will freeze the saturated snow from the top downward,
times collide, override one another, and freeze together
creating a layer of "snow ice," which will eventually
to form ice that is immediately doubled in thickness.
bond to the sea ice if low temperatures persist. Nelson
This process is known as "rafting." Another type of
(1995) reported that snow ice comprises a large amount
consolidation and thickening, known as "fingering,"
of the ice cover in Cook Inlet.
occurs when two pans merge and interlock such that
In the relatively shallow and highly turbulent wa-
the edges of one floe alternately slide over and under
ters of Cook Inlet, "frazil" ice is also a major compo-
the adjacent floe.
nent of the new ice that forms. Frazil is formed from
Collision and stress between stronger and thicker
tiny ice plates or needles that rapidly accumulate in
ice sheets result in more extreme edge deformation,
turbulent water when colder water and air near the
consolidation, and piling up of ice, known as "pres-
surface mix with warmer, deeper water. These ice crys-
sure ridging." Depending on the magnitude and di-
tals flocculate into low-density masses that float to
rection of the stresses, floating ice sheets (floes) can
the surface to form a soupy layer of unconsolidated
converge on other floes, fast ice, the shore, or the sea
ice, giving a greasy appearance to the water surface.
bottom and create large pressure ridges. In Cook In-
The next stage in the development of sea ice is
let, pressure ridges have been reported to be as thick
"shuga"--ice crystals that continue to coalesce into
as 6 m (Blenkarn 1970); no other quantitative infor-
floating pans and soon acquire a white, slushy appear-
mation concerning ice thickening, roughness, or floe
ance. Wind, wave, and current motion cause these pans
size and spacing is available.
to collide repeatedly, deforming and thickening their
Ice propelled by the tides presents the greatest dan-
outer edges to create what is known as "pancake ice."
ger to navigation and marine structures in Cook Inlet.
With further hardening, thickening, and consolidation,
Table 1 lists incidents that occurred between 1960 and
ice pans increase in size, sometimes forming larger
1986 from U.S. Coast Guard records of damage caused
floes or sheets of ice. Blenkarn (1970) reported that
by floating ice.
ice floes in the Inlet can be classified as "big" (greater
More recently, problems caused by floating ice have
than 500 m across [Appendix A]), with 400-m-wide
been reported in the news. In February 1990, cargo
floes being common. These floes and sheets some-
loading of three oil tankers at Drift River Terminal
Table 1. Vessel and structure damage in Cook Inlet caused by floating ice. (After USCG 1991.)
Date
Location
Damage
Cause of Accident
11/29/64
Port of Anchorage
Pilings torn from petroleum dock,
Winter ice and tides
approx. ,000 damage
12/12/64
Off port of Anchorage
Icebreaker Milton II caught in ice
Winter ice
3/29/67
Anchorage City Dock
Dock extension torn from pilings
Winter ice
and demolished,
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.9 million damage
5/2/67
Knik Arm Shoal
Tanker Evje IV ruptured cargo of
Winter ice ripped
oil tanks after striking shoal
out marker buoy
12/24/80
Port of Anchorage
540-ft SS Philadelphia struck under-
Ice pan pushed ship
water object, 0,000 damage
into obstacle
2/24/82
Fire Island Shoal
497-ft SS Newark grounded
Heavy ice and tide
1/10/83
Port of Anchorage
523-ft SS Galveston came loose
Ice floe and tide
from dock when ice severed mooring
lines, slight damage
3/5/86
Anchorage City Dock
540-ft SS Philadelphia collided
Large ice pan forced
with dock, ,000 damage to 7
ship into dock
fender pilings
11
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