A field experiment on the formation of beach cusps

本研究的目的是进行一项实地实验的卵石海滩，位于神奈川县的马纳苏鲁半岛的尖端附近，以检查的开始海滩尖点的卵石海滩支付特别注意的时间变化，与尖点形成的冲/冲运动。实验开始时，手动压平前滨区作为初始边界条件。本研究利用一个新研制的悬挂式摄像系统连续地记录了波浪型和尖点型的发展过程，该系统由一个小型、轻型的CCD摄像模块和一个带LCD监视器的录像机组成。摄像头模块从碳棒的顶部悬挂在空中，碳棒将以高角度站立。对视频图像的分析表明，尖点流场的冲流区存在Bagnold型流态，即：冲水流被尖角折流器折流后集中在湾内向下流动，增强了冲水流。这种环流模式的存在表明，实验中出现的尖点是由同步边缘波引起的，因此用边缘波理论计算了尖点间距。计算值与实测值的比较表明，计算值（11.8m）远大于实测值（2.3-2.9m）。这表明边缘波理论不能应用于本结果。

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.