A supplement to the Guildford Borough Strategic Flood Risk Assessment, forming part of the Guildford Development Framework evidence base - November 2010

www.guildford.gov.uk www.environment-agency.gov.uk

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Contents

Page3 Vision3 Executive summary

4 Introduction

4 Flood Zone 34 The reason for this document (policy background)6 When will this document be used and by whom?7 How was this document prepared?7 The context of this document in accordance with other documents9 Flood risk in Guildford Urban Area

9 Surface water flooding10 Critical infrastructure19 Flood risk reduction measures in existing and new developments in Guildford Urban

Area19 Risk reduction measures in Guildford Urban Area20 Existing developments26 New developments33 Tracking and monitoring34 Other sources of information (references to other documents)

34 Contact details34 Useful websites35 Glossary

Page Figure Title8 1 Reducing flood risk in Guildford Urban Area16 2a Map of Guildford Urban Area17 2b Location of land use/development examples in Guildford Town Centre18 2c Property type within 1 in 100 flood outline19 3 Table 2.7 an extract from ‘Improving the flood performance of new buildings

– May 2007’, Defra, CLG and Environment Agency22 4 Flood protection measures for existing property owners27 5 Risk reduction measures for new development

Page Annex Title41 A Explanation of terms used in Figure 1 – Reducing Flood Risk in Guildford

Urban Area

43 B Document history44 C Supplementary Note on Flood Hazard Ratings and Thresholds for

Development Planning and Control Purpose

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Vision

“To reduce the risk1 of flooding in Guildford Urban Area, using redevelopmentopportunities to provide increased safety, additional floodwater storage andimproved floodwater flows whilst making space for water and the enjoyment ofthe River Wey.”

The Thames Catchment Flood Management Plan (CFMP) addresses flood risk management over thenext 50 100 years. In accordance with this, the vision above is a long term vision which will bereviewed and updated in partnership with the Environment Agency and in accordance with thetimescales of the CFMP, the River Wey Flood Risk Management Draft Strategy 2010 and theGuildford Development Framework (GDF).

Executive summary

This Flood Risk Reduction Measures document forms part of the Guildford Development Framework(GDF) evidence base. This document provides information on the type of flood risk reductionmeasures required for all proposed redevelopment within Flood Zone 32 of Guildford Urban Area. Italso offers information to those wishing to improve the flood resilience or resistance of existingproperties. This approach recognises that due to the challenges and constraints facing the borough,an acceptance of some level of flood risk is necessary. This approach is in conjunction with theGuildford Strategic Flood Risk Assessment (SFRA), Planning Policy Statement 25: Development andFlood Risk (2006, updated 2010) and the Thames Catchment Flood Management Plan (CFMP).

1 Risk = probability x consequence2 The extent of Flood Zone 3 in Guildford Urban Area can be seen on the Environment Agency’s interactive maps, or theCouncil’s interactive maps online (www.environment agency.gov.uk/homeandleisure/floods/ orwww.guildford.gov.uk/planningpolicy)

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IntroductionThis document provides details of flood risk reduction measures to be used in Flood Zone 3 inGuildford Urban Area, taking all opportunities through new development to reduce theconsequences of flooding. It also provides advice to those people with properties at risk of floodingin Guildford Borough. Development in Flood Zone 3 must pass the sequential and exception tests, asset out in PPS25, where required before appropriate risk reduction measures can be considered.

This document supports the Guildford Strategic Flood Risk Assessment (SFRA) and sets out a uniqueapproach to the functional floodplain in Guildford Urban Area. It also forms part of the GuildfordDevelopment Framework (GFD) evidence base. This evidence base informs the preparation ofpolicies in Development Plan Documents (Core Strategy and Site Allocations) and has been approvedby the Council as a material planning consideration for the determination of planning applications(October 2010). In the longer term, this evidence base document will be progressed to become aSupplementary Planning Document (SPD).

Flood Zone 3Throughout this document, all references to Flood Zone 3 refer to the detailed 1:100 year floodoutlines of river flooding in the revised flood outlines as published by the Environment Agency (June2010). These maps are based on detailed river modelling of the River Wey catchment and supersedethose shown in the SFRA 2009. This detailed information is necessary to allow the application of theSequential Test. See the SFRA (chapter 2) for more information.

The reason for this document (policy background)Planning Policy Statement (PPS) 25: Development and Flood Risk was published by Communities andLocal Government (CLG) in December 2006, and updated in March 2010. PPS25 seeks to ensureflood risk is taken into account at all stages of planning and that allowances are made for futureclimate change. PPS25 requires Local Authorities to prepare a SFRA. The SFRA informs the planningprocess of flood risk and provides information on future risk over a wide spatial area. It is also usedas a planning tool to examine the sustainability of potential development allocations in the GDF.Guildford Borough Council published its SFRA in January 2009 and will be updated in future.

PPS25 sets out guidelines for appropriate development within specific Flood Zones. In many casesdevelopment can only be considered once the Sequential Test and Exception Test have been passed.These tests must be considered before this stage of flood risk reduction measures. PPS25 originallydefined the Functional Flood Plain relatively rigidly as the area with an annual probability of floodingof 1 in 20 or greater, or at any probability to be agreed between the LPA and the EnvironmentAgency. In August 2009, the Government consulted on a revised definition, and received support forits proposal (Summary of responses and key issues raised, 29 March 2010). The definition hassubsequently been updated in PPS25 (March 2010) and now reads;

“This zone comprises land where water has to flow or be stored in times of flood. Localplanning authorities should identify in their SFRAs areas of functional floodplain and itsboundaries accordingly, in agreement with the Environment Agency. The identification offunctional floodplain should take account of local circumstances and not be defined solely onrigid probability parameters. But land which would flood with an annual probability of 1 in20 (5%) or greater in any year, or is designed to flood in an extreme (0.1%) flood, should

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provide a starting point for consideration and discussions to identify the functionalfloodplain.” (PPS25, revised 29 March 2010).

Guildford Town Centre, and the wider Guildford Urban Area, is a sustainable location fordevelopment within the borough. The Council intends to help address housing need by providinghousing, and other forms of development, within these sustainable locations and other identifiedsustainable locations within the borough3. However, as the River Wey flows through Guildford TownCentre and Guildford Urban Area, large areas of land are identified as within the 1 in 20 year floodoutline. Using the identified ‘starting point’ in the definition in PPS25, these areas4 would becategorised as Functional Floodplain where development, other than essential infrastructure andwater compatible uses, would not be permitted.

Based on detailed river modelling, there are around 67 hectares of 1 in 100 year floodplain in theGuildford Urban Area. Of this, about 47 hectares are defined as functional floodplain. With a highproportion of Flood Zone 3 also defined as Functional Floodplain, and a large amount of existingdevelopment in this area, a solution to not only prevent urban blight, but also reduce flood risk inthe Guildford Urban Area has been explored.

The definition within the original PPS25 allowed for an alternative flood probability to be agreedwith the Environment Agency where appropriate, and was thus pursued. This approach has sincebeen supported through the revisions to the definition of the Functional Floodplain in the revisedPPS25 (March 2010). The decision to re define the Functional Floodplain is based on the Council’scommitment to directing development to the Urban Areas of the borough, the identified housingneed, the highly constrained nature of Guildford Borough (due to the extensive amount of GreenBelt land), the requirement to deliver the Catchment Flood Management Plan and, when approved,the River Wey Flood Risk Management Draft Strategy 20105. The need to allow safe and sustainabledevelopment to proceed where appropriate in Guildford Urban Area is agreed between bothGuildford Borough Council and the Environment Agency.

Therefore, as detailed in the SFRA (p.2 2),

“Within the main Guildford Urban Area the definition of Functional Floodplain has beenagreed to be delimited by the top of bank, within this area therefore the 1 in 20 year outlinedoes not equate to the area defined as Functional Floodplain. This definition has beenadopted as it is agreed it is not appropriate to classify built up areas of central Guildford asFunctional Floodplain.”

In accordance with this document, redevelopment within Flood Zone 3 (1 in 100 flood outline) inGuildford Urban Area must be supported by additional flood risk reduction measures. The measurescontained in this document are not designed to promote Guildford Urban Area, or the town centrewithin it, as a major development site. Instead, they will provide tighter controls on developmentwithin Zone 3a (Flood Zone 3) in Guildford Urban Area than those contained in PPS25. The approachapplies to any development which passes the Sequential Test and Exception Test (parts A and B) in

3 Guildford Urban Area, Ash and Tongham Urban Area, Identified Village Settlements and Guildford Town Centre aresustainable locations within the borough for development in accordance with the Local Plan 2003. Any proposal fordevelopment is subject to all relevant planning policy and material planning considerations.4 Note Guildford Town Centre is within Guildford Urban Area5 See Annex A for more information on these documents.

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Flood Zone 3. Incorporation of the measures within this document will assist in meeting part C of thePPS25 Exception Test, making development safe, without increasing flood risk elsewhere andreducing risk overall.

Rather than trying to justify development within Zone 3b (Functional Floodplain) on a site by sitebasis, this more flexible approach to development within the whole of Flood Zone 3, offers a chanceto manage flood risk in the Guildford Urban Area at a strategic level and in a more consistentmanner. As a result, re development in the floodplain will provide a net reduction in flood risk forthe people and properties currently in Flood Zone 3 in Guildford Urban Area.

This corresponds to an increase in people’s safety, and a reduction in economic damages toproperty, providing social, economic and environmental benefits in Guildford.

This unique agreement with the Environment Agency on the definition of the functional floodplain inGuildford Urban Area requires everyone to follow the advice contained in this document. If theCouncil or the Environment Agency believes this advice has not been adhered to, the agreeddefinition of the Functional Floodplain cannot be used. This may result in an objection to theplanning application from the Environment Agency and Guildford Borough Council with thesubsequent refusal of planning permission.

