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

Sediment deposited on a continuously ctnanging snhaerial beach on the Outer Banks, North Carolina, exhibits variations in size distribution rangiug from unimodal coarse through bimodal intermediate o unimodal fine. A shift from one end of this range to the other coincides with the progress of a beach cycle in which a beach, after being eroded by a storm, restores itself to a fulIy accretive state over a period'of approximately two months. In the early phase of beach recover3', coarse material with nnimodal distribution appears at the beach front in the form of a swash bar. Tt:e swash bar, because of its porous texture, tends to arrest fine materials ubsequently washing up on the beach, adding a second mode to the size distribution curve. Accretion is at(ted 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 subsequentlyercded 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 heaeh assumes a maximnm state of erosion. Under this cotldition, beach deposits continu