Quantcast Theory Theory
In the normal operation of a reservoir with a low-level intake, during the summer, the
hypolimnion is gradually withdrawn and released downstream. Thus, the cooler hypolimnetic water is
gradually replaced with warmer water from the mixed surface layer. Although thermal stratification may
be retained, the degree of stratification is weakened such that the surface-tobottom temperature
difference is continually being reduced. This reduces the available habitat for in-reservoir cool-water
fish and may threaten their survival. If the cool-water fishery is in the downstream river, the loss of cool
water for release can be likewise disastrous.
A temperature control curtain or a submerged weir can limit the withdrawal of hypolimnetic
water preserving the cool water for in-reservoir fishery or for later release to maintain cool
temperatures in the downstream river. A cool-water "sanctuary" can also be created in an embayment
of the reservoir, but fish usage may be limited. The temperature control weir or curtain modifies the
withdrawal zone of the structure's outlet to minimize release of cool water by skimming release water
from the epilimnion. If there is cool water fishery in the downstream river, then temperature control is
aimed at maintaining cool water withdrawal from the reservoir. A temperature control weir, installed in
front of the outlet, can limit withdrawal of epilimnetic water by skimming cool water from the bottom of
the reservoir.
Figure 4.3.3 Schematic of a submerged dam
The design procedure is similar to the design of any withdrawal structure:
a. determine objectives of the structure. The temperature control device be designed to skim
surface water from the reservoir and retain cool water in the reservoir or regulate the release of cool
water to meet temperature objectives in the downstream river. The latter requires the temperature
curtain or weir have operational capabilities to adjust the withdrawal zone with changes in reservoir


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