On the use of NRFA peaks-over-threshold data for design flood modelling

This study presents an initial investigation into the use of the NRFA Peaks-over-Threshold (POT) database for design flood estimation in the United Kingdom. Two new POT datasets are created from a quality controlled version of the original NRFA dataset; the POT3 and POT1 datasets, characterised by on average three and one exceedance above the threshold. Using these new datasets, the distributional assumptions underpinning the statistical POT models are assessed, showing that peak flow data are predominantly over-dispersed (tendency for temporal clustering), while the magnitude of the exceedances can mostly be considered to follow a general pareto distribution. Links between the POT model and the equivalent annual maximum distributions are considered. Finally, preliminary result are reported on the potential link between the POT data and large-scale climatic drivers, including regional rainfall and the NAO index. The implications of non-stationarity in the POT data are discussed in terms of design flood estimation.