The extent of Guildford Urban Area is shown on the Local Plan (2003) proposals maps and is theboundary used in reference to this document (see page 16 of this document for a map of GuildfordUrban Area). The text of the Local Plan (2003) can be viewed on the Council’s website atwww.guildford.gov.uk/planningpolicy. Hard copies of the proposals maps can be viewed at theCouncil’s planning helpdesk, at the Millmead Office, Guildford. The Flood Zones referred to in thisdocument are the current flood zones at August 2010. It is advisable to check if there have been anyupdates since this date using the Environment Agency’s maps online (www.environmentagency.gov.uk/homeandleisure/floods). Detailed datasets are also available from the EnvironmentAgency.

This approach will inform progression of the Council’s Core Strategy. The latest version of the CoreStrategy can be viewed online at www.guildford.gov.uk/corestrategy.

When will this document be used and by whom?

This document will be used by those considering submitting a planning application for developmentwithin Flood Zone 3 in Guildford Urban Area. It is also aimed at householders whose property iscurrently within this area and who wish to investigate which resistance or resilience measures6

6couldbe employed to reduce the consequences of flooding.

Consultants preparing a Flood Risk Assessment (FRA) are advised to consider all potential optionslisted within this document for inclusion within the FRA to ensure the consequences of flooding onsite, and elsewhere if possible, are reduced.

6Resistance measures aim to prevent floodwater entering the building and causing damage (for example door guards).Resilience measures minimise the damage that is caused by floods which enter a property (for example water proof wallplaster).

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Those seeking planning permission are advised to liaise with the Council and the EnvironmentAgency at the early stages of preparing a planning application. Pre application advice can be soughtfrom the Council and the Environment Agency, using the contact details on page 34.

How was this document prepared?

As a key stakeholder in the determination of planning applications and the preparation of the SFRA,the Environment Agency has an important role to play in assisting Local Authorities in achieving theobjectives of PPS25. This document has, therefore, been prepared in partnership with theEnvironment Agency and is recognised by both organisations as sound evidence informing thepreparation of the GDF and the determination of planning applications.

Drawing on the expertise of the Council’s planning policy, development control and engineeringofficers, and the Environment Agency’s expertise from development and flood risk, regional floodrisk management, asset system management and planning liaison, this document has beenproduced by all of the aforementioned in partnership during a series of workshops. Any futureupdates to this document will be made by the partnership group established. This is the secondpublished version (see Annex B for document history).

The context of this document in accordance with other documents

Figure 1 shows how this document relates to other work prepared by the Environment Agency andthe Council. See Annex A for an explanation of the terms used. Also see Appendix A1 of the SFRA(volume 1) for how the SFRA links with other plans and policies.

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Figure 1 Reducing Flood Risk in Guildford Urban Area (see Annex A for an explanation of terms)

Guildford Borough Council Environment Agency

Flood

Guildford Development Framework (GDF)(Local Development Framework)

Strategic policies (Core Strategy), developmentmanagement policies and site allocations

GDF evidence base: Strategic Flood RiskAssessment (SFRA)

Site specific Flood Risk Assessment (FRA)carried out by developers and submitted aspart of a planning application where required

Thames Catchment Flood Management Plan(CFMP)

Guildford Policy Unit

River Wey Flood Risk Management DraftStrategy 2010

Flood Risk Management Measures (Structuraland Non structural)

Reduced flood risk(probability andconsequence)

Flood risk reductionmeasures

GDF evidence base: Flood RiskReduction Measures document

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Flood Risk in Guildford Urban Area

Guildford is an established riverside town and the largest urban area along the corridor of the RiverWey. It has a heavily developed floodplain, with both commercial and residential development builtup to the banks of the river (see figure 2c for a map of the land uses in Flood Zone 3 in GuildfordTown Centre). The floodplain is flat but constrained by the surrounding topography. Therefore, forthe larger flood events, the extent of the flooding does not significantly increase. Instead it is thedepth of flooding that changes. There are approximately 620 properties at risk from a 1 in 100 yearevent within Guildford Urban Area.

The principle fluvial watercourse is the River Wey which flows through the town centre. This includesthe Wey Navigation, which is a combination of navigable river and man made sections maintained bythe National Trust.

The primary source of flooding within Guildford is from fluvial (river) flooding from the River Weyalthough surface water flooding, groundwater flooding and flooding from land drainage systems andsewers can also be locally significant. Any further development could potentially increase surfacewater run offs if not designed correctly.

Managing and reducing flood risk is a priority for Guildford Borough Council and the EnvironmentAgency. A bypass tunnel has been considered in Guildford, but is a high risk option that is noteconomically viable and will become less effective with flow increases due to climate change7. Inaddition, the solution, involving the construction of a tunnel, would not be the best optionenvironmentally.

Consequently, our focus is on reducing the consequences of flooding. Where possible we also wantto ensure that we reduce the probability of floods occurring through providing additional floodwaterstorage and improving floodwater flows. As the urban landscape of the town changes we need to:

make it a safer place to live,reduce the time and cost of recovery after a flooding event, andenhance the River Wey environment for wildlife and recreation.

To achieve these aims we have developed this plan for future development and land use in thefloodplain in Guildford Urban Area. We have considered the floodplain through Guildford UrbanArea and its contents as a whole rather than looking at it on a site by site basis, as has been ourpractice in the past. This has allowed us to identify strategic principles for how and where we wantto bring about the greatest change in the future.

Surface Water Flooding

Surface water flooding was recorded in the Guildford Urban Area during the flood events ofNovember 2000, August 2006 and July 2007. The main cause of the flooding in July 2007 was fromthe highway due to drains being overwhelmed by large volumes of rainfall in a short time span.

7River Wey Flood Risk Management Draft Strategy 2010 Public Consultation Document

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The Environment Agency is due to publish detailed surface water maps by the end of 2010. Thesecan be obtained from the Environment Agency once completed. Surface water flooding can becaused by an isolated incident such as poorly maintained drains, culverts and gullies. It is difficult topredict such events by modelling in the same way as fluvial flooding. The surface water maps havebeen created using ground modelling and on the ground inspections for verification. Only on theground inspection can determine where problems are likely to occur due to lack of maintenance.

Ground conditions can also affect the extent to which surface water flooding can become a problem.In the summers of 2006 and 2007, flash flooding caused by heavy rain over a short period of timeoccurred in the Guildford Urban Area. Ground conditions in August 2006 after a long dry period,meant that surfaces were dry and hard. In July 2007 the soil was already saturated with water due tohigh rainfall totals in May and June. Extremely high rainfall totals and intensities then fell on alreadysaturated soils on the 19th and 20th July 2007. As a result, in both 2006 and 2007, there was lesssoakage and infiltration of surface water into green field sites. Consequently the volumes of waterflowing over ground and directly into surface water drainage systems increased. No fluvial floodingwas experienced from the River Wey in Guildford Urban Area in 2006 or 2007.

In January 2008 the opposite scenario occurred. Continuous heavy rain caused the River Wey tocome out of bank in the Millmead area of the town centre. The extent of the flooding only affectedriverside car park areas and no properties were flooded. There was no surface water flooding inJanuary 2008. Ground conditions were soft and not fully saturated so much of the rain fall soakedinto the ground and entered streams that flow into the River Wey. This caused the river to swell andconsequently come out of bank at Millmead.

The main tool for managing surface water flooding through development is via a Sustainable UrbanDrainage Systems (SUDS) approach. Details of SUDS systems can be found in figure 5. A variety oftechniques are available for reducing runoff rates from a site, but many of these require earlyconsideration in the development proposals. For example, green roofs, permeable paving, balancingponds and a host of other SUDS features, can be used together to attenuate and convey waterbetween different features. They slow down the flow of water to sewers and watercourses duringrainfall and can provide additional amenity, water quality, water resources, nature conservation andeven energy efficiency benefits. When SUDS techniques are used before surface water drains into asewer or watercourse, the existing surface water drains are less likely to become overwhelmed. It isimportant that ownership and regular maintenance is arranged at an early stage of the developmentto ensure SUDS systems continue to function as intended.

Critical Infrastructure

Guildford Police Station is located within Flood Zone 3 and Guildford Fire Station is just outside ofFlood Zone 3. Many of the major roads which link Guildford Urban Area with other parts of theborough could potentially be blocked during a flood event. This includes the route between the firestation and Royal Surrey Hospital. The main road between Guildford Urban Area and the RoyalSurrey Hospital is prone to flooding from highway drains being overwhelmed. It is also in Flood Zone3.

Many of the churches which have been identified as emergency shelters during non floodemergencies are in fact in Flood Zone 3, and therefore, could not be used during 1 in 100 year return

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period flood event. There are a number of other emergency rest centres that do not fall in the FloodZone which could be accessible in an emergency. The main railway lines in Guildford Urban Area areelevated so would not be affected by such a flood event. Many electricity substations are situatedwithin Flood Zone 3 so potentially electricity supplies could be affected within the Guildford UrbanArea.

The Council is currently preparing a Multi Agency Flood Plan which details how the Council willsupport the emergency services during a severe flood event.

Current land uses – reducing flood risk through redevelopment

To help us understand how we need to manage flood risk in Guildford, we have looked at thefloodplain according to the current (see figure 2b) and potential future land use (based on initialworkings from the Guildford Development Framework). To inform us about where the greatestchanges in Guildford’s river corridor and urban environment are required, we have combinedexisting information on land use type, flood risk and the consequences of flooding.

Different flood risk reduction measures will apply to different areas, depending on the type ofdevelopment. For each development type there is a potential proposal given as an example of whatwe want to achieve. As redevelopment of some of these areas can assist in reducing flood risk, anysite that comes forward in the future can be classified according to the categories below and therelevant guidance applied (see figures 4 and 5 for more details).

As a general principle, along the stretch of the River Wey that flows through the Town Centre,setting development back from the river will provide more space to accommodate water during aflood. Creating additional floodplain storage has multiple benefits including reducing the frequencyof flooding to the adjacent development, providing possible community benefit by creating publicaccess to the riverside (depending on site design) and also improving the natural river environmentthrough landscaping opportunities. Our target is that when a site is redeveloped, the newbuilding should not be any closer than the existing building footprint and should be set back at least8 metres (and more where possible) from the river.

