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Assessing flood risk and estimating its economic impact

Objectives: The Environment Agency and JBA Consulting undertook this project to see how information about uncertainty around climate change can be used in the assessment of flood risk. The aim was to illustrate the use of the UKCP09 projections to estimate changes in the economic impact of flooding in a catchment. It examined two questions:

  • What are the projected impacts of climate change on flood damages?
  • How do UKCP09 projections compare with current guidance on Climate Change?

UKCP09 products used: User Interface

How were UKCP09 products used?

  1. The 10,000 sampled data of monthly changes in precipitation and temperature were downloaded from the UKCP09 user interface. UKCP09 grid number 1121 was chosen, as it contains the centre point of the catchment, for the 2080s under a Medium emissions scenario.
  1. The data was entered into the PDM rainfall runoff model to generate projected changes in peak flow for specified values of the annual exceedance probability (AEP) as follows: 1/2, 1/20, 1/30 and 1/50. 
  1. For each scenario, the projected flow, Q, was estimated for a given AEP using the outputs from the PDM.
  1. Broad scale hydraulic modelling and Multi Coloured Manual techniques provided a relationship between flood flow and economic damages.
  1. The analysis produces an estimate of the catchment flood damages for different AEP's. There is one curve for each of the 10,000 sampled future climates.
  1. The curves for all 10,000 projections reflect the range of the sampled UKCP09 climate data used to drive the PDM analysis of river flows.
  1. The range of flood damages at each AEP can be expressed as a probability density function and used to estimate the most likely outcome, conditional on the UKCP09 data, and percentiles to summarise the uncertainty.

Difficulties & limitations

  • Using the UKCP09 sampled data product as a sub-set may be misleading.
  • It is not possible to use distribution functions directly.
  • Assumptions were made in all three components. For example, the future state of flood defences and depth-damage relationships.  

Lessons learned

This process required significant additional models and data such as a rainfall-runoff model, a flood model and data on economic damages. Interpreting the results required a good understanding of both the climate change models and the limitations of the flood damage models. 

How will the results be communicated to the target audience?

Histograms have been used to show the range of projections of peak river flow just downstream of the catchment, from the UKCP09 data and PDM flow analysis. Histograms have also been used to show the economic damages associated with the flood zones produced by a broad scale hydraulic model.

Find out more

  • Contact details: Rob Lamb, JBA and Glenn Watts, Environment Agency.