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Coastal New Hampshire Communities to Get New Hydrodynamic Flood Risk Model

The state of New Hampshire is currently working off limited flood risk maps. 

The water level maps guiding decision makers on coastal storm and sea-level rise scenarios don’t account for waves, winds, or other physical features that dynamically interact with tidal floodwaters. 

The existing maps provide a rudimentary “bathtub” approach for depicting flood risk, said Kirsten Howard, resilience program coordinator for the coastal division of the Department of Environmental Services’ Watershed Management Bureau. Missing are “the complex hydrodynamics associated with our tidally influenced areas” and the different effects wind-driven waves may have in deeper waters, she said. 

The state is contracting with Woods Hole Group, an international environmental consulting organization based on Cape Cod, to create a new hydrodynamic coastal flood risk model so stakeholders can accurately assess the risk of tidal flooding, storm surge, and present and future sea-level rise for New Hampshire’s coast and estuaries.

The partnership between DES, Woods Hole Group, and a local advisory committee will develop a science-based approach to more exactly identify risk in 17 coastal communities. Woods Hole Group, selected from a competitive bid process, will receive more than $370,000 from the state in federal funds through next June, the project’s expiration date.  

The contract was originally approved last February but went in front of the Executive Council again recently, as DES requested a project timeline extension due to COVID-prompted delays and additional funding to include a sea-level rise model for 2090, in addition to 2030, 2050, and 2070.

“The 2090 scenario enables a more complete understanding of predicted sea levels throughout the 21st century at consistent timeframes,” DES wrote to the Executive Council. “The additional scenario of 2090 will also provide critical information to long-term planning efforts such as transportation and infrastructure.”

Hampton Harbor flooding
 In a 2021 report on flooding mitigation in Hampton Harbor, the water levels used include the combined mathematical representations of tides, waves, winds, storm surge, sea-level rise, and wave setup. (Screenshot)

Howard said hydrodynamic elements have been previously incorporated into flood studies for specific neighborhoods in Hampton, for example, such as Meadow Pond and Hampton Harbor

“This effort will allow us to apply that more accurate approach consistently across all tidally influenced waters in coastal New Hampshire,” she said.

The town of Seabrook worked with a consultant to develop a vulnerability assessment for its wastewater treatment facility, and though budgeting limited the work to the “bathtub” modeling, one of the report’s recommendations was to re-evaluate risk to facilities using a hydrodynamic coastal flood risk model “to help (the town) make potentially costly decisions with more certainty and precision,” Howard said. 

Sea level rise and extreme weather events are raising concern in the Granite State. In 2016, the Legislature passed a law requiring the Department of Environmental Services to convene representatives from several state agencies at least every five years – starting in 2019 – to oversee an updating of storm surge, sea-level rise, and precipitation estimates. 

In 2019, several partners published the two-part 2019-2020 New Hampshire Coastal Flood Risk Summary.

Howard said the new model designed by Woods Hole Group will be used in the next five-year update to the New Hampshire Coastal Flood Risk Summary.

According to DES, Woods Hole Group has already presented some progress made on sea-level rise projections, culvert and crossings analysis, riverine discharge analysis, and influence of historic storm events. 

When completed, the coastal flood risk model will be publicly available for communities, state agencies, residents, and other stakeholders to make decisions down to the parcel level. DES’ Coastal Program also has a grant from the National Fish and Wildlife Foundation to use the modeling results at coastal sites to inform future adaptive management decisions.

Source: New Hampshire Bulletin