detailed description of how the various data sets and information was used to asses flood risk is provided in Volume 2 of each SFRA.

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3. SUMMARY OF FLOOD RISK IN SURREY HEATH

This chapter summarises the flood risk within Surrey Heath based on the findings of the Blackwater Valley SFRA and the Woking and Surrey Heath SFRA.

PPS25 identifies six forms of flooding; five of these are relevant to Surrey Heath:

• Rivers;

• Land;

• Groundwater;

• Sewers; and

• Artificial sources.

Of these it is likely that flooding from rivers presents the greatest risk. Much of the land within the river catchments is rural or semi-rural, although some existing urban areas are affected. The main settlements within the Borough, Camberley, Frimley, Bagshot, Lightwater, West End and Chobham have areas at risk of river flooding within them. There is also an increased likelihood of surface water and drainage system flooding in urban areas. Groundwater has been identified as a potential issue, and requires consideration in planning policies.

The flood risk within Surrey Heath from the five sources of flooding identified is summarised in the remainder of this chapter. Further details are provided in the respective SFRAs.

Flooding from Rivers

Flooding from rivers occurs when the volume of water in a river exceeds the capacity of the channel. There are two different river systems within Surrey Heath. To the west, the River Blackwater poses a risk of flooding to Camberley and Frimley. To the east of the borough, flood risk is from the rivers Hale Bourne and Addlestone Bourne.

Different areas are at risk from flooding during different sized flood events. Four flood zones based on the probability of flood events occurring are defined in PPS25 and are shown in Table 3-1 below.

Flood Zone 1

Low Probability

This zone comprises land assessed as having a less than 1 in 1000 annual probability of river or sea flooding in any year (<0.1% AEP)

Flood Zone 2

Medium Probability

This zone comprises land assessed as having between a 1 in 100 and 1 in 1000 annual probability of river flooding (1% – 0.1% AEP) or between a 1 in 200 and 1 in 1000 annual probability of sea flooding (0.5% – 0.1% AEP) in any year.

Flood Zone 3a

High Probability

This zone comprises land assessed as having a 1 in 100 or greater annual probability of river flooding (>1% AEP) or a 1 in 200 or greater annual probability of flooding from the sea (>0.5% AEP) in any year.

Flood Zone 3b

Functional Floodplain

This zone comprises land where water has to flow or be stored in times of flood (and that would flood with an annual probability of 1 in 20 (5% AEP) or greater in any year).

Table 3-1 Flood Zones defined in PPS25

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A combination of EA Flood Zones, EA detailed river modelling, and historic flooding records were used to assess flood risk. The analysis is described in more detail in Volume 2 of the Blackwater Valley SFRA and the Woking and Surrey Heath SFRA.

History of Flooding from Rivers

In the relatively recent past a number of storms that occurred within the catchment area of the Hale and Addlestone Bourne resulted in the rivers overtopping their banks. A few notable events are recorded in November 2000, October 1993, April 1990, and September 1968. These storms resulted in several houses being flooded and roads being blocked. Reports from the more recent events record that deployment of sandbags alleviated property flooding in the majority of cases.

In August 2006, many properties in Windlesham, Lightwater, West End and Chobham were flooded internally and externally. The principle flooding sources were a combination of fluvial, surface and sewer water.

Areas within the catchment with known flooding problems include Bagshot, Chobham, West End, Windlesham and Lightwater.

Historical flooding on the River Blackwater can be seen in Figure 4.1 of Blackwater Valley SFRA Volume 3. A majority of the flood events have occurred in rural areas where there is little risk to people and property. However, a few noteworthy events occurred in the Surrey Heath area.

In 1990 at Frimley Green, the Sturt Road under the railway bridge experienced 3ft of water. In 2000, the A331 at Mytchett was flooded along with the end of the High Street at Frimley, and Knightsbridge in Camberley.

The most recent flood events occurred in July 2007. Table 3-2 lists the incidents within the study area that were reported.

Locality Incidents Residential Commercial Infrastructure Other Details Date of Flood

Event

Bagshot 8 6 1 1 0 - 27/07/2007

Chobham 47 38 5 3 1 School 27/07/2007

Frimley 3 3 0 0 0 - 27/07/2007

Lightwater 7 7 0 0 0 - 27/07/2007

West End 3 0 3 0 0 - 27/07/2007

Table 3-2 Flooding incidents reported to the EA during the July 2007 flood

Chobham had the most reported incidents with 47 in total, including flooding of 38 residential properties and a school.

Historic flooding across Surrey Heath from all sources is shown in Figure B.

Flood Zones

The areas at risk of flooding within Flood Zones 2, 3a, and 3b are summarised below and shown in Figure C. More detailed descriptions of flood risk from rivers are provided in the Blackwater Valley SFRA, Volume 2, chapter 4, and the Woking and Surrey Heath SFRA Volume 1, chapter 3.

