RASP - Risk Assessment of flood and coastal defence for Strategic Planning A High Level Methodology Project partners and co-authors Paul Sayers / Corina Rosu - HR Wallingford Jim Hall / Richard Dawson - University of Bristol John Chatterton - John Chatterton and Associates Rob Deakin - Halcrow DEFRA Conference 2002 Project funders Joint EA and DEFRA R&D Programme Theme 5 - Risk Evaluation and Understanding Uncertainty Theme leader: Ian Meadowcroft (EA)

Outline of presentation DEFRA Conference 2002 Project overview Overview of the High Level Methodology A forward look to more detailed methods Conclusions

Project background DEFRA Conference % of the UK population live in the Indicative Floodplain They are protected by over 34,000km of flood defences Flood risk assessment is recognised as necessary to appraise policy options, allocate resources and monitor performance However, a hierarchy of risk assessment methodologies, appropriate to a range of decisions, does not yet exist.

Project background and overview DEFRA Conference 2002 RASP aims to: Develop Test, and Support implementation a tiered set of practical risk assessment methodologies explicitly recognising that defences and flood plains perform as ‘systems’.

Project overview - Project Tasks DEFRA Conference 2002

Project overview DEFRA Conference 2002 All tiers of the RASP methodology aim to deliver: Failure probabilities for individual defences Failure probabilities for the defence “system” Total flood risk for an identified “impact zone” An indication of the risk associated with each defence Associated uncertainties

Project overview - Linkage with NFCDD DEFRA Conference 2002

Project overview - Supporting Integrated Risk Management DEFRA Conference 2002

Project overview - Supporting Integrated Risk Management DEFRA Conference 2002 RASP is a Decision support technique

Project overview - Supporting Integrated Risk Management DEFRA Conference 2002 Sources of risk include: Extreme river and tidal conditions Not groundwater or local rainfall

Project overview - Supporting Integrated Risk Management DEFRA Conference 2002 Risk pathways include: Structural failure (i.e breaching) Non-structural failure (i.e. overflow / overtopping) Floodplain flow

Project overview - Supporting Integrated Risk Management DEFRA Conference 2002 Risk receptors include: Any socio-economic dataset, e.g. – –People – –Properties – –Agricultural – –Environmental (no new research on impact valuation only evaluation)

Project overview - Project Tasks DEFRA Conference 2002 Summer 02 Autumn 03

HLM - Step 1 Identify the flooding system DEFRA Conference 2002 River centreline Fluvial IFM

HLM - Step 2 Collate defence data DEFRA Conference 2002 Gaps in NFCDD defence data filled to create a continuous “tramline” of information  Crown Copyright

HLM - Step 3: Estimate the probability of a single defence failure DEFRA Conference 2002 Defence performance described by a fragility curve Where P(failure|load) is known as the defence ‘fragility’ The probability of failure is given by:

DEFRA Conference 2002 Parrett Catchment and Bridgewater Bay 1km*1km Impact Zones HLM - Step 4 Identify Impact Zones

HLM - Step 5/6 Establish flood extent and depth DEFRA Conference 2002 A rapid flood inundation routine developed based on: Characteristics of the flood plain Defence type / likely breach size / overtopping discharge Evidence of past events/detailed studies of flood inundation

1. 1.The number of defence failures 2. 2.The mode in which those defences fail (overtopping or breaching) 3. 3.The location of failures HLM - Step 7 Estimate the probability of combinations of defence failure DEFRA Conference 2002 By combining multiple load and failure scenarios the probability of a given Impact Zone being inundated is calculated. Flood severity will depend upon not only the severity of the loading event (water level, duration etc.), but also

DEFRA Conference 2002 Parrett Catchment and Bridgewater Bay Information from Steps 6 and 7 combine to provide Flood depth v probability curves for each 1km*1km Impact Zone HLM - Step 8/9 Establish flood depth v probability and risk

DEFRA Conference 2002 Parrett Catchment and Bridgewater Bay Information from Steps 6 and 7 combine to provide Flood depth v probability curves for each 1km*1km Impact Zone HLM - Step 8/9 Establish flood depth v probability and risk Combine with required socio-economic impact descriptors to calculate “risk”

DEFRA Conference 2002 Parrett Catchment and Bridgewater Bay HLM - Step 8/9 Establish flood depth v probability and risk Contribution to risk from each defence can also be calculated

High Level Methodology - Initial case study results DEFRA Conference 2002 High Level Methodology Output showing the estuary of the river Parrett.

High Level Methodology - Initial case study results DEFRA Conference 2002 Scenario of increased maintenance

High Level Methodology - Initial case study results DEFRA Conference 2002 Scenario of climate change

Conclusions DEFRA Conference 2002 High Level Methodology A consistent approach has been developed to assess flood risk on a large scale taking account of the protection afforded by defences. Its utility has been demonstrated on a case study site, in particular its use of NFCDD enabling future improvements in defence data to feed through to improved decisions.

Conclusions DEFRA Conference 2002 High Level Methodology Turning research into applications National Flood Risk Assessment 2002 Support to the identification of Flooding Warning Risk Areas

Conclusions DEFRA Conference 2002 More detailed methodologies The more detailed tiers will be developed over the next 2 years These will support the following decisions, amongst others: – –Catchment Flood Management Planning – –Next generation of SMPs – –Scheme design – –Performance-based Asset Management

Outline of presentation DEFRA Conference 2002 Proposed application of the more detailed methods

Conclusions DEFRA Conference 2002 To find out more speak to me or visit….. or join….. the risk seminar in November 2002