Assessment of compound flood risk from the combined effects of sea level rise on storm surge, tidal and groundwater flooding, and stormwater
BACKGROUND
Long Island Sound has 600 miles of coastline and there are over 23 million people living within 50 miles of its shores. In response to water-quality issues and nitrogen pollution in the Sound, Congress created the Long Island Sound Study (LISS) in 1985. LISS is a partnership of federal, state, and local government agencies, private organizations and educational institutions working together to restore and protect the Sound. The USGS New England and New York Water Science Centers are partners in the LISS. These organizations also have historical and ongoing work with other partners in the LISS study region. Although historically the focus of LISS has been on water quality issues, a “Sustainable and Resilient Communities” theme was created by LISS as part of the 2015 revision of the Comprehensive Conservation and Management Plan (CCMP). The goal of the work in this theme is to support vibrant, informed and engaged communities that use, appreciate and help protect the Sound. This theme is a reflection of new challenges stemming from climate change, such as flooding exacerbated by sea level rise (SLR) and compound flooding, or the co-occurrence of several flood drivers, during coastal storms. These emerging 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 support of LISS objectives, the USGS is conducting an assessment of compound flood risk from the combined effects of sea level rise on storm surge, tidal and groundwater flooding, and stormwater. The study area for the LISS compound flood risk study, and a concurrent study supported by Hurricane Ida supplemental funding, includes Coastal Connecticut, New York City and Long Island Counties of Bronx, New York, Kings, Queens, Nassau, and Suffolk.
PROBLEM
The ability to accurately forecast the risk of coastal inundation from large coastal storms (hurricanes and nor’easters), as well as more chronic impacts due to tidal and groundwater flooding and heavy rain events, is necessary to quantify the vulnerability of coastal communities and infrastructure. Moreover, while coastal protection has been a subject of much focus, NOAA has developed resources on Adapting Stormwater Management for Coastal Floods describing the challenges specific to drainage infrastructure. Without an integrated approach to better understanding both coastal protection and associated drainage infrastructure, communities will increasingly be subject to risks of chronic and acute flooding from SLR and storms without the requisite information needed to make informed coastal and flood management decisions.
There currently exist models that can simulate groundwater and oceanic/estuarine processes separately, at various levels of complexity, in the coastal watershed and surface waters of Long Island Sound (LIS), respectively. To accurately predict coastal flood extents, and the impacts of SLR on stormwater infrastructure and management, these models may be coupled to better understand compound flood risk on event, seasonal, and long-term scales. Coupling these models would better represent the flow of water through the land/sea system and the dynamics connecting surface stormwater, coastal ocean, and groundwater especially as they pertain to flood risk and ecosystem response. The resulting coupled modeling framework may be used by public and private entities seeking to identify future capital-improvement and operational management needs that address increased flooding caused by SLR and groundwater table rise. This underlying framework can help agencies develop cost and benefit data associated with financing projects under future climate scenarios, including consideration for environmental justice. Additionally, the coupled modeling framework and associated flood risk products could be applied and (or) adapted for use in other areas.
Objectives and Scope
Compound flooding is flooding that results frommultiple drivers, in this project we will consider the combined effects of SLR on storm surge, (non-storm) tidal flooding, groundwater flooding, and stormwater. Understanding compound flood risk is critical to the development of resilient long-term plans of communities located in the Long Island Sound. The specific objectives of this study are to:
1. Develop a better understanding of the risks of compound flooding from the combined effects of SLR on storm surge, tidal flooding, groundwater, and stormwater over multiple timescales ranging from short-term storm events to decadal-scale SLR
2. Effectively communicate with and educate municipalities and their residents about the compound-flood risks associated with SLR (i.e., exacerbated flooding from storm surge, groundwater, and stormwater)
3. Establish tools to evaluate strategies for mitigating and adapting to effects of compound flooding
The compound flood risk modeling initiative will be a task led by USGS, given its overall depth and breadth of in-house topical expertise, with technical direction and guidance from topical experts, including from within and outside the LISS Sustainable and Resilient Communities (SRC) Working Group. Specifically, the Working Group and USGS together will identify members for a compound flood risk modeling Steering Committee, composed of relevant topical experts, modelers, and local/regional managers, who will oversee technical and programmatic aspects of the initiative. USGS will develop a roadmap to assure an appropriate complement of expertise and experience on the Steering Committee. Ultimately, USGS and partners will deliver a coupled modeling framework with user-friendly and management-oriented outputs. It is also anticipated that along the way several intermediate products will be created that can also be used by communities and managers; this set of products will be developed with regular input and guidance from the Steering Committee. This effort will be followed with a targeted outreach effort, in coordination with Sea Grant, after this three-year project has concluded. USGS will engage regularly (no less than quarterly) with the Steering Committee, and report bi-annually to the Working Group. Regular engagement with the Steering Committee and other targeted outreach efforts will occur throughout the model development process to identify scenarios of particular concern to stakeholders.
