The operation of a multilevel intake structure requires the consideration of numerous project
conditions and constraints, the most important of which is thermal stratification. As stratification
develops, the limits of withdrawal for a given port are reduced because of the density differences
imparted by the thermal stratification. Thus, flow through a port at a given elevation may not result in a
release temperature similar to that observed at the center-line elevation of the port or that desired to
meet a downstream objective.
Selective withdrawal, which is defined as the capability to describe the vertical distribution of
withdrawal from a density-stratified reservoir and then use that capability to selectively release the
quality of water that is desired, can be used to determine the appropriate or best available operation of
a release structure. It can also be used in the design of multilevel intakes as well as in the modification
of existing projects to achieve a given release water quality criteria. Theory
This technique relies on manipulation of the density-impacted withdrawal pattern to control the
release quality from a structure. The withdrawal pattern that is set up as the result of release of water
through a port during stratified conditions is defined as the withdrawal zone (Figure 4.3.1). Water
within this zone will ultimately be released through the port, even though the rate of release from
individual strata of the pool varies. By identifying the portion of each layer released under a given
operating condition, the release quality can be predicted. Selective Withdrawal Methodology
Much research has been performed in the area of water withdrawal through a point sink (such
as a single port) or through a linear sink (such as a line of penstock openings for hydropower). The
effort has focused on the withdrawal profile that is created and on the way in which density stratification
and physical boundaries affect the withdrawal limits (beyond which, water is not drawn into the port).
When water is withdrawn through a single port, water from well above and below the horizontal center
line of that port is withdrawn.
The development of these upper and lower withdrawal limits is dependent on the density
gradient present in the water column, the flow through that port, and local geometric effects. Beyond
the upper and lower limits of withdrawal, insufficient energy is available in the flow to entrain the water
from these levels into the main flow (Wilhelms 1986).


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