Pluvial and Compound Flooding in a Coupled Coastal System Modeling Framework: New York City during Post-Tropical Cyclone Ida (2021)

Coastal-urban areas, like all urban areas, are highly vulnerable to extreme pluvial flooding exacerbated by limitations in stormwater system capacity, but with the additional potential for flooding to be compounded by storm surge, tides, and waves. Understanding and simulating these coastal processes can improve prediction and flood risk management. Here, we improve the Regional Ocean Modeling System (ROMS) within the Coupled Ocean-Atmosphere-Wave-Sediment Transport framework (COAWST) to simulate post-tropical cyclone Ida (2021) pluvial flooding for the Jamaica Bay watershed of New York City (NYC). We modify the model to capture the volumetric effects of rainfall and parameterize soil infiltration and a stormwater conveyance system as a drainage rate. We generate a spatially continuous flood map of Ida with a quantified RMS error based on high water marks, useful for understanding Ida’s impacts and subsequent mitigation planning. Results show that over 37.2 km² of urban area in the watershed were deeply flooded (deeper than 0.3 m) during Ida. Sensitivity analyses are used to study the broader risk from events like Ida and compound flooding. Spatial shifting of the storm track within typical 12-hour forecast track uncertainty reveals a worst-case scenario that increases the deeply flooded area. Shifting Ida’s rainfall to coincide with high tide increases deeply flooded regions by a relatively small amount due to the lack of significant storm surge and the significant pluvial flood area. The application of COAWST to this storm event addresses a broader goal of developing the capability to model compound flooding by simultaneously representing coastal storm processes such as rain, tide, waves, erosion, and atmosphere-wave-ocean interactions. The sensitivity analysis results underscore the need for detailed flood risk assessments, showing that Ida, already NYC's worst rain event, could have been even more devastating with slight shifts in storm track.