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Regional Assessment of Compound Flood Hazard from the Combined Effects of Coastal, Stormwater and Groundwater Emergence Flooding

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By New York Water Science Center April 6, 2025

Compound flooding is flooding that results from a co-occurrence of multiple flood drivers. In this project we consider precipitation (pluvial flooding, overland flow of stormwater), coastal storm surge and tidal flooding (coastal flooding), and flooding that occurs as a shallow water table intersects the land surface or critical infrastructure (groundwater emergence flooding). These flooding hazards have forced communities to reconsider the ways they plan and manage coastal development, as well as when and where they choose to make investments. 

In 2021, the U.S. Geological Survey (USGS) began a study to assess the compound flood hazard associated with the flood drivers described above, under present-day and possible future hydrologic conditions. The project study area includes New York City and Long Island, and Long Island Sound Watersheds in Southeastern New York, Southern Connecticut, and Southwestern Rhode Island. 

The planned outcomes for the study are: 

  • Displays of the potential (hazard rank) across the study area for precipitation, coastal, and groundwater emergence flooding on a web map application. Additional maps displaying flood influencing variables for the respective drivers as well as the extent of coastal area inundation and groundwater emergence flooding under current mean sea level and future mean sea level scenarios will also be available at higher resolution.​ 
  • Analysis of how often flood drivers co-occurred in the past and the likelihood of future compound, or co-occurring, events. 
  • Development of a fully coupled model framework, at a set of locations, to represent the hydraulic connections between coastal, groundwater and subterranean infrastructure systems. The fully coupled modeling framework will be developed to model processes at the appropriate local scale (e.g., over a watershed or sewershed). The coupled model framework will be applied to study compound flood processes for flood mitigation scenarios under different hydrologic conditions. 

This work would not be possible without our partnership with the New York State Department of Environmental Conservation. Click the links below to learn more about our partners:

Long Island Sound Study Sustainable and Resilient Communities Work Group

New York Sea Grant

Compound flooding is flooding that results from a co-occurrence of multiple flood drivers. In this project we consider precipitation (pluvial flooding, overland flow of stormwater), coastal storm surge and tidal flooding (coastal flooding), and flooding that occurs as a shallow water table intersects the land surface or critical infrastructure (groundwater emergence flooding). These flooding hazards have forced communities to reconsider the ways they plan and manage coastal development, as well as when and where they choose to make investments. 

In 2021, the U.S. Geological Survey (USGS) began a study to assess the compound flood hazard associated with the flood drivers described above, under present-day and possible future hydrologic conditions. The project study area includes New York City and Long Island, and Long Island Sound Watersheds in Southeastern New York, Southern Connecticut, and Southwestern Rhode Island. 

The planned outcomes for the study are: 

  • Displays of the potential (hazard rank) across the study area for precipitation, coastal, and groundwater emergence flooding on a web map application. Additional maps displaying flood influencing variables for the respective drivers as well as the extent of coastal area inundation and groundwater emergence flooding under current mean sea level and future mean sea level scenarios will also be available at higher resolution.​ 
  • Analysis of how often flood drivers co-occurred in the past and the likelihood of future compound, or co-occurring, events. 
  • Development of a fully coupled model framework, at a set of locations, to represent the hydraulic connections between coastal, groundwater and subterranean infrastructure systems. The fully coupled modeling framework will be developed to model processes at the appropriate local scale (e.g., over a watershed or sewershed). The coupled model framework will be applied to study compound flood processes for flood mitigation scenarios under different hydrologic conditions. 

This work would not be possible without our partnership with the New York State Department of Environmental Conservation. Click the links below to learn more about our partners:

Long Island Sound Study Sustainable and Resilient Communities Work Group

New York Sea Grant

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