DYNAMICS OF TURBIDITY CURRENTS MEASURED IN KATSURAZAWA RESERVOIR, HOKKAIDO, JAPAN

The movement of turbid underflows in a reservoir is examined using three recording current meters. The density underflows, called turbidity currents, are induced by sediment discharge in the inflow system during the spring thaw. The continuous measurement near the reservoir bottom at two longitudinally located stations shows the characteristic diurnal variations of flow velocity and temperature, and decreasing temperature in the downslope direction. The former corresponds to the diurnal variations of sediment discharge and its temperature in the inflow system, and the latter is explained by the 'entrainment' of upper, cold water which turbidity currents induce. Assuming the upper water temperature to be 0°C, two mixing coefficients and turbidity-current discharge at the stations are numerically obtained. Applying Manning's equation to the critical, low turbidity-current discharge, a value of 1.51 is obtained for the roughness coefficient, n. Hence, assuming n to be constant, Reynolds number, Re, and densimetric Froude number, Fd, for the turbidity currents are evaluated at 7.1 × 104-5.8 × 105 and 1.5-4.4, respectively. The entrainment coefficient. Ee, from the equation of continuity holds a significant correlation with 'overall Richardson number', Ri0 (= 1 Fd2). This shows decreasing Ee with increasing 1 Ri0. The decreasing entrainment is due to decreasing friction at the interface with the relative increase of kinetic energy. This could be applied to turbidity currents with Re of the order of 105. The downslope movement of turbidity currents with the diurnal variations of discharge and temperature is also explained by the longitudinal propagation of long waves. These could change from kinematic to intermediate waves with increasing discharge. © 1990.