A field experiment on the formation of beach cusps

这项研究的目的是在神奈川县马纳苏鲁半岛尖端附近的卵石-卵石海滩Banbaura进行现场实验，以考察砂砾海滩上海滩尖点的形成，特别关注与尖点形成相关的冲刷/反冲运动的时间变化。实验以人工夷为平坦的前滨地带作为初始边界条件。本研究新研制的悬挂式摄像系统由一个小型、轻量化的CCD摄像模块和一个带LCD显示器的录像机组成，可连续记录斜纹和尖端的发展过程。相机模块从碳棒的顶部悬浮在空中，碳棒将以高角度站立。视频图像分析表明，在尖角场的激流带上存在着Bagnold流型，即尖锥喇叭偏转的激流集中在海湾内，向下流动，增强了反冲洗流。这种环流型的存在表明，同步边波是造成实验中出现的尖点的原因，因此利用边波理论进行了尖点间距的计算。计算值与实测值的比较表明，前者(11.8m)比后者(2.3-2.9m)大得多。由此得出结论：边波理论不能适用于目前的结果。

The study aims to conduct a field experiment on a pebble-cobble beach called Banbaura, located near th tip of the Manasuru Peninsula, Kanagawa Prefecture, in order to examine the initiation of beach cusps on a shingle beach paying a special attention to the temporal changes in swash/backwash movement associated with the cusp formation. The experiment started with a manually flattened foreshore zone as the initial boundary condition. The swash patterns and the cusp development were continuously secorded by a suspended video-camera system that is newly developed in this study.This recording system is composed of a small, light-weight CCD camera module, and a video tape recorder with an LCD monirtor. The camera module is suspended in the air from the top of a carbon rod which will be stood at a high angle. The recording system is portable and very easy to set up.The analyzes of the video images showed that there exixted the Bagnold type flow pattern on the swash zone of the cusp field, i.e., swash water flow deflected by the cusp horn concentrates in the embayment to flow down inhancing the backwash flow. The presence of this circulation pattern suggested that synchronous edge waves were responsible for the cusps appreared in the experiment ; so that the cusp spacing by use of the edge wave theory was conducted. Comparison between the calculation and the measurement indicated that the former (11.8m) was much greater than the latter (2.3-2.9m). This concludes that the edge wave theory cannot be applied to the present result.