The Intermittently Exposed, Coarse-Grained Gravel Beaches of Prince William Sound, Alaska: Comparison with Open-Ocean Gravel Beaches

Abstract

Long-term studies (1989-2007) of the gravel beaches in Prince William Sound, Alaska, conducted as part of the Exxon Valdez oil spill monitoring programs, provide the basis for understanding the geomorphology and sediment dynamics of these intermittently exposed, coarse-grained gravel beaches. There are key differences between these beaches and open-ocean gravel beaches, including bermlike ridges that occur at different levels on the beach as a result of the diurnal inequality of the tides. The 1964 earthquake, with up to 3.5 m of uplift, provided information on sediment transport patterns and rates. In less than 25 years, a stable cobble/boulder armor formed on the upper and lower platforms, and the beaches reached a steady-state or graded condition. Eighteen years after the spill, oiled sediments were still present at depths greater than about 25 cm on beaches with a stable armor in the middle and lower intertidal zones. Cleanup efforts to remove the deep oil penetration into the porous gravel of the upper intertidal zone provided an indicator of short-term rates of sediment erosion and deposition. Where the high-tide berms were pushed onto the middle intertidal zone, they quickly reformed, usually after a few storms. However, the storm berm that was relocated for 2000 m along one of the most exposed shorelines in Prince William Sound took 6 years to recover. Data on the occurrence of gravel beaches in North America show that they are most common (comprise 48% of the shoreline) on paraglacial, leading-edge coasts and least common on nonparaglacial, trailing-edge coasts (similar to 2% of the shoreline).