Three-Dimensional Beach Changes

Abstract

Beach forms, such as crescentic bars and cuspate shoreline, manifest a three-dimensional structure of rhythmic topography characterized by periodic longshore undulations of both the surf zone bed and the subaerial surface. The three-dimensional structure and the tendency for migration of rhythmic topography cause distinctive modes of beach change. Since crescentic outer bars tend to migrate parallel to shore, profile changes involving an outer-bar system generally reflect two unrelated processes, one arising from net transformation of individual profiles and the other from longshore displacement of a whole rhythmic system in response to currents. Crescentic inner bars exhibit a tendency for predominant shoreward migration during the time of swells. Under this condition, a subaerial beach will initially assume a ridge-and-runnel-type structure and eventually a uniform two-dimensional profile when full accretion is reached. Beach changes involving crescentic inner bars can produce a beach cycle in response to a succession of random storms. A model predicts the most likely cycle length, which agrees with the field observation. Crescentic bars forming between headlands appear to be generated by a standing edge wave continuously excited by an incident seiche of beat frequency, whereas rhythmic topographies on an unbounded coast appear to be caused by sand-wave trains which result from instability of the surf zone bed perturbed by longshore currents and waves.