SFRA Flood Zone 2

Developed areas currently within Flood Zones 2 include:

• Areas between the A3046 and the A319 at Chobham.

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• Land in the vicinity of Riverside, Lightwater.

• Land between Bridge Road and Freemantle Road, Bagshot.

• the area to the south of West End, which incorporates a number of properties.

• Areas to the west of Camberley, particularly the industrial/commercial area to the east of the A331, including York Town.

• Areas of open land between the Frimley Green, and Mytchett and the A331

• Frimley High Street, Frimley Station, Albany Park Industrial Estate, Lyon Way Industrial Estate and Frimley Business Park are all located within Flood Zone 2.

These flooding extents can be seen in Figure 4.3 (Blackwater Valley SFRA, Volume 3) and Appendix C (Woking and Surrey Heath SFRA, Volume 3).

SFRA Flood Zone 3a

The majority of land within Flood Zone 3a is undeveloped, i.e. open areas, rural and semi-rural land. However, some industrial and residential areas are at risk including:

• Land in Bagshot between Bridge Road (B3029) and the railway, and between the railway and Freemantle Road

• Land in Lightwater, including properties on Riverside Avenue, The Willows, and Birchwood Drive.

• Land, roads, and property in Chobham south of the A319 and around the High Street flood from the Hale Bourne.

• Area around Grants Bridge, Chobham

• Land and property between the river and Sandpit Hall Road at Rothwell Nursery, Chobham.

• Land and property at Philpot Lane west of Fairoaks Airport. This area is upstream of the confluence between the Hale and Addlestone Bournes.

• Land at Mimbridge.

• Property south of the A3046 and a school north of the road is partially within the flood extent at Chobham.

• A few properties on Epsom Close, north of Camberley and at York Town between the Wish Stream and Stanhope Road.

• Gas Holder Station and Industrial units adjacent to the A331.

• Frimley Business Park, Albany Park Industrial Estate, Lyon Way Industrial Estate, and property in Frimley Village to both the north and south of the High Street.

These flooding extents can be seen in Figure 4.3 (Blackwater Valley SFRA, Volume 3) and Appendix C (Woking and Surrey Heath SFRA, Volume 3).

SFRA Flood Zone 3b

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Similarly to Flood Zone 3a, much of the land at risk within Flood Zone 3b is not developed. Within more developed areas, the following areas have been identified as being within Flood Zone 3b:

• Properties between Bridge Road (B3029) and Guildford Road in Bagshot

• Parts of Riverside Avenue and land north of Blackstroud Lane East in Lightwater

• Land and property west of the High Street, to the north of the A319 and around Grants bridge at Chobham

• Land and property between the river and Sandpit Hall Road at Rothwell Nursery.

• Land and Property at Philpot Lane west of Fairoaks Airport, east of Chobham.

• Land near Riverside Way, Camberley

• Land near Coleford Farm, Camberley

These flooding extents can be seen in Figure 4.3 (Blackwater Valley SFRA, Volume 3) and Appendix D (Woking and Surrey Heath SFRA, Volume 3). It should be noted that where detailed river models were not available the extent of Flood Zone 3b has not been assessed.

Climate Change

Current predictions of climate change suggest river flows may increase by as much as 20% in extreme events over the next 50 years. It is therefore necessary to consider how flood risk may change and potentially increase in coming years and for the lifetime of proposed developments.

When flooding is assessed using increased flows (to simulate future flood events accounting for the predicted impact of climate change), there is an extension of the floodplain in some areas. Due to the floodplain being well defined on many of the watercourses in the study area, the increased river flow has relatively little impact on the current flood extents.

The potential impacts of climate change on flood events which currently have a return period of 1% AEP (equivalent to Flood Zone 3a) have been assessed and the resulting flood extents for the borough are shown in Appendix D of Woking and Surrey Heath SFRA Volume 3 and Figure 4.4 of Blackwater Valley SFRA Volume 3.

Figure D summarises the future flood extent (1% AEP adjusted for climate change) across Surrey Heath.

Flooding from Land

Flooding from surface water runoff in urban areas can result from under capacity of drainage systems and blockage of pipes, or alternatively due to the presence of saturated ground after prolonged wet periods. The urban drainage issue is most prevalent in the major urban centres and has been known to lead to flooding within towns in the borough including Camberley, Frimley, Lightwater, West End and Chobham.

