2Designing for Floodrisk

07 Designing for Flood Risk

Climate Change Toolkit

About this Document

This is the seventh of eight components of Climate ChangeTools, a package of guidance developed by the RIBA toencourage architects to engage with the issue of climatechange, and to help them both mitigate and adapt to thechallenges of global warming.

Designing for Flood Risk gives users a general understanding of the main issues that flood risk gives rise to, provides anoverview of current policy and legislation, and outlines potentialdesign responses.

The complete toolkit consists of:

01 Climate Change Briefing

02 Carbon Literacy Briefing

03 Principles of Low Carbon Design and Refurbishment

04 Low Carbon Standards and Assessment Methods

05 Low Carbon Design Tools

06 Skills for Low Carbon Buildings

07 Designing for Flood Risk

08Whole Life Assessment for Low Carbon Design

Each guide summarises its subject and provides links to othersources of more detailed information.

You can explore all of the RIBA Climate Change Tools atwww.architecture.com/climatechange

http://www.architecture.com/climatechange

Contents

Introduction 3

Part A: Flood Risk in Context

The Sources of Flooding 4

The Effects of Flooding 5

Risk, Vulnerability and Consequence 6

Policy Context and Responsibility 7

Integrating Place-making with Sustainable Design 9

and Flood Risk Mitigation

Part B: The Design Response

An Integrated Approach 10

Introduction to the Design Sequence 10

1.0 Flood Risk Assessment 12

2.0 Land Use Planning 17

3.0 Control: Reducing the Risk 20

4.0 Mitigation: Minimising the Consequences 23

5.0 Re-assessment of Impact 28

Further Reading 29

Glossary of Terms 30

3 Introduction

IntroductionThe Increased Risk of Flooding

Flood risk is a design challenge that is becoming increasingly relevant for a number of reasons.

Climate change is predicted to cause sea levels to rise and more extremeweather patterns, which in turn will cause greater risk of tidal flooding, inlandflooding from rivers and surface water run-off.

The pressure for new housing and current policies that prioritise theregeneration of brownfield land, much of which is already at risk of flooding,may well mean more new development within the floodplain.

Increasing urbanisation and higher densities of development have reduced the amount of natural soak-away available and strained existing drainageinfrastructure. People and properties that are not within currently recognisedand defined floodplains are increasingly at risk of flooding, usually from surface water.

The Threat to Place-making and Good Design

Standard responses to the risk of flooding include flood defences, barriers toflood pathways and raising accommodation above the potential water levelonto columns or stilts. These measures are often not well integrated with theoverall architecture and landscape design, resulting in poor quality and badlyfunctioning neighbourhoods and streetscapes.

Flood barriers limit opportunities for linkage as they are often both physicallyand visually isolating which can result in poor quality public and private spaces.Also, developments characterised by empty undercrofts or dominated by carparking at ground level tend to lack identity and a sense of neighbourhood.

Who is the Guide Aimed at?

This guide takes a design-led approach and is aimed at practicing and student architects, urban designers and landscape architects, but will also haverelevance for clients, planners and other urban professions. It is intended tocompliment other useful guidance including PPS25, Development and FloodRisk, its practice companion guide, Development and Flood Risk – Guidance forthe Construction Industry by CIRIA and the Environment Agency’s Building aBetter Environment – A Guide for Developers.

What the Guide Covers

This guide gives users a general understanding of the main issues that floodrisk gives rise to, followed by an overview of current policy and legislation. It willgive an appreciation of the range of inputs that other specialist consultants mayhave to the design process and also the value of early consultation with thelocal planning authority and the Environment Agency. Finally, whereverpossible, it will outline potential design strategies for integrating place-making,sustainable design, and the control and mitigation of flood risk.

River Yare – Seven Mile Reach to Breydon WaterRealignment and strengtheningof the flood bank has beencarried out under apartnership arrangementbetween Halcrow GroupLimited, BAM Nuttall Limitedand the Environment Agencyas part of the 20 year, £100mBroadland Flood AlleviationProject www.bfap.org

Image by Mike Page

http://www.bfap.org

4 Flood Risk in Context

The Sources of Flooding

There are six recognised sources of flooding:

Tidal Flooding

Both sea and river defences may beovertopped or breached by a combination oflow pressure weather systems and peak hightides. Storms with high wind speeds cause talland powerful waves and low pressure frontscause sea levels to rise above normal levels.High tide levels vary through the lunar andsolar cycle and when superimposed uponother tidal variations exceptionally high tides result.

The onset of flooding from the sea and tidalrivers is often sudden and the extreme forcesdriving it present a significant danger to life. The east coast storm surge of 1953 claimed307 people’s lives in the UK and 1,835 inHolland. A similar storm surge tide inSeptember 2007 came within a fewcentimetres of breaching a number of the UK’s coastal defences.

It is often possible to forecast this type offlooding with reasonable accuracy, due to thepredictability of the tide and trackability of low pressure systems. The duration of thistype of flooding is also limited by the cycle of the tides where drainage is available.

Fluvial Flooding

Flooding occurs in the floodplains of riverswhen the capacity of water courses isexceeded as a result of rainfall or snow andice melts within catchment areas furtherupstream. Blockages of water courses andflood channels or tide locking may also lead toponding and rising water levels. River defencesmay then be overtopped due to increasedwater levels, or breached by large objects of debris carried at high water velocities.

Flooding from rivers has in recent years beenexperienced in the Severn Valley, in Sheffield,in Hull from the river Humber in 2007 andCarlisle on the river Eden in 2006.

The onset can be quite slow in somecatchments with steadily rising water levels.However, flash flooding can occur in steepcatchments and is far more immediate.

Flooding from rivers, particularly in recognisedfloodplains, can usually be predicted withgood accuracy. However flash floods fromsudden downpours such as those in Carlislecontinue to challenge the capability ofdetection and forecasting systems.

Water over about 250mm in depth may carry debris particularly in urban locations and can also be very cold. Even travelling at low speeds this can make it extremelyhazardous to people caught in it.

Figure 1 The Sources of Flooding

Tidal flooding

Fluvial flooding

Pluvial flooding

Ground water flooding

Flooding from sewers

Flooding from man-made infrastructure

Part A: Flood Risk in Context


Ground Water

Low lying areas sitting over aquifers mayperiodically flood as ground water levels rise.This type of flooding is often seasonal andtherefore can be forecasted with goodaccuracy. It is often slow in its onset.

Pluvial Flooding

Surface water flooding is caused by rainwater run-off from urban and rural land with low absorbency. Increased intensity ofdevelopment in urban areas has given rise toland with a larger proportion of non-permeablesurfaces, a problem often exacerbated by overloaded and out-dated drainageinfrastructure. These circumstances,combined with intense rainfall, can give rise to localised flooding.

This sort of flooding often occurs outside of recognised floodplains and because it iscaused by quite localised weather conditionsit is very difficult to forecast. Its onset can also be very rapid, and the level of floodingvery severe.

In the summer of 2007 much of the floodingexperienced in Gloucestershire and Yorkshirewas not directly caused by rivers but bysurface water. Large volumes of rainfall early in the year saturated the ground and intenserainfall later caused both urban and rural areas to flood.

Flooding from Sewers

Flooding from sewers can occur where there are combined storm and foul sewersand their capacity is exceeded due to largeamounts of surface water run-off in a shorttime. Poor cleaning and maintenance can lead to blockages that can also cause local flooding.

This type of flooding is hard to predict, hassignificant sanitary consequences for thoseaffected, and can occur very rapidly.

Flooding from Man-made Infrastructure

Canals, reservoirs and other man-madestructures can fail causing flooding to areasdownstream. Industrial activities, water mainsand pumping stations can also give rise toflooding due to failure.

The Effects of Flooding

The effects of flooding from the sourcesoutlined above are felt by various ‘receptors’.These include, people, buildings, infrastructure,agriculture, open recreational space and thenatural world.

In extreme cases flooding may cause a loss of life. However, the social and emotional costsfrom flooding can also be significant and areoften widespread and indiscriminate in floodedareas. These costs include: displacement fromhomes, the loss of personal valuables and the ongoing fear and insecurity caused by the experience.

Potable water supplies may be lost orcontaminated in a flood and this can haveimmediate health effects upon people and animals.

The economy can also be severely affected by flooding. Businesses may losestock, patronage, data and productivity, and disruption to utilities and transportinfrastructure can have knock-on effects to a wider area. Tourism, farming and livestockcan equally be affected. The Association ofBritish Insurers has estimated the cost of theJuly 2007 flooding, in insurance claims alone, at over £3billion.

The built environment may be damaged ordestroyed as a result of flooding with highrepair costs and long periods required forreinstatement. The public realm is often badlyaffected through damage and the deposit of potentially large quantities of debris. Landcontamination may also be transported andspread during flooding.

Vital infrastructure may also be damaged or disrupted. Electricity and gas supplies canbe interrupted to individual properties but also to wider communities if sub stations andtransformers themselves are flooded. Roadlinks, railways, canals etc. may be blockedcausing disruption to the wider transportnetwork, and accessibility severely disruptedfor local inhabitants, especially amongst thoseconsidered most vulnerable.

A knock-on effect of the loss of electricityexperienced in the 2007 summer floods was the loss of communications networks.Telephones, radios, televisions and the internetare all increasingly reliant upon mains powerand without a robust means of conveyinginformation to householders, rescue and cleanup operations may be hampered.

The Pitt Review: Lessons Learnt from the2007 Floods describes in detail the specificevents that took place in June and July and theimpact of the floods on individuals, businessesand the wider community and economy. Itcontains a number of recommendations forhow flooding can be better handled in future,including recommending that design solutionsto flood risk are more widely considered andapplied across a range of developments.

Factors which Determine the Effects of Flooding

The level of predictability – this affects the timing, accuracy and communication of warnings given before a flood.

The rate of onset of the flood – how quicklythe water arrives and the speed at which itrises will govern the opportunity for people to prepare and respond effectively for a flood.

