Physics-based compound flood risk assessment in a warming climate

Abstract:

Compound flooding, characterized by the co-occurrence of multiple flood mechanisms, is a major threat to coastlines across the globe. Previous efforts to quantify compound flood hazard have typically adopted statistical approaches that may be unable to fully capture spatio-temporal dynamics between rainfall-runoff and storm surge, which ultimately impact total water levels. In contrast, we pose a physics driven approach that utilizes a large set of realistic TC events (~1000) and a simplified physical rainfall model, and simulates each event within a 2D hydrodynamic model framework. We apply our approach to investigate TC flooding in the Cape Fear River, NC under current and future climate conditions. We find TC approach angle, forward speed, and intensity are relevant for compound flood potential, but rainfall rate and time lag between centroid of rainfall and peak storm tide are the strongest predictors of compounding magnitude. Under current climate conditions, neglecting rainfall underestimates 100-yr flood depths across 28% of the floodplain, and taking the max of each hazard modeled separately still underestimates 16% of the floodplain. We then utilize a large number of synthetic TCs generated under future climate conditions from six CMIP5 models to assess evolving compound flood hazard and its impacts to the coastal floodplain. Results suggest an increase in TC rainfall hazard, resulting in heightened compound flood risk for a range of storm return periods. Our approach effectively links compound event occurrence to actual flood impacts by simulating each event within a hydrodynamic model, and illustrates how future climate warming could exacerbate total flood heights/extents due to increased compound event occurrence.