Simulations of Flow and Dye Distributions of Density Currents in a River-Reservoir System under Different Upstream Releases

A three-dimensional environmental fluid dynamics code model was calibrated and applied to simulate unsteady flow patterns, dye distributions and concentration in the Bankhead River-Reservoir System in Alabama, USA. A series of sensitivity model runs were performed under different large releases (140 m³/s) from Smith Dam Tailrace (SDT) when other input variables were kept unchanged. The cold-water current from SDT moves to MSF (~11 km) during the 4-hr single large release (SLR) and advances to ~18 km in 1 day after the release stops. In the following 4 days with constant small release only (after SLR stops), the dye moves downstream very slowly, less than 4 km. Daily repeated large releases (DRLRs) with longer durations push the dye to move further downstream. In the period without the large release, the dye spreads and mixes with the surrounding water due to turbulent diffusion and dispersion (due to backwater effect). When the next large release comes, the dye released at SDT is pushed downstream again. For the 4-hr DRLR, the dye moves to UPJ (~22 km from SDT), MJC (~33 km from SDT), Cordova (~43.7 km from SDT) and GOUS (~64.3 km from SDT) at 12 hrs, 34 hrs, 60 hrs, and 145 hrs after the dye release from SDT. For the 4-hr DRLR, the momentum effect has some impacts at Cordova but does not reach GOUS. For the 6-hr DRLR, the momentum effect definitely reaches Cordova and the dye pattern repeats daily, and also has certain impact at GOUS.