APPROACH
To resolve current and future flood risk we must understand how water flows through the land/sea system and the dynamics connecting surface stormwater, coastal ocean, and groundwater (as well as the changes to the landscape). The development of the compound flood modeling framework will be dependent on identifying the appropriate expertise and obtaining the best datasets for each of the processes. We (USGS) currently have existing groundwater models that geographically cover the coastal watershed of LIS that may be used to analyze depth to water table and changes to the location of the freshwater-saltwater interface under different SLR and storm scenarios. There have been many efforts that have already evaluated storm, wave, and SLR hazards from coastal water levels in and around LIS (e.g., NOAA, FEMA); we will leverage these efforts as well as run additional coastal modeling scenarios as needed. We anticipate exploring at least 3 SLR scenarios with a set of 2 to 4 identified storm scenarios for both a spatial analysis and individual, place-based models. Up to three locations will be identified for modeling in the coupled framework; the locations will be chosen to represent a continuum of development from urban to suburban to rural which can help us to understand how the compound flood process may be mediated or exacerbated by the type of development. While all of the processes outlined below (table 1) may not be explicitly included in the fully coupled modeling framework, existing resources will be leveraged to allow them to be considered in the compound flooding vulnerability hazard map created in the initial phase of investigation. The USGS has significant expertise on landscape processes and where there are existing tools and information; we will leverage those, but further development of landscape modeling is beyond the scope of this project.
Table 1.Processes considered in evaluating flood hazard over the land/water interface at the coast
Land Surface Urban Drainage Groundwater Coastal Water Rainfall
*Shoreline change *Storm and sanitary *Depth to water *Total coastal *Precipitation
(accretion/erosion) sewersheds table water level intensity, duration
*Land use change *Urban stormwater *Interactions with *Wave energy and frequency
*Vertical land motion BMP's subterranean *Overwash flow *Overland flow
(subsidence/uplift) *Leaky or damaged *Freshwater- rate *Infiltration
sewer systems saltwater *Infiltration *Storm types
*System capacity interface
- Source: USGS Sciencebase (id: 6172c442d34ea36449a881b0)
BACKGROUND
Long Island Sound has 600 miles of coastline and there are over 23 million people living within 50 miles of its shores. In response to water-quality issues and nitrogen pollution in the Sound, Congress created the Long Island Sound Study (LISS) in 1985. LISS is a partnership of federal, state, and local government agencies, private organizations and educational institutions working together to restore and protect the Sound. The USGS New England and New York Water Science Centers are partners in the LISS. These organizations also have historical and ongoing work with other partners in the LISS study region. Although historically the focus of LISS has been on water quality issues, a “Sustainable and Resilient Communities” theme was created by LISS as part of the 2015 revision of the Comprehensive Conservation and Management Plan (CCMP). The goal of the work in this theme is to support vibrant, informed and engaged communities that use, appreciate and help protect the Sound. This theme is a reflection of new challenges stemming from climate change, such as flooding exacerbated by sea level rise (SLR) and compound flooding, or the co-occurrence of several flood drivers, during coastal storms. These emerging 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 support of LISS objectives, the USGS is conducting an assessment of compound flood risk from the combined effects of sea level rise on storm surge, tidal and groundwater flooding, and stormwater. The study area for the LISS compound flood risk study, and a concurrent study supported by Hurricane Ida supplemental funding, includes Coastal Connecticut, New York City and Long Island Counties of Bronx, New York, Kings, Queens, Nassau, and Suffolk.
PROBLEM
The ability to accurately forecast the risk of coastal inundation from large coastal storms (hurricanes and nor’easters), as well as more chronic impacts due to tidal and groundwater flooding and heavy rain events, is necessary to quantify the vulnerability of coastal communities and infrastructure. Moreover, while coastal protection has been a subject of much focus, NOAA has developed resources on Adapting Stormwater Management for Coastal Floods describing the challenges specific to drainage infrastructure. Without an integrated approach to better understanding both coastal protection and associated drainage infrastructure, communities will increasingly be subject to risks of chronic and acute flooding from SLR and storms without the requisite information needed to make informed coastal and flood management decisions.