The speed and depth of the water – thisdictates the level of exposure of people andproperty to a flood. It is difficult to stand orwade through even relatively shallow waterthat is moving. Flood water often carriesdebris, including trees, and water over 1m indepth can carry objects the size of cars. Fastflowing water can apply devastating force to property and other receptors.

The duration of the flood – this is anotherimportant factor in determining the extent of itsimpact, particularly on individuals and affectedcommunities.


Risk, Vulnerability and Consequence

Two factors should be considered whenassessing flood risk. Firstly, the likelihood of a flood occurring, and secondly, the potentialconsequences that it might have upon thevarious receptors in its path.

The probability of a flood is normallyexpressed as a likely return period for a certaindepth of flood water. In a recognised floodplainit is possible to estimate from records the likelylevel of a one in 20 year flood, a one in 100year event, a one in 200 year flood and a onein 1,000 year event. These are the mostcommonly used design frequencies.

Climate change is now also accepted as a factor which will increase the level andfrequency of flooding in the future and anallowance for this should also be made.

The Environment Agency holds data on river and coastal flooding and has developedmodels for predicting risk. They can oftensupply information, including the probabilityand height of flood waters.

The potential consequences that a floodmight have depend upon the vulnerability of the receptor.

Clearly residents being cared for in a nursinghome are significantly more vulnerable to theeffect of a flood than visitors to an outdoorsports facility or recreational park.

Certain building uses such as hospitals, police and fire stations and other emergencyservices can be considered more vulnerablethan others because of their importance todisaster relief and recovery.

Equally, infrastructure items such as watersupply and mains electricity are also morecritical than others due to the far reachingconsequences of their failure on health, and social and economic activities.

Risk is a product of probability andconsequence, it is for example possible to reduce the probability of a flood with new defences but still increase the overall risk by placing vulnerable receptors behind the defences thereby increasing the potentialconsequences.

Surface water flooding


Policy Context and Responsibility

National policy and guidance is set in PlanningPolicy Statement 25 (PPS25): Developmentand Flood Risk.

The main aims of PPS25 are:

• To ensure that flood risk is considered as an integral factor in the planning process

• To prioritise new development away fromareas of flood risk

• To stop inappropriate development takingplace in areas at risk

• To make sure that new development takesclimate change into consideration

• To ensure new development does notincrease the risk of flooding elsewhere.

It obliges local authorities to assess the risk of flooding by carrying out Regional FloodRisk Assessments (RFRAs) or StrategicFlood Risk Assessments.

PPS25 then requires local authorities to carryout a ‘sequential test’ when allocating land.The test is intended to encourage authoritiesto consider land at risk of flooding fordevelopment only if there are no other suitable locations.

An ‘exceptions test’ requires the authority to demonstrate that the sustainable benefits in allocating development on land at risk of flooding outweigh the costs and thatappropriate measures have been taken to address residual flood risk. For instance, the social and environmental advantages ofregenerating previously developed brown field land.

Finally, once land has been allocated fordevelopment, any planning applicationsubmitted must be supported by a sitespecific Flood Risk Assessment (FRA). This must quantify the extent of any flood risk and then demonstrate that proposals:

• Fully mitigate the risk of flooding

• Do not place occupants at greater risk

• Do not increase the risk of flooding toneighbouring areas.

The Practice Guide Companion to PPS25is aimed at those involved in the planningprocess and is likely to be regularly updated toaddress findings following the 2007 summerfloods, the Pitt Report and subsequentdevelopments in best practice.

Making Space for Water is the cross-Government programme taking forward thedeveloping strategy for flood and coastalerosion risk management in England. The LifE (Long-term Initiatives for Flood-riskEnvironments) project led by Baca Architectswith the BRE and funded by DEFRA is part ofthe programme. The project is an integrateddesign approach to achieving sustainable,adaptable (to climate change) and saferdevelopment. Through three conceptualmaster plans the LifE project shows how this can be achieved, what it might cost and how it might look.

At present the Building Regulations do notcontain requirements that relate directly toflood risk, however the recent Pitt Review:Lessons learnt from the 2007 Floods, calls for an update to the regulations. Flood riskmitigation measures to achieve resistance

and resilience to properties at risk are likely to become increasingly necessary, and this is likely to be reflected in future changes to the building regulations.

Improving the Flood Performance of NewBuildings, Flood Resilient Construction waspublished by CLG and provides guidance for developers and designers on how toimprove resilience of new properties in low or residual flood risk areas.

The Code for Sustainable Homes, which is currently a voluntary standard, sets downrequirements for water conservation andmanagement. Higher levels within the coderequire increasing amounts of on-siteattenuation of rainwater run-off as a way of reducing the drivers of pluvial flooding.

Responsibility

Responsibility for flood risk management isdivided amongst a large number of parties.These include the following:

The Department of Communities and LocalGovernment has overall policy responsibilityfor land use planning and Regional or LocalPlanning Authorities have responsibility forimplementation of these policies taking intoconsideration the requirements of PPS25.

Regional and Local Authorities areresponsible for preparing Regional andStrategic Flood Risk Assessments and also for preparing Surface Water ManagementPlans intended to co-ordinate individualresponsibilities for multiple sources, pathwaysand systems.

Overloaded drainage infrastructure


The Environment Agency (EA) hasresponsibility for preparing strategies, plansand programmes to reduce risk to existingcommunities, properties and other assetsfrom coastal and fluvial flooding.

The EA models and maps river and coastalflooding and in conjunction with the METOffice issues flood warnings. Not all river andcoastal defences are in the ownership or arethe responsibility of the EA to maintain. Manyare in private ownership or in that of the localauthority. There are other flood and coastaldefence operating agencies too, including localauthorities and internal drainage boards.

The EA is funded by Government to proposeand implement new flood risk managementmeasures. Funding is finite and projects areevaluated and prioritised according to therelative costs and benefits. The EA is also astatutory consultee on all new developmentwithin floodplains. However, they are notcurrently responsible for monitoring groundwater or surface water flood risks. It is arecommendation of the Pitt Review that their remit is extended to cover this, and the Government is likely to take this forwardthrough a proposed Bill during the 2008/9Parliamentary Session. Surface water floodrisk maps are likely to be made available in the near future.

Internal Drainage Boards (IDBs) areindependent bodies responsible for landdrainage in areas of special drainage need that extends to 1.2 million hectares of lowlandEngland. They are long established bodiesoperating predominantly under the LandDrainage Act 1991 and have permissivepowers to undertake works to securedrainage and water level management of their districts. They may also undertake flooddefence works on ordinary watercourseswithin their district (that is, watercourses other than ‘main river').

Riparian Land and Property OwnersSections of flood defences are often ownedand maintained by private land owners, andproperty owners are responsible for specificresistance and resilience measures to theirindividual properties.

British Waterways is responsible formaintaining the physical infrastructure of manycanal networks and some rivers. They can insome cases provide emergency assistanceby managing these waterways to createadditional storage capacity to allow pumpingto alleviate flooding.

Water Companies and SewerageUndertakers are responsible for theinstallation and maintenance of public sewers,the design, maintenance and capacity ofwhich can affect levels of flooding from land sources.

The Highways Agency is responsible for thedrainage of major trunk roads and motorways.Local authorities manage local road drainage.

Local Resilience Forums co-operate inpreparing for and responding to emergencies.Such forums sit at the apex of local civilprotection arrangements. Their overallpurpose is to ensure that there is anappropriate level of preparedness to enable an effective multi-agency response toemergencies, which may have a significantimpact on local communities.

Emergency Responders those that respondin an emergency include the police, fire andrescue services, ambulance and healthservices, the armed forces, local authorities,the Maritime and Coastguard Agency, andmany voluntary organisations.

However, it is important to note that the fire and rescue services are not formallyresponsible for flood rescue and, althoughthey often provide this service, they havevariable levels of training for this work. TheCoastguard and RNLI have no legal obligationto provide rescue services inland.

Insurance Companies Currently there is an agreement between the Association ofBritish Insurers and the Government that itsmembers will maintain insurance cover whereit is currently in place for households that areat risk of flooding. However they generallywant evidence that measures are in hand orplanned to reduce risk below 1:75. Insurers areunder no obligation to provide insurance onnew homes built within the floodplain.


Integrating Place-making withSustainable Design and FloodRisk Mitigation

There are a number of potential challenges to place-making and sustainable design thatarise from possible flood risk control andmitigation measures.

Flood defences are by their very naturebarriers which can often physically divide and segregate one area from another. Animportant urban design goal is to promotepermeability and linkage through and across schemes.

Whilst the function of a flood defence is to separate the source of the risk from thepotential receptors, it is often desirable from a place-making point of view to link the inhabitants and visitors of a newneighbourhood with the river or coastline that poses the risk. The link should ideally be both visual and physical, providing access if possible.

One of the requirements when designing new properties within the floodplain is toensure that occupants are not placed at risk.The Environment Agency will normally insistthat all habitable rooms within residentialaccommodation be raised above the height of the design flood level or other practicalmeasures be taken that provide the samestandard of safety.

An obvious response to this is to place car parking and garages at ground level with residential accommodation at first floorand above. However this can often result in buildings with poor quality unanimatedelevations at street level and leave both thepublic and private realm dead and lifeless. It can also be a challenge to provide equalaccess to accommodation raised aboveground level.

An additional requirement is that the floodstorage capacity of the land should not bedecreased as a result of new development.New buildings will in effect restrict flood waterto the remaining space left between them. Inorder to try and avoid this, buildings on stiltsmay be proposed. The empty undercrofts andaerial walkways that result from this approach,however innovative the architectural design,usually leave the public realm at ground levelwithout good passive surveillance and with no sense of ownership. This in turn can lead to security problems and attract antisocial behaviour.

Retro-fitted flood ‘proofing’ measures such as demountable barriers by their very natureoften fail to compliment the original design.Measures fully integrated into the functionality of a building will also tend to be more userfriendly than afterthoughts.