In more rural areas flooding from surface water occurs when intense or prolonged rainfall is unable to enter the ground or enter drainage systems. It is often exacerbated when soils are saturated from prolonged rainfall events and are unable to absorb any more water or the rain falls on impermeable surfaces such as concrete where there is no scope for infiltration. This water tends to collect in low points, overflows or minor drainage lines that are usually dry. These flood events are often very localised and short in duration but there is often only a limited warning before it occurs.

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In developed areas, surface water runoff is also likely to result in increased water levels within the local watercourses compared to natural catchment river levels. Although it has not been quantified it is generally accepted that a positive drainage system associated with development increases the peak flow rate from a development area and therefore in the receiving watercourses.

Surface water flooding has been assessed as part of the Blackwater Valley SFRA although the extent of the assessment was limited by data availability. High-risk areas within Surrey Heath have been identified as Camberley, Frimley and Frimley Green. Historic data shown in Volume 3, Figure 5.1 of the Blackwater Valley SFRA, indicates a large number of past incidents of surface water flooding within Camberley. Generally, it has been determined that risk is lower towards the south of the Borough. Volume 2, chapter 9 of the Woking and Surrey Heath SFRA, discusses the risk of flooding from sources other than rivers. From an analysis of historic data areas at risk from surface water flooding include but are not limited to Windlesham, Bagshot, West End, and Lightwater.


Sustainable Drainage systems have been proven to reduce the impact of surface water flooding. Further information can be found in Chapter 9 of Woking and Surrey Heath SFRA Volume 2 and in Blackwater Valley SFRA (Volumes 1 and 2).

Climate Change

No specific research has been carried out to consider the impact of climate change on surface water flooding. However, with future climate change predictions indicating that more frequent high-intensity, short duration rainfall and increased periods of long duration rainfall, there is likely to be significant implications for flooding from land. Indirect impacts of climate change on land use and land management may also change future flood risk and must be considered.

Flooding from Sewers

Where rainwater is drained into surface water sewers or combined sewers, flooding can result if the volume of water received by the sewer exceeds it’s capacity. This can be due to under capacity, blockage or the occurrence of an event greater than size of event the sewer network was designed to convey. When flooding occurs from combined sewers there is a high risk of the flood water being contaminated with raw sewage.

Sewer flooding is often combined with surface water flooding when combined sewerage and drainage systems overfill. Therefore, this type of flooding is likely to occur within the denser urban areas, which include Camberley, Frimley, Bagshot and Lightwater. Due to the type of material contained within the sewers, and the after effects associated with inundation, it can be a particularly damaging source of flooding.

Sewer systems are managed and maintained in this area by Thames Water. Historic Incidents of flooding cannot be relied upon to indicate areas currently at risk, because maintenance and improvement programmes are likely to improve the situation in areas flooded from sewers in the past.

Climate Change

With the predicted increase in rainfall intensity due to climate change, it is expected that there will be an impact on sewer flooding. If an increased volume of water is experienced within the sewerage system, it is likely that existing capacities may be exceeded. Therefore, new and upgraded infrastructure may be required.

Flooding from Groundwater

Groundwater flooding is a consequence of a rise in groundwater level sufficient for the water table to rise higher than that of the ground surface resulting in inundation of low lying land. The

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groundwater can emerge either at a single point location or over a large area. The duration of such a flood event can be long lasting up to days or weeks until the water level lowers.

Groundwater flooding has been assessed on a broad-scale using historical, geological, soil and other topographical data. Figure 6.1 (Blackwater Valley SFRA, Volume 3) shows the possible areas at risk, and should be used to identify areas where further more detailed study is required as part of a detailed Flood Risk Assessment. It is important to recognise that the assessment was limited due to data availability and was only completed on a broad-scale. As such the results are only indicative of areas where there is potential for groundwater, and therefore where the risk of flooding from groundwater should be considered. Based on the geology of the eastern sector of Surrey Heath, the area within the Woking and Surrey Heath SFRA is considered to have less potential for groundwater flooding.


The western sector of Surrey Heath is identified as having greater potential for groundwater flooding and consideration should be given to the possibility of groundwater emergence. In the medium to low-risk areas, some consideration should be given to the emergence of groundwater due to geology, soils, elevation and existence of aquifers.

Climate Change

The impact of climate change on groundwater flooding is relatively uncertain. However, if the predicted increased rainfall intensities and durations do occur, it is possible that groundwater levels may increase. This may be balanced, however, by a lower recharge during the predicted hotter and drier summers.

Flooding from Artificial Sources

Artificial sources of flooding include reservoirs, canals or lakes that retain water above the natural ground level. It can also include operational or redundant industrial processes such as mining, quarrying and sand and gravel extraction where water depths and velocities have been increased in the vicinity. Flooding may occur as a result of any impoundment structure (such as a dam, sluice or raised bank) being overtopped or failing. This can cause significant threat to life and property as flooding can occur very quickly with little warning and can result in deep fast flowing floodwaters.

