ASSESSING THE POTENTIAL FOR INLAND MIGRATION OF A NORTHEASTERN SALT MARSH

It is often assumed that, as sea level rises, salt marshes will maintain their area by migrating onto adjacent uplands. Anthropogenic barriers, such as sea walls, roads, and other impervious surfaces can prevent this translation by blocking water movement and inhibiting the growth of marsh vegetation. To date, most marsh migration studies have focused on inhibiting anthropogenic structures and little research has been done on the role of upland topography. If the slope of the surrounding area is very flat, similar to that of the marsh surface, the marsh would be able to expand horizontally and maintain its areal extent. It is much more common, however, for marshes to be backed by steeper slopes, so that rising sea level will produce a much smaller areal gains. Given this barrier to marsh expansion, inland migration may not be a viable solution to marsh habitat loss with rising sea level.

This project seeks to quantify the potential changes in areal extent of the Great Marsh in Massachusetts that will occur under future sea-level rise, taking into account inland topography and anthropogenic barriers. Using LiDAR and SRTM data to determine elevation and calculate slopes, aerial photographs to map anthropogenic barriers, local accretion rates (observed and estimated based on future inundation levels), and NOAA tide gauge data to determine local rates of relative sea-level rise, we assess the percent change in marsh area with varying degrees of sea-level rise. Developed areas (i.e. areas covered by impervious surfaces) and area covered by salt marsh are identified and digitized (using ArcGIS and Matlab), following a standard method and resolution. Gradients are determined by analysis of elevation data. For the purposes of this study, it is assumed that marsh vegetation will not expand to cover impervious surfaces. Changes in marsh area at 10 and 50 years into the future are assessed based on rates of mean sea-level rise predicted under the four Representative Concentration Pathway (RCP) scenarios used in the IPCC’s most recent report. We predict that area loss will increase rate of sea-level rise increases and marshes encroach upon increasingly steep slopes and populated areas.