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

Gang Chen,Xing Fang

doi:10.3390/w7116244

Abstract

A calibrated three-dimensional Environmental Fluid Dynamics Code model was applied to simulate unsteady flow patterns and temperature distributions in the Bankhead river-reservoir system in Alabama, USA. A series of sensitivity model runs were performed under daily repeated large releases (DRLRs) with different durations (2, 4 and 6 h) 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 a cooler bottom water temperature. For the 6-h DRLR, the momentum effect definitely reaches Cordova (~43.7 km from SDT). Positive bottom velocity (density currents moving downstream) is achieved 48.4%, 69.0% and 91.1% of the time with an average velocity of 0.017, 0.042 and 0.053 m/s at Cordova for the 2-h, 4-h and 6-h DRLR, respectively. Results show that DRLRs lasting for at least 4 h maintain lower water temperatures at Cordova. When the 4-h and 6-h DRLRs repeat for more than 6 and 10 days, respectively, bottom temperatures at Cordova become lower than those 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.

Highlights

A density difference can exist between two fluids because of a difference in temperature, salinity, or concentration of suspended sediment
When the large flow is released for 1 h from Smith Dam, the flow momentum begins to affect the velocities at middle cross section of Sipsey Fork (MSF) where water at all layers moves downstream with a maximum velocity of
The density currents are clearly showed in the bottom layers moving downstream at UPJ and Gorgas upstream cross section (GOUS) for all three duration daily repeated large releases (DRLRs), and at Cordova for

Summary

Introduction

A density difference can exist between two fluids because of a difference in temperature, salinity, or concentration of suspended sediment. Many laboratory and numerical model studies of density currents have been conducted in the last several decades. Various simplifying assumptions were made to develop analytical models with laboratory data to understand density currents [10,11,12,13]. Fang and Stefan [14] developed an integral model for a discharge from a river channel over a horizontal or a sloping bottom into a reservoir or a lake to determine dilution up to plunging for density current computations. Using a series of laboratory experiments in a two-layered ambient stratification, Cortes and colleagues [15] developed a theory to predict the partition of the buoyancy flux into the interflow and underflow and how a gravity current splits in two upon reaching the sharp density step

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