Undeveloped natural floodplain

These are areas where the basic floodplain function remains intact and there are no significantbuildings. Open land uses such as playing fields and parkland are permitted with only necessaryessential infrastructure or water compatible development ensuring there is no loss of floodplain orobstruction of floodplain flows.

These areas are shown as Area 1 and Area 2 in figure 2b (p.17). They are the fields to the north ofthe A3 (Area 1 over 20Ha/0.2km) and Shalford Meadows (Area 2), an extensive area ofundeveloped floodplain to the south of the town centre.

The priority here is to safeguard undeveloped floodplain for flood storage purposes. This will ensurethat the floodplain continues to provide flood risk benefits locally and also further downstream.

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Non residential re development

Industrial areas consisting of warehouses, workshops and storage uses have high consequences interms of economic damages. These have been identified as high priority because re development ofthese sites can provide a large reduction in the total property damages in Guildford Town Centre.

For example, Walnut Tree Close, highlighted as Area 3 in figure 2b, contains a number of industrialproperties that have very high economic damages due to their use and floor area.

The aim for non residential re development is to reduce overall flood risk within these areas andbeyond. The probability of flooding can be reduced by creating space for flooding to occur within asite. Methods include restoring floodplain and flood flow pathways, setting properties back from thewatercourse, raising floor levels and by identifying open space for flood storage8 which create spacefor flooding to occur safely. Car parks, if designed appropriately, can contribute to these goals.

Any re development of these sites also provides an ideal opportunity to reduce the consequences offlooding. This can be done through layout and design. For example incorporating appropriateresistance or resilience measures, depending on how frequently the site floods and to what depth.See figures 4 and 5 for more detail about individual resistance and resilience measures.

Permanent resistance measures are preferable here rather than temporary ones to avoid issues suchas incorrect deployment (who is responsible, training requirements, staff turnover) and ability todeploy (flooding may occur outside of working hours). Additional investment in permanentresistance (e.g. permanent flood proof doors and windows) reduces the costs of damage by between65% and 84% per property compared to about 50% for temporary measures93.

Our target is to reduce property damages to individual properties by employing a range of riskreduction measures, as described above, depending on what is most appropriate on site. As anexample of what could be achieved, if all new non residential development in Area 3 was eitherflood resistant up to 0.6 metres or resilient up to 2 metres (depending on the average flood depthfor each area), this could reduce the total property damages in Guildford Urban Area byapproximately 15% for the 1 in 10 year flood event.

Increase in residential units

These comments apply to areas where an intensification of residential use is proposed, for example,replacing a row of town houses with flats. It equally applies to the splitting of one residential unitinto multiple separate units.

The risk to life and property are the top priorities for these forms of development. Consequently, theidentification of potential opportunities to relocate existing more vulnerable properties to an area oflower flood risk, ideally outside the floodplain, must be sought. For example, the swapping of aresidential site in Flood Zone 3 with a commercial site in Flood Zone 2 (land swapping).

8 See PPS25 Practice Guide (CLG,2009) for useful case studies.9 Developing the evidence base for flood resistance and resilience: summary report (Joint EA/Defra R&D Summary Report,June 2008).

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Where it is not possible to relocate to areas of lower risk, careful consideration to the design andlayout will be required. The aim is to provide a development which is safe to occupy and escapefrom in a flood event, whilst reducing flood risk to others.

For this to be achieved, new buildings should be laid out and designed so that no properties will beinundated by future flood events (i.e. all properties are made resistant), future residents have safeaccess and egress to their homes, and also, that the wider urban environment (streetscape, openspaces, car parking) is designed to be resilient to flooding.

As cars are easily moved prior to a flood event, given sufficient warning, damages to cars should beconsidered in emergency plans for residential sites. Where residents’ parking is provided on theground floor of a development, there must be measures to prevent them being swept away in aflood event.

Mixed use re development

Areas where there currently is mixed use development and any potential redevelopment is likely tocontribute to that mix, consisting of both residential and commercial uses in the area.

An example of this is the area marked as Area 4 in figure 2b where there are commercial uses (forexample the Bus Depot and Pembroke House) but also residential uses (for example Leas Road andMary Road area). All of these properties should be considered as part of any re development of thearea around the bus depot to achieve an overall increase in the resilience of the local area toflooding.

The objective in these locations is to reduce risk to life, reduce property damage from the currentestimated amount, and increase the speed at which people can return to their properties following aflood. This can be achieved by following the specific guidance set out in this document for this typeof development. For example, by applying the sequential test at a site level or introducing resistanceand resilience measures as properties are redeveloped or refurbished by individual property owners.In the case of the Bus Depot and surrounding areas, this is an ideal location to increase the resilienceof an area of the town to flooding. As a minimum we would require all new properties here to eitherbe flood resistant up to 0.6 metres or resilient up to 2 metres depending on the average flood depth.

As cars are easily moved prior to a flood event, given sufficient warning, damages to cars should beconsidered in emergency plans for residential sites. Where residents’ parking is provided on theground floor of a development, there must be measures to prevent them being swept away in aflood event.

Re development that includes a change of use to a more vulnerable land use

This relates specifically to sites where re development would result in a change of use that would beconsidered ‘highly vulnerable’ or ‘more vulnerable’ development.

For example, this would be the case should re development of the car park in Mary Road beconsidered (shown as Area 5 in figure 2b).

Where this type of re development is shown to be necessary to progress in accordance with theGDF, it will need to comply with all material planning considerations. The SFRA (volume 1) provides

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useful decision support information to assist in determining whether a development will be safe.Development needs to be designed to be safe with both safe access and egress. The developer willneed to show that future users of the development will not be placed at risk from flood hazardsthroughout the lifetime of the development (taking into account climate change). Mitigationmeasures will be needed and developers should ensure that appropriate evacuation and floodresponse procedures are in place. Where possible, developers should also demonstrate flood riskbenefits to nearby properties.

The objective in these locations is to reduce risk to life, reduce property damages from the currentestimated amount and increase the speed at which people can return to their properties following aflood. This can be achieved by following the specific guidance set out in this document for this typeof development. Resistance and resilience measures should be applied to all new buildings since thisprovides benefit both to the developer in terms of reduced re establishment costs following a floodand for the occupier in terms of minimal damage to the property and their possessions and a shorterrecovery time.

As cars are easily moved prior to a flood event, given sufficient warning, damages to cars should beconsidered in emergency plans for residential sites. Where residents’ parking is provided on theground floor of a development, there must be measures to prevent them being swept away in aflood event.

Existing residential (no re development planned in the short to medium term)

This document has identified where, considering the nature of the existing flood hazard, redevelopment would deliver a reduction in flood risk, and could be promoted through GDF policy.The objective of such a policy, in line with the Catchment Flood Management Plan (CFMP), would beto prioritise redevelopment where it achieves a net reduction in flood risk by providing a floodresilient urban environment and safer properties to live in. However, when the age of existingproperties is considered and where the likelihood of redevelopment in the next 25 years is low,policies to promote resistance and resilience during refurbishment of existing properties should beprioritised.

Examples include the Leas Road/Mary Road/Wharf Road area where there are approximately 70residential properties in the 1 in 100 year flood extent and approximately 75 residential properties inthe floodplain on the other side of the river in Walnut Tree Close.

Different resistance or resilience products can be retro fitted to these properties depending on flooddepths and cost benefits. For example, flood resilience is more effective than resistance for deeperfloods (above 60 90cm) which would overwhelm barriers such as door guards and could causestructural damage to the property if the water was held back.

The lower cost of resistance measures mean they are likely to be economically worthwhile, if theannual chance of flooding is 2% or more (1 in 50 years) for temporary measures and 4% or more (1in 25 years) for permanent measures (as they are more expensive). To be effective, these measuresdo need to be applied to the whole house at once whereas resilience measures could be installedgradually over time, for example when re fitting a kitchen or during repair following a flood event.

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To introduce packages of resilience measures in homes not already in need of refurbishment is onlycost beneficial in areas with a 4% or greater annual chance of flooding10.

For every residential property in the 1 in 20 year floodplain that is:

(a) in an area where the maximum depth of flooding is 0.6 metres or less, homeowners shouldlook to install permanent resistance measures to reduce property damages by up to 80%.

(b) in an area where flood depths are over 0.6 metres, resilience measures should beincorporated during repair or major renovation. Where refurbishment is not being carriedout, there may be a possibility of funding through the Defra Property level Flood Protectionand Resilience Grant.

The Council will apply for these grants when and if they become available. The Environment Agencywill let the Council know when grants become available and open to bids.

Guildford Borough Council is actively applying for central government grant funding for floodprotection projects within the borough. Although outside of Guildford Urban Area, £60,000 of Defrafunding has already been awarded through the Environment Agency for a £120,000 project toprotect the village of Ash Green from flooding. The scheme is scheduled for completion in 2011.

Existing commercial (no re development planned in the short to medium term)

Properties at high risk but with lower consequences (in terms of costs) than the sites identified inArea 3 above.

When re development opportunities arise in the longer term (in 25 years or more), look to returnthese areas to less vulnerable land uses or those that are flood compatible, for example throughland swapping opportunities. In the short to medium term, consider retro fitting with resilience orresistance measures. For the average office based business that floods more than once every 10years, the benefits of installing resistance measures outweigh the up front investment by betweensix and eleven times1112(over the lifetime of the resistance measure).

10consultation on policy options for promoting property level flood protection and resilience (Defra, July 2008)

11Developing the Evidence Base for Flood Resistance and Resilience. Summary Report. R&DTechnicalReportFD2607/TR1http://randd.defra.gov.uk/Default.aspx?Menu=Menu&Module=More&Location=None&ProjectID=14738

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Figure 2a Guildford Urban Area

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Figure 2b – Location of land use/development examples in Guildford Town Centre

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Figure 2c Property Type within 1 in 100 Flood Outline

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Flood Risk Reduction Measures in Existing and New Developments in Guildford Urban Area

Risk Reduction Measures in Guildford Urban Area

The probability of fluvial flooding can be reduced but never eliminated. For sustainable developmentto take place in towns that are within the floodplain, it is likely that some infrastructure andbuildings will be at risk from floodwater.

