Geology Reference
In-Depth Information
A specifi c cause of thermokarst activity relates to the movement of vehicles over
permafrost terrain. If this occurs in summer, when the thawed surface (active layer) is
soft and wet, vegetation can be destroyed and trenching and rutting can result. Some of
the worst examples of this sort of activity occurred on the Alaskan North Slope in the
old US Naval Petroleum Reserve No. 4 during the late 1940s and early 1950s, when
uncontrolled movement of tracked vehicles in summer led to widespread disruption of
the surface organic layer. Subsequent thaw subsidence and thermal erosion along vehicle
tracks formed trenches as much as 1m deep and between 3m and 5m wide (Figure
8.15A). In Canada, a similar error was made when a summer seismic exploration
program was authorized in the Mackenzie Delta in the late 1960s. Approximately
300km of seismic line were bulldozed and long strips of vegetation and soil, approxi-
mately 4.2 m wide and 0.25 m thick, were removed. These lines were subsequently trans-
formed into prominent trenches and canals over much of their length (Kerfoot, 1974).
Another example of vehicle track disturbance occurred in 1970 on the Sabine Peninsula
of Eastern Melville Island (Figure 8.15B). At that time a blow-out occurred at a wildcat
well and vehicles were moved, of necessity, across tundra in summer. Sensitive lowlands
underlain by soft ice-rich shale were crossed, and substantial and dramatic trenching
occurred.
8.8.2. Case Studies
Two case histories from Banks Island, Arctic Canada, illustrate the nature and progression
of human-induced thermokarst and provide insight into the time constraints involved.
They suggest that terrain stabilization only begins 10-15 years after initial disturbance
and probably is not complete until 30-50 years have passed. In both cases, disturbance
was associated with the removal of surface material for construction purposes.
In the fi rst example, disturbed terrain was monitored over a four-year period following
initiation disturbance in August 1973 at an exploratory well site on Banks Island (French,
1978). Disturbance was caused during site restoration when surface material was scraped
from an adjacent gravel ridge and used to infi ll a waste-disposal pit. A total of approxi-
mately 3000 m
2
was disturbed. Two years later, a crude polygonal system of gullies had
developed at the site, refl ecting erosion along underlying ice wedges. After two further
years, the area consisted of unstable and irregular hummocks interspersed with standing
water bodies.
The second example is the human-induced thermokarst terrain which formed adjacent
to the airstrip, constructed during the summers of 1959-1962, at Sachs Harbour (French,
1975a). In order to grade the proposed strip, thawed material was removed each summer
from adjacent terrain and transported, via access ramps, to the site. In all, a total of
50 000 m
2
was disturbed and as much as 2.0 m of material removed in places. When fi rst
examined in the summer of 1972, the borrow pits portrayed actively subsiding thermokarst
mound topography (see Figure 8.14A). By 1998, the terrain showed signs of stabilization,
with vegetation beginning to re-establish.
Efforts to reduce terrain damage due to human-induced thermokarst activity focus
upon regulatory procedures and geotechnical methods aimed at minimizing surface dis-
turbance and preserving the thermal equilibrium of the permafrost. Canada was one of
the fi rst countries to implement such procedures through the Territorial Arctic Land Use
Act and Regulations of the Federal Government in the early 1970s. Procedures include
the introduction of vehicles with low-pressure tires, the restriction of heavy equipment
movement to winter months, the implementation of terrain and biophysical mapping
