Physical Process Variables
|Figure 8. Rate of relative sea-level rise for Virgin Islands National Park. Click on figure for larger image.|
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 the US Virgin Islands is 0.5 +/- 0.74 mm/yr based on 25 years of data at Charlotte Amalie, St. Thomas (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 Virgin Islands falls within very low vulnerability based on water elevation data on St. Thomas (Figure 8).
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 1990-1999 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 the Virgin Islands, wave heights ranged from 1.0 m (moderate vulnerability) to 1.9 m (very high vulnerability) (Figure 9).
|Figure 9. Mean significant wave heights for Virgin Islands National Park based on WIS data. Click on figure for larger image.|
Tidal range is linked to both permanent and episodic inundation hazards. Tide range data were obtained from a NOAA/NOS published benchmark on St. John Island in Coral Harbor. Mean tidal range here is 0.262 m; therefore Virgin Islands National Park is classified as very high vulnerability (> 1meter) with respect to tidal range (Figure 10).
|Figure 10. Mean Tidal Range for Virgin Islands National Park. Click on figure for larger image.|