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

Abstract

A three-dimensional flow and temperature model was applied for a 124 km 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 (2.83 m 3 /s) and large intermittent releases (~140 m 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°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.

Sensitivity Analysis of Flow and Temperature Distributions of Density Currents in a River-Reservoir System under Upstream Releases with Different Durations

“…In addition to the calibration results previously presented [43], Figure 2b shows the time series plot of observed and modeled water surface elevation (WSE) at GOUS from 8 September to 18 October 2010. The agreement between observed and modeled WSEs at GOUS is very good, with a median difference of 0.008 m. The average difference between observed and modeled WSEs at GOUS is −0.020 m. Flow velocities at Cordova measured from 8 September to 18 October 2010 by USGS were compared with modeled results from the EFDC model.…”

“…In the previous study [43], the 3D hydrodynamic EFDC model was applied to simulate unsteady flow patterns and temperature distributions under various upstream releases and variable atmospheric forcing in BRRS in 2011. The calibrated EFDC model provided simulated water surface elevation, temperature, velocity, and discharge at different layers (depths) for all grids in different cross sections.…”

“…Detailed information on the governing equations for the heat exchange processes in a water body is given in the CE-QUAL-W2 manual [20]. The EFDC hydrodynamic and temperature model applied to BRRS was previously calibrated and validated using extensive field data measured in 2010 and 2011 [43,52].…”

“…In this study, a three-dimensional (3D) Environmental Fluid Dynamics Code (EFDC) model [44] configured for BRRS [43] was applied to simulate unsteady flow patterns and temperature distributions in BRRS including the intake canal and the discharge canal of a power plant (Figure 1c) near Parrish, AL. This study mainly focuses on the EFDC simulations and result analysis from SDT to GOUS, the upstream portion of BRRS, under different large releases from Smith Dam.…”

“…In our previous study [43], preliminary simulation results using particle tracking show that particles of water released from Smith Dam could take several days to reach Cordova and Gorgas upstream cross section (GOUS); the momentum developed by the releases pushed the cooler water near the river bottom downstream. The momentum or pushing effect from the large release provides driving force for the density current movement along the river bottom in the river-reservoir system.…”