Regulated Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 51

• Deine the present conditions, including point and nonpoint pollution sources, expected

lows and levels, and concentrations of pollutants.

• Deine the total maximum daily loads (TMDLs) of pollutants and compare them with

existing conditions.

• Develop corrective action implementation plans for impaired uses, excess loadings,

and the protection of species or habitats.

A typical reservoir and reservoir storage zones are illustrated in Figure 3.4. Typically, there is
a dead zone that cannot be managed, and the surcharge zone is to accommodate extreme events
to prevent failure. The lood control and conservation pools are the managed pools. Conservation
purposes include municipal, industrial, and agricultural water supplies, hydroelectric power, rec-
reational uses, and instream low maintenance. A reservoir may be operated only for conservation
purposes or only for lood control or to designate a certain reservoir volume, or pool, for conser-
vation purposes and a separate pool for lood control. In many cases, however, the construction
of a reservoir cannot be justiied by a single project purpose, and many are operated for multiple
purposes. The conservation and lood control pools in a multipurpose reservoir are ixed by a desig-
nated top of conservation (bottom of lood control) pool elevation (Tibbets et al. 1985).
A reservoir's storage capacity and operating policies are generally established prior to its
construction and tend to remain constant thereafter. For federal projects that are authorized by
Congress, changing the operation usually requires an additional act by Congress. The operating
procedures are usually implemented based on a rule or guide curve derived from historical data
on tributary inlows and water demands. The rule curve shows the minimum water level elevation
(and consequently storage) requirement in the reservoir over the year at a speciic time to meet the
particular needs for which the reservoir is designed and it controls the reservoir management. The
goal of the management is typically to follow the rule curve, with exceptions allowed only under
extreme conditions, such as during periods of extreme drought. The rule curve is usually bounded
by an upper rule curve for handling surcharge and a lower rule curve based on the minimum level
at which the design uses are satisied (Figure 3.5).
Reservoir volumes typically increase in the spring and late fall or winter due to increased runoff
during those periods. If a reservoir is operated to supply water, then the rule curve can be similar to that
shown in Figure 3.6 where the goal would be to maintain as much water in the reservoir as possible.
Similarly, for a peaking hydropower facility, the goal would be to have the maximum elevation possible,
particularly during those periods of the year with the highest demand. For a lood control reservoir, as
illustrated in Figure 3.7, the goal may be to have the reservoir as empty as possible during periods of
high lows, to accommodate those lows and attenuate the downstream peak low. For a facility operated
for navigation purposes, the goal may be to maintain a constant water-surface elevation (Figure 3.8).
The designated uses of reservoirs are often in conlict. For example, the ideal rule curves are in
conlict for reservoirs operated for both lood control and water supply, so the resulting rule curve

Freeboard

Spillway
crest

Surcharge

Flood control

Conservation

Sedimentation allowance/reserve

Inactive/dead

FIGURE 3.4 Typical reservoir zones.