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 periods to increase the unit discharge may be enhance release dissolved oxygen (DO) by concentrating
the flow to pass through a minimal number of gates (perhaps only one). This would increase turbulence
and air entrainment in the stilling basin and enhance reaeration. As an example, a structure with six
gates may operate with all 6 gates open at 0.5 ft, but this would like provide very little reaeration in the
stilling basin. Modifying the operation to a single gate with a 3.0-ft opening concentrates the flow
through and will likely increase turbulence and reaeration downstream (Figure 4.4.4).
Figure 4.4.4. Example of effect of flow concentration through a single gate
4.4.4.2 Evaluation Methodology
Design Criteria. Concentrating flow through a small number of gates, as an operational
alternative, can be implemented at nearly any structure where multiple gates extend horizontally across
the dam. However, its application must be determined on a case-by-case basis. The specific objective
of this technique is to increase gas transfer in the stilling basin of the spillway, usually to improve the
dissolved oxygen, although other volatile solutes are also transferred. Although this technique has not
been widely used, it has been applied at navigation dams on the Ohio River (Price 1990). A general
method to evaluate the application of this technique is given
4.4-12
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