Physical Process Variables
The relative sea-level change variable is derived from the increase or decrease in annual mean water elevation over time as measured at tide gage stations along the coast. The rate of sea-level rise in Beaufort, NC (about 75 km SW of Ocracoke Island) is 3.71 +/- 0.64 mm/yr based on 27 years of data (Zervas, 2001)(Table 2). This variable inherently includes both global sea-level rise as well as regional sea-level rise due to isostatic and tectonic adjustments of the land surface. Relative sea-level change data are a historical record, and thus only portray the recent sea-level trend (<150 years). The rate of relative sea-level rise for Cape Hatteras is very high (5), approximately 2 times the global average, based on water elevation data at Beaufort, NC (Table 1).
Mean significant wave height is used here as a proxy for wave energy which drives the coastal sediment budget. Wave energy is directly related to the square of wave height;
E = 1/8 ρgH2
where E is energy density, H is wave height, ρ is water density and
g is acceleration due to gravity. Thus, the ability to mobilize and transport coastal sediments is a function of wave height squared. In this report, we use hindcast nearshore mean significant wave height data for the period 1976-95 obtained from the U.S. Army Corps of Engineers Wave Information Study (WIS) (see references in Hubertz and others, 1996). The model wave heights were compared to historical measured wave height data obtained from the NOAA National Data Buoy Center to ensure that model values were representative of the study area (Table 2). For Cape Hatteras, mean significant wave heights are between 1.2 and 1.3 m, which is categorized as high (4) and very high vulnerability (5), respectively (Table 1).
Tidal range is linked to both permanent and episodic inundation hazards. Tide range data were obtained from NOAA/NOS for ocean tide gauges at Cape Hatteras fishing pier and the Duck, NC Field Research Facility pier (Table 2). All of Cape Hatteras is classified as very high vulnerability (< 1 m) with respect to tidal range (Table 1).