MAPPING RELATIVE COASTAL VULNERABILITY TO FUTURE SEA-LEVEL RISE IN THE NATIONAL SEASHORES

A coastal vulnerability index (CVI) was used to map the relative vulnerability of the coast to future sea-level rise within the Cape Cod National Seashore (CACO) and Gulf Islands National Seashore (GUIS). The CVI ranks the following in terms of their physical contribution to sea-level rise-related coastal change: geomorphology, regional coastal slope, rate of relative sea-level rise, shoreline change rates, mean tidal range and mean wave height. The rankings for each variable were combined and an index value calculated for 1-minute grid cells covering the parks. The CVI highlights those regions where the physical effects of sea-level rise might be the greatest. This approach combines the coastal system's susceptibility to change with its natural ability to adapt to changing environmental conditions, yielding a quantitative, although relative, measure of the park's natural vulnerability to the effects of sea-level rise.

CACO consists of high glacial cliffs, beaches, sand spits, and salt marsh wetlands. In CACO, areas most vulnerable to sea-level rise are those with the lowest regional coastal slopes, geomorphologic types that are susceptible to inundation, and the highest rates of shoreline change. These variables are the controlling factors at this scale because other variables (tide range and rate of sea level rise) do not change significantly within the park.

GUIS is a low-lying barrier island system. Within GUIS, washover-dominated barrier islands with high erosion rates are at the greatest risk. The few dune-ridge barrier island segments with the lowest erosion rates have the least risk. As with CACO, it is the shoreline change, geomorphology, and coastal slope data that control the variability in vulnerability to long-term sea-level rise.

In some cases, areas within the parks that are at the greatest risk are also the most popular and historically significant. In GUIS, Fort Massachusetts is at great risk due to high rates of erosion, a low-sloping coastal plain and washover island morphology. In contrast, most of CACO's infrastructure lies on high elevation uplands away from the shore, with most high use areas accessible by foot only. It is in planning for the long-term evaluation of such sites that the CVI becomes useful as an objective tool for scientists and park managers.