A comparison of FEH22 and FEH13 rainfall datasets for reservoir safety flood estimation

The Flood Estimation Handbook (FEH) rainfall depth-duration-frequency (DDF) model is routinely used in combination with event-based rainfall-runoff models when estimating design floods with return periods up to 10 000 years. The FEH22 model is the latest version, superseding FEH13 and the original FEH model. This paper examines the difference between design rainfall and peak flow for the 150-year, 1000-year and 10 000-year events across 71 of Welsh Water’s reservoir catchments. Rainfall depths (for the catchment specific critical duration) were found to be up to 37% lower when using the FEH22 model compared with the FEH13 model. This increased to 44% when estimating the 10 000-year peak flow, using the default parameters in ReFH2.3. This illustrates how significant the choice of DDF model can be when carrying out flood conveyance assessments and designing schemes to comply with reservoir safety legislation/guidance. As the consequences of a dam failure could be catastrophic, a precautionary approach is often applied within reservoir safety. Care should however be taken to ensure the FEH13 DDF model is still reliable, especially where erroneous data was known to have been used for calibration. Using the FEH22 model and adding an allowance for uncertainty and/or climate change may be more appropriate.