For properties vulnerable to flooding, it is important to limit the risk to life, damage costs and theamount of time that the property becomes uninhabitable. In Guildford Urban Area, implementingflood risk reduction measures has been accepted as the most sustainable way of reducing theconsequences of flooding (see Thames CFMP and River Wey Flood Risk Management Draft Strategy2010).

The range of flood risk reduction measures outlined in figures 4 and 5 are based on evidence fromlaboratory tests, technical evidence and industry experience. They are designed to help developersidentify the best option or design strategy for managing flood risk at both a property and site level.The techniques that reduce flood risk and provide resistance and resilience to flooding can also beretrofitted to existing properties, as described in figure 4. They must, however, be considered for allnew developments, as described in figure 5. The majority of measures, particularly flood resilienceand resistance (also defined as flood proofing) involve any combination of structural or nonstructural changes or adjustments incorporated in the design, construction or alteration of individualproperties that will reduce flood damage. The type of damage which some of the flood riskreduction measures can prevent are listed in figure 3.

Figure 3 Table 2.7 An extract from ‘Improving the flood performance of new buildings – May 2007’,Defra, CLG and Environment Agency

Depth ofFloodwater

Damage to the building Damage to services and fittings

Below groundfloor level

Possible erosion beneath foundations,causing instability and settlement

Possible corrosion in metal components (e.g.joist hangers)

Excessive moisture absorption in timber,causing warping

Cracking of ground floor due to upliftpressures

Accumulation of contaminated silt

Structural and material weaknesses frominappropriate drying

Rot and mould

Damage to electrical sockets andother services to basements andcellars

Damage to fittings in basementsand cellars

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Existing Development

Figure 4 lists some examples of the possible flood resistance and resilience measures that can beretro fitted to existing properties. These measures may reduce the damage, cost and time to repair aproperty if it is flooded.

The probability of flooding and the likely depth of water are both key factors determining the scaleand nature of the technique chosen. The type of measure selected will depend on the depth of floodwater around the property. The maximum return period for which resistance and resiliencemeasures are cost effective for existing properties is 1 in 50 years. The 1 in 50 flood outline isavailable from the Environment Agency, however, the 1 in 20 flood outline, as shown in the SFRA, isa useful guide in the first instance.

N.B. The flood depths outlined in figure 4 should be taken as a general guide specified by the DCLG‘Improving the flood performance of new buildings – May 2007’. For further specific information onflood depths, please refer to the Guildford SFRA Annex E.

Ground levelto half ametre abovefloor level

Build up of water and silt in cavity walls, withpotential reduction in insulating properties,for some materials

Immersed floor insulation may tend to floatand cause screeds to debond

Damage to internal finishes, such as wallcoverings and plaster linings

Floors and walls may be affected to varyingdegrees (e.g. swelling) and may requirecleaning and drying out

Timber based materials likely to requirereplacement

Damage to internal and external doors andskirting boards

Corrosion of metal fixings

Rot and mould

Damage to water, electricity andgas meters

Damage to low level boilers andsome under floor heatingsystems

Damage to communicationwiring and services

Carpets and floor coverings mayneed to be replaced

Timber based kitchen units arelikely to require replacement

Electrical appliances may need tobe replaced

Insulation on pipe work mayneed replacing

Half a metreand above

Increased damage to walls (as above)

Differential heads of greater than 0.6m acrosswalls could cause structural damage,although this will vary depending on thestructure of the building. Damage to windowscan be caused by much smaller differentialpressures

High speed flow around the buildingperimeter can lead to erosion of the groundsurface; there is also the potential risk ofdamage to the structure from large items offloating debris, e.g. tree trunks

Damage to higher units,electrical services and appliances

21

Guidance on the range of measures available and estimated costs is available on the EnvironmentAgency and Defra websites13.14If unfamiliar with the different techniques it is recommended to seekprofessional advice.

13 Developing the Evidence Base for Flood Resistance and Resilience. Summary Report. R&D Technical ReportFD2607/TR1http://randd.defra.gov.uk/Default.aspx?Menu=Menu&Module=More&Location=None&ProjectID=14738http://www.environment agency.gov.uk/homeandleisure/floods/31644.aspx

22

Figure 4 Flood Protection Measures for Existing Property Owners

FloodDepth Measure

Type ofMeasure

Description Benefits Design / Operational Considerations

Over 0.6mall floodswhichenterproperty

Movingservices andelectricalsockets abovethe flood level

Permanent

FloodResilience

Place fittings such as electricalappliances, utility meters wellabove ground level so that theyare out of reach of flood water

Extremely effective andrelatively inexpensive,compared to the cost of flooddamageCosts no extra if undergoing aplanned refurbishment

Ensure that the property is completelydry before working with electrics

0.3 – 0.6m

ResilientInternal walls

Permanent

FloodResilience

Walls coated with internal cementbased renders; apply tanking onthe inside of every internal wall

Permanenteffective if applied correctlyMinimises damages to walls andfloorsDries quicker (back in propertysooner)

Requires good workmanshipMust be regularly checked andmaintained

0.3 – 0.6m

Resilientfloors

Permanent

FloodResilience

Ground supported floors withconcrete slabs coated withimpermeable membrane

Permanent, effectiverelatively inexpensive comparedto damage cost

Requires quality productsSkilled workmanship

Up to 0.6m

Basement orcellar tanking

Permanent

FloodResilience

Tanking basements, cellars orground floors with water resistantmembranes

Permanent, effective if appliedcorrectly. Minimises damage towalls and floorsDries quicker (back in propertysooner)

Can only be applied when property isfully dryUse in conjunction with resistant doorsand other resilient measuresRequires good quality workmanship byspecialists

All

Resilientinternal doors

Temporary

FloodResilience

Water resistant plastic doors sothat flood damage is minimised.Can be fitted with adjustablehinges so that doors may be easily

Easily removableLightweight

Storage may be difficult

23

removed before floodwater entersproperty

Up to 0.6m

Floodresistant/water proofexternaldoors andwindows

Permanent

Floodresistance

Use of materials and constructionswith low permeability

Proven in floodsReduces flood ingress anddamage

Extensive groundwork may be requiredMust not be used in isolationMay require regular maintenance

Up to 0.6m

Floodresistancegates

Permanent

Floodresistance

Solid gates with discreetwaterproof seals

Prevents floodwater reachingpropertyTried and testedVery effective and unobtrusive

May require ancillary pumparrangementsRegular maintenance must be carried out

Up to 0.3m

Air brickcovers

Temporary

Floodresistance

Sealed covers fitted outside of airbricks to stop ingress of floodwater through vents in the airbricks

Easy and quick to applyEffectiveEasy to remove

Requires occupant to remove cover whenflood subsides

Up to 1m

Local Bunds Permanent

Floodresistance

Low earth bund to provide aneffective local flood defence

Prevents floodwater reachingpropertyCommunity protection

Suitable where this will not impact onflood water flows or storageMay need to provide pumpingarrangements to remove potentialrainwater or floodwater seepage.May need to incorporate flood resistantgatesWill require expert engineering design toensure breaching will not occurWill require assessment to show thatflood risk is not increased elsewhereMay require floodplain compensation

Up to 0.3m

Sump andPump

Permanent

Floodresistance

A system which could be installedfor domestic properties so thatfloodwater may be pumped awayfaster than it enters

If fitted correctly, and ofsufficient capacity, can removefloodwater in an emergency

Must be positioned and sized correctlyMay require external or ancillary powersupply

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Up to 0.3m

Storm Porch Permanent

Floodresistance

Constructing porches to providean added barrier to flood waterreaching the front or back door.The porches are built with a higherdoor step level than the existingentrance

Cost is equivalent to a standardporchRamps can be built for disabledaccessUnobtrusiveDesigned to complimentproperty design

More effective if water resistantmaterials were to be used.Requires good workmanshipMay need similar work at rear ofproperty

Up to 1.2m

BoundaryWalls andfencing

Permanent

Floodresistance

Designed to create flood resistantbarriers. Solid gates with discreetwaterproof seals, or fencing wherethe lower elements areconstructed to be more resistantto flooding

Prevents floodwater reachingpropertyGates are tried and testedvery effectivemodern, aesthetic designsunobtrusive

May affect flood risk to neighboursRequires regular maintenance

All

RainwaterHarvesting

Permanent

SustainableDrainageSystem

Capture of rainwater from roofs,and reusing for domestic purposes

Reduction in the volume andrate of storm water runoffAdditional benefit of achievingwater efficiency in newbuildings

For detailed design requirements, referto CIRIA C609

All

PermeablePaving

Permanent

SustainableDrainageSystem

Infiltration of surface waterthrough the hard standing intounderlying layer

Reduction in the volume andrate of storm water runoffSystem allows forsedimentation, filtration,adsorption, biodegradation andvolatilisation of pollutants

For detailed design requirements, referto CIRIA C609 (Section 3.6 in particular)

All

On siteattenuation(e.g.Detentionbasins andponds)

Permanent

SustainableDrainageSystem

On site storage techniques,usually above ground, designed tocapture surface water runoff

Provide storm waterattenuation and volumereductionPollutant removal viaadsorption and biodegradationProvide green open space

For detailed design requirements, referto CIRIA C609

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All

ImprovingFloodWarning

Permanent Sign up to the free FloodlineWarnings Direct service operatedby the Environment Agency bycalling Floodline on 0845 988 1188or applying on the website:

www.environmentagency.gov.uk/floodline

Saves lives by providingadvance warning of a floodevent so people can evacuateproperties and find safe shelterProvides vital time to putdamage prevention measures inplace, should it be necessary,such as storing valuables,sentimental items andimportant documents upstairsor well above expected floodlevels and installing temporaryresistance measures

Requires an awareness of what thedifferent levels of flood warning are andwhat the appropriate action is to takeDoes not include surface water floodingdue to the speed at which it can occurand the unexpected places that can beaffected

All

EmergencyPlanning andresponse

Permanent Ensure your household has anemergency flood plan in place soyou know what action to takeafter receiving a flood warning.This involves preparing a flood kitof essential items (e.g. torch,bottled water, first aid kit),knowing how to turn off gas,water and electricity supplies,what possessions need to bemoved above the flood level andknowing who to contact.

