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 4.4.2 INFLOW ROUTING
4.4.2.1 Description
Inflows to reservoirs can create problems with reservoir water quality. If the inflow quality is
poor, containing high concentrations of nutrients, suspended solids, or other undesirable constituents,
poor reservoir water quality may result. Depending on the volume of the inflow and the retention time
of the reservoir, the inflow constituents may settle and be trapped in the reservoir. This could
contribute to eutrophication processes as well as filling of the reservoir with sediment. If inflow quality
is identified as a concern, it may be possible to route the inflow through the reservoir for release
downstream, without significantly impacting the quality of the reservoir, or hold the inflow in the project
for treatment while releasing unaffected water downstream.
Routing of inflows is a technique based on the water control operation or plan for the reservoir.
Undesirable inflows are identified and routed through the reservoir to minimize impacts to the existing
reservoir water quality, similar to routing of flood flows through a basin. Upon identification of an
undesirable inflow, the route the inflow will take in the reservoir as well as the volume of the inflow must
be predicted. This can be accomplished through a combination of upstream flow gauging and density
flow calculations similar to the techniques derived by Akiyama and Stefan (1985).
Since the inflow will seek its layer of neutral density in a density stratified reservoir, a density
current will develop and proceed through the reservoir on the surface or at some elevation, depending
on the stratification. Using the existing release structure and operation within the existing water control
plan, the project is operated to move the undesirable water through the pool as quickly as possible
(Figure 4.4.2).
Selective withdrawal capability greatly enhances this technique if the inflow occurs at some
intermediate depth. However, depending on the equilibrium elevation of the inflow, it may be possible
to use other outlet works' (i.e., spillways or sluiceway) to release the inflow. Because no structural
modification or addition is involved in this technique, costs are associated only with evaluation and
change of operation.
4.4.2.2 Design Criteria
The methodology for this technique involves definition of the goals to be achieved relative to the
reservoir water quality. This will usually involve determination of the reservoir water quality that exists
prior to some inflow event, predicted quality of the inflow, and its impacts on the reservoir. A general
procedure for routing of undesirable inflows is given below.
The specific water quality of the reservoir and inflow as well as the resulting reservoir quality
must be determined or predicted. This will usually involve inflow temperature, turbidity,
4.4-5
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