Mapping Dependence Between Extreme Rainfall and Storm Surge

Dependence between extreme storm surge and rainfall can have significant implications for flood risk in coastal and estuarine regions. To supplement limited observational records, we use reanalysis surge data from a hydrodynamic model as the basis for dependence mapping, providing information at a resolution of approximately 30 km along the Australian coastline. We evaluated this approach by comparing the dependence estimates from modeled surge to that calculated using historical surge records from 79 tide gauges around Australia. The results show reasonable agreement between the two sets of dependence values, with the exception of lower seasonal variation in the modeled dependence values compared to the observed data, especially at locations where there are multiple processes driving extreme storm surge. This is due to the combined impact of local bathymetry as well as the resolution of the hydrodynamic model and its meteorological inputs. Meteorological drivers were also investigated for different combinations of extreme rainfall and surgenamely rain-only, surge-only, and coincident extremesfinding that different synoptic patterns are responsible for each combination. The ability to supplement observational records with high-resolution modeled surge data enables a much more precise quantification of dependence along the coastline, strengthening the physical basis for assessments of flood risk in coastal regions. Plain Language Summary Flood events in coastal and estuarine regions can be caused by a combination of extreme ocean levels due to factors such as high tides and storm surges, and extreme rainfall occurring on the upstream catchments. This study looked at a measure of the likelihood that storm surges and extreme rainfall events occur at the same time (the dependence'), and the synoptic weather features such as tropical cyclones that cause these events to co-occur. The analysis examined historical rainfall events from 5,300 rain gauges located throughout Australia, and combined this with information on historical surge events from 79 separate tide gauges distributed along the Australian coastline. The instrumental data was supplemented by outputs from a mathematical model of storm surge, which enabled an assessment of dependence at high resolution and at locations where observational data was sparse. The results indicated that the dependence can change significantly along the Australian coastline, and depended on local weather patterns. The improved understanding of the interaction between storm surge and extreme rainfall events strengthens the physical basis for assessment of flood risk in Australia's coastal and estuarine regions.