Development of Image-based field observation system for behavior of infragravity waves along the surf zone

风暴潮引起的海岸灾害往往集中在局部地区，这一特点可能部分是由于亚重力波的存在，而不是风致风暴潮，亚重力波具有更长的波浪周期。由于现场观测的困难，特别是在风暴波下的碎波带附近，几乎没有获得定量的数据，以进一步研究在风暴波下的碎波带附近的这种长波的行为。本研究成功地开发了基于图像的监测系统，以捕捉这样的冲浪水动力。将该系统应用于西正海岸，发现在风暴波条件下，长波主要在沿岸方向上形成部分驻波，这一特征合理地解释了2007年台风袭击西正海岸时灾害发生后观察到的淹没高度。

Coastal disasters due to stormy waves often locally concentrate and this feature may be partially due to existence of infragravity waves, which have longer wave period rather than wind-induced stormy waves. Due to difficulty of field observation especially around the surf zone under the stormy waves, there are nearly no quantitative data obtained for further investigation of the behaviors of such long waves around the surf zone under the stormy waves. This study successfully developed image-based monitoring system to capture such surf one hydrodynamics. The system was applied to Seisho coast and found that long waves dominantly formed partial standing waves in the alongshore directions under the stormy wave conditions and this feature reasonably explains the inundation heights observed just after the disaster when the typhoon hit the Seisho coast in 2007.