Coastwide - Forecasting Compound Floods in Complex Coastal Regions

Managers are despaerately in need of models to better understand highlighted the urgency and exposed gaps in our understanding of coastal compound flooding processes. To address these shortfalls, researchers from several NOAA offices—including the National Weather Service, Oceanic and Atmospheric Research, and the National Ocean Service (NOS)—along with model developers and other partners from the wider scientific community are collaborating to develop analysis and prediction capabilities for performing studies of coupled inland-coastal flooding. In particular, they are linking the National Water Model (NWM)—NOAA’s inland hydrology model—to NOAA’s coastal ocean models.

For coastal communities, it’s bad enough when a river overflows its banks and floods nearby roads, homes, and buildings. Now imagine a scenario in which a rainstorm not only overfills the river channel but also causes a landslide that dams the river and causes further flooding. Add to that a high tide and storm surge, possibly made worse by sea level rise and human-caused land subsidence. Welcome to the world of compound flooding events, where multiple factors combine and compound to inundate coastal regions.

Recent examples of costly inland and coastal compound floods in the United States occurred during Hurricanes Katrina in 2005 and Irma in 2017. In Katrina, heavy rainfall and a record-setting storm surge inundated 80% of New Orleans for several weeks. Irma became the costliest hurricane in Florida’s history when high discharge from the St. Johns River combined with a storm surge. As the climate warms, many models predict more frequent occurrences of increasingly “wet” storms in the coming decades [Knutson et al., 2010], creating conditions favorable for compound flooding. Yet the processes that couple inland and coastal factors to drive compound flooding have not been carefully studied, largely because of their inherent complexity. This complexity is due in large part to the transitional nature of the physical interfaces between inland and coastal areas.

The increasing number of compound flooding events in recent years, including during the record-setting 2020 Atlantic hurricane season, has highlighted the urgency and exposed gaps in our understanding of compound flooding processes. These events also accentuate a major pitfall of the current hurricane intensity scale, which is entirely based on wind speed. This singular focus overlooks potential rainfall and inland flooding impacts, leaving hurricane initial forecasts short of vital information about hazards and risks to people and infrastructure.

To address these shortfalls, researchers from several NOAA offices—including the National Weather Service, Oceanic and Atmospheric Research, and the National Ocean Service (NOS)—along with model developers and other partners from the wider scientific community are collaborating to develop analysis and prediction capabilities for performing studies of coupled inland-coastal flooding. In particular, they are linking the National Water Model (NWM)—NOAA’s inland hydrology model—to NOAA’s coastal ocean models.

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