There are a number of artificial water sources located within the Surrey Heath area, including the Basingstoke Canal, Tomlins Pond and a number of smaller ponds. Within the borough the greatest residual risk (breaching) comes primarily from the Basingstoke Canal which is currently well maintained by the Basingstoke Canal Authority however it has been known to breach in the past when left in a state of disrepair. The possibility of a breach of canal embankments therefore has to be taken in to account when assessing flood risk.

History of Flooding From Artificial Sources

SHBC have reported a breach on the canal at the Deepcut Locks between two locks in 1984. About 150m of embankment breached releasing water, which surcharged through the drainage system in Guildford.

On September 15th 1968, lack of routine maintenance and a period of exceptional rainfall resulted

in the canal breaching its banks in two locations, in Farnborough and Aldershot. Limited damage occurred during this breach, but it is believed that if a similar event were to occur today, the damage could be potentially more severe.

There are also reports from SHBC that Tomlins Pond in Frimley has breached in the past, which may have flooded properties between the pond and Alphington Road.

Figure E shows the Basingstoke Canal alignment and indicates embanked reaches, which may be susceptible to breach.

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Flood Risk

Tomlins Pond in Frimley is elevated significantly above the surrounding ground level and thus poses a threat to the surrounding area. It is surrounded by 2.5m high embankments and owned and maintained by SHBC.

Basingstoke Canal has now been fully restored and is regularly maintained by Basingstoke Canal Authority. There have been four areas identified in Surrey Heath as being at risk from breaching from the canal. These are: the Frimley Aqueduct; Deepcut flight of locks (between locks 15-19 and 25-28); the railway embankment between Guildford Road Bridge and Mytchett Place Road Bridge; and the Mytchett Lake embankment. Locations and the potential extent of flooding from a canal breach are displayed visually in Appendix H (Woking and Surrey Heath SFRA, Volume 3) and Figure 7.1 (Blackwater Valley SFRA, Volume 3).

Breaches at any of these locations will potentially impact upon the railway, surrounding residential properties and roads. The most serious breach could be the failure of the Mytchett Lake embankment, which could discharge approximately 100,000m

3 of water. This risk is recognised

and the embankment is therefore inspected twice yearly. The lake is also registered under the Reservoirs Act 1975.

The flood risk from the canal is considered a Residual Risk. Therefore, whilst new developments will be required to manage this risk, they may not be required to fully mitigate the risk. Therefore, the sites identified at risk of flooding are still considered suitable for most development types.

Climate Change

The predicted increase in intensity and duration of rainfall events associated to climate change has the potential to affect artificial water bodies. As these water bodies are often managed, the impacts may be minimised through water level management systems, or improvements. The storage requirements during intense rainfall may increase, until water levels are lowered. However increased rainfall may increase the volume of water held within the lakes, ponds, reservoirs and canals, even if only for a relatively limited period of time. This increase in volume increases the potential damage should a breach occur. It may also increase the risk of embankment breaching unless these structures are effectively managed and maintained.

The Basingstoke Canal can be isolated into discrete sections in the event of extreme rainfall or a canal emergency. Therefore it may be possible to manage the quantity of water within the canal.

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4. SFRA DECISION SUPPORT GUIDANCE

The information in Strategic Flood Risk Assessments is relevant and required by a number of different users, including planners, policy makers, development control officers, emergency planners, highways teams, engineers, and developers. In addition to these end users, the SFRA is also likely to be referenced by consultants and the public. Guidance has been provided in Volume 1 of both the Blackwater Valley SFRA and the Woking and Surrey Heath SFRA to facilitate the use of the technical data by all end users.

The guidance describes how the SFRA can be used to inform the sequential test and adopt a sequential approach to the location of development. Information is provided for both development control and developers on the application of the SFRA, with respect to land allocated in the local development documents and windfall sites. Guidance is also included on the application of SuDS within the borough. The guidance in Volume 1 of the SFRAs explains how information in the SFRA may be useful in emergency planning. Table 4-1 below summarise where to find guidance in the Blackwater Valley SFRA and Woking and Surrey Heath SFRA.

Table 4-1 Where to find guidance in the SFRA

User Woking and Surrey Heath SFRA

Blackwater Valley SFRA

Planners Volume 1, Chapter 3 Volume 1, Chapter 3 and 4

Development control and Developers

Volume 1, Chapter 4 Volume 1, Chapter 3 and 4

Emergency Planners and Engineers

Volume 1, Chapter 5 Volume 1, Chapter 3

It should be noted that the guidance provided in the SFRA does not supersede guidance provided in PPS25 or other plans or policies. The information and procedures are simply provided as an interpretation of this guidance for the preparation of the Local Development Framework, and use of the SFRA in development control and emergency planning.

