Spatio-Temporal Variability of Compound Flood Characteristics in the Washington, DC Metropolitan Region

Abstract:

Coastal cities are often exposed to multiple flood drivers, including high riverine flow, runoff from intense rainfall, local wind, and coastal storm surges. The complex interaction between different flood drivers can result in compound flooding conditions with extremely high flood depths and extents over a longer period of time. This study has identified the major flood drivers and the hotspots of compound flooding events, quantified the impacts of the flood drivers, and investigated the spatio-temporal variability of compound flood characteristics around the Washington DC metropolitan area using a 2D hydrodynamic model. Locations along the tidal Potomac River can be divided into three zones based on the origin of the flood driver: highly riverine flow dominated zone, transition zone with impact from both riverine flow and surges, and coastal surge dominated downstream zone. However, storm-surge driven coastal floods are more common in the region with longer duration compared to riverine or compound flood. Compound floods mainly generate Moderate and Major flooding stages. Among the four Major flood events during 1980-2019, three were a result of compound flooding driven by a combination of riverine flow and coastal water levels and only one was a result of a riverine flow. Moreover, the local Southward winds with speed >5.5 m/s can significantly increase the water levels at Washington, DC, potentially playing a significant role impacting flood levels in the area. Furthermore, the riverine flow and coastal surges can inundate the streams connected to the Potomac River. However, the influence of flow and surge propagates only up to a certain length of the streams after which, rainfall becomes the only driver for flooding in the surrounding areas. Impact of flow and surge is higher in the low gradient streams compared to the steeper streams. Results also suggest that this metropolitan area will face increased flooding due to sea-level rise in the long term future, especially during surge dominated events. The outcomes provide scientific insight into the spatially and temporally variable interaction of the compound flood drivers and the consequences of such flooding in the metropolitan areas along the estuaries which will lead to an accurate estimation of flood water levels.