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Investigating climate impact on peak river flows

Objectives:  The Environment Agency and CEH undertook a project to investigate the impact of climate change on peak river flows across the UK and provide nationwide assessment of change under UKCP09. The study assesses the suitability of the Communities and Local Government PPS25 guidance (now superseded by the National Planning Policy Framework) given the advances in climate change science since its publication. In particular it investigates the use of a 20% addition to peak flows applied to any period between 2025 and 2115 for any location across Britain.

UKCP09 products used: 11-member RCM.

How were UKCP09 products used?

  1. The data was used in two ways, one based on the use of the Grid-to-Grid (G2G) hydrological model, the other using the catchment-based modelling results from the FD2020 project. For both applications, the 25 km RCM data was used for the whole of the UK and for each of the 11 ensemble members. The baseline (1961-1990) and 2080's, under a Medium emissions scenario were selected.
  1. To drive the G2G model, the rainfall time series data was downscaled from the 25 km resolution of the RCM to the 1 km resolution of the G2G. The downscaling procedure uses high-resolution (1 km) information on standard average annual rainfall (SAAR).
  1. The time series data produced 11 different estimates of river flow for locations across the UK for each time period. From these flow time-series, statistics of changes in flow indices at all points of the UK river network could be calculated.
  1. For the FD2020 catchment analysis, the rainfall time-series data was used to derive monthly changes in rainfall for each of the 11 ensemble members.
  1. The 11-member RCM data was overlaid onto a map of the modelled response pattern (plot summarising catchment sensitivity) for each of the 154 (FD2020) catchments at the 20-year return period. Maps were produced for the 10, 50 and 90th percentiles to show whether the 20% was still a precautionary percentile to apply.

Difficulties & limitations

One of the main difficulties was that the need for a national analysis required spatially consistent scenarios. The 11-member RCM was the only UKCP09 product capable of providing such data. The use of this tool has a number of limitations, not least that it uses a limited number of simulations and is not probabilistic.

Other difficulties included accounting for localised, intense storm events and large-scale 'blocking high pressure' situations.

Lessons learned

Using the 11-member RCM data presents its own problems. The dataset is large and difficult to manipulate. The RCMs include climate variability as well as a climate change signal, which means that it can be difficult to interpret time slices from them. 

Similar work could be done using the spatially coherent scenarios, but these were not available when this work was completed.

How will the results be communicated to the target audience?

This work was used to inform planning allowances for flood defence as part of Defra's project FD2020, available from Defra's R&D website.

Find out more

  • Contact details: Nick Reynard, Centre for Ecology and Hydrology and Glenn Watts, Environment Agency.