THREE-DIMENSIONAL TOPO/BATHY MODELING AND SHORT-TERM EVOLUTION OF ISABEL’S INLET, NORTH CAROLINA

On September 18, 2003 Hurricane Isabel made landfall along the Outer Banks of North Carolina and breached the barrier island chain south of Cape Hatteras. The breach isolated the community of Hatteras Village by washing out a 500 m section of NC State Hwy 12. To rapidly assess breach geomorphology, document short-term morphological evolution, and to collect critical data in support of coastal modeling efforts, a series of high-density topographic and bathymetric surveys was performed. The unique survey strategy made use of real-time kinematic GPS and ultra shallow-water singlebeam and multibeam sonar technologies. Approximately 1.5 million data points were collected over two surveys. These high-resolution topographic and bathymetric data were seamlessly merged in the surfzone with customized spatial interpolation algorithms to create detailed 3D digital elevation models (DEM’s). Through spatial surface analysis, cutting planes and 3D visualization the DEM’s are providing a comprehensive insight into the coastal morphology and short-term evolution of what some have dubbed Isabel’s Inlet. Surface analyses from the first survey revealed three separate openings to the ocean with a main northern channel that was approximately 100 m wide with depths averaging 4.5 to 6 m. A well-defined ebb-shoal complex also formed within a week of the breach, and extended up to 365 m offshore from the former location of NC 12 within three weeks. The main channel migrated 12 m toward the southwest during the course of a two-week period, eroding approximately 12,000 m³ of land while maintaining the average channel depths. Data from the southern-most channel indicate an average scour of approximately 1.5 m around the central portion of the channel that contained dense Hwy 12 debris and exposed bridge pilings constructed to span a breach that occurred in 1933. Results from 3D DEM analysis indicate rapid morphology change with extensive datum-derived shoreline adjustment as the breach attempted to reach a state of equilibrium.