Assessing the spatial correlation of potential compound flooding in the United States

Abstract. When coastal and river floods occur concurrently or in close succession, they can cause a compound flood with significantly higher impacts. While our understanding of compound flooding has improved over the past decade, no studies to date have assessed the spatial correlation of compound flooding. To address this gap, we develop a framework that captures dependence between coastal total water level and river discharge across a set of locations along the U.S. coastline. Using 41 years of observed data from 41 station combinations, we stochastically model 10,000 years of spatially-joint events of extreme sea level and river discharge based on their dependence structure and cooccurrence rate. We define potential compound flooding as events in which both drivers exceed their respective 99^th percentile thresholds. Results based on our simulated large event set show that the U.S. West coast shows high spatial correlation of potential compound flooding. Among all three coasts, the West coast has the highest frequency of widespread potential compound flooding, with around 50 % of compound events arising at multiple locations simultaneously. We identify two clusters with mutually high joint occurrence rates of simultaneous compound events on this coast, namely 1) Charleston – Cresent City – North Spit, and 2) Santa Monica – Los Angeles – La Jolla. Widespread compound events are less frequent on the East coast where approximately 30 % of potential compound flooding may affect multiple locations. Moderate spatial dependence is observed in the central region and weaker spatial dependence for the remaining locations on this coast. In contrast, the Gulf coast shows the weakest spatial correlation, where over 82 % of compound events only affect single locations. Our findings highlight the importance of accounting for spatial dependence in compound flood assessments. Our large set of stochastic spatially-joint events can be used as boundary conditions for the hydrologic-hydraulic models to simulate the surface inundation and further assess risks of compound flooding in low-lying coastal and estuarine areas.