CCTV 영상과 수치모의를 통한 중문 해수욕장의 이안류 규명

Abstract

To investigate rip currents generated due to sandbars at Jungmun beach of Jeju island, the variation of the sandbar was monitored by CCTV for 2 months from July to August of 2015. The CCTV image was rectified using the projective transformation to get GCP (i.e., Ground Control Point) data. The shape and position of the sandbar were qualitatively analyzed using this rectified CCTV image data. As a result, it is found that the sandbar moved offshore when large waves were incident on Jungmun beach during the Typhoon Chan-Hom. On the other hand, the sandbar moved onshore when the relatively small waves with long periods were incident on the beach during the Typhoons Halola, Soudelor, and Goni. In this study, in order to analyze the likelihood of rip currents corresponding to the position of the sandbar, numerical simulations were performed for three conditions of sandbar: without a sandbar, with an offshore sandbar, and with an onshore sandbar. To investigate the relationship between rip currents and incident waves, various wave conditions including the wide range of frequency peakedness and directional spreading of the wave spectra were tested in the numerical simulations. Additional numerical simulations were also conducted to investigate the effect of various tidal conditions on the generation of rip currents at the beach. The results of numerical simulations for the case of a low water tidal level show that the intensity of rip current at the beach with sandbars is stronger than the case of the beach without a sandbar. Among two cases of sandbars, the offshore sandbar gives stronger rip currents than the onshore sandbar does, because the onshore sandbar is exposed to the air due to low tidal water level. However, as the tidal water level increases, the onshore sandbar becomes submerged and gives stronger rip currents than the offshore sandbar does. This can be explained by the fact that, under the high tide condition, the water depth over the offshore sandbar is too deep to break the waves, while the onshore sandbar becomes submerged with a water depth shallow enough to break waves. From these numerical results it can be concluded that the breaking of waves over the sandbar plays an important role for the generation of rip currents. In addition to the location of sandbars the effect of wave properties on the generation of rip currents are also understood from this study. When the band widths of the frequency and directional wave spectra are narrower, the intensity of the rip currents becomes stronger. This fact agrees with the rip current cases observed at Haeundae beach.