Spatio-temporal clustering of extreme floods in Great Britain

Quantifying and attributing the tendency of flood events to demonstrate clustering in time and space is crucial for flood risk assessments. We analyse the temporal (TC) and spatial coherence (SC) of floods in 554 catchments over Great Britain. TC was assessed using the Dispersion Index and the Conway-Maxwell-Poisson regression, with both methods applied from 1 to 5 years aggregation windows. SC was investigated using the flood susceptibility index which quantifies how susceptible a geographical area is to widespread flooding. Results for TC showed that: i) most of the UK peak floods are over dispersed and ii) a positive relationship exists between winter mean North Atlantic Oscillation anomalies and annual number of peak floods across western Britain. Results for SC showed that susceptibility to widespread floods is higher for the south-east parts of Britain and for the Clyde-Forth valleys and it increases with catchment permeability and with the influence of lakes/reservoirs. The findings of our analysis are relevant to the enhancement of existing methods of flood hazard estimation and, in turn, will lead to more realistic flood risk quantification.