Physical Process Variables
The relative sea-level change variable is derived from the change in annual mean water elevation over time as measured at tide gauge stations along the coast. The rate of sea-level rise for Key West in FL is 2.27 +/- 0.09 mm/yr based on 87 years of data (Maul and Martin, 1993; Zervas, 2001). This variable inherently includes both eustatic 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 portray only the recent sea-level trend (< 150 years). Relative sea-level rise for Dry Tortugas falls within low vulnerability based on water elevation data at Key West in Florida.
Mean significant wave height is used here as a proxy for wave energy which drives coastal sediment transport. 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) (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 Dry Tortugas, mean significant wave heights are between 0.5 and 0.8 m, which represents very low and low vulnerability, respectively.
Tidal range is linked to both permanent and episodic inundation hazards. Tide range data were obtained from NOAA/NOS for a tide gauge at Key West, FL. Dry Tortugas is classified as very high vulnerability (less than 1 meter) with respect to tidal range.