Once the crest elevation has been selected, the crest length must be determined. Although the
shorter weir lengths are less expensive to construct, their effectiveness can also be reduced. For
example, a weir crest with the same length as the intake port may develop a withdrawal zone very
similar to the port because of the withdrawal of hypolimnetic water over the weir. Therefore, some
reasonable length must be determined to begin design. Using an initial length, the numerical simulations
are repeated as in the initial evaluation of a submerged weir. Examination of the results for the depth of
the withdrawal zone and the resulting release water quality will determine the optimum weir length.
Some discretion in the length of the weir must be used, as well as site-specific knowledge of the
area in front of the intake. In most applications, the weir crest should tie into the same elevation on
either bank in front of the structure to prevent the withdrawal of hypolimnetic water from around the
18.104.22.168 Applications of Submerged Skimming Weirs
Submerged skimming weirs have been used at several CE projects to prevent the release of
low-DO hypolimnetic water. Linder (1986) described the design and evaluation process for
submerged skimming weirs at two hydropower projects in the Kansas City District. These projects,
Stockton and Harry S. Truman Dams, were predicted to have low dissolved oxygen in the hydropower
releases. During construction, provisions were made to construct weirs as part of the project.
Evaluation of this technique at these projects indicates that the release quality is improved, with some
minor elevation of release temperature in the fall and some cooling in the spring.
Another concern that developed after construction of the projects was that, during long periods
of nongeneration, stratification of the area between the weir and the dam can occur. Upon start-up of
generation, some low-DO water is released downstream. This problem has also been documented at
Clarence Cannon Dam in the St. Louis District (Wilhelms and Furdek 1987). However, the
submerged weir at Clarence Cannon functions as designed during generation and maintains adequate
levels of dissolved oxygen in the hydropower releases.
An evaluation of a skimming weir for temperature control at Meramec Park Dam, a project
planned for the Meramec River in Missouri, indicated that a weir would be effective for controlling
cool-water releases during the stratified period (Bohan 1970). However, the project was deauthorized
and never constructed. An evaluation of a submerged skimming weir at Richard t. Russell was
performed to assist in release water quality enhancement (Smith et al. 1981). Results indicated that the
weir, which was proposed as a modification of an existing cofferdam, would interfere with the
hydrodynamics of proposed pumpback operations and therefore was not recommended for
The US Bureau of Reclamation (USBR) has also considered the use of submerged weirs to
enhance release water quality. Shasta Dam, located on the Sacramento River, was evaluated for