Bimodal Composition and Cyclic Characteristics of Beach Sediment in Continuously Changing Profiles

Abstract

ABSTRACT Sediment deposited on a continuously changing subaerial beach on the Outer Banks, North Carolina, exhibits variations in size distribution ranging from unimodal coarse through bimodal intermediate to unimodal fine. A shift from one end of this range to tile other coincides with the progress of a beach cycle in which a beach, after being eroded by a storm, restores itself to a fully accretive state over a period of approximately two months. In the early phase of beach recovery, coarse material with unimodal distribution appears at the beach front in the form of a swash bar. The swash bar, because of its porous texture, tends to arrest fine materials subsequently washing up on the beach, adding a second mode to the size distribution curve. Accretion is aided by the loss of backwash into runnels behind the swash bar, as well as into the swash bar itself, which leads to landward migration of the swash bar and the consequent spreading of fine materials on the beach. Thus, the second mode, representing fine fractions, gains in prominence relative to the first mode, representing coarse fractions, and finally becomes the sole mode in the size distribution curve by the time when the swash bar reaches the upper limit of run-up. As this beach is subsequently eroded by a storm, the swash bar tends to disintegrate and, correspondingly, the zone of coarse materials diminishes seaward, disappearing eventually into the sea bed when the beach assumes a maximum state of erosion. Under this condition, beach deposits continue to be unimodal fine sediment until a swash bar emerges at the horeline to initiate the process of beach recovery.