SLAMM: Modeling Future Tidal Wetland Conditions to Support Present Day Decisions

 

About this project

In 2014, the Great Bay National Estuarine Research Reserve (GBNERR) and NH Fish and Game led an effort to run the Sea Level Affecting Marshes Model (SLAMM) for all of New Hampshire to understand how estuarine habitats could shift (or “migrate”) in response to sea level rise. SLAMM simulates the dominant processes involved in wetland conversions and shoreline modifications under different scenarios of long-term sea level rise. 

The SLAMM model was rerun in 2022 with improved input data, including high resolution land surface elevation data through LiDAR  and high resolution tidal wetland data for habitat mapping.  The 2022 model rerun also included updated data on salt marsh ditches, impervious surfaces, and sea level rise scenarios.

The state was divided into eight regions, and model inputs were tailored to those areas. Local data from GBNERR and partners was entered into the model, including habitat mapping and vertical marsh accretion data. The diagram below shows factors that influence marsh migration and are included in SLAMM; for more information on methods, refer to this brief NH 2022 SLAMM Summary.

A webinar recording from March 2023 provides a detailed explanation of GBNERR’s 2022 SLAMM model methodology and results.

Factors influencing marsh migration that are included in SLAMM

Highlights of model results

The following image provides an example of the 2022 SLAMM results for southeastern Great Bay, with yellow showing persistent salt marsh (predicted to remain marsh) and red showing the potential salt marsh in the year 2100 with 1.5 meters of sea level rise.  

Example of the 2022 SLAMM model output from southeastern Great Bay.

Statewide: The SLAMM model can be used to understand how the area of salt marsh will change across the entire state.  Here is a summary of the model results for the NH coast under the highest sea level rise scenario (1.5 meters rise by 2100):  

  • Salt marsh persisting (currently salt marsh and will still be salt marsh in 2100): 4,086 acres
  • Potential salt marsh (i.e., migration space): 3,917 acres
  • Impervious area that would otherwise transition to salt marsh by 2100: 642 acres
  • Salt marsh lost by 2100: 1,571 acres

Regional comparisons: The model also predicts the following regional trends:

  • The Hampton-Seabrook area will see the highest percentage of persistent salt marsh by 2100.  
  • The Cocheco River will see the highest percentage of salt marsh lost by 2100.
  • The Squamscott River has the highest percentage of potential salt marsh by 2100.

Town-by-town:  The SLAMM model can also be used to understand the salt marsh migration potential within each municipality, as demonstrated by the following two charts.  The top chart shows the current salt marsh area (in acres) within each community, while the bottom chart shows the potential migration space for salt marsh within each community in the year 2100. 

Current Salt Marsh Acreage by Town (acres)

Potential Salt Marsh (Migration Space) by Town in the Year 2100 (acres)

How to access the data

The 2022 SLAMM data can be accessed in two ways:

  • GIS datasets of the model results, including various sea level rise scenarios, can be downloaded from NH GRANIT, the New Hampshire Geodata Portal. 
  • Users can view the data via the NH Coastal Viewer, a web-based mapping tool that brings spatial data within New Hampshire’s 42 coastal watershed communities together in one place. (SLAMM results are found under: Oceans and Coasts / Predicted Marsh Migration)

How this project is helping

SLAMM results enabled Great Bay NERR to develop the NH Salt Marsh Plan, a spatial planning resource that provides a comprehensive look at the resiliency of New Hampshire’s tidal marshes through a geospatial analysis of current conditions, vulnerability, and adaptation potential.

SLAMM results are guiding planning for salt marsh migration. The analysis has been used to identify undeveloped areas where marshes are likely to expand inland as sea rises. These future marsh areas were used to identify priority conservation opportunities in the recently updated NH Coastal Watershed Conservation Plan and to scope marsh migration projects for the Pathways for Salt Marsh Resilience Project.

In addition, the SLAMM model results are helping GBNERR and partners understand how sea level rise will impact tidal wetlands in the future and has a variety of practical applications for resource management. For example, the SLAMM outputs were directly incorporated into the vulnerability assessments done for Rockingham and Strafford counties and are part of the site suitability criteria for living shorelines.

Partners

  • Great Bay National Estuarine Research Reserve
  • NH Fish and Game
  • NH Dept of Environmental Services Coastal Program

Contact

Katie Callahan
NH Fish and Game
catherine.b.callahan@doit.nh.gov

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