HURRICANE IMPACTS ON A BARRIER ISLAND MARSH: MASONBORO ISLAND, NC AND THE IMPLICATIONS FOR SURVIVAL OF SEA LEVEL RISE

The intent of this study is to monitor the effect of major hurricane deposition in the marshes of Masonboro Island, North Carolina and to compare the event driven deposition patterns and rates with long-term trends. In 1996, Masonboro Island was impacted by 2 major hurricanes (Bertha and Fran) which dramatically altered the island landscape including substantial beach erosion and overwash into the back barrier marsh. Aerial photography from 1994 (pre-storm), 1996(directly after hurricane Fran), and 2006 (10 years post-storm) georectified and analyzed in ArcGIS 9.2 to track changes in marsh surface area directly after the hurricanes, and ten years later. The 10 year period between 1996 and 2006 is a good control to monitor marsh change as this is a period of relatively low storm incidence in the study area. Sediment tiles deployed along nine transects were used to determine non-storm sedimentation rates. These rates were then compared to data collected via push cores which contained layers deposited during the 1996 storms.

Preliminary results indicate that, although the rates vary along the island, non-storm tidal deposition ranges from (0.000001 – 0.007) g cm^-1 day^-1. Storm deposition resultant from Hurricanes Bertha and Fran in 1996 was several orders of magnitudes higher than the background tidal deposition rate and formed layers of several mms to cms thick in the marsh. The contribution of these storm derived sediments accounts for roughly 3 to 70,000 years of non-storm deposition, depending upon location on the island. Although the initial event may cause plant burial or conversion of marsh habitat to back barrier flat, the long term effect is that storm deposition increases marsh surface area by converting subtidal habitat to vegetated marsh and by allowing existing marsh to maintain its elevation with respect to sea level rise. The non-storm rates of deposition measured by this study appear to be insufficient to keep pace with local sea level rise; thus storm deposition is critical to the maintenance of tidal marshes on Masonboro Island.