Geoscience Reference
In-Depth Information
mainly in a clockwise direction with the currents in the
Beaufort Sea. These ice rafts have calved from five
small iceshelves on Ellesmere Island. Because they
have little chance of escaping south to warmer waters,
individual blocks can persist for 50 years or more.
In the southern hemisphere, icebergs pose little
threat to navigation because they do not frequently
enter major shipping lanes. However, it is not unknown
for shipping between South America and Australia,
and between Australia and South Africa, to be diverted
north because of numerous sightings of icebergs
around 40-50°S. North Atlantic icebergs do enter the
major shipping lanes between North America and
Europe, especially off the Grand Banks, east of the
Newfoundland coast. In severe years, between 30 and
50 large icebergs may drift into this region. Histori-
cally, they have resulted in an appallingly high death
toll. Between 1882 and 1890, icebergs around the
Grand Banks sank 14 passenger liners and damaged 40
others. The iceberg hazard climaxed on 14 April 1912
- a year of a strong El Niño - when the 'indestructible',
46 000 tonne
Titanic
scraped an iceberg five times its
size. The encounter sheared the rivets off plates below
the waterline over two-fifths the length of the ship.
So apparently innocuous was the scrape that most
passengers and crew members did not realize what had
happened. Within three hours, the
Titanic
sank dra-
matically, bow first. Only a third of the passengers
survived in lifeboats, while over 1500 people died as
the ship sank. Immediately, the United States Navy
began patrols for icebergs in the area. In the following
year, 13 nations met in London to establish and fund
the International Ice Patrol, administered by the
United States Coast Guard. The Coast Guard issues
advisories twice daily and all ships in the north Atlantic
are obliged to notify authorities of the location of any
icebergs sighted. Since the Second World War, regular
air patrols have also been flown. With the advent of
satellites, large icebergs can be spotted on satellite
images, while the more threatening ones can be
monitored using radio transmitters implanted on their
surface. Individual ships can also spot icebergs using
very sophisticated radar systems.
While the iceberg hazard to shipping has all but
disappeared, the hazard to oil exploration and drilling
facilities has increased in recent years, especially in the
Beaufort Sea and along the Labrador coastline.
Because over 80 per cent of their mass lies below sea
level, large icebergs can gouge the seabed to a depth of
0.5-1.0 m, forming long, linear grooves on the conti-
nental shelf. The icebergs could therefore gouge open
a pipeline carrying oil or gas across the seabed. Drifting
icebergs also have enough mass and momentum to
crumple an oil rig. Not only could there be substantial
financial loss with the possibility of deaths, but the
resulting oil spill could also pose a significant ecological
disaster. Attempts have been made to divert icebergs
by towing them away using tugboats; however, the large
mass of icebergs gives them enormous inertia, and the
fact that they are mostly submerged means that they
are easily moved by any ocean current. At present,
waves pose the most serious threat to oil and gas
activities in ice-prone waters, but it is only a matter of
time before an iceberg collides with a drilling-rig or
platform. Present disaster mitigation procedures for
this hazard consist simply of evacuating personnel from
threatened facilities. Such procedures have been used
on several occasions over the last few years.
Ice a
t shore
(Taylor, 1977; Kovacs & Sodhi, 1978; Bruun, 1985)
Nowhere is the destructive effect of ice displayed more
than at the shoreline. During winter, the icefoot, which
is frozen to shore at the waterline, protects the beach
from storm wave attack. Pack-ice, which develops in
the open ocean or lake, also attenuates the fetch length
for wave development. The middle of winter sees the
whole shoreline protected by ice. In spring, the pack-
ice melts and breaks up, whereupon it may drift
offshore under the effect of wind. At this time of year,
shorelines are most vulnerable to ice damage if the
wind veers onshore and drives the pack-ice shoreward.
At shore, pack-ice has little problem overriding the
coastline, especially where the latter is flat. This
stranding ice can pile up to heights of 15 m at shore,
overriding wharfs, harbor buildings, and communica-
tion links. Ice can actively drive inland distances up to
250 m, moving sediment as big as boulders 3-4 m in
diameter. On low-lying islands in the Canadian Arctic,
shallow pack-ice, 1-2 m thick, can push over the shore-
line along tens of kilometers of coastline (Figure 8.8).
Large ridges of gravel or boulders - between 0.5 m and
7 m in height, 1-30 m in width and 15-200 m in length
- can be plowed up at the shoreline. The grounding of
ice and subsequent stacking does not require abnor-
mally strong winds. Wind velocities measured during
ice-push events have rarely exceeded 15 m s
-1
(54 km hr
-1
). Nor is time important: ice-push events
