Defining Flood Zone Transitions in Low-Gradient Coastal Regions

Worldwide, coastal, and deltaic communities are susceptible to flooding from the individual and combined effects of rainfall excess and astronomic tide and storm surge inundation. Such flood events are a present (and future) cause of concern as observed from recent storms such as the 2016 Louisiana flood and Hurricanes Harvey, Irma, and Maria. To assess flood risk across coastal landscapes, it is advantageous to first delineate flood transition zones, which we define as areas susceptible to hydrologic and coastal flooding and their collective interaction. We utilize numerical simulations combining rainfall excess and storm surge for the 2016 Louisiana flood to describe a flood transition zone for southeastern Louisiana. We show that the interaction of rainfall excess with coastal surge is nonlinear and less than the superposition of their individual components. Our analysis provides a foundation to define flooding zones across coastal landscapes throughout the world to support flood risk assessments. Plain Language Summary Flooding in coastal communities can be caused by a variety of events, such as tides, hurricane storm surge, and intense rainfall. Large amounts of rain over inland regions can cause rivers, creeks, and canals to overflow their banks and flood neighboring areas. Waters around and within these rivers, creeks, and canals flow into the ocean. However, these inland water levels can interact with a high tide or surge from the ocean. This can complicate matters when rivers are draining rainwaters to the ocean, while at the same time, the ocean is pushing water inland. When these types of events occur, water levels and the resulting floodwaters in the surrounding communities are influenced by both rainfall and coastal processes. We define this region as a flood transition zone. Our analysis provides a foundation to define flooding zones across coastal landscapes to support flood risk assessments. In addition, we show that the combination of rainfall excess with coastal surge is less than the addition of their individual contributions. Identifying the transition zone for coastal communities worldwide can result in better planning and preparedness before these types of natural disasters occur, which will protect property and save lives.