Another strategy to overcome these issues is to raise land levels to a point at which theground level is effectively above the flood

design level. However when considered froman overall long-term flood risk managementpoint of view the sustainability of this strategyis questionable. The effect of raising landlevels in the event of a flood is to force theflood waters to go elsewhere, effectivelytransferring the risk to neighbouring land. In the extreme this approach is likely to, byreducing flood storage capacity, raise potentialflood levels and bring land and properties thatare not currently at risk into the floodplain.

There are clearly arguments againstwholesale land raising from the point of view of encouraging sustainable construction,due to the large amounts of resources it consumes.

New or existing properties and landscapesthat are not designed with adequateresistance and resilience in areas of flood risk are unsustainable on a number of levels.Repair and refurbishment in the event of aflood may currently be viewed simply as an insurable risk, however this stance isincreasingly unsustainable, both economicallyand practically. It is unlikely that the insuranceindustry will provide such cover indefinitely. On a social level the disruption caused to the continuity of people’s lives by a floodingevent involving temporary relocation is notcompatible with the goal of creatingsustainable communities andneighbourhoods.

Flood defences and parking at ground floor challenge place-making

10 The Design Response

An Integrated Approach

Assuming it is unavoidable when designing for flood risk the primary aim is to integratecontrol and mitigation of the risk seamlesslyinto an overall design that in turn successfullyachieves the goals of place-making andsustainable development. In order to achievethis it is essential that the risk of flooding isrecognised as a key constraint and the driversof the risk as potential opportunities from the outset.

A multi-disciplinary approach is needed todevelop integrated control and mitigationmeasures. The assessment of risk andhydraulic modelling of flood events in manycircumstances requires the input of specialistconsultants using sophisticated software. The design of flood defences, barriers andunderground drainage systems requires civiland structural engineering expertise. Also,landscape architecture has a great deal tooffer in the control of flood risk and themitigation of its effects through the design of the public and private realm.

The danger that poorly integrated engineeringsolutions or retro-fitted flood proofing devicesposes to good quality design is illustrated inPart A of this guide. However, it is possible to develop holistic design responses byidentifying flood risk at the outset, byunderstanding the range of issues this givesrise to and the potential contribution thatanalysis, engineering, hydraulics andlandscaping can make.

Introduction to the Design Sequence

In keeping with the sequential approach taken by PPS25 a step-by-step designsequence is illustrated opposite in Figure 3.

The sequence starts with an assessment ofthe risk (Assessment) and prioritises spatialplanning (Land Use Planning) to avoid placingnew development in risk areas or at leastsubstitute vulnerable uses wherever possible.

It then considers strategies to minimise theprobability and severity of a flood (Control),followed by a review of strategies to minimisethe potential consequences of a flood onoccupants and properties (Mitigation).

Finally proposals are re-assessed (Re-assessment) to check their impact uponfuture occupants’ safety, neighbouring areas,wildlife and ecology.

The design process is iterative, with thepotential and type of flooding understood first, proposals then developed, their impactassessed and if necessary alterations and revisions considered.

Place-making

Flood Control/Mitigation

Sustainable Design

Figure 2 An Integrated Approach

Part B: The DesignResponse


Figure 3 The Design Sequence

Design Sequence

1.0 Flood RiskAssessment

2.0 Land Use Planning

3.0 Control:Reducing the Risk

4.0Mitigation: Minimising theConsequences

5.0Re-assessment of Impact

Key Issues

• Flood risk assessment• Risk = probability xconsequence

• Risk/vulnerability matrix

Flood sources:• Tidal/storm surge• Fluvial (from rivers)• Surface water • Sewers and infrastructure

• Occupants

• Property• Public realm

• Infrastructure

• Safety of occupants• Surrounding neighbourhoods• Environmental impact• Climate change

Strategies

• Flood risk zones• Factor for climate change• Flood design height• Flood storage volume

• Avoid or substitute

• Primary and secondary flood defence

• Raise floor levels

• Flood storage space

• Attenuation

• Safety strategy

Robust designs:• resistance• resilience

• Design for recovery

Robust designs:• resistance• resilience

• Design for recovery

• Proposals do not fully mitigate flood risk

• Proposals do not place occupants at unacceptable risk• Proposals fully mitigate the risk of flooding• Proposals do not increase risk elsewhere

The Design Response

• Spatial planning to zone uses

• Stacking vulnerable over robust uses

• Temporary or time limited uses

• Coastal defences• River walls• Barriers/barrages• Raised areas

• Managed retreat• Flood storage in public realm• Integrate flow paths into

landscape design

• SuDS attenuation of run-off• Permeable surfaces• Detention ponds

• Awareness and warning• Safe escape routes• Safe refuge areas• Rescue strategy

• Resistant/resilient building structures

• Amphibious buildings• Resilient interiors• Route for water to retreat• Clean-up plans

• Essential/high impactutilities to be resilient

• Maintain essential supplies• Locate plant above flood level• Design transport infrastructure

to be resilient

• Return to design sequence step 2.0


Are You at Risk?

The various different sources of flood riskhave been described in Part A of this guide.

It is possible to make a very basic appraisal ofthe potential that a site has to flood by simplylooking at the context and topography withinwhich the land is set (see Part A).

A site adjacent to the sea and or within a rivervalley is obviously at potential risk from thesesources. Sites in the vicinity of man-madeinfrastructure such as reservoirs or canalsmay also be at risk.

The Environment Agency website is avaluable initial tool for assessing whether aproperty is within a recognised flood zone.Sites can be searched by post code and floodrisk maps viewed which define the extent ofeach of the flood risk zones.

PPS 25 defines four flood zones based uponthe annual probability of flooding:

• Flood zone 1 with a less than 0.1% (one in1,000) chance of flooding

• Flood zone 2 between 0.1% (one in 1,000) and1% (one in 100) for river flooding and 0.5%(one in 200) chance for flooding from the sea

• Flood zone 3a with a greater than 1% (one in100) chance for rivers and greater than 0.5%(one in 200) for the sea

• Flood zone 3b with a greater than 5% (one in 20) chance of flooding or areas designedto flood in an extreme one in 1,000 floodevent or land required for flood conveyanceor storage. This zone is defined as functionalfloodplain.

Information may also be available in the form of Regional and Strategic Flood RiskAssessments prepared by the local authorityor regional assembly.

Flooding from the land is more difficult to predict but the surrounding topographymay give clues along with enquiries to localoccupants or the LA to ascertain any recentflood history. Insurance providers carry out their own risk assessments and theirwillingness to provide cover may be a usefulguide to a potential risk. Finally, early enquiriesto local drainage boards may also help definethe capacity of existing storm sewers and anyhistory of flooding.

What is a Flood Risk Assessment?

Flood risk assessments (FRAs) aim to identify,quantify and communicate to decision makersand other stakeholders the risk of flooding toland, property and people. The purpose is toprovide sufficient information to determinewhether particular actions (such as proposeddevelopment, the construction of a floodprotection scheme or the installation of a floodwarning scheme) can be undertaken withoutcausing an increase in risk elsewhere andadequately reduced risk at the site.

The flood zones are normally the startingpoint of any assessment but are indicativeonly and the actual risk may be lower orhigher than indicated as a result of thepresence of defences, inaccuracies in themethod of estimation or other forms offlooding, such as surface water and sewerflooding. The Flood Zones maps essentialpurpose is to indicate the main areas whereflooding needs to be considered in decisionson land use and land management.

The flood zones are based on the probabilityof river and coastal flooding. These are thecommonest and most obvious forms offlooding. They are also more amenable toprediction than other forms of flooding. Floodzones are defined ignoring the presence offlood protection structures because defendedareas still carry a residual risk of flooding fromovertopping or breach of defences and thereis no guarantee that defences will bemaintained in perpetuity.

The assessment of flood risk requires athorough understanding of the sources offlood water (e.g. high sea levels, intense orprolonged rainfall leading to run-off andincreased flow in rivers and sewers), thepeople and assets affected by flooding(known as the receptors) and the pathwaysby which the flood water reaches thosereceptors (e.g. river channels, river and coastalfloodplains, drains, sewers and overland flow).

This is achieved through the source-pathway-receptor (S-P-R) model, which has becomewidely used to assess and inform themanagement of environmental risks. This is illustrated in Figure 4. FRAs thus requireidentification and assessment of all threecomponents:

• The probability of the source(s) (e.g. high river levels, sea levels and wave heights)

1.0Flood RiskAssessment


Groundwater flooding

Sewer flooding

Overland flooding

Pathway (eg defence)

Source(river or sea)

Receptor(eg people in the floodplain)

Figure 4 Source Pathway Receptor Model

• The performance and response ofpathways and barriers to pathways such asfloodplain areas and flood defence systems

• The consequences to receptors such as people, properties, environment.

The ultimate aim of a FRA is to combine thesefactors and map or describe the risks on aspatial scale, so that the consequences canthen also be analysed. Comprehensiveguidance is available from research in the UK, in the Practice Guide that accompaniesplanning guidance (PPS25), CIRIA Report C624and Defra/Environment Agency Guidance on flood-risk assessment (FD2320) and on flood risks to people (FD2321). Seewww.hydres.co.uk for further information.

Actual and Residual Flood Risk

It is important that a FRA should consider both the actual and the residual risks.

Actual flood risk is the risk posed to an area,whether it is behind defences or undefended,at the time of the study. This should beexpressed in terms of the probability offlooding occurring, taking into account thelimiting factors, both natural and man-made,and preventing water from reaching thedevelopment.

Residual risks are the risks remaining after all risk avoidance, substitution and mitigationmeasures have been taken. It is in

management of residual risk that urban designand innovation in use of materials has thegreatest impact. Examples of residual floodrisk include:

• The failure of flood managementinfrastructure such as a breach of a raisedflood defence, blockage of a surface waterchannel or drainage system, failure of a flapvalve, overtopping of an upstream storagearea, or failure of a pumped drainage system

• A severe flood event that exceeds a flooddesign standard such as, but not limited to, a flood that overtops a raised flood defence.