Being prepared saves lives andreduces the damage to yourproperty and possessions

There may not be the time available toreturn to your house and implement theplan before the onset of flooding occurs

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New Developments

Development pressure in Guildford Town Centre and the surrounding urban area is increasing. Asthe main settlement in the borough, the Town Centre is a focal point for new development, and animportant location for sustainable growth.

Much of Guildford has a heavily developed floodplain, where flooding from the River Wey occursfrequently. Flood risk is also predicted to increase. Due to changing weather patterns it is likely thatwe will see increasing winter river flows and more frequent and intense summer rainfall. This,coupled with the competing demand for land in the urban area within and around Guildford TownCentre means that an acceptance of some level of flood risk is necessary.

Subject to the Sequential Test, and the Exception Test, redevelopment can occur in Flood Zone 3. Ithas, however, been agreed that such a flexible approach to the Functional Floodplain in GuildfordUrban Area will not be employed until complementary flood risk reduction measures areimplemented.

Figure 5 outlines a range of flood risk reduction measures which could potentially be applied to passpart C of the PPS25 Exception Test. For the purpose of this document, the flood risk reductionmeasures are allocated according to the land use type (as outlined in figure 2b and correspondingtext). PPS25 Practice Guide: December 2009 should be used to supplement the information in figure5.

The table provides possible methods with advice on the benefits they could bring. It is not necessaryto include all methods listed under a required risk reduction measure however the table indicateswhich are essential for different development types. Some measures will be beneficial depending onthe individual site design. This is indicated in the table. The FRA must address which measures havebeen included and the reasoning for this. A FRA must show that all possible flood risk reductionmeasures have been considered and a satisfactory amount included in the development proposal.The Environment Agency and Guildford Borough Council shall jointly decide whether all possibleflood risk reduction measures have been included in the FRA.

The flood risk reduction measures in Figure 5 should be worked through as development proposalsare prepared. Some measures need to be considered at the early stages of the design process andwould be difficult to address retrospectively. It is advised that each flood risk reduction measurelisted in Figure 5 is considered and incorporated into the design process as required.

NB: Guidance on the range of measures available and estimated costs is available on theEnvironment Agency and Defra website15.

If unfamiliar with the different techniques it is recommended to seek professional advice.

15 Developing the Evidence Base for Flood Resistance and Resilience. Summary Report. R&D Technical ReportFD2607/TR1http://randd.defra.gov.uk/Default.aspx?Menu=Menu&Module=More&Location=None&ProjectID=14738http://www.environment agency.gov.uk/homeandleisure/floods/31644.aspx

27

Figure 5 Risk Reduction Measures for New Development

DevelopmentProposal

Flood RiskReductionMeasures /Approach

Description Requirements When to include

Benefits

F = Flood Risk

EC = Environment &

Community

P = Property Owner

* Depending on design

All Redevelopment

All redevelopmentSustainable

Drainage Systems(SUDS)

Techniques which caneither increase surfacewater attenuation orencourage infiltration andreduce the rate of surfacewater run off

Permeable paving, infiltration trenches, swales, wetlands, rainwaterharvesting and green roofs all contribute to sustainable drainagedesign

These systems reduce the amount of water, or delay water, enteringrivers in the build up to a flood

Essential for alldevelopment F, EC*, P*

All redevelopment Non structural Improving Flood Warning

Encourage all residents and businesses to sign up to the FloodWarnings Direct service operated by the Environment Agency

Warning provides time to prepare for a flood, including setting uptemporary defence measures and moving valuables

Essential F, EC, P

All redevelopment Non structuralEmergency Planning andresponse

Ensure robust emergency (evacuation) plans are implemented for newdevelopments

An emergency plan assists people in reacting to a flood warning,allowing the right actions to be undertaken at the right times

Essential F, EC, P

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All redevelopmentMake Space for

WaterMaintain and improvefloodwater flows

Remove boundary walls, or replace with other forms ofenclosure/screening (hedges or fencing with gaps allowing floodwater flows)

Make provisions for the development to allow flood water to flow orprovide diversion routes

Essential for alldevelopment F, EC*

All redevelopmentFlood ResistantConstruction

Techniques which aim toreduce the amount ofwater that enters aproperty and maintainsthe property’s structuralintegrity

Where possible, raise the finished floor level so that the underside ofthe suspended floor is 300mm above the 1 in 100 with climate changeflood water level (data available from the Environment Agency) Thiswill allow flood water with debris to flow easily under the buildingduring a flood maximising flood storage, This is the most effective wayof protecting a property from flooding. If this cannot be achieved thenevidence must be provided in the FRA. .

These techniques prevent the disruption caused by floodwaterentering a property:

Constructing porches to provide an added barrier to flood waterreaching the front or back door. Build with a higher door steplevel than the main entranceInstall flood resistant, solid gates with discreet waterproof sealsLow earth bund to provide an effective local flood defenceSump and pump for floodwater to extract water faster than itenters

Must be consideredalongside FloodResilientConstructionMeasures Acombination maybe the best solution

F, P

All redevelopmentand Change of Use

Flood ResilientConstruction

Methods which allowfloodwater to enter thebuilding, but the amountof damage caused by anywater reduced, due togood design and fittings.May be more appropriate

These measures allows the property to become fully usable farquicker after a flood event:

Raised fittings such as electrical appliances, utility meters so theyare out of reach of flood water.Walls coated with internal cement based renders; apply tanking onthe inside of every internal wallGround supported floors with concrete slabs coated with

Must beconsideredalongside FloodResistantConstructionMeasures Acombination may

F, P

29

for refurbishment orchange of use.

impermeable membraneTanking basements, cellars or ground floors with water resistantmembranesWater resistant plastic internal doors minimising flood damages

be the bestsolution

All redevelopmentnear the River Wey

Make Space forWater

Set development backfrom river

A zone adjacent to the river must be created or expanded for sitesadjoining the River Wey. This will create space for floodwater storageand flow

For sites adjoining the River Wey, all existing buffer zones adjacent tothe river must be maintained and not reduced in width. Wherepossible these buffer zones should be made wider. For sites wherecurrently there is no buffer zone, new buffer zones should be created.These buffer zones will create space for floodwater storage and flow.

A natural or semi natural buffer zone, at least 8 metres wide whenmeasured from the top of the river bank, should be included betweendevelopments and the River. This zone can be designed for publicaccess to the river and to enhance biodiversity.

Essential for sitesadjoining the RiverWey

F*, EC, P*

All redevelopmenton sloping oruneven sites

Make Space forWater

Additional floodplainstorage

The lowering of ground levels on a site provides additional space forfloodwater to be stored. These measures work best when applied touneven sites and sloping sites, where it is possible to extend thefloodplain without creating ponds which fill with water.

Can be combined with other measures, provided the lowered areaallows water to flow back to the river when the flood water retreats.

Where possible, ground levels must be lowered on site to provideadditional flood storage

Land raising will not be permitted

Must be exploredthrough levelsurveys

F*, EC*

30

All redevelopmentnear existing

residential areas

Community SafeAccess

To ensure that allresidents can be safelyevacuated in times offlood

Options to provide or improve the safety of access/egress for existingresidents must be explored for redevelopment sites close to existingresidential areas. Where land raising cannot provide safe access thenother ways of helping the local community escape should be used.This could include signage to show evacuation routes, bollards toindicate pavement edge etc. The provision of safe access and escapein times of flood substantially reduces the risk to life

Explained in DEFRA Research and Development document FD2320and supplementary documents (See Glossary for more details)

Must be exploredand providedwhere practical

F, EC

All multi storeyredevelopments Make Space for

WaterReduction in ground floorfootprint

Create undercover car parks. Where ground floor car parking is notfeasible, a reduction in ground floor footprint will need to beincorporated into the design. This provides space for floodwaterstorage.

Where insufficient height for car parking, an open area should becreated under the building to allow flood water storage.

Whether the space is used for paring or just kept open, there must besuitable flood openings to allow floodwater under the building.

Essential F, EC*

All redevelopments Spatial Planning Land swap

Seek opportunities to ‘swap’ more vulnerable allocations and siteswith those either:

a) outside the floodplain, or

b) with lower vulnerability uses

Reduces risk by placing more vulnerable developments ain areas oflower flood risk

Must be explored F, EC*

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Residential and/or mixed use Redevelopments

Residentialredevelopment

Safe AccessTo ensure that residentscan be safely evacuated intimes of flood

This is an absolute minimum for new residential developments. Theprovision of safe access and escape in times of flood substantiallyreduces the risk to life.

Explained in DEFRA Research and Development document FD2320and supplementary documents (See Glossary for more details)

Essential fordevelopments witha residentialelement

F, P

Residentialredevelopment

Flood ResistantConstruction

Techniques which aim toreduce the amount ofwater that enters aproperty and maintainsthe property’s structuralintegrity

The finished floor level for new residential developments must beraised so that the underside of the suspended floor is 300mm abovethe 1 in 100 with climate change flood water level (data available fromthe Environment Agency) This will allow flood water with debris toflow easily under the building during a flood maximising flood storage,This is the most effective way of protecting a property from flooding.If this cannot be achieved then evidence must be provided in the FRA..

