QUANTIFYING RAPID CHANGES IN COASTAL TOPOGRAPHY USING MODERN MAPPING TECHNIQUES AND GRASS GIS

An innovative methodology integrating LIDAR, RTK-GPS, and sonar data was developed for spatio-temporal analysis of evolving coastal/nearshore topography. The study area at Bald Head Island, NC displays complex interactions between anthropogenic activities and natural processes. Quantification of short-term spatial change in this dynamic environment is crucial for sustainable coastal management. A GIS-based, nested grid model of topography was created at resolutions: 20m - entire area, 5m - beach, and 1m - active areas.

Analysis at the island's south beach before and after nourishment exemplifies the methodology. Annual LIDAR data gridded at 5m document pre-nourishment shoreline evolution. Between the 1997 and 2000 the beach lost 400000 m3 of sand, of which about 15% was transported to the subaerial beach dune, increasing its elevation up to 1m; the remainder was lost offshore. LIDAR data from 1997 define a wide, convex beach in the west and concave with a steep scarp in the east. Intense erosion gradually changed the shape of the entire beach to concave with a scarp. In 2000, the dune at the east end eroded completely, changing the beach shape to uniform low slope.

The beach was nourished in 2001, substantially changing its morphology. Based on RTK-GPS Dec-2001 data, beach shape was reversed compared to 1997, with a concave/scarp in the west and a wide, low-slope in the east section. The 2002 data indicate that the intense erosion observed before nourishment continued in the west section, where bathymetry changes rapidly from shallow depth to deep navigation channel. The east section was widening, following the natural trend started in 2000 and enhanced by nourishment. The analysis indicates that the beach will likely need repeated nourishment under current conditions. A better understanding of nearshore processes and new nourishment approaches may be necessary to return the beach to self-sustaining dynamic equilibrium.