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

本研究的目的是通过实地考察(1)日本长滨坝的存在对陆上海滩形态的影响；(2)面向日本海或太平洋的暴露海滩上长滨坝的形成条件；(3)通过实验室波浪水槽研究长滨坝的形成机制。对190个滩地野外资料的分析结果表明，后滩高度的向陆极限不仅受滩地沉积物粒度的控制，还受长沙洲数量的控制，沙洲数量越少，后滩越低。对面向日本海或太平洋的25个暴露海滩的数据进行了量次分析，结果表明，沙洲的数量以破碎型指数的1.5次幂表示。酒吧是由溢出的断路器的作用。日本海海岸的风浪的盛行导致了海滩上常见的多个沙洲的存在。单洲海滩的存在主要是在太平洋沿岸，这与膨胀为主的环境密切相关。室内实验表明，横条的形成不仅受到斜涡的作用，斜涡是已经报道过的由溢流器引起的大规模湍流，而且还受到下爆的作用，这是一种新发现的由溢流器引起的湍流，后者比前者强。因此，发现下冲是砂坝形成建模的一个关键因素。

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.