Mixing of density-stratified impoundments with buoyant jets
Creators: name: Ditmars, John D.
Publisher: name: California Institute of Technology
Year: 1970

description: 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.
description: To Dr. Norman H. Brooks, who was a continuous source of guidance, kind advice, and encouragement throughout this study, the writer expresses his sincere gratitude. 

The writer is also indebted to Dr. Vito A. Vanoni and Dr. Fredric Raichlen for their comments and encouragement during this study and to Dr. E. John List for his comments during the writing of the thesis. 

For their ready assistance in constructing and modifying the experimental apparatus, the writer extends sincere thanks to Mr. Elton F. Daly, Supervisor of the Shop and Laboratory, and to Mr. Robert L. Greenway. Appreciation is also due Mr. Carl A. Green, Jr., who prepared the drawings; Mrs. Arvilla F. Krugh, who typed this manuscript and earlier drafts, Mr. Carl T. Eastvedt, who did the photographic work; Mrs. Patricia A. Rankin, who provided many secretarial services for this study; and Messrs. Yoshiaki Daimon, Steven S. Watkins and Paul T. Wegener, who assisted in performing the experiments and reducing the data. 

This research was supported by the Federal Water Pollution Control Administration through Grants No. 16000 DGY and No. 16070 DGY and was conducted in the W. M. Keck Laboratory of Hydraulics and Water Resources at the California Institute of Technology. The writer gratefully acknowledges the financial assistance received as a U. S. Public Health Service Trainee (1966-70) and a National Science Foundation Trainee (1965-66). 

This report was submitted by the writer in August, 1970, as a thesis with the same title to the California Institute of Technology in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Civil Engineering.