地水環境学

Lake Koyama in Tottori Prefecture has various water environmental problems due to organic pollution and eutrophication. As part of the fundamental research aiming at preserving and improving the water environment in Lake Koyama, the authors examined the characteristics of lake currents using the two-dimensional one-layer model incorporated with Altenative Direction Implicit method. Numerical simulations were excuted in the three cases of westerly wind, north-westerly wind and northerly wind based on wind directions representative of the area surrounding Lake Koyama. First, the changes over time in current velocity and water level were examined. The results showed that current velocity and water level at all locations converged and that a steady state was achieved at about 40 hours after the start of calculations. The water level at the center of the lake had a small range of oscillation. At close vicinity to the both banks, the seiche with large oscillations continued for a long time. Therefore, this model could effectively recreate the seiche generation in the lake. Next, the patterns of overall lake currents in Lake Koyama were deliberated. As a result, patterns of horizontal circling drift differed depending on differences in wind direction. Also, the following currents occurred owing to wind-driven currents in shallow water areas and the reverse currents arised due to slope currents in deep water areas. These calculation results clearly reflected the mechanisms of the horizontal circling drift and the bottom topography in Lake Koyama. Finally, the bottom sediment environment in Lake Koyama was investigated from the perspective of the currents characteristics. The comparison of the patterns of horizontal circling drift with the distribution of grain size composition of the sediment in the three cases clarified that the clay accumulated in the areas with low current velocity. Also, the ratio of clay in the bottom sediment was low in the areas where the current is fast. These results reflected the process of sedimentation and accumulation of sludge, and indicated that the currents characteristics in the Lake were closely related to the bottom sediment environment.