Estimation of Vertical Mixing Based on Water Current Monitoring in the Hypolimnion of Lake Biwa(<Special Issue>Environmental Flows)

Using continuous monitoring data over last two years in the North Basin of Lake Biwa, we showed the time change of water currents and temperature from May 2003 to May 2005. Two acoustic current meters (ACMs) were deployed at 1.8m and 54m height from lake bottom respectively, at the observation point with the depth of 90m near the deepest place (104m depth). We measured horizontal current velocity, current direction, and water temperature at every 20 minutes, and estimated the bottom stress with horizontal current velocity and water temperature. Average water current velocity near the lake bottom through the observation was weak as reported before, however, maximum current velocity due to strong typhoon in October 2004 reached almost 30cm/s. Moreover, comparatively strong water current velocity was observed from January to February in each winter. The estimation of the bottom stress with current velocity suggested us that strong currents could re-suspend bottom sediments.