해운대 이안류 발생 메커니즘과 특성

Abstract

해운대에서 발생하는 이안류의 주요 메커니즘을 확인하기 위하여 Boussinesq 파랑모형(FUNWAVE)을 관측 해상자료와 지형을 이용하여 수치모형을 수립하고, 2012년 8월 10일 발생한 이안류를 수치모의 하였다. 수치모의결과는 영상보정기준점(GCP) 측량 및 이를 이용한 영상분석을 통해 관측한 유속과 비교, 검증하였으며, 이안류 발생 위치와 흐름패턴이 잘 일치하였고 이안류 유속이 타당한 정도에서 유사함을 확인하였다. 또한 수치모의 결과로부터 해운대 앞바다의 수중 천퇴에 따른 파랑굴절로 파향이 서로 약간 다른 파랑들이 해안에 전파되고 이에 따라 해운대 해안에 설치된 CCTV 영상자료와 유사한 벌집구조가 형성되어 절점선 영역을 따라 이안류가 발생함을 명확히 확인할 수 있었다. 본 연구에서 해운대 이안류에 대하여 정도 높은 결과를 나타내는 것으로 검증된 수치모형을 이용하여 이안류의 발생 가능성을 정량화 시킬 수 있는 방법을 제안하고 해상상태특성 변수에 따른 해운대 이안류의 발생정도의 분포를 도출하였다. 동일한 파고, 주기, 파향을 갖는 파랑조건에 대하여 스펙트럼의 광협도를 변화시킨 경우, 스펙트럼의 광협도가 좁은 협대역의 스펙트럼에서 이안류가 더 잘 발생하였다. 파고는 클수록, 주기가 길수록, 조위가 낮을수록, 그리고 파향은 남동쪽보다 남서쪽 파향에서 이안류가 더 잘 발생하는 것으로 나타났다. 추가적으로, 양빈 계획과 잠제 설치계획에 따른 해운대의 지형변화가 이안류 발생에 미치는 영향을 파악하고자 현재상태, 양빈상태, 잠제설치상태, 양빈 및 잠제설치 상태의 수심지형을 이용하여 이안류 수치모의를 수행하였다. 수치모의 결과, 양빈에 의한 지형의 불규칙성을 제거(지형적 이안류 수로의 제거를 의미함)하거나 잠제를 설치하더라도 파고의 불균등성을 제거하지 못하므로 이안류의 발생을 저감하지 못하는 것으로 나타났다. 이상과 같이 해운대 앞바다의 수중 천퇴에 따른 파랑굴절로 해변에 형성되는 벌집구조 파봉선 현상이 해운대에서 발생하는 이안류의 가장 중요한 메커니즘으로 판단할 수 있었고, 이안류 발생정도를 정량화시키고 정량적인 분포를 제시함으로서 실시간 관측정보와 연계한 이안류 예측기법을 발전시킬 수 있는 토대를 마련할 수 있었다.| To investigate a dominant mechanism of rip current occurring at Haeundae beach, the numerical simulation of a rip current event occurred on the 10th of August, 2012 was performed using a fully nonlinear Boussinesq equation model (FUNWAVE) with the observed wave and topographical data. The numerical results were compared to the rip current velocity measured based on the imagery data taken into account a coordinate correction by GCP (Ground Control Point) surveys on Haeundae beach, and their velocity magnitudes and patterns agreed with each other in a reasonable manner. From the numerical results, it was found that wave refraction due to topographical characteristics (i.e., submerged shoal) of Haeundae gave rise to several wave trains propagating with slightly different directions toward the beach, and consequently rip currents were well developed through the nodal line area of honeycomb patterns of wave crest which was seen similarly to the CCTV images. The method for quantifying the likelihood of serious rip current varied according to incident wave characteristics and tidal elevation was suggested based on numerical simulations. The quantification method applied into various numerical simulations shows the followings. The narrow-banded spectral wave condition (i.e., a swell spectrum) increases the likelihood of rip current more than the broad-banded spectral wave condition. And the higher wave height, the longer wave period and the lower tide increase the likelihood of rip current. In case of wave direction, the southwest increases likelihood of rip current more than the southeast. In addition, in order to investigate the effects of the beach nourishment and the submerged breakwater installation plan on rip current of Haeundae beach, the numerical simulations have been carried out by using the topographies according to the plans. As a result, even though the topographical irregularity was removed due to the beach nourishment or the submerged breakwater was installed, the rip current likelihood was not reduced because of the non-uniformities of wave energy. In conclusion, it was confirmed that the honeycomb pattern of waves near the shore, which is caused by refracted waves due to submerged shoals in the offshore, is one of the most important mechanism of the rip current generation in Haeundae beach. And this study presented the quantification method for rip current likelihood and its distributions according to various sea states to construct a future rip current forecasting system utilizing the real-time observation data.; To investigate a dominant mechanism of rip current occurring at Haeundae beach, the numerical simulation of a rip current event occurred on the 10th of August, 2012 was performed using a fully nonlinear Boussinesq equation model (FUNWAVE) with the observed wave and topographical data. The numerical results were compared to the rip current velocity measured based on the imagery data taken into account a coordinate correction by GCP (Ground Control Point) surveys on Haeundae beach, and their velocity magnitudes and patterns agreed with each other in a reasonable manner. From the numerical results, it was found that wave refraction due to topographical characteristics (i.e., submerged shoal) of Haeundae gave rise to several wave trains propagating with slightly different directions toward the beach, and consequently rip currents were well developed through the nodal line area of honeycomb patterns of wave crest which was seen similarly to the CCTV images. The method for quantifying the likelihood of serious rip current varied according to incident wave characteristics and tidal elevation was suggested based on numerical simulations. The quantification method applied into various numerical simulations shows the followings. The narrow-banded spectral wave condition (i.e., a swell spectrum) increases the likelihood of rip current more than the broad-banded spectral wave condition. And the higher wave height, the longer wave period and the lower tide increase the likelihood of rip current. In case of wave direction, the southwest increases likelihood of rip current more than the southeast. In addition, in order to investigate the effects of the beach nourishment and the submerged breakwater installation plan on rip current of Haeundae beach, the numerical simulations have been carried out by using the topographies according to the plans. As a result, even though the topographical irregularity was removed due to the beach nourishment or the submerged breakwater was installed, the rip current likelihood was not reduced because of the non-uniformities of wave energy. In conclusion, it was confirmed that the honeycomb pattern of waves near the shore, which is caused by refracted waves due to submerged shoals in the offshore, is one of the most important mechanism of the rip current generation in Haeundae beach. And this study presented the quantification method for rip current likelihood and its distributions according to various sea states to construct a future rip current forecasting system utilizing the real-time observation data.