Yuta Namba

Proposal of Indicators for Safe Activities in Rip Currents

Naoyuki Inukai

In this research group, Yamashita et al. created an estimation formula for the rip current flow velocity in the past and estimated the rip current flow velocity. But, in the estimation formula, weak rip currents were not evaluated. However, there was an accident in the Zenibako coast due to a weak rip current. In addition, water depth at the accident point was not deep. So, We thought that it would be possible to obtain knowledge that contributes to the prevention of water accidents by investigating the cause of water accidents caused by rip currents from weak rip currents and accident occurrence mechanisms.
From the results of wave deformation calculation using NOWT-PARI, it was found that the water depth at the accident point was within reach of the feet, but changed to a depth that the feet could not reach due to fluctuations in the water level. At that time, it was thought that he was in a position like a standing swimming and swept away by a weak rip current. The speed of standing swimming in the indoor pool was measured and found to be 0.25 m / s.
For the parameters of the rip current flow velocity estimation formula, we used more observed values than before and reviewed the calculation method, the accuracy of the formula has improved.
From the above, it was possible to quantitatively grasp the moving speed and the wave height at which swimming can be safely performed when actually entering the water and moving. In addition, on the coast where the seafloor gradient is known, it is possible to estimate the rip current velocity from the wave conditions, so we were able to obtain information for safe operation in the rip current field.