Essential fordevelopments witha residentialelement

F,P

Mixed useredevelopments

ReduceVulnerability

Raise the residentialaspect

Place low risk developments at ground floor (for example in a mixeduse development, place the commercial development at ground floorand residential on higher floors)

Protection is provided for the more vulnerable properties, which willbe located above the flood water levels

Essential F*, P*

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Commercial or Industrial Redevelopment

Commercial orIndustrial

redevelopmentSafe Access

Assisting evacuation intimes of flood

These developments must explore safe access routes. Wherepractical, links to existing safe routes shall be provided. Where notpractical, the safety of the access shall be improved

This provides occupants with a safe evacuation route in times of floodand to maintain operations in times of flood

Explained in DEFRA Research and Development document FD2320and supplementary documents (See Glossary for more details)

Must be exploredand providedwhere practical

F, P

33

Tracking and Monitoring

This document will be updated when new information becomes available. This is the secondpublished version of this document. It has been updated to incorporate feedback from its pilot useand to include the revised River Wey modelling (June 2010). Any new sources of data/informationwill be reviewed through the Annual Monitoring Report (AMR) produced by the Council each year,and will trigger a review/update of this document when required.

Local Authorities are currently required to monitor a series of National Indicators16. The EnvironmentAgency is responsible for reporting local authority performance against one of these indicators: NI189: Flood and Coastal Erosion Risk Management, which is defined as:

Percentage of agreed actions to implement long term flood and coastal erosion riskmanagement plans that are being undertaken satisfactorily.

NI 189 focuses on Local Authority progress in delivering agreed actions from Catchment FloodManagement Plans (CFMP) and Shoreline Management Plans (SMP). The Council worked with theEnvironment Agency to agree specific NI 189 actions to implement in Guildford Borough for the year2009. The Flood Risk Reduction Measures document formed an important part of theCouncil’s NI189 actions for the year and its response to the Thames CFMP. The Council made itsNI189 return in April 2010. As the document was completed, published and monitored through theCouncil's Annual Monitoring Report in 2009, the Council has, therefore, successfully met andimplemented one of its key NI 189 actions.

16 Introduced by Communities and Local Government on 11 October 2007.

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Other sources of information (references to other documents)

Improving the flood performance of new buildings (Defra, CLG and Environment Agency, May 2007)

PPS25 Practice Guide (Communities and Local Government, December 2009)

Repairing buildings following flooding, (ciria, 2007)

River Wey Flood Risk Management Draft Strategy 2010, public consultation document (EnvironmentAgency)

Thames Catchment Flood Management Plan (Environment Agency, July 2008)http://www.environment agency.gov.uk/research/planning/114391.aspx

Hallmarks of a Sustainable City, (CABE 2009)

River Basin Management Plan, January 2010

Contact Details

Guildford Borough Council Planning Policy Team – 01483 444471

Guildford Borough Council Development Control – 01483 444609

Guildford Borough Council Engineers – 01483 444506

Local floodline/emergency sandbag line 01483 444499 (daytime)

Local floodline/emergency sandbag line 01483 455091 (out of hours). This should only be used in anemergency.

Environment Agency 08708 506 506 (Mon Fri, 8am 6pm)

For planning enquiries at the Environment Agency: planning.frimley@environment agency.gov.uk

Environment Agency Floodline 0845 988 1188

Useful websites

Environment Agency http://www.environment agency.gov.uk

Environment Agency Planning Pre Application Enquiry Form – http://www.environmentagency.gov.uk/research/planning/33580.aspx

Environment Agency, Green roof toolkit http://www.environmentagency.gov.uk/business/sectors/91967.aspx

Environment Agency, Guide for Developers http://www.environmentagency.gov.uk/business/sectors/32695.aspx

Communities and Local Government (to view PPS25 and its practice guide) –http://www.communities.gov.uk

35

Royal Institute of Chartered Surveyors http://www.rics.org

Royal Institute of British Architects (RIBA) http://www.architecture.com

Institution of Civil Engineers (ICE) http://www.ice.org.uk/homepage/index.asp

Guildford Development Framework (GDF) – www.guildford.gov.uk/planningpolicy

Guildford Core Strategy – www.guildford.gov.uk/corestrategy

Met Office http://www.metoffice.gov.uk/

Websites with information on resilience and resistance measures:

Defra (department for environment food and rural affairs) http://ww2.defra.gov.uk/

Environment Agency (preparing for a flood) http://www.environmentagency.gov.uk/homeandleisure/floods

Glossary

Actual Risk The risk from flooding based on best available informationand representing the influence of flood defences and thedistribution of risk within the Flood Zones.

Annual Exceedance Probability(AEP)

The probability of fluvial (river) flooding is described usingthe Annual Exceedance Probability (AEP). This is sometimesknown as the ‘annual probability’ of flooding. A flood eventdescribed as a 1% AEP has a 1% (or 1 in 100) chance ofoccurring in any given year. This could alternatively bedescribed as an event that is likely to occur, on average,once every 100 years.

Buffer zone An undeveloped area between the watercourse and thedevelopment footprint which allows appropriate access forroutine maintenance and emergency clearance.

CFMP Catchment Flood Management Plan (see Annex A for moreinformation)

CLG Communities and Local Government (previously Office ofthe Deputy Prime Minister ODPM)

DEFRA Department for Environment, Food and Rural Affairs

DEFRA Research andDevelopment documentFD2320 and supplementarydocuments. See Annex C forSupplementary Note

To assist with flood risk assessment for new propertydevelopments in England and Wales, Defra and EA hadcommissioned a Research and Development project underFlood and Coastal Erosion Risk Management’s RiskEvaluation and Understanding Uncertainty Theme. This

36

project (FD2320) started in December 2003 and wascompleted March 2005.

The main deliverable of FD2320 was to provide anoverarching risk science based framework, simple tools andguidance with a nationally consistent approach to assessingand managing flood risk for new development acrossEngland and Wales. This involved understanding theindicators and defining what an appropriate assessment offlood risk should be at all scales of development planningand all types of development. This has been achieved byintegrating and simplifying existing guidance documents andthe latest findings from an extensive range of researchprojects and government/Agency initiatives.

Technical Reports of the Project is available from thefollowing website:

http://sciencesearch.defra.gov.uk/

Environment Agency (EA) Environment Agency an Executive Non DepartmentalPublic Body responsible to the Secretary of State forEnvironment, Food and Rural Affairs and an AssemblySponsored Public Body responsible to the National Assemblyfor Wales.

Its principal aims are to protect and improve theenvironment, and to promote sustainable development. Itplays a central role in delivering the environmental prioritiesof Central Government and the Welsh AssemblyGovernment through our functions and roles.

Exception Test Designed to test suitability of a development in a high floodrisk zone

Flood Flow Pathway The route that floodwater takes during a flood

Flood and Water ManagementAct 2010

The aim of the Act is to give the Environment Agency anoverview of all flood and coastal erosion risk managementand unitary and county councils the lead in managing therisk of all local floods.

Flood Risk Regulations 2009 The Flood Risk Regulations 2009 (Regulations) transposedthe European Floods Directive into law for England andWales and came into force on 10 December 2009. TheFloods Directive sets out requirements of the Lead LocalFlood Authority with help from the borough councils tomanage flood risk from all sources in order to reduce theconsequence of flooding on human health, economicactivity and the environment.

Flood Resilience Also known as wet proofing, flood resilience methods allowfloodwater to enter the building, but the amount of damage

37

caused by any water that gets into the house is reduced,due to good design and fittings. This means no permanentdamage is caused, the structural integrity of the property ismaintained and drying and cleaning are facilitated.

Any resilience measures should be designed so that thebuilding can be occupied safely over its proposed lifetimetaking climate change into account.

Flood Resistance Also known as dry proofing, flood resistance is concernedwith constructing a building in such a way to preventfloodwater entering it in the first place and damaging itsfabric. This technique aims to reduce the amount of waterthat enters a property and maintains the property’sstructural integrity. Trying to limit deeper water getting intothe property would be difficult and could lead to structuraldamage. The materials and construction methods chosenshould help drying and cleaning.

Flood Warning Improving flood warning services to residents and businessis an important measure to manage flood risk in Guildford.The Environment Agency issues flood warnings to propertiesat risk of flooding from main rivers, with Floodline WarningsDirect (FWD). FWD is a free service that provides earlywarnings of potential flooding to homes or business. It is theUK's first integrated multi channel warning system, wherebythe public, businesses and other professional partners,including Local Authorities, the Emergency Services, utilities,as well as local radio stations are informed directly bytelephone, mobile, email, SMS text message and fax, whenflooding is expected. Register by calling Floodline on 0845988 1188

FRA Flood Risk Assessment (see Annex A for more information)

Flood Zones (EA) This refers to the Environment Agency’s Flood Zones. Theseare areas, also known as floodplains, which could beaffected by flooding from rivers and the sea. In the case ofriver flooding Flood Zone 3 indicates the extent of a floodwith a 1 per cent (1 in 100) chance of happening in any year.

In the case of flooding from the sea Flood Zone 3 indicatesthe extent of a flood with a 0.5 per cent (1 in 200) chance ofhappening in any year.

The Flood Zones also indicate the extent of an extremeflood from rivers or the sea with a 0.1 per cent (1in 1000)chance of happening in any year (Flood Zone 2).

These Flood Zones are defined in the government’s planningpolicy for England. They ignore the presence of existingflood defences, since defences can be ‘overtopped’ if a floodoccurs which is higher than the defences are designed to

38

withstand. Defences can even fail in extreme events.

Fluvial Flooding Flooding from rivers

Guildford Borough Council(GBC)

The local authority or council that is empowered by law toexercise planning functions. Often the local borough ordistrict council. County councils are the authority for wasteand minerals matters

GDF Guildford Development Framework. See Annex A for moreinformation.

LLFA Lead Local Flood Authority.

LLFAs are county or unitary authorities, and are thecompetent authorities for all other sources of floodingwhilst these sources will predominantly be 'local' forexample, surface water, groundwater and ordinarywatercourses, the Regulations also means it includes canalsand flooding from lakes within their areas.

Surrey County Council is the LLFA for Guildford BoroughCouncil.

LPA Local Planning Authority

Main River As defined by the Environment Agency main rivers areusually larger streams and rivers, but also include smallerwatercourses of strategic drainage importance. A main riveris defined as a watercourse shown as such on a main rivermap, and can include any structure or appliance forcontrolling or regulating the flow of water in, into or out ofthe main river. The Agency’s powers to carry out flooddefence works apply to main rivers only. Main rivers aredesignated by the Department for Environment, Food andRural Affairs in England.

