Coastal Erosion and Engineering Structures: The Oregon Experience

Abstract

Seawalls and riprap revetments are used on the Oregon coast to protect properties threatened during erosion of seacliffs and sand spits. The erosion processes involve the combined effects of swash from storm waves and embayments cut through the berm by rip currents. Most shore protection structures on the Oregon coast have not been designed by engineers, and therefore commonly do not follow sound design practices. They often are grossly overbuilt and do not take into consideration the processes of erosion and beach morphology. Particularly difficult is the protection of high cliffs where the structures can defend only the toe of the slope. Questions have been raised as to whether the structures might contribute to erosion of the beach and adjacent unprotected properties. Although such adverse impacts have been demonstrated in our laboratory wave-basin experiments, the role of rip currents in Oregon-coast erosion produces significant longshore variability in property losses and tends to mask impacts by structures. We are presently monitoring beach-topography changes adjacent to seven structures, including both seawalls and riprap, with the objective of determining whether the structures do cause erosion.