Flood risk and coastal erosion

A2.22 Flooding and coastal erosion can lead to social costs (e.g. harm to people and damage to property, infrastructure and the environment). Typical damage per property, per flood event varies from around £7,000 to £10,000 for a flood of less than 0.1 metres in depth, to between £37,000 and £42,000 for a flood in excess of 1.2 metres in depth.

A2.23 To estimate the changing risk of flooding and coastal erosion over time, risk is generally measured in terms of changes to Annual Average Damages (AAD). This is the probability-weighted resource cost of flood damage to property and infrastructure, plus adverse health impacts and the resource costs of disruption. Estimating AADs for large scale flood and coastal erosion requires complex hydraulic modelling to estimate the probability and severity of flooding and/or coastal erosion, and its likely impact on people and assets in a defined spatial area.

A2.24 Generic national Weighted Annual Average Damage (WAAD) estimates are available for broader-scale, high-level scoping analysis. These are national average, per property, annual damage estimates and have been developed for residential properties across flood events with different probabilities and levels of flood warning service. The estimates for an average house in 2017 prices range from the following:

a property with no flood protection and no flood warning service - £5,054 per property, per annum

a property with existing protection against a "1 in 200 chance" (0.5% annual probability) and a flood warning service of more than 8 hours - £39 per property, per annum

A2.25 Local economic output and employment impacts of flooding can be material, though as with other local impacts, the potential for displacement at the national level should be recognised. Defra provides a toolkit for assessing such impacts.30 For further guidance see the Environment Agency's Flood and Coastal Erosion Risk Management Appraisal Guidance (Environment Agency 2010) and the online Flood and Coastal Erosion Risk Management Handbook and Data for Economic Appraisal 2017.

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30 "Flood and Coastal Erosion Risk Management (FCERM) and the Wider Economy" Frontier Economics for Defra, 2014. Available at http://randd.defra.gov.uk by searching on "FD2662".