The operation of navigation lock causes much backward flow of salt water, which prevents the freshening reservior from its replacement of salt water by fresh water. Here, some behavior of density currents in the concave basement that stores the backward flow of salt water, the diffusion of salinity in the concave basement, and the instability of two layer flow were investigated by hydrauric model experiments with width ratios of 0.2, 0.33, 0.66, and 1.0, and a length ratio of 0.1 between the lock and the concave basement.
The results obtained are as follows :
A width ratio of 0.2 made it possible to restrain most of the density currents in the concave basement, and then some swinging internal waves with decreasing velosity and amplitude were observed. Then, the salinity transported into the freshening reservior was very little, and after operation the salinity recorded was satisfactorily low (below 200 ppm.).