Compound Urban Flooding: The Emerging Hazard for Large Metropolitan Areas Located Along Tidal Rivers

Compound floods from the co-occurrence of multiple flood drivers like riverine flow, storm surges, sea-level rise, and extreme rainfall have impacted several cities across the United States (US) in recent years. Metropolitan areas located in the coastal regions of the US have become increasingly vulnerable to such flooding conditions due to their exposure to multiple flood hazards, rapid urbanization, proximity to major rivers or lakes, and the risk of sea level rise. This study has identified the major flood drivers impacting Washington, DC, and the surrounding communities in Maryland and Virginia and quantified the impacts of those flood drivers on compound flooding. The region is often exposed to various flood hazards that overlap spatially and temporally, leading to the potential of compound flooding conditions. Historical data analysis from 1931 to 2019 provided strong evidence that compound flooding in the region can be caused by a combination of high coastal water levels, riverine flow, local wind, and urban runoff. Compound flood events had the highest magnitude and duration during the period of 1931-2019. Among the four Major flood events during this time, three were a result of compound flooding, and one event resulted from high riverine flow. Storm-surge driven coastal floods were more common in Washington, DC compared to river floods when the flood stages were either at Moderate or Minor stage. Local wind and urban runoff also play a significant role in impacting the flood levels in the area. The locations around the study area can be locally divided into three zones based on the impact of flow and surges: highly riverine flow dominated upstream zone, transition zone with impact from both flow and surges, and coastal water level dominated lower zone. Flood depths during surge dominated events were significantly higher than flow dominated events. Small urban streams in the area were significantly impacted by flow, surge, and rainfall. Low gradient streams were highly impacted by compound flooding compared to steep streams. The comprehensive analysis of the historical flooding in the region has provided scientific insights into the physical characteristics and spatial variability of compound flooding in urban areas located along the tidal rivers and estuaries.