PPS 3 Planning Policy Statement 3: Housing. Planning PolicyStatement 3 (PPS3) underpins the delivery of theGovernment's strategic housing policy objectives. PPS3 wasupdated in June 2010.

PPS 25 Planning Policy Statement 25: Development and Flood RiskGuidance replacing PPG 25 in December 2006 and wasupdated in March 2010. It outlines how flood risk should beconsidered at all stages of the development process.

PPS 25 Practice Guide(December 2009)

This practice guide is complementary to Planning PolicyStatement 25: Development and Flood Risk (PPS25) andprovides guidelines on how to implement development andflood risk policies by the land use planning system. The

39

guide also includes working examples through case studies.

Previously Developed Land Land which has been previously developed (see PPS3 p.26for a detailed definition – www.communities.gov.uk).

Residual Risk A flood event more severe than that for which particularflood defences of structures have been designed to provideprotection.

River Basin Management plan River Basin Management Plans are plans for protectingand improving the water environment and have beendeveloped in consultation with organisations andindividuals. They contain the main issues for the waterenvironment and the actions needed to deal with them.

Safe Access Enable safe access and escape in an emergency.

Sequential risk basedassessment

Priority in allocating or permitting sites for development, indescending order to the Flood Zones set out in Table D1 ofPPS 25, including the sub divisions in Zone 3. Thoseresponsible for land development plans or decidingapplications for development would be expected todemonstrate that there are no reasonable options availablein a lower risk category.

SFRA Strategic Flood Risk Assessment (see Annex A for moreinformation)

Study Area Refers to the Guildford Borough council area.

Sustainable Urban DrainageSystem (SUDS)

Conventional drainage practises, where piped systemsdominate, remove water as quickly as possible from a site,thereby increasing the risk of surface water flooding. Unlikeconventional drainage, sustainable urban drainage systems(SUDS) minimise the risk of flooding by managing rainwaterrunoff by replicating natural drainage patterns of a site.

SUDS are a more environmentally friendly way of managingsurface water runoff as close to the source as possible usingnatural processes to deliver water quality and amenitybenefits. Techniques such as permeable paving, infiltrationtrenches, swales, wetlands, rainwater harvesting and greenroofs can either increase surface water attenuation orencourage infiltration and reduce the rate of surface waterrun off.

Sustainable Urban Drainage Systems (SUDS) should be

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implemented to ensure that runoff from the site (postdevelopment) is reduced, with space set aside within theconfines of the site to allow its implementation. The use ofSUDS techniques and attenuation should take into accountthe local geological and groundwater conditions.

Thames Region Land DrainageByelaws (1981)

Thames Region Land Drainage Byelaws 1981 are made bythe Thames Water Authority under Section 34 of the LandDrainage Act 1976. The Land Drainage Byelaws wereamended by Thames Region Flood defence Byelaws 1991.

Byelaw 4, Control of structures pipes and cables statesspecifically in part (b); No person shall, without the previousconsent of the Authority: ...(b) erect or construct, or causeor permit to be erected or constructed, any fence, post,pylon, wall, or any other building or structure within 8metres measured horizontally from the foot of any bank ofthe river on the landward side or, where there is no suchbank, within 8 metres measured horizontally from the topedge of the batter enclosing the river.

1 in 20 year return periodflood event

The flood event that is predicted to occur with an annualprobability of 5% (there is a 1 in 20 (5%) chance in any yearthis event will happen).

1 in 100 year return periodflood event

The flood event that is predicted to occur with an annualprobability of 1% (there is a 1 in 100 (1%) chance in any yearthis event will happen)

1 in 1000 year return periodflood event

The flood event that is predicted to occur with an annualprobability of 0.1% (there is a 1 in 1000 (0.1%) chance in anyyear this event will happen).

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Annex A – Explanation of terms used in Figure 1 – Reducing Flood Risk in Guildford Urban Area

Local (Guildford) Development Framework (GDF)

The Local Development Framework (LDF) is a non statutory term used to describe a folder ofdocuments, which includes all the local planning authority's local development documents. An LDF iscomprised of:

Development Plan Documents (which form part of the statutory development plan)Supplementary Planning Documents

The local development framework will also comprise of:

the Statement of Community Involvementthe Local Development Schemethe Annual Monitoring Reportany Local Development Orders or Simplified Planning Zones that may have been added

Core Strategy

A Development Plan Document (DPD) setting out the spatial vision and strategic objectives of theplanning framework for an area, having regard to the Community Strategy. It is a statutorydocument forming an essential part of the Local Development Framework.

All DPDs must be subject to rigorous procedures of community involvement, consultation andindependent examination, and adopted after receipt of the inspector's binding report. Onceadopted, development control decisions must be made in accordance with them unless materialconsiderations indicate otherwise.

Development Management DPD

The Core Strategy contains spatial policies for future development. A Development ManagementDPD will contain detailed Development Control policies. This document will be prepared after theCore Strategy.

Site Allocations DPD

The Site Allocations DPD will identify the sites that will help achieve the aims of the Core Strategy. Itwill include:

Areas for preservation and enhancement (for example, Sites of Nature ConservationImportance and areas of open space)Sites which will meet the broad spatial strategy for locating development set out in theCore Strategy (including land for housing, employment, business, retail, leisure,transport or a mix of these and other uses).

The Site Allocations DPD has to comply with the Core Strategy and national planning policy. It is ouraim to find the most sustainable sites for development, to preserve and enhance the distinctcharacter of Guildford Borough while at the same time meeting local needs and national targets (forexample, providing affordable housing).

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GDF Evidence Base

The information and data gathered by local authorities to justify the "soundness" of the policyapproach set out in Local Development Documents, including physical, economic, and socialcharacteristics of an area.

Strategic Flood Risk Assessment (SFRA)

The Strategic Flood Risk Assessment (SFRA) informs the planning process of flood risk and providesinformation on future risk over a wide spatial area. It is also used as a planning tool to examine thesustainability of the proposed development allocations in the Guildford Development Framework.

The Guildford Borough SFRA considers the River Wey, the River Blackwater and the Tillingbourne.The SFRA is available to view on the council’s website (www.guildford.gov.uk/planningpolicy).

Flood Risk Assessment (FRA)

An assessment of the probability of flooding in a particular area so that development needs andmitigation measures can be carefully considered.

Thames Catchment Flood Management Plan (CFMP)

The Thames CFMP is one of 76 CFMPs for England and Wales. It gives an overview of the fluvial floodrisk in the Environment Agency’s Thames Region and sets out the preferred plan for sustainableflood risk management over the next 50 to 100 years.

One of the key messages from the CFMP is that floodplain management is a sustainable way ofmanaging flood risk and this is particularly relevant in and around Guildford.

The vision of this document is in line with the recommendations for the Guildford Policy Unit in theThames CFMP and is an excellent example of how they can be implemented through the planningsystem. The key messages for urban areas such as Guildford with no major river flood defences are:

A clear vision for land use is the most effective way of managing flood risk. In some places itwill be through adaptation of the urban environment to make it more resilient to flooding,and in others it will be about putting new development in low risk areas.We want to adapt what is in the floodplain to increase the resilience of what is at risk. Insome cases re locating areas of development may be the best option. Our work with LocalAuthorities will help to determine this.These areas are located on the floodplains of large rivers where increasing the river channelcapacity is constrained by development. Most of the technically feasible defence options areexpensive to implement. We will investigate those that are viable.Reducing the consequences of flooding will be important, particularly in those areas whereredevelopment rates are low or flood defences are not viable. We will work withcommunities and our partners to establish how this can be achieved.

See the summary document:http://www.environment agency.gov.uk/research/planning/114391.aspx

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Guildford Policy Unit

Through the CFMP, broad policies and approaches for the current and future management of floodrisk have been recommended. The Thames region has been divided up into 43 geographical areascalled policy units, of which Guildford is one. For each policy unit there is a broad flood riskmanagement policy

The River Wey Flood Risk Management Draft Strategy 2010

The River Wey Flood Risk Management Draft Strategy 2010 has been completed. This contains theEnvironment Agency's recommended options for sustainable flood risk management in the Weycatchment over the next hundred years. The public and key stakeholder consultation was between22 March and 10 May 2010. The Environment Agency considered the outcomes of the consultationin the Strategy. The strategy has been submitted to the National review Group (NRG) for approval.

The Environment Agency will consider the comments received and submit the final strategy to theEnvironment Agency's National Review Group. Once this is signed off, the Environment Agency willput the plan into action.

Flood Risk Management Measures (Structural and Non Structural) and Flood Risk ReductionMeasures

See figures 4 and 5.

Annex B

Document history

Version 1 – publication September 2009

Version 2 – publication 26 November 2010

Annex C

Supplementary Note on Flood Hazard Ratings and Thresholds for Development Planning and ControlPurpose – Clarification of the Table 13.1 of FD2320/TR2 and Figure 3.2 of FD2321/TR1.

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SUPPLEMENTARY NOTE ON FLOOD HAZARD RATINGS AND THRESHOLDSFOR DEVELOPMENT PLANNING AND CONTROL PURPOSE

– Clarification of the Table 13.1 of FD2320/TR2 and Figure 3.2 of FD2321/TR1.

Suresh Surendran and Geoff Gibbs (Environment Agency), Steven Wade and Helen Udale-Clarke (HR Wallingford) May 2008

Introduction

This document is a supplementary note to reconcile information provided in the ‘Flood Risks to People Methodology’ (FD2321/TR11) and the ‘Framework and Guidance for Assessing and Managing Flood Risk for New Development’ (FD2320/TR22) reports about the Flood Hazard Rating. It has been produced because both PPS25 in England and TAN15 in Wales require that people should be appropriately safe around new development. The document emphasises that for FRAs and FCAs at all levels to inform development allocations and proposals the simplified approach of FD2320 with regard to flood hazard rating should be used rather than the approach in FD2321. Although the final version of FD2321/TR1 post-dates FD2320/TR2, the work presented actually pre-dates the guidance in FD2320/TR2. This supplementary guidance is issued for those involved in development planning and control and to clarify the detail or difference of the Table 13.1 of FD2320/TR2 and Figure 3.2 of FD2321/TR1.

