Factors controlling initial deposition and long-term fate of spilled oil on gravel beaches

Abstract

Several major oil spills of record (including the 1974 Metula spill, the 1978 Amoco Cadiz spill, and the 1989 Exxon Valdez spill) occurred along shorelines with abundant gravel beaches. Observations of these spills help in understanding facts controlling the initial deposition and long-term fate of spilled oil in such locations. Gravel beaches are most common on rocky, glaciated coasts (Holocene and/or Pleistocene). For example, leading-edge coastlines in subpolar to polar regions, of which the Alaska coast is a prime example, may have gravel beach deposits along more than 50 percent of their length. Spilled oil coming onshore in such gravel areas is likely to remain for a tong time (up to decades), because of the potential for deep penetration and burial of the oil in the coarse sediments. The detailed, three-dimensional configuration of gravel beach deposits is affected by the internal characteristics of the waves shaping the beach (reflective or dissipative). Reflective waves typically produce steep, coarse, cuspate berms, which allow for deep penetration and burial in the beach face/berm areas. Dissipative waves typically build intertidal swash bars that my move landward and bury oil deposits, such as asphalt pavements. It would be useful for future contingency mapping projects to include this distinction on maps of gravel beaches. The formation of armoring (structural strengthening) of a gravel beach surface impedes erosion and sediment transport. Therefore, such beaches are likely to retain buried oil longer than those without armoring. Gravel beaches, because they are commonly located along plate margins, are usually subject to tectonic readjustment. The Exxon Valdez spill occurred along a shoreline out of equilibrium as a result of the 1964 earthquake. Uplift and downwarp is on the order of 1 to 3 meters throughout most of the area of the spill. Almost every coastal location affected by the spill is undergoing readjustment, causing many site-specific interpretive problems related to burial, penetration, and hydraulic flushing. For example in many areas, a thin gravel veneer of armor overlies uplifted rock platforms and/or fine-grained bay bottom, or downwarped soil horizons, which served as either zones of oil accumulation or pathways of hydraulic flushing, depending on local conditions.