Wind-induced current and water exchange in Lake Kasumigaura

This paper presents a study on wind-induced water exchange between a closed bay and the main part of Lake Kasumigaura, a shallow eutrophicated lake in Japan, under the development of diurnal thermocline. The long-term field measurements show that the diurnal thermocline is developed in a daily circle in the lake. When there is a diurnal thermocline, the wind-induced current becomes a two-layered flow because the vertical momentum flux is suppressed by the densimetric stability at the thermocline. As a result, the water exchange is more active in daytime when the diurnal thermocline exists than at night when the temperature is uniform in vertical. Because this phenomenon appears every day when solar radiation is strong enough to develop the thermocline, it is considered to yield a large volume of water exchange between the bay and the main part of the lake. Since the bay is much more contaminated, it could be considered the two-layered flow may cause pollutants exchange and has effect on the water quality of the lake. A 3-D model, in which k-epsilon two-equation model is included, is adopted to simulate the diurnal thermal stratification and density current. Heat flux at water surface is estimated by a existed semi-empirical formula. The coefficients of the formula are calculated by regression analysis with meteorological data from center of the lake in long time period. The simulated temperature and velocity distributions are compared with the data obtained from the field experiment. The annual volume of water exchange is estimated by both field data and simulated one and the difference arc discussed.