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4.23
INTAKE FACILITIES
TABLE 4.9 Frazil Ice Characteristics

Type Remarks
Initial phase of formation
Active
Rapid growth in size
Readily adheres to intake facilities
Short-lived phase
System clogging within a few hours
Inactive (passive) Static or declining size
Lost adherence and characteristics
Less troublesome

the water mass. Frazil ice particles remain in an active, adhesive state for only a short
time after their formation. With the reduction of supercooling and the return of the water
to 32 ° F (0 ° C), frazil ice crystals stop growing and change to an inactive, or passive,
state. Passive frazil ice is ice that has lost its adhesive properties and is therefore less
troublesome.
Some confusion exists concerning the relationship between frazil ice and anchor ice.
It has been suggested that anchor ice occurs rarely and consists of sheetlike crystals that
adhere to and grow on submerged objects. Accumulations of frazil ice may closely re-
semble anchor ice. Some investigators designate all ice attached to the bottom as anchor
ice regardless of how it is formed. Anchor ice may form in place on the bottom and grow
by the attachment of frazil crystals. On the other hand, according to Giffen (1973), ice
crystallization and growth directly on the surface of a shallow intake structure in open
water is commonly termed anchor ice. Anchor ice normally does not form at depths greater
than 40 to 45 ft (12.2 to 13.7 m), although the depth associated with anchor ice forma-
tion ultimately depends on water turbidity.
Predicting Frazil Ice Formation. The climatological conditions that encourage frazil
ice formation are a clear night sky, an air temperature of 9.4 ° F (-12.5 ° C) or less, a
day water temperature of 32.4 ° F (0.222 ° C) or less, a cooling rate greater than 0.01 ° F
(0.0056 ° C) per hour, and a wind speed greater than 10 mi/h (16.1 km/h) at the water sur-
face. Frazil ice generally accumulates in the late evening or early morning hours and sel-
dom lasts past noon. Conditions favorable to frazil ice formation vary considerably from
site to site, making it difficult to use weather data alone as a forecasting toot.
Frazil ice formation can be illustrated graphically by plotting water temperature vari-
ation versus time, as shown in Figure 4.15. When the original constant cooling rate (A
to B) of water undergoing supercooling deviates, frazil ice begins to form. The process
illustrated in Figure 4.15 takes only a few minutes. The rate of ice production at point C
is equal to the cooling rate divided by the product of the ice density.

Design Features. Research and experience on the Great Lakes and elsewhere indicate
that location and design features of submerged intakes can significantly reduce intake ice
problems, but probably not completely eliminate them. Submerging lake intakes in deep
water and sizing inlet ports for a velocity of 0.3 ft/s (9.1 cm/s) or less minimize the amount
of frazil ice transported downward to the structure. However, during winter storms, strong
wind and wave action can carry ice crystals and supercooled water to considerable depths,
making accumulation of ice on and around the intake likely. Some of the procedures to
alleviate frazil ice problems at submerged intakes are summarized in Table 4.10.