FRAs need to consider the situation as it isnow and also how it might change in thefuture. Such consideration should includechanges in climate (which impact largely onsources), the construction of flood protectionor drainage schemes (which modify thepathways) and the introduction of receptorsinto areas at risk of flooding.

At what Scale Should Flood Risk be Assessed?

FRAs are required at different scales bydifferent organisations for many differentpurposes. A hierarchy of assessments isnecessary to ensure a proportionate responseto the needs of organisations by avoiding theneed for detailed and costly assessments

prior to making strategic decisions. It is usual in the design of new development for reference and information to be gainedfrom the Strategic Flood Risk Assessmentundertaken by the LPA. These assessmentsdo vary, but may include more risk data in key areas, and some specific guidance fordesigning new development in flood riskareas. They will never obviate the need for a site specific flood risk assessment.

Site specific flood risk assessments establishwhether and to what extent a site is likely to be affected by flooding. This will includeevaluation of the effects that any changes,such as proposed new development, will have on flood risk elsewhere and whetherproposed mitigation measures andmanagement of residual risks will reducethose risks to an acceptable level. Theseassessments may need to be undertaken by a competent engineering consultancy, and is more than likely it will use existing flood models prepared by the EA.

To ensure a proportionate methodology toassessing flood risks for a given land area for a particular decision, a staged approachshould be adopted comprising appropriatescreening, scoping and detailed assessments.This will ensure all the issues are identified andre-use of data and model information providesfit-for-purpose risk assessments.


Key Steps in the FRA Process

Broad assessment of flood levels, flowroutes, depths on the proposed site.

Simplistic assessment of the likely impacts ofreducing the risks by removing that area offloodplain from the river system.

Detailed assessment of current risks at thesite with the development in place and itsmitigation measures.

Design of compensation works to reducethe impacts of the mitigation measures.

Design of flow balancing for surface waterrun-off from the site.

Decision and Influence Point in Design

The scale of flood risk would then influencethe detail applied to the next steps, butcritically would identify early whether floodrisk would be a significant issue to manage. It will also identify whether flood risk wouldalter not only layout, but place-making andurban form. Will also guide whether floodinundation will need to be managed withinthe site, as impacts of keeping water out of the site are too significant on adjacentproperty.

Scale of impacts and what potentialcompensation is possible within or outsidethe site. How could the compensation works(flood storage or flood alleviation works) beincorporated into the urban layout andlandscaping?

Confirm mitigation measures are suitable for proposed development and integrate into the design.

This is the critical phase and can be a show-stopper if significant works are required andlittle remaining land is available.

Landscaping and layout issues, that mayconflict or integrate with the floodcompensation measures.

General Principles and Key Issues

With all FRAs undertaken for land-use changeand new developments there are somegeneral principles that are applicable andshould be followed. These are noted below:

• Properly prepared assessments of flood riskshould use the S-P-R model and describethe flood risks in a transparent manner

• A greater focus on the receptors and theinundation patterns within the floodplain isnow common. Managing the flood eventwithin a development is essential, and to dothis effectively two dimensional inundationmodelling is now a standard technique usedin FRAs to assess residual flood risks

• Any organisation or person proposing a change in land use or development must consider its impacts on flood risk elsewhere. Any such changes ordevelopments must not add to and should,where practicable, reduce flood risk. Wherean impact is identified and cannot bemitigated the development may not begranted permission.

• Future users of the land or developmentmust not be placed in danger from floodhazards and should not be exposed tounacceptable risks throughout the lifetime of proposed development and land use.Understanding human vulnerability andbehaviours in design of any mitigation works is essential to describe in the FRA.

Figure 5 Flood risk assesment and the design process


Minimum Requirements

As with the general principles above, at allstages of the planning process there areminimum requirements for FRAs. They should:

• Be proportionate to the risk and appropriateto the scale, nature and location of thedevelopment

• Consider the risk of flooding from thedevelopment as well as the risk of floodingto the development

• Take the likely impacts of climate change intoaccount and identify a strategy of adaptationof the design or climate change proofing

• Be undertaken by competent people, asearly as possible in the particular planningprocess to avoid misplaced effort and toinfluence the master planning process

• Consider the effects of proposed flood-riskmanagement infrastructure including raiseddefences, flow channels, flood-storage areasand other artificial features together with theconsequences of their failure

• Consider the vulnerability of those that couldoccupy the development, taking account ofthe sequential approach and ExceptionsTest and the vulnerability classification,including arrangements for safe access

• Consider and quantify the different types of flooding (whether from natural and man-

made sources and including joint andcumulative effects) and identify flood riskmitigation measures so that assessmentsare fit for the purpose of the decisions being made

• Consider the effects of a range of floodingevents including extreme events on people,property, the natural and historic environmentand on river and coastal processes

• Include the assessment of the remaining(known as ‘residual') risk after risk-mitigationmeasures have been taken into account

• Consider how the ability of water to soakinto the ground may change withdevelopment, along with how the proposedlayout of development may affect drainagesystems

• Be supported by appropriate data andinformation, including historical information on previous events.

How are FRAs Undertaken?

Key to a good FRA is to start the assessmentas early as possible in the design process and where necessary involve a specialistconsultant. The FRA is undertaken in keysteps, which allows influencing decisions to be taken by the design team with the fullknowledge of the actual and residual risks.

Illustrative Scheme –Flood Risk Assessment

To demonstrate the step-by-step designprocess a brownfield site which is at risk of flooding has been illustrated.

A flood risk assessment of the site showsthe extent of areas of high, medium and lowrisk across the current site. The source andtype of flooding is identified as primarilyfluvial and the design flood level will eitherbe calculated or taken from assessmentscarried out by the EA or local authority.

Further work may be done such as a flood hazard assessment if appropriate todetermine the level of hazard as a functionof water speed and depth across the site,water quality and duration.

The assessment will also estimate thecurrent capacity of the site to store floodwater in the event of a flood, allowing forthe footprint of existing buildings andstructures within the flooded areas.

Figure 7 Flood risk assessment

Low risk

Medium risk

High risk

Very high risk

Figure 6 Brownfield site at risk of flooding


Case Study –Flood Risk AssessmentLower Broughton

Project TeamAssessment: JBA consultingClient: Countryside PropertiesA flood risk assessment for the area identifiedthat overtopping and breaching of existingdefences would be the prime risks thatrequired consideration in the layout and thedesign of its proposed redevelopment. Adetailed river model was supplied by theEnvironment Agency and the FRA set aboutadding the floodplain elements using 2Dhydrodynamic software to represent the flowroutes present, known from previous events.The model assessed the current and revisedoverland flow routes, identified where and howstorage was utilized, and how much flow returned to the river. A preliminaryassessment of flood depths and flow intensitywithin the floodplain was used to inform aflood compatible strategic masterplan of the area.

The analysis was an iterative one wheredifferent layouts and building types weretested against the present case, ensuring no impact on flood depths or flood extentacross the floodplain. High hazard areas(based upon velocity and depth output) were identified and either moderated oravoided in the emergency plan.

Figure 9 Predicted flood depths

Figure 8 Constraints plan

Low flood hazard:limited mitigation required

Moderate flood hazard:some mitigation required

High flood hazard:significant mitigation required

Potential evacuation routes


PPS 25 indicates compatible uses for each of the flood zone levels 1 to 3b. Figure 10 belowdeals with vulnerability classification. Thisapproach seeks to locate the most vulnerablecategories of land use in the areas of leastflood risk and vice versa. Only the most robustuse categories are appropriate for areas ofhigh flood risk. Figure 11 overleaf deals withvulnerability classification.

As an expansion of this approach land usecategories, as defined by the National LandUse Database, have been ranked in order oftheir vulnerability. Categorised in this way theyhave been set against low, medium, high andvery high levels of flood risk to form a ‘landuse ready reckoner’ to assist in the selectionof compatible uses (see Figure 1 1 overleaf).

The table highlights that some uses may beviable only if appropriate flood risk mitigationmeasures or a combination of mitigation andcontrol measures are put in place.

The matrix should only be used as a ‘readyreckoner’ because ultimately individual landuse proposals will be assessed against the specific set of local circumstances inconjunction with the control and mitigation

measures proposed.2.0Land UsePlanning

Figure 10 Flood risk vulnerability and flood zone compatibility taken from PPS25

Risk:Vulnerability Land use Zone 1 Zone 2 Zone 3a Zone 3bWater-compatible development Water infrastructure and pumping stations

Sewage infrastructure and pumping stationsSand and gravel workingsDocks, marinas and wharvesMOD defence installationsShipbuilding, fish processingAmenity open space, outdoor recreation

Less vulnerable Buildings; shops, professional, restaurants, storage, distribution, assembly and leisureLand and buildings for agriculture and forestryWaste treatment plantMineralsWater treatment and sewage plants

More vulnerable Hospitals, health and educational;Residential institutions and dwellingsHotels and nightclubsLandfill sites and hazardous wasteHoliday caravans and camping

Highly vulnerable Emergency services and command centresEmergency dispersal pointsBasement dwellings, permanent caravansInstallations of hazardous substances

Essential infrastructure Essential transport and evacuation routesStrategic utility infrastructure, power stations and primary substations

Development appropriate Exception test required Development should not be permitted

Opportunities –Land Use Planning

•Make efficient use of land at risk offlooding by matching a mix of uses with the levels of risk. At the same time create sustainable communitieswith local centres and reduced needfor travel

•Work with the natural topography for cost effective and sustainabledevelopments that minimiseengineering land movement

• Stack vulnerable uses over robust uses to create active frontages and a positive public realm at street level

• Provide new outdoor amenity space, areas of biodiversity, and new recreational uses within areas of higher flood risk.


