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4.22 CHAPTER FOUR

Surface Ice. In some locations, surface ice and ice floes create a structural hazard to
exposed intakes. On lakes, an accumulation of wind-driven ice floes near a shore intake
can produce a deep, nearly solid layer of ice capable of restricting or completely block-
ing intake ports. Under such conditions, reliable intake operation is virtually impossible,
and water supplies obtained from lakes and reservoirs subject to severe ice problems are
typically served by offshore intakes.
Ice jams can cause partial or complete blockage of river intakes. Jams below a river
intake can also cause extremely high river stages, and an upstream jam can produce low
water levels at the intake location, reducing its capacity.
Frazil Ice. Frazil ice is small, disk-shaped ice crystals. Frazil ice has been aptly called
"the invisible strangler." When conditions favor its formation, the rate of buildup on un-
derwater objects can be rapid; frazil ice accumulation can reduce an intake's capacity sub-
stantially and can, in some cases, clog it completely in only a few hours.
Ice starts forming when water temperature is reduced to 32 ° F (0 ° C) and water con-
tinues to lose heat to the atmosphere. For pure water, supercooling to temperatures well
below the freezing point is necessary to start ice formation, but with natural water, the re-
quired supercooling is much less. Two types of ice formation are recognized: static ice
and dynamic ice (Table 4.8).
Static ice forms in quiet water of lakes and river pools. Dynamic ice formation occurs
in turbulent water such as areas of high flow in rivers and in lakes mixed by wind action.
Frazil ice formed under dynamic conditions adversely affects hydraulic characteristics of
intakes.
When natural water loses heat to the atmosphere and a condition of turbulence exists,
uniform cooling of a large fraction of the water body occurs. If the initial water temper-
ature is slightly above the freezing point and cooling is rapid, a small amount of super-
cooling occurs, and small, disk-shaped frazil ice crystals form and are distributed through-
out the turbulent mass. These small crystals are the initial stage of ice production. Other
ice forms can develop from this initial ice production in sizable quantities.
Where there is little or no mixing, supercooled water and existing surface ice crystals
are not carried to a significant depth, and the result is the formation of a layer of surface
ice rather than a mass of frazil. Surface ice formation reduces heat loss from the water
and usually prevents formation of frazil ice.
Two kinds of frazil ice have been identified, as shown in Table 4.9: active and inac-
tive, or passive. Freshly formed frazil crystals dispersed in supercooled water and grow-
ing in size are in an active state. When in this condition, they will readily adhere to un-
derwater objects such as intake screens or rocks. Frazil ice production and adhesiveness
are associated with the degree of supercooling, which is related to the rate of cooling of

TABLE 4.8 Ice Formation Conditions
Type Remarks
Static ice Quiet waters
Small lakes and river pools
Dynamic ice Turbulent water
Rivers in area of great flow
Reservoirs with significant wind action
Frazil ice formation occurs first
Massive ice formations may follow