With the popularization of diving, its safety is an important issue. One of the factors of serious diving accidents is an abrupt downflow called “down current” by divers. However, its reality and mechanism are poorly understood. In order to make them clear, a continuous observation of current, wave, temperature, and salinity at the fatal accident site near the coast of Kumano Nada was conducted.
In the 15 days of observation, we succeeded in observing downflows with vertical velocity up to 22cm/s. Some of the major downflows were accompanied by oscillations of velocity, temperature, and salinity, which were associated with long-period waves. By analyzing an aerial photograph and satellite images, the frequent existence of a density interface between river water and seawater on the accident location was discovered. It is concluded that this interface, along with the steep slope topography, would convert horizontal velocities into vertical ones, and thereby be a cause of “down current.”
