Mixing of density-stratified impoundments with buoyant jets

This study is an investigation of the mixing of density-stratified impoundments by means of buoyant jets created by a pumping system. The deterioration of water quality which often occurs in density-stratified lakes and reservoirs may be counteracted by mixing. The physical aspects of the mixing process are the primary concern of this study, although several implications regarding changes in water quality are indicated. A simulation technique is developed to predict the time-history of changes in the density-depth profiles of an impoundment during mixing. The simulation model considers the impoundment closed to all external influences except those due to the pumping system. The impoundment is treated in a one-dimensional sense, except for the fluid mechanics of the three-dimensional jet and selective withdrawal of pumping system. The numerical solution to the governing equations predicts density profiles at successive time steps during mixing, given the initial density profile, the area-depth relation for the impoundment, the elevations of intake and jet discharge tubes, and the jet discharge and diameter. The changes due to mixing in the profiles of temperature and of a conservative, non-reacting tracer can be predicted also. The results of laboratory experiments and two field mixing experiments in which density-stratified impoundments were mixed using pumping systems show that the simulation technique predicts the response of the impoundment reasonably well. The results of a series of simulated mixing experiments for impoundments which have prismatic shapes and initially linear density profiles are given in dimensionless form. For these special conditions, the efficiency of the pumping system increased as the jet densimetric Froude number decreased, and the time required for complete mixing was a fraction of the characteristic time, T ≤ V-/Q (where V- is the impoundment volume included between intake and jet elevations and Q is the pumped discharge). Recommendations are made for the application of the generalized results and for the use of the simulation technique for lakes and reservoirs which are not closed systems