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A Partnership of UNH Cooperative Extension and NH Fish and Game

Modeling the Fate of NH Salt Marshes

As the sea level changes, coastal dynamics and ecosystems change with it. Natural resource managers and community conservation commissions will need to consider how different habitats will be impacted by sea level rise and extreme weather events as they consider what to protect and how.  Salt marshes, one of the most important habitat types in coastal New Hampshire, pose unique challenges in the future. Salt marshes are critical habitat for commercially and recreational important fisheries, they filter runoff to lessen the impact of non-point pollution on our waterbodies, sequester more carbon than mature forests, and they act as sponges to absorb water in flood conditions; protecting adjacent property and adjacent habitats.

Salt marshes are naturally resilient to changes in temperature, salinity, and water level but are limited by elevation and sediment supply.  As sea levels rise, the marsh at the water edge will be drowned. But under the right conditions, salt marshes have the capacity to migrate inland with a rising sea level. The problem in New Hampshire, as in many coastal areas, is that the same low and level land that marshes could migrate to has been developed with roads, homes, and businesses.

The Sea Level Affecting Marshes Model (SLAMM) is a tool that models the processes that affect how coastal wetland systems naturally respond to sea level change over time and where natural and man-made barriers will inhibit these changes. This tool can help users visualize and understand how coastal wetland systems will likely shift and adapt over time under a range of projected sea level rise conditions. In the 2014 New Hampshire Fish and Game Department (NHFG) and Great Bay National Estuarine Research Reserve (GBNERR) ran SLAMM for all of coastal New Hampshire using accurate, updated, local data, including recently updated National Wetlands Inventory data, and on the ground measurements of marsh elevation at four sites using Sediment Elevation Tables paired with  detailed marsh elevation data. These model outputs are accessible through the UNH GRANIT’s Coastal Viewer where users can track the projected shift of coastal wetlands from current conditions to the time points 2025, 2050, and 2100, under different sea level rise scenarios. As a part of a project, partners worked with four communities, Hampton, Hampton Falls, Seabrook, and Portsmouth to explore how SLAMM can aid local decision-making.

The map on the top represents current habitat conditions for the Hampton-Seabrook Estuary and the map on the bottom represents habitats under a high sea level scenario in 2100. The aqua and yellow colors indicate low and high marsh, the purple is mudflat and the teal color is open water.

The maps produced by the SLAMM can be used to support municipal and state decisions about citing future infrastructure, habitat restoration, land conservation, and the development of land use regulations (e.g. buffers and setbacks).  Models do not provide definitive answers, but they can be powerful tools to encourage a dialogue about what the future might look like, and what actions we can take now to be proactive in achieving our conservation goals. 

Mapping products from Sea Level Affecting Marsh Migration (SLAMM) modeling can be viewed in the GRANIT: NH Coastal Viewer, an online mapping tool that brings coastal resources spatial data, hazards-related spatial data, and other spatial data sets within NH's 42 coastal watershed communities together in one place.

By Cory Riley, NH Fish & Game, Great Bay National Estuarine Research ReserveTaking Action for Wildlife Spring 2015 Newsletter