SUGGESTION OF A NUMERICAL ANALYSIS METHOD FOR SALINITY TRANSPORT IN RIVERS DURING A TSUNAMI ATTACK

Abstract

 The Nankai Trough Earthquake Tsunami is predicted to occur in the near future in Japan. One of the measures to be undertaken against a tsunami attack is to predict the duration for which the water intakes of a nearby water purification plant in the estuary region are to be shutdown to prevent machinery damage due to salinization. In the tidal area of the Yodo River in Osaka, western Japan, there is an estuary barrage and many intakes upstream. The taken water is purified and supplied for drinking and industrial usage over a large urban area. When a tsunami overtops the barrage, it is necessary to prevent a high level of salinity from entering into the purification plant, as salt causes a breakdown of the machinery. Therefore, prediction of the maximum value and duration of salinity is important. In this study, we developed a new model that could simulate the propagation of a tsunami from the wave source in horizontal two-dimension and the flow in the estuary area in three-dimension, where the density current was considerable, and the interaction between these domains was also considered. This model enabled us to conduct a tsunami simulation from the time when the tsunami occurs to the time when the overtopped salinity is washed away by the river flow; this was impossible to achieve in extant studies. We made predictions for five conditions of the river flow rate. It was determined that the water intakes of the purification plant must remain shut for at least five hours under the condition of normal river flow rate. We also conducted a few simulations with operational modifications made to the barrage to develop methods to diminish the salinity influence. Concepts that could shorten the intake shutdown periods by more than an hour have also been proposed.