Nursing home

Housing

Apartments

Recreation

Figure 11 Land use ready reckoner

Risk:Land use category Low Medium High Very high

Agriculture and fisheries

Forestry

Utilities and infrastructure – renewable energy production and distribution

Recreation and leisure – outdoor amenity and open spaces

Defence

Transport – car parks, vehicle storage, goods and freight handling

Minerals (extraction)

Utilities and infrastructure – refuse disposal

Industry and business – storage and wholesale distribution

Industry and business – manufacturing

Industry and business – offices

Recreation and leisure – indoor

Retail

Transport – tracks, ways, terminals and interchanges

Residential – hotels, boarding and guesthouses

Residential – dwellings

Utilities and infrastructure – energy production and distribution, water storage and treatment

Residential – residential institutions

Community services – medical, healthcare, education, and community (emergency) services

Suitable use Requires mitigation Requires defence and mitigation Avoid use

Illustrative Scheme –Land Use Planning

The illustration shows how the spatialarrangement of the scheme has, to a largedegree, worked with the lie of the land.

Robust recreational uses have been locatedin the most hazardous part of the siteadjacent to the water course, whilst landlevels have been adjusted to create a lowrisk area of the site for a nursing home withvery vulnerable occupants. In adjusting thetopography of the site care has been takento retain at least the same level of storagecapacity of flood water as the initialassessment identified. Ideally this capacityshould be increased.

In the area of high risk apartment buildingswith control and mitigation are proposedand in the area of medium flood riskhousing with mitigation.

Low risk

Medium risk

High risk

Very high risk

Figure 12 Land use planning responds to the risk


Case Study –Land Use PlanningRowan Road, Merton

Architect: Sheppard RobsonLandscape architect: MacFarlane WilderClient: Crest Nicholson The Design for Manufacture Competition set out to achieve new levels of sustainabilityand energy efficiency in affordable housingand to establish a delivery mechanism usingMMC (Modern Methods of Construction) forsustainable communities. In their approach to all of the sites The SIXTYK consortia havestrived to achieve schemes which set highstandards for sustainability in both thearchitecture and the landscape. The RowanRoad site in particular provides an opportunityto provide a significant landscape thatprovides both visual amenity and plays anactive role in surface water management and bio-diversity.

The necessary 2.6 hectares of public openspace which have been provided for theRowan Road development have beendesigned to provide maximum benefit to all residents from the surrounding area. Thespace has been used to incorporate amenityspace as well as sustainable drainage, floodattenuation, habitat creation and a closeintegration with the urban design of thedevelopment. Strong visual connections andnatural surveillance is coupled with regularaccess points to ensure that the landscapedspace is both accessible and safe to use. Theoverall quality of the environment stems froma diversity of treatments that provide wildlifehabitat without compromising the usability of the space for leisure and recreation.

Sustainable design has been embodied as a concept throughout the scheme. Thelandscape whilst acting as a green lung forthe development also has a vital role to play in the water management strategy of the site.Run-off from hard surfaces is directed into

surface swales via petrol and oil interceptorswhere reed beds provide filtration and removalof suspended solids. Detention basins ornormally dry ponds provide space for theattenuation of rainwater run-off during peakstorm events and provide a contribution to the amenity space when they are dry. Thebalancing pond provides a more significantstorage capacity for attenuation of rainfallduring peak storm events and makes apositive contribution to the amenity andecological value of the site. The reed shelvesthat provide habitat for wildlife also providefiltration and final polishing of rainwater run-offquality prior to discharge off-site.

The site also provides flood attenuation to the River Wandle basin and the constructionof a flood alleviation basin in the westerncorner of the site ensures that the housingdevelopment is not at risk. This area alsoprovides an opportunity to use wetland andmarginal plant species to further enhance the biodiversity of the site.

Petrol/oil interceptor

Hard surface channels

Surface water collectors – hard surface

Grass swale

Surface water collection – soft areas

Balancing pond

Rainwater collection

Foul water

Irrigation from rainwater storage tank

Hard surface run-off

Petrol/oil interceptor

Drainage swale collection from surface run-off

Balancing pond Discharge into ditch

Figure 13 Figure 14

Figure 15


Having attempted to avoid placing vulnerableuses in hazardous areas the next step is toreduce the probability and severity of a floodto any parts of the scheme still at risk.

The control measures most appropriate willdepend largely upon the source of flood risk.

Tidal and Fluvial Sources of Flood Risk

Tidal and fluvial flooding can be controlled with costal defences, river walls, barriers andbarrages. These measures provide a primarydefence against the risk.

There will, however, remain a residual risk of a breach or failure of the primary defencemoreover these defences can still beovercome by a severe flood. In order to controlthis risk it will be necessary to raise the level of habitable accommodation above the designflood level and provide safe access.

Stilts or columns should be avoided insituations where flood water flows. Debriscarried by the flood can build up and form a dam-like obstruction which may suddenlycollapse releasing water and exacerbating the consequences of the flood.

Where it is not possible to provide safe accessit may be possible to design accommodationin such a way as to allow occupants to remainsafely in place for the duration of the flood.Sufficient emergency utility supplies includingpotable water will need to be maintainedthroughout the period of the flood.

This approach will only be viable if the durationof the flood is limited. Tidal flooding caused bya breach in defences can be of short durationprovided it is possible to repair the failure in the time between high tides.

Even if this strategy can be justified buildingsshould be designed to allow rescue ofoccupants by boat or air as an option of last resort.

As previously discussed, it may be possible to adjust land levels in such a way as toprotect some areas whilst providing floodstorage capacity in other areas. The overallflood storage capacity of the site must bemaintained or improved and there should be no adverse effects on neighbouringproperties.

On a larger scale making additional space for water within overall river catchment areasmay contribute to a reduction in overall level of flood risk. It may be possible to justify

‘reclaiming’ land in one area by providingcompensatory flood storage elsewhere in the catchment area.

Pluvial Flood Sources and Flooding from Sewers

The risk of flooding from rainwater run-off and exceedance of drainage capacity can be reduced in a number of ways.

Sustainable Drainage Systems (SuDS)integrated with the building and landscapedesign of a scheme will help control run-offgenerated by the development and alsomanage run-off from adjacent land.

SuDS range from green roofs, rainwaterharvesting systems and permeable pavingthrough to networks of swales, filter beds and detention ponds. In many cases rainwaterharvesting can be incorporated intosustainable drainage systems to help reducevolumes of water entering the storm water system.

3.0Control:Reducing the Risk

Opportunities – Control

• Integrate river and coastal flooddefences with new areas of publicrealm such as riverside walkways,cycle routes and pocket parks

• Design defences to promotebiodiversity and create wildlifecorridors

• Provide additional flood storagecapacity by creating new wetland areas

•Maximise visual and physicalconnections to rivers, coasts, wetlandsetc. to increase the amenity andcommercial value of newdevelopments

• Provide safe refuge on site for theduration of a short flood by designinglow carbon self sufficient schemeswith their own renewable energy,water harvesting and purification

• Enhance the public and private realmwith high quality robust landscapedesigns which promote waterconservation and surface watermanagement

• Integrate SuDS with systems for the rapid retreat of flood waters.


In situations where soak away is not possible and confined urban areas it may benecessary to supplement the limited amountof rainwater storage that green roofs canprovide with over-sized drainage pipes, raincrate systems and underground attenuationtanks. However even in urban developments it is often possible to design sacrificial areasfor water detention into public and privateopen space by working with the naturaltopography of the land or by making smalladjustments to it. In this way the flood storagecapacity of the land may be directed awayfrom accommodation and even increased at the same time as providing high qualitylandscape designs and amenity space.

With all types of flooding, consideration shouldbe given to how the design and layout canassist the retreat of flood water and clearanceof debris. An integrated approach to rainwaterharvesting, bio-filtration and attenuation can be linked to areas of flood alleviation thatimprove the quality and quantity of rainwaterrun-off discharged into surface water or sewer networks.

There is a clear synergy between SuDSsystems of swales and detention pondsprimarily designed to manage surface waterand the removal of pluvial flood water. Floodalleviation networks built into sustainabledrainage design can help absorb the impactof tidal or fluvial surges during peak storm events.

A wide range of technical advice andguidance is available on SuDS from the UKenvironment agencies, from the ConstructionIndustry Research and Information Association(CIRIA) www.ciria.org.uk/suds and others.

The following resources are available free of charge from the Environment Agency:

• Designs that Hold Water25 minute video (or DVD) on SuDS and their benefits

• Sustainable Drainage Systems: An introduction20 page A4 booklet

• Sustainable Drainage Systems – A Guide for Developers8 page A5 booklet

Illustrative Scheme – Control

The illustrative scheme shows how thecontours of the site have beenadjusted to provide additional floodstorage in the recreational area next tothe river. Hard and soft landscaping andfixtures such as play equipment andseating are designed to be resilient toinundation and easy to clean up andrecommission. Signage to raiseawareness and flood warnings will be provided.

Many areas at risk from fluvial flooding have traditionally turned theirback on the river, choosing to put upimpermeable flood defences that onlyexacerbate the problem downstream.The process illustrated here of ‘river repatriation’ relinks the river to tidal/fluvial expansion zones and sets development back from the edge to create opportunities for floodexpansion, greater biodiversity andamenity value along river floodplains.

Living accommodation in theapartment buildings has been raisedabove the design flood level and they have been orientated to takeadvantage of views towards the river.Safe access is provided via the low riskarea higher up the gradient for all butthe most severe flood for which saferefuge within the buildings is provided.

To control surface water run-off and provide a route for flood waters toretreat, a system of drainage corridorsin the form of swales planted with treeshave been proposed. These lead towater detention ponds that will in turnslowly discharge back into the river.

Green roofs to the nursing home,permeable surfaces to parking areasand access drives are also illustrated.These devices slow down the passageof rainwater towards the river therebyhelping to control flooding on site andpotentially further downstream too.

