Understanding flow dynamics and density currents in a river-reservoir system under upstream reservoir releases

A three-dimensional flow and temperature model was applied for a 124km river-reservoir system from Lewis Smith Dam tailrace to Bankhead Lock & Dam, Alabama. The model was calibrated against measured water levels, temperatures, velocities and flow rates from 4 May to 3 September 2011 under small constant release (2.83m(3)/s) and large intermittent releases (similar to 140m(3)/s) from an upstream reservoir. Distributions of simulated flow and temperatures and particle tracking at various locations were analyzed which revealed the complex interactions of density currents, dynamic surface waves and solar heating. Flows in the surface and bottom layers moved in both upstream and downstream directions. If there was small constant release only from Smith Dam, simulated bottom temperatures at Cordova were on average 4.8 degrees C higher than temperatures under actual releases. The momentum generated from large releases pushed bottom density currents downstream, but the released water took several days to reach Cordova.