RHYTHMIC PATTERN OF LONGSHORE BARS RELATED TO SEDIMENT CHARACTERISTICS

Abstract

Despite a number of valuable contributions by many predecessors, namely F. F. Shepard (Ref. 1) and Q. H. Keulegan (Ref. 2), our knowledge regarding the effect of a longshore bar on the sedimentary process of a coast has long remained a plausible, qualitative understanding that a longshore bar constitutes a zone of active migration of bottom deposit due to agitation of breakers and currents. This was probably due mainly to the difficulty of performing an accurate hydrographic survey near the breaker zone. On the other hand, the geometrical characteristics along a single bar profile, which was sounded off either from a stable pier (Ref. 1) or a suspended cable (Ref. 3) resulted in a hopeless scatter. An aerial photograph, if taken under favorable conditions, may show an Interesting picture of submerged topographies In a distinct contrast made by the bright tone of a shallow bar crest or a shoal, against the dark background of a deep trough or a rock bottom. By comparing such photographic records with convenient soundings derived from some of the Japanese coasts, an Interesting topographical feature of a longshore bar has been disclosed. A longshore bar may attain a rhythmic pattern consisting of echelons of arcuate (or lunate) bar unit, which in entire appearance strongly resembles that of a honeycomb. It has also been discovered that a rhythmic bar pattern is correlated with other Important factors either dynamic or static, which participate in the general processes of a coast, namely the shoreline configuration, the shoreface slope and deposit, the topographies on the offshore bottom, transformation of Incident waves, the longshore currents and the littoral drifts. The authors have further attempted to develop a hypothetical concept on the origin of littoral rhythms as well as the behaviors of alongshore movement of sediment, and to consider their engineering lmplications on the basis of such findings. Although our success which has been achieved so far is yet incomplete due to lack of available data, it is believed that the approach and concept as proposed in the present paper may suggest an encouraging line of research toward formulating a unified macroscopic view on the mechanics of the littoral process.