Sediments, Beach Morphology and Sea Cliff Erosion within an Oregon Coast Littoral Cell

Abstract

The 24-km long beach of the LincoIn City littoral cell on the central Oregon coast is bound by pronounced headlands that prevent bypassing of beach sands, in effect making this a large pocket beach. There is a seasonal reversal in the sand transport along this embayed shoreline, but the net littoral drift is zero. The wave energy is extreme with storms generation 7-meter significant wave breaker heights. This wave energy is essentially uniform along the Lincoln City littoral cell; in spite of t this uniformity there is a marked longshore variation in grain sizes of the beach sediment and an accompanying change in beach morphology from dissipative to reflective beaches. These variations in the beach sediment grain sizes parallel those found in the sea cliffs which consist of Pleistocene beach and dune deposits apparently formed in environments similar to those found today. Gravel and coarse sand layers are exposed in the sea cliff along the south-central portion of the littoral cell, and these constitute the major source of coarser sizes in the modern beach. By dissecting the multimodal grain size distributions of beach sediments into individual modes having log-normal distributions, we have been able to trace the coarse modes back to their sea-cliff sources. The proportions of these modes within the beach rapidly decrease away from their sources indicating a relatively small degree of longshore dispersal in spite of the high energy wave environment. The persistence of the dispersal pattern suggests a period of time that has been insufficient to produce the longshore homogenization of the sediments. It is suggested that the sea cliff erosion which introduces the coarse modes into the beach sediments did not begin or was insignificant until about 300 years ago, at which time a major subduction earthquake caused subsidence of this portion of the coast and rapid cliff recession. The longshore variation in beach sand grain sizes and accompanying beach morphology are playing an important role in the continuing sea cliff erosion with cliffs backing the coarse-grained reflective beaches eroding more rapidly than the cliffs buffered by the fine-grained dissipative beaches.