EP 1110-2-11
30 Nov 94
Chapter 3
Background
3-1. Types of Ice
a. Ice forms in freshwater bodies whenever the surface water cools to 0 oC (32 oF) or a fraction of a degree
lower. There are many types of ice, depending on the precise mode of formation and evolution (Ashton 1986).
b. Sheet ice forms in calm water, such as lakes or reservoirs, or in slow-moving river reaches where the flow
velocity is less than 0.5 m/s (1.5 ft/s). Ice crystals formed at the water surface freeze together into skim ice that grad-
ually thickens downward as heat is transferred from the water to the air through the ice layer. Sheet ice usually origi-
nates first along the banks and expands toward the center of the waterbody. In slow rivers, the sheet ice cover may also
be created by the juxtaposition of incoming frazil pans generated in upstream faster reaches. Sheet ice that grows stati-
cally in place is often called black ice because of its appearance. An ice cover may also thicken at the top surface when
water-soaked snow freezes to form snow ice that has a milky white appearance.
c. Frazil ice (Figure 3-1) consists of small particles of ice formed in highly turbulent, supercooled water, such
as river rapids or riffles, during cold, clear winter nights when the heat loss from the water to the atmosphere is very
high. As the frazil particles are transported downstream, they join together to form flocs that eventually rise to the sur-
face where they form frazil pans or floes. Frazil is often described as slush ice because of its appearance.
d. Fragmented ice is made up of ice pieces that originated as consolidated frazil ice pans or from the breakup of
sheet ice growing at the surface of slow-moving water.
e. Brash ice is an accumulation of ice pieces less than 1.5 to 2 m (5 to 6 ft) in maximum dimension resulting
from the breakup of an ice cover by increasing water flow or by vessel passage. It is of particular concern in navigation
channels and lock approaches.
3-2. Types of Ice Jams
a. An ice jam is a stationary accumulation of ice that restricts flow. Ice jams can cause considerable increases
in upstream water levels, while at the same time downstream water levels may drop, exposing water intakes for power
plants or municipal water supplies. Types of ice jams include freezeup jams, made primarily of frazil ice; breakup jams,
made primarily of fragmented ice pieces; and combinations of both.
b. Freezeup jams. Freezeup jams are composed primarily of frazil ice, with some fragmented ice included, and
occur during early winter to midwinter. The floating frazil may slow or stop due to a change in water slope from steep
to mild because it reaches an obstruction to movement such as a sheet ice cover, or because some other hydraulic occur-
rence slows the movement of the frazil (Figure 3-2). Jams are formed when floating frazil ice stops moving downstream,
forms an "arch" across the river channel, and begins to accumulate. Freezeup jams are characterized by low air and
water temperatures, fairly steady water and ice discharges, and a consolidated top layer.
c. Breakup jams. Breakup jams occur during periods of thaw, generally in late winter and early spring, and are
composed primarily of fragmented ice formed by the breakup of an ice cover or freezeup jam (Figure 3-3). The ice
cover breakup is usually associated with a rapid increase in runoff and corresponding river discharge due to a significant
rainfall event or snowmelt. Late season breakup is often accelerated by increased air temperatures and solar radiation.
d. The broken, fragmented ice pieces move downstream until they encounter a strong, intact downstream ice
cover or other surface obstruction to flow, or other adverse hydraulic conditions such as a significant reduction in water
surface slope. Once they reach such a jam initiation point, the fragmented ice pieces stop moving, begin to accumulate,
and form a jam (Figure 3-4). The ultimate size of the jam (i.e., its length and thickness) and the severity of the resulting
3-1
Previous Page