The information contained within the SFRA should be used with an understanding of its limitations and the scale to which flood risk has been assessed. The uncertainties in flood risk assessment are outlined in Volume 2, chapter 11 of the Woking and Surrey Heath SFRA. Within the Blackwater Valley SFRA Volume 2, chapters 4 to 7 describe the methods used to assess flood risk and the uncertainties associated with the assessment.

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5. SFRA MAINTENANCE AND MANAGEMENT

Data Collection

The datasets used in the SFRAs were derived from:

The Environment Agency - existing flood studies and hydraulic models such as River Blackwater Flood Risk Mapping Project, Addlestone and Hale Bourne Flood Risk Mapping Study, non flood risk datasets including topographic data (LiDAR and SAR), historic flood incidents, and the National Flood and Coastal Defence Database (NFCDD). Data was issued for the SFRAs under guidance provided by the Environment Agency pertaining to the provision of data to Local Authorities for SFRAs.

Surrey Heath BC, Hart DC, and Woking BC - these datasets are held under a variety of licence and copyright agreements including historic flooding incidents.

Thames Water – details of historical sewer flooding.

Parish Councils – Including Bramshill, Crookham Village, Dogmersfield, Eversley, Heckfield, Hook, Mattingley, Rotherwick, South Warnborough, Odiham and North Warnborough.

Basingstoke Canal Authority - details of operation procedures and historic breach events.

Details of the key data sets received from various organisations in order to develop the SFRAs are given in the Volume 1, chapter 6 of the Blackwater Valley SFRA and Volume 2, chapter 6 of the Woking and Surrey Heath SFRA. The SFRAs are living documents and as such should be updated when new data become available. A record of data used in the SFRAs should be maintained.

Data Ownership

The datasets obtained for use in the SFRA have come from a number of sources under licence agreement. These datasets can not be passed to external sources without permission from the owner. Datasets produced during the SFRA are held by the Local Authorities and can be passed to external parties at their discretion. The key datasets are summarised in Table 5-1.

Table 5-1 Key Datasets

List of Key Data Sets Ownership Contact

Floodplain topography – LiDAR, and topographic survey

Environment Agency Flood Mapping & Data Environment Agency

Flood Warning Areas Environment Agency Flood Mapping & Data Environment Agency

Detailed Flood Risk Mapping Study’s and models

Environment Agency Flood Mapping and Data, Environment Agency

Existing Defences and Structures (Strategy Studies once completed)

Environment Agency System Asset Management, Environment Agency

Historic flood information Environment Agency and/or Local Authority

Flood Mapping & Data Environment Agency/or Local Authority

OS 10k and Landline data Ordnance Survey SHBC

SFRA Reports, Maps, and GIS data

Surrey Heath BC, and Woking BC or

SHBC, HDC, WBC

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Hart DC

Emergency Flood Plans Surrey Heath BC, and Woking BC or Hart DC

SHBC, HDC, WBC

Basingstoke Canal Basingstoke Canal Authority

Basingstoke Canal Authority (BCA)

Sewer Flooding Thames Water Thames Water

It is recommended that information on all sources of flooding continues to be collected and that where possible more resources are invested in determining the source and pathways of flooding. When more detailed or updated hydraulic modelling becomes available from the EA or other sources this information should be incorporated into the SFRA. More detailed information may also be collected for FRAs carried out by developers and land owners at the local site scale. Information from site level FRAs will be submitted to the councils and the Environment Agency as part of the development control process and this information should be used to inform the SFRA in the future.

SFRA data management system

The data management strategy developed for the SFRA is designed to account for likelihood that external parties will seek to make use of the information within the SFRA in preparing flood risk assessments and assessing sites. The SFRA is also a “live” document, and as such it is necessary to ensure at regular intervals that the information within it remains valid.

The final deliverables of the SFRA are delivered in two forms:

Hardcopies of the SFRA reports – the SFRA contents are divided into several volumes and chapters to allow easier update during future iterations.

Electronic datasets including:

• Raw GIS data - SFRA flood outlines and additional GIS data layers used to provide the SFRA maps and figures. These were obtained under licence from the Environment Agency. All data is provided in a format compatible with Surrey Heath Borough Councils existing corporate GIS infrastructure.

• Electronic document management system - PDF versions of all maps and reports produced during the SFRA

To ensure that the SFRA remains ‘live’ it is important to nominate a Management Group with responsibility for monitoring, managing and maintaining the SFRA, as shown in Figure 5-1.

By following this process of information dissemination and review, the management team can ensure a consistent and up to date supply of strategic flood risk information to all levels of planning process.

