A basic study on the stimulation of the tidal exchange of an enclosed coastal sea by the configuration of the bay opening or the installation of offshore structures is presented. Tidal current is obtained by the usual ADI method in two-dimensional plane. Lagrangian movement of water particles is performed by the Euler-Lagrange method. The remaining ratio of water particles placed inside the bay to the initial number of particles is calculated for many tidal periods. Model tests corr~ponding to the numerical simulations are conducted in a tidal tank, where the movements of the floating balls are analyzed by an image processor. Advection patterns of the particles and the water exchange ratios in some configurations with/without offshore structures are presented graphically and are discussed by comparing numerical simulations with experiments.
The total length of the coastline of Japan is about 34,000 km and 30 % of it is along a bay or an enclosed sea. The pollution of the water quality in the closed sea areas is still serious, though the effluent sewage has been curtailed in recent years by law. Shortterm improvement is difficult, however despite curtailment of the discharge from land, because of the enormous amount of sewage accumulated from the past. We must take measures in addition to the regulations on discharge, because if the present condition is allowed to continue for a long time, it can exert a dire influence on the ecosystem. There are several programs researching ways of improving the water quality of an enclosed sea or a bay by stimulating the tidal exchange 1), a), s),4), 5). We here consider the tidal exchange characteristics of a small bay using numerical simulation and a model experiment on the breadth, position and shape of the opening of the bay.