Tsuyoshi�@TAKEBAYASHI

A Numerical Study on the Rip Current in Niigata shore

Tokuzou�@HOSOYAMADA


The beach, not only in the Pacific Ocean but also in the Japan Sea, becomes to be leisure area in the summer. But the accident, the swimmer is carried away to the offshore, is sometimes happened. There are some reasons why the accident is happened. One of the reasons is that there is the especial current, called as the rip current.
Rip currents, commonly called rip tides, and erroneously called undertows, affect many of the surf areas along the coast, and pose a life threatening situation to the unsuspecting beach goers. Rip currents are strong surface current of water flowing out past the surf zone that can pull the strongest swimmer, like as an Olympic swimmer, into deeper water. Most deaths occur when people caught in the current try to swim toward the shore directly against the current; they become totally exhausted and drown. Rip currents occur around the world at "surf" beaches, including the Pacific coasts, and the Japan Sea.
Therefore, the mechanism of the rip current has to be known to expect its occurrence, and to prevent the accident. This study simulated the rip current by using the two-dimensional finite difference model, based on the Boussinesq equation, firstly in a simplified geography, and secondary, in the modeled bathymetry, Shell�fs law, easily to occur the rip currents.

The results show that the rip current occur at near the beach, the stream is narrow (10-30 meters wide) with the length extending up to 200-300 meters offshore, and the water velocity of the offshore direction becomes to be about 1.0m/s.