FD2321/TR1 was a research project based on the detailed literature review and analysis of empirical evidence related to flood hazard, derived mainly from theoretical assumptions and some basic laboratory experiments. Factors that affected flood hazard and vulnerability were combined in a form of multi-criteria analysis that was be used to identify the hot-spots and broadly estimate the probability of people seriously harmed and fatalities during the event of a flood. The multi-criteria method was calibrated to actual events, validated using data from seven flood events and shown to work well. The FD2321 (Risk to people) methodology illustrates the fundamental concepts and demonstrate how the approach could be used for different applications - it did not set a policy for flood hazard thresholds.

(Nevertheless there are a number of assumptions used in the FD2321 methodology, particularly with respect to the impact of debris and people’s behaviour during flood events. There is a requirement for further research to collate more evidence on flood hazard, particularly the impacts of debris, and vulnerability in order to refine assumptions made in the flood hazard calculations, flood hazard thresholds and risks to people guidance. The study recommend more laboratory and field based tests on the impact of physical water quality aspect such as debris, mudflow; chemical and biological water quality that cause seriously harm or fatalities to people.)

1 Defra and Agency (2006) The Flood Risks to People Methodology, Flood Risks to People Phase 2, FD2321 Technical Report 1, HR Wallingford et al. did the report for Defra/EA Flood and Coastal Defence R&D Programme, March 2006. (http://sciencesearch.defra.gov.uk/Document.aspx?Document=FD2321_3436_TRP.pdf) 2 Defra and Agency (2005) Framework and Guidance for Assessing and Managing Flood Risk for New Development, Flood Risk Assessment Guidance for New Development, FD2320 Technical Report 2, HR Wallingford et al. did the report for Defra/EA Flood and Coastal Defence R&D Programme, October 2005. (http://sciencesearch.defra.gov.uk/Document.aspx?Document=FD2320_3364_TRP.pdf)

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FD2320/TR2 (FRA guidance for new development) provides guidance that is a specific interpretation of the methodology developed under FD2321, within the context of development planning and control. Based on FD2320 consultation workshops, the project board (key users and experts) advised the project team to provide a simple methodology. Due uncertainties and limitations related to estimating risks to people, FD2320 adopted a precautionary approach, particularly with respect to the selection of debris factors and flood hazard thresholds

Risk to People (Ninj)

Ninj = Nz x Flood Hazard Rating x Area Vulnerability x People Vulnerability

where,Ninj (Risk to People) = number of injuries within a particular hazard ‘zone’; Nz = number of people within the hazard zone (at ground/basement level); Flood Hazard Rating = HR = function of flood depth/velocity (within the hazard zone being

considered) and debris factor; Area Vulnerability = function of effectiveness of flood warning, speed of onset of flooding

and nature of area (including types of buildings); and People Vulnerability = function of presence of people who are very old and/or

infirm/disabled/long-term sick

Flood Hazard Rating (HR) and thresholds

The revised ‘hazard rating’ expression based primarily, on consideration to the direct risks of people exposed to floodwaters.

HR = d x (v + n) + DF

where, HR = (flood) hazard rating; d = depth of flooding (m); v = velocity of floodwaters (m/sec); and DF = debris factor ( 0, 0.5, 1 depending on probability that debris will lead to a

hazard) n = a constant of 0.5

This final revised Flood Hazard Rating formula from the Flood Risks to People project is presented on page 10 (section 3.5) of FD2321/TR1. The formula is identical in both FD2320 and FD2321 reports.

Based on Table 3.2 of FD2321, the Figure 3.2 of FD2321 illustrates the “Hazard to People Classifications” as a function of depth, velocity and debris factor. Such categorisation and the look-up table with flood hazard threshold could be useful for a range of application as an initial indication of Risks to People.

In this case (Figure 3.2 of FD2321) the calculation takes a debris factor as zero(HR = d x (v + 0.5) + 0).

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However FD2321 strongly recommends the use of the debris factor and the formulas described in the Guidance Document for further calculation. The Table 3.1 of FD2321/TR1 (Table 1 of this note) suggests appropriate debris factors for different depths, velocities and the dominant land use.

Table 1: Guidance on debris factors for different flood depths, velocities and dominant land uses. (Source FD2321 Table 3.1): Depths (d) Pasture/Arable Woodland Urban 0 to 0.25 m 0 0 0 0.25 to 0.75 m 0 0.5 1 d>0.75 m and\or v>2 0.5 1 1

The way that Flood Hazard Rating and thresholds have been presented in Table 13.1 in FD2320/TR2 compared to Figure 3.2 of FD2321/TR1

A concern was raised in the FD2320 consultation workshops and by the FD2320 Project Board during discussions on FD2321, that the methodology was complex and the results presented in the Figure 3.2 of FD2321 were not reflecting the potential risk to people (as this table was of hazard rating for different depths and velocity without debris). There was a need for further work to include debris, area vulnerability and people vulnerability aspects. They requested a simpler single table to represent the risk to people.

For example Figure 3.2 of FD2321 did not reflect the fact that there is a risk from drowning even at low depths and velocities. In reality FD2321/TR1 recognises this but only in the subsequent “people vulnerability” calculation (risk to children, old, sick and disable). For still water up to 1.25m depth, the Figure 3.2 of FD2321/TR1 assumes that there is low hazard, if there are no debris or vulnerable group. However to avoid further calculation, but include the vulnerability aspect the Table 13.1 of FD2320 for still water with the depths between 0.25–1.25m were reclassified as “danger to some”, which was felt to be more appropriate for development planning and control, where users may make use of flood hazard without completing he more complex full calculations including people and area vulnerability.

Similarly Figure 3.2 of FD2321/TR1 shows that at the depth of 0.25m, if there is no debris then up to the flow velocity of 2.0 m/sec there would be low hazard. However FD2321/TR1 suggests the usage of an appropriate debris factor dependent on depth, velocity and the dominant land use. To make the process simpler (whatever the land use), FD2320/TR2 includes a default debris factor. In the Table 13.1 of FD2320/TR2 a debris factor of 0.5 has been applied for depths less than and equal to 0.25m and a debris factor of 1.0 has been used for depths greater than 0.25m. Therefore, in the Table 13.1 of FD2320/TR2 at the depth of 0.25m, up to the flow velocity of 0.30 m/sec is treated as low hazard.

Table 3.2 of FD2321/TR1 (Table 2 of this note) provides thresholds for classifying the hazard to people. In the FD2321/TR1 report the threshold between “danger for most” and “danger for all” is 2.5 and it was used as an initial indication of Risk to People (further calculation is recommended using the formulas). However as there is no further analysis in FD2320 but the Project Board decided that the threshold between “danger for most” and “danger for all” should be more precautionary and a Flood Hazard Rating of 2.0 is selected as a key threshold. i.e. In FD2321 the threshold for “danger for all” is 2.5 and it lowered to 2.0 in FD2320. Therefore, the Flood Hazard Rating between 2.0 to 2.5 in FD2320 is not classified as it is in FD2321.

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Table 2: Hazard to People (Source Table 3.2 in FD2321/TR1)

Thresholds for Flood Hazard Rating H = d x (v + 0.5) + DF

Degree of FloodHazard

Description

FD2321 FD2320 <0.75 <0.75 Low Caution - “Flood zone with shallow flowing water or deep

standing water” 0.75 - 1.25 0.75 -

1.25 Moderate Dangerous for some (i.e. children) - “Danger: Flood zone

with deep or fast flowing water” 1.25 - 2.5 1.25 - 2.0 Significant Dangerous for most people - “Danger: flood zone with deep

fast flowing water” >2.5 >2.0 Extreme Dangerous for all - “Extreme danger: flood zone with deep

fast flowing water”

The final difference between Table 13.1 in FD2320/TR2 and Figure 3.2 of FD2321/TR1 is the use of smaller increments of depth, so that lower depths are presented more fully in FD2320/TR2. This was felt to be more helpful for identifying what might be judged as acceptable depending on site specific circumstances.

Conclusions

Table 13.1 of FD2320 and Figure 3.2 of FD2321 look very similar but there are significant differences (see Table 3 of this paper). Either Table/Figure can be used as the basis for assessing the risks to people associated with different flood depths velocities and debris factors.

Table 3: comparison of Table 13.1 of FD2320/TR2 and Figure 3.2 of FD2321/TR1

In Table 13.1 of FD2320/TR2 In Figure 3.2 of FD2321/TR1 The depths above 0.25m

Danger for some, most or all For still water, up to 1.25m the hazard is low (In addition to hazard rating further calculation to include vulnerability aspect is recommended)

Debris factor Debris factor of 0.5 has been applied for depths <0.25m and a debris factor of 1.0 has been used for depths > 0.25m.

In this case a Debris factor of zero applied(in addition to this further calculation is recommended using debris factor and the formulas)

HR Thresholds for “Dangerous for all” hazard classification

>2.0 (precautionary due to uncertainties and to avoid further calculation as FD2321)

>2.5

Increments of depth Small increments at lower depths Every 0.25 m

Table 13.1 of FD2320/TR2 is a simple method applies the precautionary principle and uses suitable assumptions (so that there is no need for further calculations) for application in the development planning and control context (see Table 4 of this paper - an extended version of table 13.1).

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This table is recommended for development planning and control use.

Table 4 – Hazard to People Classification using Hazard Rating (HR= d x (v + 0.5) + DF) for (Source Table 13.1 of FD2320/TR2 - Extended version)

Flood Hazard Rating (HR)

ColourCode

Hazard to People Classification

Less than 0.75 Very low hazard - Caution0.75 to 1.25 Danger for some – includes children, the elderly and the infirm 1.25 to 2.0 Danger for most – includes the general public More than 2.0 Danger for all – includes the emergency services