National-scale quantified analysis of future flood risk in

the UKPaul Sayers

Head of Floods and Water ManagementHR Wallingford, UK

Jim HallProfessor of Earth Systems Engineering

School of Civil Engineering and GeosciencesNewcastle University, UK

The traditional response to floods in the Thames(picture courtesy: Rachael Hill, Environment Agency)

Contents

1. The principles of quantified flood risk analysis

2. National-scale flood risk analysis in the UK

3. Foresight analysis of scenarios of future change in flood risk

4. Foresight analysis of potential responses to changing flood risk

5. Approximate analysis of the costs of responding to flood risk

The aim of the quantified analysis

• To quantify Risk: f(Probability.Consequence)

• To differentiate high risk from low risk areas and its change in time

• To support the identification of the:– Most important factors influencing risk

– Drivers of future change in risk

– Most effective responses

….. exhibits spatial complexity

The flood risk system

And exhibits temporal change….

Pathway(e.g. beach, defence and floodplain)

Source (River or sea)

Receptor(e.g. people in the floodplain)

The RASP system risk modelUtilises a structured definition of the flood system…….

Pathway defence (overtopping, overflow,

breaching)

Pathway floodplain (flood inundation modelling)

Limit of the undefended floodplainSource - Extreme water levels in rivers

Source – Extreme water levels on coast Defence system 1 (defences 1 to n)incl. raised and non-raised defences and gates

Defence system 2 (defences 1 to n) – incl. raised and non-raised defences and gates Receptor Impacts

The system risk modelComponents of the RASP model…….

Data used in national-scale analysis• Indicative Floodplain Maps (IFM)

• 1:50,000 maps with 5m contours (Composite DTM – 50m)

• National dataset of the centreline of all watercourses

• National Flood and Coastal Defence Database (35,000km of flood defences)

• National database of locations of residential and business properties (National Property Dataset)

Wales

Midlands

East Anglia

North-east

South-west

Thames

North-west

South-east

IFM as percentage of 10km x 10km grid cell<1%1 - 5%5 - 20%20 - 50%> 50% Drawing numberJob number

Date

Percentage of Indicative Flood Plainper 10km x 10km grid cell

Revision

Foresight - Future Flood Risks

June 2003 1.01

CDS 0438

0 50 10025Kilometres±

This map is reproduced from Ordnance Survey materialwith the permission of Ordnance Survey on behalf ofthe controller of Her Majesty's Stationary Office.(c) Crown Copyright. Unauthorised reproductioninfringes Crown Copyright and may lead to prosecutionor civil proceedings.Department of Trade and Industry. 100037208. 2003.

IFM as percentage of 10km x 10km grid cell<1%1 - 5%5 - 20%20 - 50%> 50% Drawing numberJob number

Date

Percentage of Indicative Flood Plainper 10km x 10km grid cell

Revision

Foresight - Future Flood Risks

June 2003 1.01

CDS 0438

0 50 10025Kilometres±

This map is reproduced from Ordnance Survey materialwith the permission of Ordnance Survey on behalf ofthe controller of Her Majesty's Stationary Office.(c) Crown Copyright. Unauthorised reproductioninfringes Crown Copyright and may lead to prosecutionor civil proceedings.Department of Trade and Industry. 100037208. 2003.

Distribution of the “Indicative” Flood Plain 10km/6m x 10km/6m grid)

Flood Defence

Fluvial Coastal

Type 1:Vertical Wall

Type 2:Slope or

Embankment

Type 3:High Ground

Type 4:Culvert

Type 5:Vertical Seawall

Type 6:Sloping

seawall or dyke

Type 7:Beach

Narrow Wide

Front slope protection

Front and crest

protection

Front, crest and rear

protection

Front slope protection

Front and crest

protection

Sheet piles & other

materials

Concrete structures

Bricks and masonry

Sheet piles & other

materials

Concrete structures

Bricks and masonry

The system risk model

Board scale representation of levee type and

performance

The system risk model

Representing levee performance

The system risk model

Representing levee performance

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

Overflow head (Water level - crest level)

P (

bre

ac

hin

g¦f

ee

bo

ard

)

Condition grade1Condition grade2Condition grade 3Condition grade4Condition grade 5

Limited no. of parameters in the NFCDD support the definition of a High level fragility curve All sloping embankments

The system risk model

Representing levee performance

The system risk model

Determining flood depth versus probability –

accounting for system behaviour

All inundation scenariosForesight 02 – we used a generalised statistical spreading technique...

….more recently…

National flood risk assessment 2008 – we now use a physical based rapid spreading model (RFSM)

The national system risk model

Spreading flood water….

Example results from the national assessment 2008 (NaFRA, 08)

Results of national-scale analysis

Foresight scenarios of future socio-economic change

Medium-low emissions

High emissionsand

Low emissions

Medium-high emissions

Low emissions

Foresight Futures + climate change scenarios

• .

System state variablesPathways

urban surfacesfields, drains

channelsflood storage

flood defences

floodplains

Receptorspeoplehouses

industriesinfrastructure

ecosystems

Sourcesrainfall

sea levelmarine storms

etc.

Risk

economic, risk to life, social, natural environment etc

System analysis

DriversProcesses that change the state of the

systemChange in risk

Translating drivers of change into the risk model….

Translating drivers of change into the risk model….some examples

Results:Change in

probability of flooding

Present day values

The four scenarios for the 2080s: change in risk

World Markets 2080s

Present Day

Global Responsibility 2080s

2080sDecrease

Negligible change

Low increase

Medium increase

High increase

Probability of inundation

Present DayNegligible (less than 1:1,000)

Low (between 1:200 and 1:1,000)

Medium between (1:75 and 1:200)

High (greater than 1:75)

Results:Change in annual

economic flood risk

Present day values

The four scenarios for the 2080s: change in risk

SUMMARY: A very significant increase in flood risk

• .

System state variablesPathways

urban surfacesfields, drains

channelsflood storage

flood defences

floodplains

Receptorspeoplehouses

industriesinfrastructure

ecosystems

Sourcesrainfall

sea levelmarine storms

etc.

Risk

economic, risk to life, social, natural environment etc

System analysis

DriversProcesses that change the state of the

systemChange in risk

ResponsesInterventions that change the state of the

system

Change in risk

Quantified analysis of theresponse portfolios

Realignment of Flood Defence Infrastructure + + ++

+++

Quantified analysis of theresponse portfolios

Reduce Current Exposure to Flood Loss Through Flood Proofing ++

+ ++

++++

Responses:Probability of inundation

Responses: Expected Annual Damage

Analysis of implementation costs

Analysis of implementation costs

• Only the costs of engineering measures have been estimated (excluding non-structural interventions)

• Exclude land purchase, compensation or significant environmental mitigation

• Without non-structural measures the costs roughly double

ConclusionsNational-scale flood risk analysis...

• has been shown to be feasible in the UK, and is steadily improving

• yields results that are approximate, but do correspond to recent observed flood damages

• provides the basis for quantified policy analysis

• was used in the Foresight study to demonstrate that:

– the potential for future increases in risk is considerable, and due to a combination of factors

– application of a portfolio of measures can bring the risk down