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 change for Yakutat, AK is -5.75 +/- 0.27 mm/yr based on 60 years of data (Zervas, 2001). This variable inherently includes both eustatic sea-level change as well as regional sea-level change 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 change for GBNPP falls within the very high change-potential category based on water elevation data at Yakutat, AK (fig. 10). A higher resolution study of uplift rates not including sea level change along southeast Alaska was conducted using the tide records mentioned here as well as GPS measurements, raised shorelines, and earth models (Larsen and others, 2003; Larson and others, 2005).

Mean significant wave height is used here as a proxy for wave energy which drives coastal sediment transport. Wave energy is directly related to 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. Historical measured significant wave height values were available for eastern Gulf of Alaska through the National Data Buoy Center. Station 46083 was located 170 km southeast of Yakutat, AK and average mean significant wave heights were 3.55 m (very high change-potential). Wave heights for Icy Straits were determined from NOAA nautical charts (low change-potential). While wave heights within Glacier Bay were assumed to be within the very low change-potential category because of the limited fetch length. Mean significant wave heights within GBNPP ranged from very low to very high change-potential (fig. 11).

Tidal range is linked to both permanent and episodic inundation hazards. Tide range data were obtained from NOAA/NOS published benchmarks from Graves Harbor, Dixon Harbor, Lituya Bay, Dry Bay, Lemesurier Island Light, Bartlett Cove, Willoughby Island, Muir Inlet, and Composite Island (NOAA, 2005). Mean tidal ranges fell between 2.31 and 4.25 m, therefore classified as moderate change-potential (2.0 - 4.0 meters) and low change-potential (4.01 - 6.0 meters) with respect to tidal range (fig. 12).