WELDING SWASH BARS AND PROGRADATION DURING A TRANSGRESSION: CAPE LOOKOUT NATIONAL SEASHORE, NORTH CAROLINA

National Seashores can provide unique natural laboratories for studying coastal processes that may be unencumbered by anthropogenic effects. A spit at Cape Lookout National Seashore, North Carolina, has been rapidly prograding since the early part of the 1900s, while adjacent littoral cells have experienced recurrent shoreline erosion. The chief mechanism for spit growth, welding of swash bars onto the beachface, has significantly increased the surface area and volume of this coastal landform and provided a "textbook" example of one process by which spits can prograde. Because of its particular location in the complicated Cape Lookout region, the spit plays an integral role in the sedimentary processes of the area. The mechanisms responsible for this progradation have implications for regional sediment budgets and beach nourishment needs that extend beyond the seashore's boundaries.

In order to provide estimates of the sediment budget, rates of longshore transport and extent of progradation, data were collected at several scales of temporal and spatial resolution. Photogrammetric analyses of aerial photographs from 1940-1998 were used to quantify surficial area change and draw inferences concerning coastal morphodynamics. Bathymetric surveys were conducted to document the volume of sediment incorporated into the spit. Monthly beach profiles were taken using RTK-GPS technology from October 2000 to June 2001 and, during one fortnightly spring-neap cycle in June 2001 daily profiles were taken. Sediment samples were collected twice at 6-month intervals, and trenches were dug on two separate occasions to document swash bar stratigraphy.

During constructional phases, nearshore bars move toward the beachface and attach at the updrift end. Ridge and runnel topography begins to develop on the beachface. One continuous welding bar can be hundreds of meters in length and incorporate tens of thousands of cubic meters of sand onto the beachface. As the ridge is overwashed by sediment during high tides the runnel is filled in, completing the welding process. This entire sequence can occur within a few weeks. The above process adds approximately 300,000 m3 of sediment to the spit each year.