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-gauge stations along the coast. The rate of sea-level rise in Lewes, DE, is 3.16 +/- 0.16 mm/yr and 3.59 +/- 0.27 mm/yr in Kiptopeke, VA, based on 81 and 49 years of data, respectively (Zervas, 2001). 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). Relative sea-level rise for Assateague Island falls within high vulnerability based on extrapolation from water elevation data at Lewes, DE, and Kiptopeke, VA.

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 1976-1995 obtained from the USACE 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. For Assateague Island, mean significant wave heights are between 1.2 and 1.3 m, which represents high and very high vulnerability, respectively.

Tidal range is linked to both permanent and episodic inundation hazards. Tide range data were obtained from National Ocean Service (NOS/NOAA) for an ocean tide gauge at the Ocean City Inlet fishing pier. All of Assateague Island is classified as high vulnerability (1-2 m) with respect to tidal range.