Raised floor levels

Safe access

Dry access

Drainage, corridors and swales

Green roofs and permeable paving

Additional flood storage capacity (for fluvial flooding)

Detention ponds (for surface water)

Figure 16 Control measures to reduce the probability and severity of a flood


Case Study – ControlAlbion Quayside, Gravesend

Urban design and architecture: KCA ArchitectsCivil Engineers: Walsh AssociatesAssessment: Halcrow Group LimitedClient: Feabrex Ltd

A brownfield site next to the River Thames.The scheme is in flood zone 3a, protected bya flood defence wall that isolates the inlandareas from the river. A range of control andmitigation proposals are integrated into theurban, architectural and landscape designs.The existing flood defence wall is replaced by a new terraced flood defence which opensup the river frontage to the public with a newlandscaped promenade and also creates anew river bank rich in estuarine flora andfauna. New access to the river allows thescheme to be organised around a series ofcut through that run from the interior of the

site through to the river, a feature characteristicof many riverside towns which alsomaximises the benefit of the riverside setting.Surface water is managed using green andbrown roofs, permeable paving with rain crateunderground storage and a new green canalacting as a swale which reopens the historictow path. In case of a breach the canal, theexcess capacity of the marina and a sacrificialarea of the quayside square will provide floodstorage capacity to assist the retreat of floodwaters. Residential accommodation is stackedover more robust uses. Active commercialuses line the public spaces and ground withparking courts to the interior of each urbanblock. Roof gardens over these parking courtsprovide amenity space and access foremergency evacuation points. As the land isisolated from higher ground, all buildings aredesigned to provide safe refuge at higherlevels and infrastructure is designed to berobust and plant placed above the flood level.

Top Albion Quayside, Gravesend

Above Riverside promenade acting as new flood defence


Having put in place measures to control the flood risk, the next step is to consider how design proposals can minimise thepotential consequences of a flood uponoccupants, property, the public realm and the utilities infrastructure.

Design flood risk levels are based uponhistorical data and modelling of complexhydraulic systems. At best they define anacceptably low risk of a certain flood event or worse happening. Obviously, however lowthat probability is, there is still a chance that a considerably worse flood will happen thanthat designed for.

The consequences of climate change are as yet not fully understood. It is, however,generally accepted that warmer winters, drier summers and unseasonal and frequentlyextreme storm events will put even areas notnormally prone to flooding increasingly at risk.

Although allowances are made for climatechange in estimating the design flood heightwe are unarguably entering a period of greateruncertainty when it comes to predicting futureevents. Schemes should therefore as far aspossible plan for the worst case in orderreduce the impact of a flood.

Safety

Clearly the most important consideration is the safety of people: residents, workers and visitors.

Safe escape routes should be designed andsafe refuge areas above the level of the floodfor those that are unable to escape provided.Planning for the worst case a strategy foremergency access by rescue services inboats or helicopters should be designed into balconies, roof gardens or other suitable points.

Flooding has a disproportionate impact uponvulnerable groups and these should beconsidered and planned for. Early warningsystems and procedures must be put in place.

Identify the relevant emergency respondersand consult with them to agree viable plans at an early stage in the design process.

As previously described, emergency suppliesof potable water, heat, light, communicationsand temporary storage of sewage may alsobe required.

4.0Mitigation:Minimising theConsequences

Opportunities – Mitigation

• Put in place early warning systems,increase awareness of occupants and residents

• Integrate escape and rescue points into landscape and building design

• Integrate barriers and baffles into thelandscape design in the form of streetfurniture to reduce water speeds andlevels of hazard

• Secondary defences can be integratedwith SuDS systems

• Raising ground floor levels andproviding flood storage undercrofts

• Resistant construction for shallow flood depths to keep water out

•Amphibious buildings designed to floatin the event of a flood for watersidelocations

• Resilient buildings designed to minimiseinternal damage and reinstatementcosts in the event of a flood.


Resistant Designs

Property and internal fittings should bedesigned to be resistant and/or resilient.These measures are particularly relevant torefurbishing existing properties which are atrisk of flooding and in some circumstanceswhen developing properties in areas of lowflood risk. Design of resistant and resilientbuildings is not currently covered by theBuilding Regulations however they are likely to in future revisions.

A resistant building attempts to keep anywater from entering property. This is normallyonly viable if the potential flood water is lessthan about 600mm in depth. Above this theforce exerted by the water is likely to causestructural failure and on balance it will be lessdamaging to allow the water to pass throughthe interior.

Traditional cavity masonry construction withventilated suspended timber floors is notinherently resistant or indeed resilient. To createproperly resistant buildings more inherentlyrobust forms of construction such as tankedreinforced concrete or reinforced block workmay be more appropriate.

Creating acceptable access into a resistantstructure presents a design challenge.Elevated access over the resilient thresholdmay be possible if it can be designed tocomply with the principles of equal access,

the Disabilities Discrimination Act and LifetimeHomes guidance.

Otherwise creating access through anotherwise resistant enclosure introduces a weak point.

Demountable or automatic shutter systemsare available to protect doors and openings of buildings. However, these are reliant uponbeing properly maintained and installed oractivated prior to a flood, leaving them open to human error.

Taking a similar approach to the design ofpermanently moored house boats, floating or amphibious houses have been developedin Holland that rise up on pontoons alongmooring posts with rising flood water.

These fall into two basic categories – a hulltype and a raft type. The hull type excludeswater in the same way that a boat does andat a certain depth of water becomes buoyant. The raft type incorporates buoyancy into adeep supporting foundation slab. Access and servicing is via flexible walkways and connections.

Demonstration projects are located onwaterfront locations or in polders and areasdesignated for controlled flooding. This type of resistant design is only appropriate forsituations where there is no flow in floodwaters and where debris will not betransported onto the site.

Paalwoningen ‘stilt houses’, Haarlemmermeer, WaterstudioArchitects In this study, based in a water-retention area in northernHolland, houses have been raised above the floodplain, exploiting asite which would otherwise be uninhabitable. www.waterstudio.nl.

http://www.waterstudio.nl


Resilient Design

Resilient buildings are designed in such a way as to reduce the cost and time requiredto reinstate the property should it be flooded.Robust materials and finishes should be used,including hard floor surfaces that can bewashed down, and solid wood rather thanparticle board or MDF for doors and fittedfurniture in kitchens and bathrooms.Alternatively, finishes can be designed to be removable or sacrificial and easily replaced in the zone affected by flooding.

Special vent covers can be used to closeventilation bricks to prevent underfloor voidsand cavities becoming flooded. However, solidfloors rather than suspended timber floors arepreferable. Insulation should be closed cell inorder to reduce water take-up and minimisethe time needed for drying out.

Electrical and heating systems should bedistributed at high level rather than under theground floor and drop down within walls tosockets and radiators. Electrical sockets are to be set above the flood level at dado ratherthan skirting level.

Solid walls finished in cement render systemsor tiling, at least up to dado level, should beused in preference to timber stud partitionsfinished in plaster board.

Building fabrics and components should be designed and specified to be inherentlyresistant or resilient in order to achieve an end product that integrates both the functionsrequired by the user and the need to mitigatethe risk of flooding.

Detailed guidance on the design andspecification of resilient buildings is available inthe CIRIA document Development and FloodRisk – Guidance for the Construction Industryand Improving the Flood Performance of NewBuildings, Flood Resilient Construction by CLG.

Design of Resistant Infrastructure

The infrastructure serving a scheme or widerneighbourhood should also be designed to beresistant or at least resilient to flooding.

Main utility supply routes for power, data andwater should, if possible, be run in the area oflowest flood risk. Any electrical sub-stationsand transformers should also be placed in thearea of least risk and should be protected byflood defence enclosures if necessary.

Within buildings pump sets for water suppliesshould be raised above the design flood levelalong with electrical switch rooms, meterrooms, oil storage tanks, lift motor rooms andany other vulnerable plant. Sewerage systemsin areas of flood risk should be fitted with non-return valves in order to prevent foul water backing up into properties.

Public Realm and LandscapeThe public realm and landscapes too shouldbe designed to withstand greater fluctuationsin rainfall run-off and flooding. Native plantsable to withstand periods of dry as well aswet, such as Salix caprea (Goat Willow), Alnusglutinosa (Alder) and Hippophae rhamnoides(Sea Buckthorne) should be planted in or nearswales to prevent scouring and wash-out of surface drainage systems and to preventdebris from washing into and blocking storm water culverts.

Combining a Hierarchy of MitigationMeasuresThe effective mitigation of flood risk isgenerally a combination of several differentmeasures. A hierarchy of measures shouldsuccessively reduce the impact of a series of worsening flood scenarios.

The first measure should be to raiseaccommodation levels above the flood level. If it is not possible to raise all theaccommodation above the design flood level because it is necessary to provide levelaccess to some parts of the building thenthese areas should be raised above the levelof more frequent floods and designed to be resistant.

Should resistant measures fail to exclude flood waters parts of the building below theworst case flood level should be designed tobe sacrificial in terms of the use and functionof the property. In addition they should bedesigned to be resilient in order to minimisethe cost of any damage and to speed uprecovery time.

Within a building, whether commercial orresidential, high-value fittings and essentialfunctions should be located well above theflood level to give a degree of future proofing.

Finally safe refuge and access for theemergency services should be incorporatedinto the design.

Watergaten (Waterholes) project, ‘s-Hertogenbosch, RondayWinkelaar Architects The Waterhole houses do not employconventional foundations but instead respond to fluctuating waterlevels. www.rondaywinkelaar.nl.

http://www.rondaywinkelaar.nl


Illustrative Scheme –Mitigation

The illustrative scheme shows housingwithin the area of low flood risk designedwith a combination of resistant and resilientconstruction measures.

Primary utility infrastructure has been run in the lowest risk area. An electrical substation has been made resistant to floodingwith a flood proof enclosure.

Within the apartment buildings pump setsfor water and other items of plant havebeen located at high level and meter roomsabove the flood design level.

