Understanding Unsteady Flow Dynamics, Temperature, and Dye Distributions of Density Currents in a River-Reservoir System under Different Upstream Releases and Meteorological Scenarios

Abstract

Three dimensional hydrodynamic Environmental Fluid Dynamics Code (EFDC) model was applied to simulate flow, temperature and dye distributions in the river-reservoir system. The study is in a river-reservoir system (124.2 km) from SDT (upstream flow boundary) to BLD (downstream water level boundary) in Walker County, AL, USA. The model was calibrated against measured water levels, temperatures, velocities, and flow rates from 2010–2011 data under small constant release (2.83 m3/s) and large intermittent releases (~140 m3/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 oC higher than temperatures under actual releases. The momentum generated from large releases pushed bottom density currents advancing towards downstream, but the released water took several days to reach Cordova. To understand and quantify formation and propagation of density currents which are caused by daily large release of different durations and solar heating, a series of sensitivity model runs were performed under daily repeated large releases (DRLRs) with different durations (2, 4, and 6 hrs) from Smith Dam Tailrace (SDT) when other model input variables were kept unchanged. The density currents in the river-reservoir system form at different reaches, are destroyed at upstream locations due to the flow momentum of the releases, and form again due to solar heating. DRLRs (140 m3/s) with longer durations push the bottom cold water further downstream and maintain the bottom water temperature cooler. For the 6-hr DRLR, the momentum effect definitely reaches Cordova (~43.7 km from SDT). There are 48.4%, 69.0%, and 91.1% of time with positive bottom velocity (density currents moving downstream) with average velocity of 0.017, 0.042, and 0.053 m/s at Cordova for the 2-hr, 4-hr, and 6-hr DRLR, respectively. Results show that DRLRs lasting for at least 4 hrs can maintain lower water temperatures at Cordova. When the 4-hr and 6-hr DRLRs repeat more than 6 and 10 days, respectively, bottom temperatures at Cordova become lower than ones for the constant small release (2.83 m3/s). These large releases overwhelm the mixing effects due to inflow momentum and maintain temperature stratification at Cordova. Both the duration of large releases and weather conditions affect and control the formation and spread of density currents and then affect the bottom-layer temperatures. A series of model scenario runs were performed to further understand the bottom-layer water temperature changes at downstream corresponding to hypothetical meteorological changes under different release scenarios. The daily drop rate of bottom-layer water temperature under 6-hr DRLRs are 0.3, 0.45 and 0.4 °C/day for 2, 4, 6 days of 2 °C/day air temperature drop. Average bottom temperature at the river intake under 4-hr DRLRs is 2.3 oC lower than one under 2-hr DRLRs and only 1.1 oC higher than one under 6-hr DRLRs in the whole simulation period. The daily drop rate and dropping duration of bottom temperature are almost same for 2-, 4- and 6-hr DRLRs due to the same drop and rise pattern for weather conditions. The maximum differences between the constant weather scenario and the 11-day drop and rise weather scenario range from 3.1 to 4.2 °C under different releases. The lower bottom-layer water temperatures at GOUS and the river intake are primarily due to the lower air temperatures and solar radiations during the 11 days and less affected by the release pattern. To identify more efficient release operations should be further studied.