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Figure 5-1 Conceptual SFRA management process

Continuous

improvement of

SFRA

Informs

strategic

planning

Informs

feasibility

studies/

design

development

Informs

outline

planning

application

Informs full

planning

application

SFRA

documents and

data

Audit

process

New data generated

Technical

guidance of

SFRA

Support to

control data &

manage

predictions

New data

New data

from other

sources

Supply of SFRA data

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Monitoring the SFRA

It is in the interest of Surrey Heath Borough Council that the SFRA remains current and up-to-date. It is recommended that the following maintenance checks be undertaken:

Review the currency of datasets used in the SFRA.

Consider whether a formal review of the SFRA is necessary.

Whilst all datasets should be checked for updates and key organisations contacted, Table 5-2 contains a list of datasets that are likely to be updated regularly.

Table 5-2 Datasets that are known to be updated regularly

Dataset Owner Comment

Flood Zones Environment Agency Updated quarterly

Catchment Flood Management Plans

Environment Agency Updated every five years

National Flood and Coastal Defence Database (NFCDD)

Environment Agency Ongoing updates

System Asset Management Plans

Environment Agency Unknown

Historic flood incidents

Environment Agency, Water companies,

Fire Brigade,

Highways Depots

Unknown

To assist in identifying the requirement for a SFRA update the following questions should be answered:

1. Have the EA completed or updated any studies within the boundaries of Surrey Heath (e.g. CFMP, Strategy, Detailed Flood Risk Mapping, Feasibility studies, SAMPs)?

2. Have there been any flood incidents since the last update of the report?

3. Have there been any alterations made to the physical form of the rivers, drainage systems or artificial water sources?

4. Have there been any large new developments constructed within the boundaries of Surrey Heath for which a FRA was prepared?

5. Have any adjacent local authorities made significant changes or updates to their SFRA, which may impact on Surrey Heath?

If significant changes in the available information are identified it may be necessary to arrange a meeting with the relevant data owner.

Incorporating new datasets

The following tasks should be undertaken when reviewing datasets in the Surrey Heath SFRA:

• Identify new dataset.

• If necessary request the data set from the data owner

• Save new dataset/information.

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• Record new information in the log so that the next update can review and incorporate this information.

Updating SFRA reports and figures

The following tasks should be undertaken when updating SFRA reports and maps:

• Undertake further analyses as required after SFRA review

• Document all new technical analyses by rewriting and replacing relevant Volume 2 chapter/s.

• Amend and replace relevant SFRA Maps in Volume 3.

• Review and if required, amend Volume 1.

• Reissue to Surrey Heath Borough Council (and Hart District Council, or Woking Borough Council), the Environment Agency and other stakeholders.

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6. CONCLUSIONS AND RECOMMENDATIONS

The Surrey Heath SFRA consists of three documents, this report, the Woking and Surrey Heath SFRA, and the Blackwater Valley SFRA. The Surrey Heath SFRA provides an assessment of flood risk in accordance with PPS25 and the Practice Guide Companion to PPS25. The SFRA should be used to inform the Sequential Test and Sequential Approach to planning and LDD. The SFRA can also be used to inform development control, developers, emergency planners, policy makers, engineers, and the public.

This SFRA has utilised the information available from a range of sources including the Environment Agency, local authorities, parish councils, Thames Water and the Basingstoke Canal Authority. The SFRA has been based on the best available information and is limited by the quality and extent of the data.

The Surrey Heath SFRA provides information on flood risk and guidance for applying the information in the SFRA. Sites where the SFRA Flood Zones indicate a medium to high probability of flooding require particular attention and consideration by the LPA in determining whether the site should be allocated for development, and the type of land use that should be permitted at the site.

Flooding from other sources should be considered at all sites using a Sequential Approach, as advocated by PPS25. In applying the Sequential Approach to other sources of flooding, development should be steered toward areas considered to be at a lower probability of flooding. As such careful consideration should be given to the suitability of sites where there has been a higher concentration of incidents reported or identified.

The SFRA makes the following recommendations, which should be considered when developing policies and using the SFRA. The recommendations have been divided into categories in Table 6-1. The organisations likely to be involved in implementing the recommendations have also been identified in Table 6-1.

Table 6-1 Recommendations

Recommendations Organisation

General Flooding

1.1 Developments adjacent to watercourses with catchment areas less than 3 km

2 should consider flood risk from the watercourse as part of a FRA as

the Flood Zones (EA) do not provide information for such small catchments.

SHBC

EA

Developer

1.2 On watercourses that have not been included within detailed river models, and as such do not have a defined Flood Zone 3b, it is recommended that the guidance in the Practise Guide Companion to PPS25 is followed. That is ‘all areas within Flood Zone 3 should be considered Flood Zone 3b (Functional Floodplain) unless or until, an appropriate FRA shows to the satisfaction of the EA that it can be considered as falling within Flood Zone 3a (High Probability).