Case Study – MitigationThe Turnaround House

Architects: Nissen Adams LLPQuantity Surveyors:MeasurServices Engineering Consultants:Mendick Waring Ltd

Norwich Union, together with the RIBA,launched a design competition in the Summerof 2008 to encourage innovative solutions tobalance development needs with a responseto increased flood risk. Architects were askedto design a house for the future, which could be built on a floodplain, and took theassociated flood risk into account via itsarchitectural features and design. Overall, wesought designs that would help ensure thatproperties built on floodplains are designed to mitigate the flood risk, are workable, and, of course, insurable.

Concentrating on resilience, Nissen Adams’solution applied three design principlesresulting in their design for ‘The TurnaroundHouse’. These principles were:

• Creating an adaptable house that respondsto a flood, without compromising livingduring the rest of the year

• Ensuring that occupants’ needs are met at all times

• Acting as a physical link to the communityand its support networks.

For most of the year the flood housefunctions as a typical house and only in aflood does it transform and a ‘turned around’living arrangement is adopted. Should waterpenetrate the ground floor occupants relocateupstairs. A robust concrete dado extendingfrom the foundations allows easy cleaningafter the flood subsides.

Provisions have been made to tailor to eachfamily member’s needs, right down to those of the pets and car. The typical arrangementof bedrooms upstairs, living downstairs isinverted to raise the family’s most expensivebelongings away from damage by floodwaters. Desks in the storage wall provideoffice space in case occupants cannot get to work or school. A sunken concrete pontoonallows for the family car to be floated out ofharms way. A small green space is providedfor pets when the garden is flooded. Gabionwalls act as a filter to limit the amount ofdebris entering the garden where children and pets play. Timber shutters providesecurity to the first floor and fold down likedrawbridges to join neighbouring balconies ina flood, thereby creating a continuous raisedwalkway that replaces the flooded pavementbelow to reconnect the community.

Image by permission of Nissen Adams LLP

Figure 17Mitigation measures to minimise the consequences of a flood

Resistant utilitiesResilient buildingsResistant services


Thickened floor structure provides an easily accessible deep storage space for mattresses, temporary bedding andemergency drinking water

Wall hooks allow for furniture and toy boxes to be raised out of harm’s way during a flood

Electrical sockets above flood line

A concrete dado extends to a height of 1,000mm above floor finish, creating a resilient surface

Hard-wearing screed flooring maintains itsintegrity and is easy to clean after a flood

Operable roof light to facilitate drying after a flood

Water storage tanks above WC allow for toilet flushing during a flood if water mains are shut down

Storage wall – emergency supplies are stored in deep cabinets and accessed in times of flood

Storage in drawers forming steps of staircase for shoes, board games andemergency supplies

Timber storage shelves below staircase to a height of 1,000mm above floor finish, water resistant storage below

Concrete steps to a height of 1,000mm above floor finish with timber steps above

Removable water resistant concrete plank flooring

Sloped concrete trough below floor creates a path of least resistance for floodwater. Also acts as a reservoir in the event of a minor flood. Sloped floor allows water to draintowards WC and exit at the rear of house

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Finally, having developed design proposals, anassessment of the scheme should be madein the event of a flood. This assessment willultimately form part of the site Flood RiskAssessment in support of the development.However, the process is iterative, and followingan initial appraisal of the proposals it may benecessary to revise designs if they fail to fullymitigate the flood risk.

The assessment must first demonstrate that the occupants would not be placed at an unacceptable level of risk.

It must then demonstrate that the designproposals to control and mitigate the risk of flooding are effective.

It must also show that the proposals wouldhave no adverse impact upon flood risk toareas outside of the site.

There are also other important factors toconsider when assessing the impact ofproposals.

Climate change is likely to cause greaterfrequency and severity of flooding largely as aresult of unsustainable development. Thereare strong arguments to support the idea thatall new development and not just

development within the floodplain should aim for carbon neutrality to avoid furthercontribution to the risk.

Consideration should also be given to theimpact of proposals on local ecology andecosystems. It is not just a case of protectingthe development against flood waters, but a case of protecting rising rivers againstcontamination from development.

5.0Re-assessmentof Impact

Opportunities –Re-assessment of Impact

• New development in the floodplaincan be used to increase flood storagecapacity thereby progressivelyreducing overall risk

• New development in the floodplaincan set high standards of sustainabilityshowing by example how drivers ofclimate change can be reduced

• Sensitively designed regenerationschemes within the floodplain candeliver overall environmental andecological benefits.

Illustrative Scheme –Re-assessment of Impact

The illustrative scheme shows that througha combination of land use planning, controland mitigation measures the risk tooccupants of the site has been reduced toan acceptable level. It demonstrates thatthe flood storage capacity of the site hasbeen maintained in the most severe floodevent and actually increased for morefrequent but lesser flood events.

The assessment also confirms thatneighbouring land and properties are not adversely affected.

Figure 18 Assessment of the scheme’s impact

29 Further Reading

RIBA climate change homepage: download guidance and further information on the RIBA’s climate change programmewww.architecture.com/climatechange

Association of British Insurers: information on flooding and insurancewww.abi.org.uk/flooding (X)

Ciria flooding homepage: advice on the repair and restoration of flooded buildingswww.ciria.org.uk/flooding

Chartered Institute of Water and Environmental Managementwww.ciwem.org

Department for Environment, Food and Rural Affairs: flood and coastal erosion riskmanagement www.defra.gov.uk/environ/fcd

Department of Energy and Climate Changewww.decc.gov.uk

Environment Agency flooding homepagewww.environment-agency.gov.uk/subjects/flood/

Flood Protection Association: information about manufacturers and installers of flood protection productswww.floodprotectionassoc.co.uk

Flood Resilient Home www.floodresilienthome.com

Geographical Association: flood risk assessment and managementwww.geography.org.uk/resources/flooding

Improving the flood resilience of new buildings (CLG)www.planningportal.gov.uk/uploads/br/flood_performance.pdf

Institution of Civil Engineerswww.ice.org.uk

The LifE Project and Handbookwww.brebookshop.comwww.lifeproject.info

National Flood Forumwww.floodforum.org.uk

The Pitt Report: ‘Learning Lessons from the 2007 Floods’www.cabinetoffice.gov.uk/thepittreview

Planning Policy Statement 25: Development and Flood Risk (Department for Communities and Local Government)www.communities.gov.uk/documents/planningandbuilding/pdf/planningpolicystatement25.pdf

PPS25 Practice Guide (CLG)www.communities.gov.uk/publications/planningandbuilding/pps25practiceguide

Thames Estuary 2100 project (EA)www.thamesweb.com

UK Climate Impacts Programmewww.ukcip.org.uk

FurtherReading

The LifE project promotes innovativearchitectural and landscape solutions for situations where it is not possible tolocate development in areas of lowerflood risk. The project presents a shift inthinking by investigating the permitting ofwater into sites in a controlled manner.

The LifE Handbook summarises theprincipals of the LifE approach, illustratingstrategies for sites at Hackbridge,Peterborough and Littlehampton.

The handbook illustrates issues at a range of development scales andprovides possible solutions for manydevelopment situations.

Author: Baca Architects and the BuildingResearch Establishment in collaborationwith Cyril Sweett, Halcrow Group Ltd,Fulcrum Consulting and LDA Design.Funded by DEFRA.

http://www.architecture.com/climatechange

http://www.abi.org.uk/flooding

http://www.ciria.org.uk/flooding

http://www.ciwem.org.uk

http://www.defra.gov.uk/environ/fcd

http://www.decc.gov.uk

http://www.environment-agency.gov.uk/subjects/flood

http://www.floodprotectionassoc.co.uk

http://www.floodresilienthome.com/index.asp

http://www.geography.org.uk/resources/flooding

http://www.planningportal.gov.uk/uploads/br/flood_performance.pdf

http://www.ice.org.uk

http://www.brebookshop.com

http://www.lifeproject.info

http://www.floodforum.org.uk

http://www.cabinetoffice.gov.uk/thepittreview

http://www.communities.gov.uk/documents/planningandbuilding/pdf/planningpolicystatement25.pdf

http://www.communities.gov.uk/publications/planningandbuilding/pps25practiceguide

http://www.thamesweb.com

http://www.ukcip.org.uk

duffymactwo

Note

This web address not currently working

30 Glossary of Terms

Breach of defencesA structural failure at a defence allowing waterto flow through.

Catchment The area that is drained by a river or artificialdrainage system.

Climate changeLong-term variations in global temperatureand weather patterns, which occur bothnaturally and as a result of human activity,primarily greenhouse gas emissions.

Coastal erosionThe gradual wearing away of the coastlinethrough a combination of wave attack and, inthe case of coastal cliffs, slope processes (e.g.high groundwater levels). This may include cliffinstability, where coastal processes result inthe periodic reactivation of landslide systemsor promote rock falls.

Coastal floodingFlooding from the sea. Coastal flooding iscaused by higher sea levels than normalresulting in the sea overflowing onto the land.

Consequence of floodingHealth, social economic and environmentaleffects of flooding, some of which can beassessed in monetary terms, while other, lesstangible, impacts are more difficult to quantify.Consequences depend on the hazardsassociated with the flooding and thevulnerability of receptors.

Conveyance functionWhen a river overflows its banks, it continuesto flow over the floodplain, conveying waterdown-stream, as well as storing water wherethe floodplain may be obstructed andreleasing it slowly.

Detailed assessmentTo assess flood-risk issues in detail and toprovide a quantitative appraisal of potentialflood risk to a proposed or existingdevelopment, of its potential impact on floodrisk elsewhere and of the effectiveness of any proposed mitigation measures.

Environmental Impact Assessment (EIA)A technique used for identifying theenvironmental effects of developmentprojects. As a result of European UnionDirective 85/337/EEC (as amended 1997), thisis a legislative procedure to be applied to theassessment of the environmental effects ofcertain public and private projects which arelikely to have significant effects on theenvironment.

Estuarial floodingFlooding from an estuary, where water levelwill be influenced by both river flows and tidalconditions, with the latter usually beingdominant.

