Morphological characteristics of Japanese beaches viewed from the number of long shore bars: field investigations, laboratory experiments, and modeling

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The study aims to examine (1) the influence of the presence of long shore bars upon the subaerial beach morphology in Japan through field investigations, (2) the formative conditions for long shore bars on exposed beaches facing the Sea of Japan or the Pacific Ocean, and (3) the mechanism of the formation of long shore bars through a laboratory wave flume.The result of the analysis of the field data obtained from 190 beaches showed that the landward limit of the backshore height is not only controlled by the grain size of beach sediment but also by the number of long shore bars: lower backshore is found at a beach with smaller number of bars. Dimensional analysis applied to the data from 25, exposed beaches facing the Sea of Japan or the Pacific Ocean indicated that the number of bars is expressed in terms of the breaker-type index to the 1.5th power. Bars are originated by the action of spilling breakers. The prevalence of wind waves on the Sea of Japan coast is conductive to the presence of multiple bars commonly found on the beach. The presence of one-bar beaches mostly on the Pacific coast is closely associated with the swell-dominated environment. The laboratory experiment showed that bars are formed by the action not only of oblique vortexes that are the already reported, large-scale turbulence induced by spilling breakers, but also of down bursts, a newly found, spilling-breaker induced turbulence, with the latter being stronger than the former. Thus, the down burst is found to be a crucial element in modeling for bar formation.