SPATIAL VARIABILITY AND TEMPORAL STABILITY IN THE GEOMORPHOLOGY OF A BARRIER ISLAND: SHACKLEFORD BANKS, NORTH CAROLINA

The components of the beach-dune-nearshore system of an undeveloped barrier island are being studied in order to characterize the features of each, and to understand ways in which they change and interact.

The field site, Shackleford Banks, North Carolina, forms the downdrift limb of the wave-dominated Cape Lookout cuspate foreland. The development of a new RTK-GPS survey design and data collection strategy has made it possible to create detailed multi-dimensional maps of beach and foredune environments and monitor changes in topography on a variety of scales. Morphologies along the 15km long island are highly variable spatially, yet preliminary results indicate that they are fairly consistent temporally at any given point.

The western half of the island is wide and dominated by dunes. Relict peat (C-14 age of 674 years BP) outcrops intermittently for 3.5 km along the shoreface in a transitional section, and water runs up higher on the beach where it is present than in adjacent areas. The eastern half is narrow and dominated by extensive overwash, with the exception of the far eastern end, where large shoals have been welding onto the beach and result in the island’s only apparent area of accretion.

Regionally, Shackleford is a transition zone between the morphologies of bordering islands. The eastern half may be sediment starved, as indicated by previous seismic data, the paucity of dunes, a drop in beach elevation from west to east along the length of the island, and the lack of sediment cover on the relict peat. Cape Lookout and a large spit to its NW, which are both sediment sinks, may reduce the availability of sediment to the island and simultaneously dampen and/or diffract waves, making it possible for areas in their ‘shadow’, on the far eastern end of Shackleford, to grow by trapping sand carried along shore.

Recovery from large storm events can be slow. Dune scarping is most common where the foredunes are continuous, and scarped dunes are more prone to new scarping. For about 2 km on the western part of the island, foredunes are ‘perched’ above the backbeach as a result of a beach scarp formed several years ago. Although ridges and runnels are consistently present in the nearshore here, the beach has not accreted to regain its former elevation. Although coupling of the beach and dunes seems to occur, many of the linkages are subtle and remain to be identified.