SHBC

EA

Developer

1.3 It is recommended that policies are developed to manage the impact of developments on flood risk.

SHBC

1.4 It is recommended that SHBC liaise with Hart DC, Rushmoor BC and Guildford BC regarding development policies in their boroughs, to ensure flood risk is not increased along the Blackwater Valley.

SHBC

HDC

RBC

GBC

1.5 It is recommended that SHBC liaise with Woking BC regarding development policies in their boroughs, to ensure flood risk is not increased in the Addlestone and Hale Bourne catchments.

SHBC

WBC

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River Flooding

2.1 The Environment Agency is planning to model flood risk on some tributaries of the River Blackwater in the future. The results of this study should be incorporated into the SFRA once completed.

SHBC

EA

2.2 The use of data in the SFRA is appropriate to the level of detail required in the study, when applied to more detailed studies it may be necessary to re-visit and complete more detailed assessments. Where limitations in data or the scale of assessment have been identified information should be improved through more detailed study. (e.g. photogrammtery used in the Addlestone and Hale Bourne study and the hydrology for the 0.1% event on the Blackwater).

SFRA Users

Groundwater, Surface Water, and Sewer Flooding

3.1 It is recommended that a policy is developed regarding areas at risk of groundwater flooding taking into consideration the limitations of the assessment made in the SFRA and available data. It may be appropriate for FRAs to complete more detailed groundwater analysis, given the local nature of this source of flooding.

SHBC

EA

Developer

3.2 Policies should be developed to ensure appropriate surface water management and mitigation is provided for developments.

SHBC

3.3 Flood risk from the Windlesham Ditch needs to assessed should development be proposed within the vicinity of the watercourse.

SHBC

Developer

3.4 Due to limitations and lack of information available on other sources of flooding; and in some cases the local nature of problems it is recommended that these issues are considered in detail on a site by site basis.

SHBC

Developer

3.5 It is recommended that information on all sources of flooding continues to be collected and that where possible more resources are invested in determining the source and pathways of flooding.

SHBC

EA

Artificial Sources of Flooding

4.1 It is recommended that the council consult with the Environment Agency and the Basingstoke Canal Authority to agree policies for development at risk from canal breach, this may include agreeing raised floor levels, or developing evacuation plans

SHBC

EA

BCA

4.2 It is recommended that FRAs for developments at risk of flooding from breach of the canal consider this in their assessment.

Developer

4.3 It is recommended that planning applications continue to be sent to the Basingstoke Canal Authority for consultation.

SHBC

BCA

4.4 It is recommended that the council consult with the Environment Agency to agree appropriate policies for development adjacent to artificial water bodies where there is a risk of flooding

SHBC

EA

Links to the Thames Catchment Flood Management Plan

5.1 It is recommended that Surrey Heath BC resolve the following key points identified in the CFMP with Environment Agency:

1. Is the redevelopment of the commercial and industrial sites within the Blackwater floodplain justified on wider sustainability grounds, or can they be relocated in the long-term. The sequential test is of crucial importance.

2. If on the whole, redevelopment is justified, policies in LDF’s will need to be very clear on the requirements for the layout and design of sites and buildings.

3. The functional floodplain will need to be agreed as redevelopment within this catchment does offer the long-term prospect of restoring a more continuous and natural river corridor.

SHBC

EA

5.2 It is recommended that Surrey Heath BC also consider objectives within

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the Thames CFMP when developing policies on restoration of the river corridor and reducing flood risk through layout and design of redeveloped sites.

Links to the Addlestone Bourne strategy

6.1 It is recommended that policies are developed that are sympathetic to the recommendations made in the Addlestone Bourne Inception Strategy:

1. Continue with current watercourse maintenance

2. Object to inappropriate development within the floodplain;

3. Promote use of SuDs and encourage Greenfield run-off rate for attenuation in all new developments within the catchment;

4. Encourage people at risk to flood proof their homes; and

5. Improve the flood warning system to better reflect catchment hydrology.

SHBC

EA

Maintenance and management of the SFRA

7.1 It is recommended that the SFRA is updated to ensure it remains a robust and current document. Therefore it is further recommended the SFRA Management and Maintenance strategy is adopted.

SHBC

7.2 Information from site level FRAs will be submitted to the councils and the Environment Agency as part of the development control process and this information should be used to inform the SFRA in the future.

SHBC

7.3 It is likely that the council will receive multiple requests for copies of the SFRA, it is therefore recommended that the SFRA is made available for viewing and download through the council webpage.