There currently exist models that can simulate groundwater and oceanic/estuarine processes separately, at various levels of complexity, in the coastal watershed and surface waters of Long Island Sound (LIS), respectively. To accurately predict coastal flood extents, and the impacts of SLR on stormwater infrastructure and management, these models may be coupled to better understand compound flood risk on event, seasonal, and long-term scales. Coupling these models would better represent the flow of water through the land/sea system and the dynamics connecting surface stormwater, coastal ocean, and groundwater especially as they pertain to flood risk and ecosystem response. The resulting coupled modeling framework may be used by public and private entities seeking to identify future capital-improvement and operational management needs that address increased flooding caused by SLR and groundwater table rise. This underlying framework can help agencies develop cost and benefit data associated with financing projects under future climate scenarios, including consideration for environmental justice. Additionally, the coupled modeling framework and associated flood risk products could be applied and (or) adapted for use in other areas.
Objectives and Scope
Compound flooding is flooding that results frommultiple drivers, in this project we will consider the combined effects of SLR on storm surge, (non-storm) tidal flooding, groundwater flooding, and stormwater. Understanding compound flood risk is critical to the development of resilient long-term plans of communities located in the Long Island Sound. The specific objectives of this study are to:
1. Develop a better understanding of the risks of compound flooding from the combined effects of SLR on storm surge, tidal flooding, groundwater, and stormwater over multiple timescales ranging from short-term storm events to decadal-scale SLR
2. Effectively communicate with and educate municipalities and their residents about the compound-flood risks associated with SLR (i.e., exacerbated flooding from storm surge, groundwater, and stormwater)
3. Establish tools to evaluate strategies for mitigating and adapting to effects of compound flooding
The compound flood risk modeling initiative will be a task led by USGS, given its overall depth and breadth of in-house topical expertise, with technical direction and guidance from topical experts, including from within and outside the LISS Sustainable and Resilient Communities (SRC) Working Group. Specifically, the Working Group and USGS together will identify members for a compound flood risk modeling Steering Committee, composed of relevant topical experts, modelers, and local/regional managers, who will oversee technical and programmatic aspects of the initiative. USGS will develop a roadmap to assure an appropriate complement of expertise and experience on the Steering Committee. Ultimately, USGS and partners will deliver a coupled modeling framework with user-friendly and management-oriented outputs. It is also anticipated that along the way several intermediate products will be created that can also be used by communities and managers; this set of products will be developed with regular input and guidance from the Steering Committee. This effort will be followed with a targeted outreach effort, in coordination with Sea Grant, after this three-year project has concluded. USGS will engage regularly (no less than quarterly) with the Steering Committee, and report bi-annually to the Working Group. Regular engagement with the Steering Committee and other targeted outreach efforts will occur throughout the model development process to identify scenarios of particular concern to stakeholders.
APPROACH
To resolve current and future flood risk we must understand how water flows through the land/sea system and the dynamics connecting surface stormwater, coastal ocean, and groundwater (as well as the changes to the landscape). The development of the compound flood modeling framework will be dependent on identifying the appropriate expertise and obtaining the best datasets for each of the processes. We (USGS) currently have existing groundwater models that geographically cover the coastal watershed of LIS that may be used to analyze depth to water table and changes to the location of the freshwater-saltwater interface under different SLR and storm scenarios. There have been many efforts that have already evaluated storm, wave, and SLR hazards from coastal water levels in and around LIS (e.g., NOAA, FEMA); we will leverage these efforts as well as run additional coastal modeling scenarios as needed. We anticipate exploring at least 3 SLR scenarios with a set of 2 to 4 identified storm scenarios for both a spatial analysis and individual, place-based models. Up to three locations will be identified for modeling in the coupled framework; the locations will be chosen to represent a continuum of development from urban to suburban to rural which can help us to understand how the compound flood process may be mediated or exacerbated by the type of development. While all of the processes outlined below (table 1) may not be explicitly included in the fully coupled modeling framework, existing resources will be leveraged to allow them to be considered in the compound flooding vulnerability hazard map created in the initial phase of investigation. The USGS has significant expertise on landscape processes and where there are existing tools and information; we will leverage those, but further development of landscape modeling is beyond the scope of this project.
Table 1.Processes considered in evaluating flood hazard over the land/water interface at the coast
Land Surface Urban Drainage Groundwater Coastal Water Rainfall
*Shoreline change *Storm and sanitary *Depth to water *Total coastal *Precipitation
(accretion/erosion) sewersheds table water level intensity, duration
*Land use change *Urban stormwater *Interactions with *Wave energy and frequency
*Vertical land motion BMP's subterranean *Overwash flow *Overland flow
(subsidence/uplift) *Leaky or damaged *Freshwater- rate *Infiltration
sewer systems saltwater *Infiltration *Storm types
*System capacity interface
- Source: USGS Sciencebase (id: 6172c442d34ea36449a881b0)