Iwasa and Matsuo (1981) discussed the problem of turbid inflows and the impacts to the
reservoir water quality. When the reservoir lacks thermal stratification, turbid inflows mix the entire
reservoir and create a linear turbidity profile from surface to the bottom. When thermal stratification is
present, the turbid inflow remains near the thermocline and does not easily settle into the hypolimnion.
The authors indicated that mathematical modeling techniques could be used to control reservoir
turbidity by withdrawing the turbid layer.
In some reservoir systems, inflows occurring in the absence of thermal stratification may be
routed through the reservoir without mixing the entire reservoir. Although published accounts of this
phenomenon are unavailable, this has been reported through personal communications.1
Murota and Michioku (1986) described a selective withdrawal technique for reservoirs that have a
three-layer stratification. Their discussion also indicated the usefulness of removing layers in the
reservoir that are highly turbid and may contribute to sedimentation in the reservoir.
Inflows to reservoirs can create water quality problems, if they contain significant
concentrations of suspended solids, turbidity, or other undesirable constituents. By quickly routing
these undesirable constituents through the reservoir, the impacts can be minimized.
A simple concept is presented to evaluate the effectiveness of routing of inflows for water
quality purposes. Although few references are available for this technique, the approach may be useful
for some projects. A summary of information concerning this technique is presented in Table 4.4.2.
4.4.3 SUPPLEMENTAL RELEASES FOR WATER QUALITY
Many multipurpose projects incorporate several types of release structures, designed for flood
control, hydropower, water quality control, and navigation. In most cases, water control plans do not
call for simultaneous operation of the various release structures during the stratification season. For
example, flood control and water quality releases will not generally be made at the same time. It may
be possible to simultaneously operate two or more release structures and thereby create a selective
withdrawal configuration, provided the intake center-line elevations for the various structures are not the
Thus, selective withdrawal techniques may be used to enhance reservoir release water quality
by reducing the stability of stratification through hypolimnetic withdrawal, enhancing mixing conditions in
Pete Juhle, Headquarters, US Army Corps of Engineers, Washington, DC.