Toward Forecasting the Coastal Compound Hazard Caused by River Flooding and Storm Surge during Extreme Weather Events

Hurricanes Joaquin (2015), Matthew (2016), Irma (2017), Florence (2018) and Michael (2018) hit the U.S. East Coast and their associated heavy rainfalls and strong winds caused extensive river flooding and storm surge. Hurricane-caused coastal river flooding is an example of compound hazards in which the combination of storm surge and rainfall-induced freshwater river flooding amplify the hazardous impacts from individual events. The improvement of coastal compound flooding has been hindered by the fact that the traditional modeling approaches use a hydrologic model for rainfall-induced freshwater flooding and an oceanic model for storm surge and seawater inundation, and as such, these two processes have been treated separately without considering their interactions. The hydrologic model’s downstream boundary conditions are usually set from only astronomical tidal ocean water elevations; and the oceanic model’s upstream conditions are typically set to the climatological river flow data. However, both of these criteria are not physically realistic. The capability of correctly forecasting coastal river flooding is critical as 40% of U.S. population live in coastal areas. A hydrologic-oceanic coupled model has been developed and an algorithm has been implemented that allows for a more realistic representation of physical processes during a compound flooding event given the interactions of storm surge inundation and freshwater river flooding. In this study, this coupled model is employed to investigate coastal river compound flooding processes and to evaluate the model system’s capability of forecasting such compound hazards during the several recent hurricanes in coastal river basins in South Carolina.