Quantifying the dependence between extreme rainfall and storm surge in the coastal zone

The interaction between extreme rainfall and storm surge can be critical in determining flood risk in the coastal zone. This paper investigates the presence of dependence between these two processes along the Australian coastline using the most extensive observational records of rainfall and storm surge events currently available. A bivariate logistic threshold-excess model was employed to conduct the dependence study. Statistically significant dependence was observed for the majority of locations that were analysed, although regional variations as well as seasonal variations of the dependence strength are also apparent. The dependence remains significant even at distances of several hundred kilometres between the tide gauge and the rainfall gauge, indicating that dependence arises largely due to synoptic scale meteorological forcings. The strength of dependence varies as a function of storm burst duration, with an increase in dependence when going from one hour through to 24 h storm bursts. The dependence strength also varies with the lag between the extreme rainfall and the storm surge event, with the greatest level of dependence when extreme events occurred in the same time step for storm burst durations exceeding six hours, or for lags up to ±10 h for storm bursts durations below six hours. These findings have important implications for flood risk assessments in the coastal zone, showing that the two processes must be considered jointly if flood risk is to be quantified correctly.