SHBC

The emerging Surrey Heath Local Development Framework and other users of the SFRA need to take into consideration the recommendations within the SFRA. It is important to recognise that the allocation of future development may impact flood risk, and should be managed carefully.

In the longer term there is a need to maintain and manage the information in the SFRA so that further and future decisions are made using the best available data. It is important the liaison is maintained between the Local Authority, Thames Water, the Environment Agency, Highways Agency, and other stakeholders to work towards sustainable management of flood risk now and in the future.

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7. GLOSSARY

Actual Risk The risk from flooding from all sources based on best available

information and representing the influence of flood defences,

large infrastructure features, and the distribution of risk within

the Flood Zones.

AEP Annual Exceedence Probability

BCA Basingstoke Canal Authority

EA Environment Agency

Flood Zones This refers to the Flood Zones in accordance with Table D1 of

PPS 25 derived for this Blackwater Valley SFRA

Flood Zones (EA) This refers to the Environment Agency’s Flood Zones.

Flood Zone 3b This zone comprises land where water has to flow or be stored in times of flood. Land which would flood with an annual probability of 1 in 20 (5%) or greater in any year.

Flood Zone 3a This zone comprises land assessed as having a 1 in 100 or greater annual probability of river flooding (>1%) or a 1 in 200 or greater annual probability of flooding from the sea (>0.5%) in any year.

Flood Zone 2 This zone comprises land assessed as having between a 1 in 100 and 1 in 1000 annual probability of river flooding (1% – 0.1%) or between a 1 in 200 and 1 in 1000 annual probability of sea flooding (0.5% – 0.1%) in any year.

FRA Flood Risk Assessment

GBC Guildford Borough Council

HDC Hart District Council

iSIS River Modelling software

Km2 Square kilometres

LDF Local Development Framework

LiDAR Light Detection and Ranging survey method used to collect data

for construction of a ground model.

m Metres

m/sec Metres per second

mAOD Metres Above Ordnance Datum

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Main River As Defined by the Environment Agency main rivers are usually

larger streams and rivers, but also include smaller watercourses

of strategic drainage importance. A main river is defined as a

watercourse shown as such on a main river map, and can

include any structure or appliance for controlling or regulating

the flow of water in, into or out of the main river. The Agency’s

powers to carry out flood defence works apply to main rivers

only. Main rivers are designated by the Department for

Environment, Food & Rural Affairs in England and by the Welsh

Assembly Government in Wales.

mm Millimetres

Ordinary Watercourse As Defined by the Environment Agency an ordinary watercourse

is every river, stream, ditch, drain, cut, dyke, sluice, sewer

(other than public sewer) or passage through which water flows

which does not form part of a main river. On ordinary

watercourses, the local authority and, where relevant, Internal

Drainage Boards (IDBs) have similar permissive powers as the

Agency has on main rivers.

PPG 25 Policy Planning Guidance Note 25: Development and Flood

Risk - Guidance explaining how flood risk should be considered

at all stages of the planning and development process in order

to reduce future damage to property and loss of life.

PPS 25 Planning Policy Statement 25. Development and Flood Risk

Guidance replacing PPG 25 in December 2006 and outlining

how flood risk should be considered at all stages of the

development process.

Precautionary Principle “Where there are threats of serious or irreversible damage, lack

of full scientific certainty shall not be used as a reason for

postponing cost effective measures to prevent environmental

degradation’’. The precautionary principle was stated in the Rio

Declaration in 1992. Its application in dealing with the hazard of

flooding acknowledges the uncertainty inherent in flood

estimation.

RBC Rushmoor Borough Council

Residual Risk Risk remaining due to the possibility of an event more severe

than that for which particular flood defences have been

designed or more severed than that which significant

infrastructure features, such as motorway embankments are

able to provide protection. A more severe event includes future

flood events resulting from the impacts of climate change.

Return Period The average time until the next occurrence of a defined event.

SAR Synthetic Aperture Radar used to collect data for construction of

a ground model.

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Section 105 Environment Agency Floodplain Modelling produced from

hydrological and hydraulic modelling.

SFRA Strategic Flood Risk Assessment.

SHBC Surrey Heath Borough Council.

Study Area Refers to the combined Hart District and Surrey Heath Borough

Area.

WBC Woking Borough Council

1 in 20 year return period

flood event

(5% AEP)

The flood event that is predicted to occur with an annual

probability of 5.0% (there is a 1 in 20 (5%) chance each year

this event will be witnessed).


flood event

(1%AEP)


probability of 1.0% (there is a 1 in 100 (1%) chance each year

this event will be witnessed).


flood event

(0.1% AEP)


probability of 0.1% (there is a 1 in 1000 (0.1%) chance each

year this event will be witnessed).

Documents

SURREY HEATH STRATEGIC FLOOD RISK ASSESSMENT