VULNERABILTY TO OCEANSIDE OVERWASH ALONG THE NORTH CAROLINA COAST – A PILOT STUDY

A pilot study to assess vulnerability to oceanside overwash and potential impacts was conducted the 375 km portion of the North Carolina barrier island system that is populated or contains key infrastructure elements. The study used multiple newly created and existing data layers including: a high-resolution shoreline layer digitized from 1998 aerial photography and the state 1998 long-term erosion rate shoreline; state LiDAR elevation data obtained as 20 ft DEM's with 3.5 cm vertical resolution; several sets of rectified aerial imagery; generalized geomorphic mapping interpreted from these layers, and a data layer containing key infrastructure elements.

The fore-island dune was judged to be the single feature most relevant to providing overwash protection that could be analyzed on a coast-wide basis within the scope of the study. LiDAR elevation data enabled quantitative analysis of the fore-island dune system. Parameters measured and classified include: average elevation, minimum elevation, continuity, and volume. These were combined into an overall vulnerability based on a weighted average of the vulnerability ranking for each individual parameter.

The analyzed barrier island system was subdivided into 1-km-long segments that were characterized in terms of vulnerability to overwash from the ocean side. To represent the results of the analysis and the vulnerability rankings determined for the various parameters analyzed, shore parallel “bands” were created and divided into segments representing the 1-km barrier island segments. Each band represents a given parameter and its segments are filled with colors indicating the vulnerability ranking (five levels) as classified with GIS software for the corresponding island segment.

The GIS software classified 57 coastal segments into the highest overall vulnerability class. The majority of the these segments occur in the Long Bay and southern Onslow Bay coastal areas. Examination of barrier island key infrastructure indicated that numerous elements were present within the highest risk island segment or an adjoining segment. Seven police stations, five fire stations, three schools, three highway bridges, and approximately 19 miles of highway segments are in this potentially vulnerable zone.