Naoki TAKAHASHI

Study on wave behavior in a pocket beach located in the shaded portion of peninsula 

Naoyuki INUKAI

 An accident that four people including two children in total were exiled to the offing in Koga Beach in Koga-shi, Fukuoka on August 11, 2017 and died occurred. 
 I grasped the change of the  by data of the nearest NOWPHAS Genkainada for the cause in a long-term short-term mainly. I grasped the change of the long-term and short term  for the cause by data of the nearest NOWPHAS Sea of Genkai mainly and paid my attention to topography characteristics around Koga Beach.
 In the case of incidence from NNE for excellence waves, Koga Beach is located in the shadow part of the islands and it is with a calm walrus at Koga Beach in whole year because the surge which damped diffraction is incident. However, sea condition may turn worse at a stretch because a surge is incident on Koga Beach directly when it changes in NNW. I solved an equation of the group speed to get ‚”he time required from NOWPHAS Genkainada to Koga Beach.As a result, the time is approximately 70 minutes.I carried out a calculation by surge estimation software SWAN.I did making wave at the spot as an N side border at a spot of NOWPHAS. By the case of NNE, significant  wave is 0.38m maximum wave is 0.26m.By the case of NNW, significant  wave is 0.80m maximum wave is 1.11m.Thus, the result that the wave height at Koga Beach increased to approximately 3 times by  changing wave direction.
 Low pressure proceed east coast of Chugoku district slowly, and the direction of the wind changes nearer N, and wave height in the Genkainada increases, and wave direction concentrates on NNE on the accident day.It is thought that this accident occurred by topography characteristics of the Genkainada and the transition that it is like wave height increase by the passage of the low pressure and burst for waves.