Page 74 -

INTAKE FACILITIES 4.25

peratures slightly above the freezing point. For larger installations in cold climates, a pro-
hibitive amount of energy would probably be required for heating intake screens.
Control Methods. An indication of icing problems at an intake is abnormal drawdown
of the intake well. If the intake has a screen, excessive drawdown can rupture the screen.
Methods most commonly used to control frazil ice at an intake structure include in-
jecting steam or compressed air at the intake opening, backflushing the intake with set-
tled water, and manual or mechanical raking to remove the ice. If backflushing is to be
routinely practiced, provisions must be included when the intake is designed. It is some-
times possible to clear partially clogged intake ports by a method termed control draw-
down, which involves throttling the intake well pumps and maintaining reduced intake
flow. Under some conditions, this flow may be sufficient to erode ice bridges at the ports
and restore intake capacity.
If the plant has an alternate intake or sufficient storage capacity to operate without
source water for a few hours, the simplest solution is to shut down the intake and wait
for the ice to float off.

Zebra Mussel Considerations
The zebra mussel (Dreissena polymorpha) is a small bivalve mollusk native to Europe
that has alternating light and dark stripes on its shell. Zebra mussels average about 1 in.
in length and may typically live for 4 to 8 years. The mussels are most frequently found
at water depths between 6 and 12 ft (1.8 and 3.7 m). However, they have been found at
depths up to 360 ft (109.7 m) in the Great Lakes (Lake Ontario). The mussel grows illa-
mentlike threads (byssus filaments) from the flat side of its shell that allow it to attach to
hard surfaces, such as the bottoms of ships and the components of water facility intakes.
Zebra mussels have some natural predators, including some diving ducks and bivalve-
eating fish such as the common carp and the freshwater drum. However, these predators
appear to be of little help in controlling the zebra mussel population. The zebra mussel is
well established in the Great Lakes region of the United States and Canada and prolifer-
ates under a relatively wide range of water conditions, as illustrated in Table 4.11. Pro-
jections indicate that they will eventually infest all freshwaters in two-thirds of the United
States and the majority of southern Canada. The magnitude of the zebra mussel threat can
be illustrated by the rate at which the mollusk can reproduce. A single female can pro-
duce between 30,000 and 40,000 eggs per year. Although biologists estimate that only

TABLE 4.11 Zebra Mussel Requirements for
Reproduction

Water characteristics Remarks
Water pH Basic, 7.4 minimum
Calcium content of water 28 mg/L minimum
Water temperature
Maximum 90 ° F (32.2 ° C)
Minimum 32 ° F (0 ° C)
Velocity of flow 5.0--6.5 ft/s (1.5-2.0 m/s)
Location Prefer a dark location