The influence of inlet modifications, geologic framework, and storms on the recent evolution of Masonboro Island, NC

Masonboro Island is a 13 km long undeveloped barrier island located within a sand deficient southwestern portion of Onslow Bay. The island is situated along major storm tracks and is chronically impacted by tropical and extra-tropical storm events (HOSIER and CLEARY, 1977). The response of the island to storms is a function of the pre-storm condition of the island, the lack of sand in the offshore environment, the influence of adjacent inlets, and the underlying geology (CLEARY et al., 1999). Masonboro Island provides an exemplary setting to study changes in barrier island morphology and shoreline change in response to inlet modifications, geologic framework, and storm impact because the island is undeveloped, remains relatively unnourished, and is flanked by two modified inlets. A GIS based study utilizing aerial photography was used to ascertain shoreline change occurring during 1938 – 2002. The island was divided into a northern, central, and southern segment based on distinct erosional characteristics. Masonboro Island experienced island wide erosion during the 64-year study period. The northern segment, which includes the fillet adjacent to Masonboro Inlet, eroded an average distance of 43 m. The central segment had net shoreline erosion of 110 m, while an area of shoreline within the central reach that was perched on a paleo-interfluve eroded only 106 m. The greatest amount of shoreline change was 164 m of erosion, which occurred along the southern segment of the island adjacent to Carolina Beach Inlet. Short-term analysis (1.3 yr.) of a 2.5 km section of the island/estuary related the influence of a small Pleistocene interfluve to morphological changes within the area. Vibracore data from the estuary behind this area indicate that the Holocene fill is relatively shallow and ranged in thickness from greater than 4 m over the infilled valleys to less than 1.5 m over the interfluves. The network of interfluves extends from the mainland, beneath the estuary, and is overridden by the island. Survey data indicate that the shoreline perched on the interfluve was restricted to ~17 m of erosion compared to an average of ~30 m to either side. The profile of this entire section was lowered in elevation, and approximately 53,000 m3 of material were removed from the area. The lowered profile made this section of the island increasingly vulnerable to overwash. The coarse grained nature of existing washover fans prevented the regrowth of new dunes. Petrologic analyses indicated that the source of the coarse grained material was a sandy limestone found along the shoreface of the southern two-thirds of the island. Further analyses indicated that the limestone was similar in nature to the coquina facies of the Neuse Formation intermittently exposed in the region.