FloodingOverflowing of water onto land that isnormally dry. It may be caused by overtoppingor breach of banks or defences, inadequate orslow drainage of rainfall, underlyinggroundwater levels or blocked drains andsewers. It presents a risk only when peopleand human assets are present in the areawhich floods.

Flood Alleviation Schemes (FAS) A scheme designed to reduce the risk offlooding at a specific location.

Flood defenceA man-made structure (e.g. embankment,bund, sluice gate, reservoir, and barrier)designed to prevent flooding of areas adjacentto the defence.

Flood-detention reservoirsAn embanked area designed to holdfloodwater upstream from areas at risk and release it slowly. Embankments may be constructed across the river or off-line, with flood flows being diverted into thereservoir area.

Flooding from artificial drainage systemsThis occurs when flow entering a system,such as an urban storm water drainagesystem, exceeds its discharge capacity, itbecomes blocked or it cannot discharge due to a high water level in the receivingwatercourse.

Flood hazardThe features of flooding which have adverseimpacts on receptors such as the depth ofwater, speed of flow, rate of onset, duration,water quality etc.

Flood hazard assessmentAn assessment of the hazards that wouldarise from flooding, e.g. to identify whereflooding would occur, how deep the waterwould be, how fast it would flow, how rapidlywould it rise and how long it would remain.

Flood riskAn expression of the combination of the floodprobability and the magnitude of the potentialconsequences of the flood event.

Glossary of Terms


Flood Risk Assessment(covers all scales of assessment) A study toassess the risk of area flooding under bothpresent and potential future circumstances,such as changes in climate, land-use,development or flood-risk management.

Flood Risk Management (FRM)Combines the function of mitigating andmonitoring flood risks and may include pre-flood, flood-event or post flood activities.

Flood Risk Management Plans (FRMP)Developed in accordance with national floodpolicy and the EU Floods Directive, providesthe strategic direction for flood-riskmanagement decisions in a catchment. Thesewill describe a range of techniques fromtraditional river defences to non-structuralresponses, such as flood warning andresilience measures at property level. Thecoastal strategies produced by DAFF will alsofollow this current thinking in managing therisks associated with coastal erosion andcoastal flooding.

Flood storageThe temporary storage of excess run-off orriver flow in ponds, basins, reservoirs or on the floodplain.

Flood zonesA geographic area within which the flood risk is in a particular range as defined within PPS25.

Flooding DirectiveAn EU Directive (Directive 2007/60/EC of theEuropean Parliament and of the Council of 23 October 2007 on the assessment andmanagement of flood risks) to integrate theway flood risk is managed throughout theEuropean Union.

Fluvial floodingFlooding from a river or other watercourse.

Groundwater floodingFlooding caused by groundwater escapingfrom the ground when the water table rises to or above ground level.

Indicative Floodplain Map (IFM) A map that delineates the areas estimated to be at risk of flooding during an event ofspecified flood probability. ‘Indicative’acknowledges that such maps give anindication of the areas at risk but, due to thescale of the exercise, they cannot be reliedupon to give precise information in relation to individual sites.

Inland floodingAny flooding away from the sea, the primarycause of which is prolonged and/or intenseprecipitation (or the failure of water-retaininginfrastructure, such as burst water pipes ordam-breaks).

InundationOverflowing of water onto land that is normally dry.

Exception testAn assessment of whether a developmentproposal within an area at risk of floodingmeets specific criteria for proper planning andsustainable development and demonstratesthat it will not be subject to unacceptable risknor increase flood risk elsewhere.

Likelihood (probability) of floodingA general concept relating to the chance of an event occurring. Likelihood is generallyexpressed as a probability or a frequency of a flood of a given magnitude or severityoccurring or being exceeded in any given year. It is based on the average frequencyestimated, measured or extrapolated fromrecords over a large number of years and is usually expressed as the chance of aparticular flood level being exceeded in anyone year. For example, a 1 in 100 or 1% flood is that which would, on average, be expectedto occur once in 100 years, though it couldhappen at any time.

Mitigation measuresRefers to an element of development designwhich may be used to manage flood risk to adevelopment, either by reducing the incidenceof flooding both to the development and as aresult of it and/or by making the developmentmore resistant and/or resilient to the effects of flooding.

Overtopping of defencesFlood defence failure or exceedancemechanism. Flood water reaches levels thatare higher than the flood defence level andflows over the top of the crest of the structure.The structure may remain stable howevererosion of the land ward face of the defencemay cause the defence to collapse.

PathwaysProvides the connection between a particularsource (e.g. high river or tide level) and thereceptor that may be harmed (e.g. property). In flood risk management pathways are often‘blocked’ by barriers, such as flood defencestructures, or otherwise modified to reducethe incidence of flooding.


Precautionary approachThe approach, to be used in the assessmentof flood risk, which requires that lack of fullscientific certainty, shall not be used as areason for postponing cost-effectivemeasures to avoid or manage flood risk.

River basin management planA management plan for all river basinsrequired by the Water Framework Directive.These documents will establish a strategicplan for the long term management of theRiver Basin District, set out objectives forwater bodies and in broad terms whatmeasures are planned to meet theseobjectives, and act as the main reportingmechanism to the European Commission.

River floodingSee fluvial flooding.

ResilienceConstructing the building in such a way thatalthough flood water may enter the building, its impact is minimised, structural integrity ismaintained, and repair, drying & cleaning andsubsequent re-occupation are facilitated. This is sometimes known as wet-proofing.

ResistanceConstructing a building in such a way toprevent flood water entering the building or damaging its fabric. This is sometimes known as dry-proofing.

ReceptorsRefers to those things that may be harmed by flooding (e.g. people, houses, buildings orthe environment).

Residual riskThe risk which remains after all risk avoidance,substitution and mitigation measures havebeen implemented, on the basis that suchmeasures can only reduce risk, not eliminate it,and they may not be maintained in perpetuity.

Run-offThe flow of water, caused by rainfall, from an area, which depends on how permeablethe land surface is. Run-off is greatest fromimpermeable areas such as roofs, roads andhard standings and least from vegetated areas– moors, agricultural and forestry land.

Sequential testA risk-based approach to consideringdevelopment in relation to flood risk, appliedthrough the use of flood zoning that givespriority to development in areas of low risk but if no such land is reasonably available,

the type of development that is acceptable ina given zone is dependent on the assessedflood risk of that zone (i.e. the probability offlooding and the vulnerability of thedevelopment).

Scoping assessmentA generally qualitative or semi-quantitativeassessment to confirm sources of floodingthat may affect a plan area or proposeddevelopment site, to appraise the adequacy of existing information and to provide aqualitative appraisal of the risk of flooding and potential impact of a development onflooding elsewhere and of the scope ofpossible mitigation measures.

Screening assessmentA broad-brush assessment to identifywhether there is any flooding or surface watermanagement issues related to a plan area orproposed or existing development site thatmay warrant further investigation.

SourceSource refers to a source of hazard (e.g. strong winds, heavy rainfall).

Source Pathway Receptor ModelThe representation of the components offlood risk. There are three components: thesource of the hazard, the receptors affects by the hazard and the mechanism of transferbetween the two. For there to be flood risk all three, source pathway and receptor must exist.

Strategic Environmental Assessment (SEA)The application of Environmental ImpactAssessment to earlier, more strategic, tiers of decision-making policies, plans andprogrammes.

Strategic Flood Risk Assessment (SFRA)The assessment of flood risk on a widegeographical area against which to assessdevelopment proposed in a district orcommunity.

Sustainable Drainage Systems (SuDS) A philosophy of drainage which aims tocontrol run-off as near its source as possibleusing a sequence of management practicesand control structures designed to drainsurface water in a more sustainable fashionthan some conventional techniques.

Water Framework Directive (WFD) A European Community Directive(2000/60/EC) of the European Parliament and Council designed to integrate the way

we manage water bodies across Europe. Itrequires all inland and coastal waters to reach‘good status’ by 2015 through a catchmentbased system of River Basin ManagementPlans (RBMP) incorporating a programme ofmeasures to improve the status of all naturalwater bodies.

VulnerabilityRefers to the resilience of a particular group,people, property and the environment, andtheir ability to respond to a hazardouscondition. For example, elderly people may be less able to evacuate in the event of a rapid flood than young people.

Printed by Seacourt Ltd on Revive recycled paper, using thewaterless offset printing process (0 per cent water and 0 percent isopropyl alcohol or harmful substitutes), 100 per centrenewable energy and vegetable oil-based inks. Seacourt Ltdholds EMAS and ISO 14001 environmental accreditations.

Acknowledgements

This document is based upon work undertaken for the RIBA by Kiran Curtis of KCA architects,www.kcaarchitects.co.uk with contributions by Jonathan Cooper of JBA consulting,www.jbaconsulting.co.uk

Project Steering Group:Peter Borrows (NGM Sustainable Developments Ltd)Jonathan Cooper (JBA Consulting)Ken Glendinning (London Thames GatewayDevelopment Corporation)Oliver Grant (Environment Agency)Rachel Hill (Environment Agency)Chris McCarthy (Battle McCarthy)Duncan McKinnon (MacCormac Jamieson Pritchard)Peter Phipps (Mott MacDonald)Andy Stanford (Walsh Associates)Peter Wilder (Macfarlane Wilder)

Edited by Ewan Willars (RIBA)Design by Duffy www.duffydesign.comProduced with the kind support of CABE and the Energy Saving Trust

Royal Institute of British Architects66 Portland PlaceLondon W1B 1ADT 020 7580 5533www.architecture.com

Energy Saving Trust21 Dartmouth StreetLondon SW1H 9BPT 0845 120 7799www.energysavingtrust.org.uk/housing

Second editionISBN 978-0-9561064-6-9

2Designing for Floodrisk

Documents

flood risk assessment

flood risk mitigationpart

greater risk of tidal

flood defences

design sequence

landscape design

sustainable design

design challenge