Use of peak over threshold data for flood frequency estimation: an application at the UK national scale

Thomas Kjeldsen, Ilaria Prosdocimi

Research output: Contribution to journalArticlepeer-review


This study investigates choices of statistical distributions to represent the threshold exceedance frequency and magnitude of peaks-over-threshold (POT) series from a national dataset of extreme hydrological events from 842 gauging stations in the UK. From the initial POT series, two new series were created, POT1 and POT3, representing POT series with an average of, respectively, one and three exceedances per year. Using a χ 2 goodness-of-fit test, the choice of distributions for both the annual exceedance counts and the magnitude of threshold exceedances were explored for both the POT1 and POT3 datasets. The results show that the negative binomial and geometrical distributions provide a better fit to the annual exceedance count than the Poisson distribution typically assumed. These results are particularly pronounced when considering the POT3 dataset and in the South-East of the UK where many river flow series are dominated by slow responding groundwater dominated catchments. Finally, estimates of design floods with a return period of 2 and 100 years were obtained for each POT series and compared to the equivalent estimates obtained from direct at-site analysis of the annual maximum series. The results show a good alignment between the magnitude of the design floods estimated by the two methods, but a generally lower standard deviation of estimates obtained from the POT data, as quantified using a bootstrap procedure. The results presented here show that the POT series could beneficially replace the current operational guidelines based on annual maximum series for design flood estimation in the UK.

Original languageEnglish
Article number130235
JournalJournal of Hydrology
Early online date25 Sept 2023
Publication statusPublished - 30 Nov 2023

Bibliographical note

Funding Information:
The Authors are grateful to the University Ca’ Foscari and the University of Bath for supporting the work through the Ca’ Foscari visiting researcher programme and the University of Bath international collaboration grant. The National River Flow Archive (NRFA) maintains the national Peak Flow Data which contains the Annual Maxima records and the POT records which form the basis for the data used in this study. The construction of the POT record was done using the winfapReader R package ( Prosdocimi and Shaw, 2022 ). The authors also wish to thank the associate editor and the reviewers for their insightful comments which led to an improved version of the manuscript.


  • Design flood estimation
  • Goodness of fit
  • Peak-over-threshold
  • Uncertainty

ASJC Scopus subject areas

  • Water Science and Technology


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