For example, an outlet structure placed at the end of a long, narrow channel will have a greater
vertical withdrawal zone as compared to a similar structure that is located on the main body of a lake
and is able to withdraw water from a full 180 degrees or greater around it.
For existing projects for which selective withdrawal is being considered, the existing release
quality must then be simulated. This may involve the use of a numerical model, such as SELECT, or a
one-dimensional model such as WESTEX or CE-QUAL-Rl. For proposed projects, simulation will
require a one-dimensional model. In some cases where local topography may influence the withdrawal
zone, a physical model of the intake structure and local topography may be necessary to determine
impacts. Simulations should then be verified against project conditions.
All of these options and scenarios have been incorporated into the computer program SELECT
(Davis et al. 1987). Given a particular water surface elevation, temperature profile and center-line
elevation, and the dimensions and withdrawal flows of a port, the withdrawal zone can be predicted.
The results predict the upper and lower withdrawal limits as well as a withdrawal velocity profile and
the resulting average outflow parameters (i.e., temperature).
If profiles of other conservative parameters (such as dissolved oxygen and algal profiles) are
used in combination with the temperature profile, SELECT will also predict the resulting average
outflow qualities. It is also possible to adjust the program to account for many differing entrance
conditions.
At this point in the design, the location of potential new ports should be determined. The
numerical model used above to verify existing conditions is then used to predict release conditions with
the new port(s) locations. If multiple ports are necessary, optimization routines are available to
determine the optimum number and location for new ports.
By knowing the temperature profile as well as profiles of any other important qualities in front of
the outlet structure, and having ports at different elevations throughout the water column, it is possible to
withdraw a wide range of qualities provided they are available in the lake. By operating multiple ports,
it becomes possible to mix water of different qualities to produce a desired quality.
4.3.1.4 Applications of Selective Withdrawal
The selective withdrawal phenomenon has been of interest to researchers for the past 40 years.
As such, there are many reports detailing lakes and reservoirs in which this technique has been tested
and used successfully to solve water quality concerns. In addition, several other techniques discussed
in this report utilize selective withdrawal for identification or evaluation purposes.
Much work has been done toward describing and predicting how water will behave when
withdrawn from a stratified environment. Cariel (1949), Bohan and Grace (1969), Monkmeyer et al.
4.3-3
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