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ABSTRACT

This short note reports a series of density current experiments designed to model turbidity underflows caused by flood-stage discharge of lake-tributaries. In a 5.8 m long tank filled with freshwater, saltwater was fed in continuously, flowing down a 15°‘delta’ slope onto a horizontal floor. These density currents maintained steady state characteristics. The main objectives of this investigation were to determine (1) the flow regime of the density currents and (2) the underflow-induced movements in the freshwater. Reynolds numbers for thirty-five runs ranged from 70 to 4100. Experiments with laminar flow reproduced kinematic (Froudian) models of underflows measured in the Walensee (Switzerland). Flow was rapid on the slope (Froude number, Fr > 1) and tranquil (Fr<1) on the floor. Turbulent flow experiments yielded velocity profiles (with a maximum at the flow interface) which approximate natural conditions. Movements in formerly stagnant water body are induced by interfacial shear stress: a layer of freshwater is dragged along by the density current and replaced by the backward flow of an equal amount of overlying water (mass conservation). Extrapolated to a natural setting, circulation induced by underflows is probably an important mechanism for oxygenating deep lacustrine basins.