琵琶湖南湖の湖流解析

Abstract

It is an important problem for us to analyze the three dimensional structure of the current in ashallow lake such as the South Basin of Lake Biwa. Assuming the coefficient of vertical eddyviscosity to be 0.1-10 cm2/sec, the frictional layer depth is evaluated to be within a range of 1-15 meter and is no more than a few times of the mean depth of that basin.We developed the two-dimensional three-level model considering the surface and the bottomEkman layer so as to simulate the currents under weak wind conditions, and conducted severalnumerical calculations under the transient boundary conditions of the measured wind and theoutlet discharges. The calculated current velocity is reasonarably in accordance with the mea-sured one if the relation of the eddy viscosity with wind velocity and water depth is appropriatelygiven. The effect of the non-uniformity of the wind field on the currents is also investigated.Drift currents and resultant topographic gyre patterns in the shallower part of the South Basin, slope currents with longitudinal convective circulation and its deflective revolution under the Cor-iolis effect due to decreasing eddy viscosity after the wind ceased are examined. The distributedwind tends to weaken the the secondary circulation compared with the result under the uniformwind velocity.