Environmental Engineering Reference
In-Depth Information
the 7300-m-high peak, Nevada Huascaran. Triggered by a thaw, 3 million tons of ice fell
and flowed down a narrow canyon picking up debris and spilling out onto the fertile val-
ley at an elevation 4000 m lower than the glacier and a distance of 15 km. The debris
remaining in the towns ranged from 10 to 20 m thick.
Event No. 2
: The catastrophe was almost duplicated on May 32, 1970, when the big
Peruvian earthquake caused another avalanche from Nevada Huascaran that buried
Yungay, adjacent to Ranrahirca, as well as Ranrahirca again, taking at least 18,000 lives
(Youd, 1978). During the 1962 event, Yungay had been spared. The average velocity of the
avalanche has been given as 320 km/h (200 mi/h) (Varnes, 1978), and the debris flowed
upstream along the Rio Santa for a distance of approximately 2.5 km. As with the 1962 fail-
ure, the avalanche originated when a portion of a glacier on the mountain peak broke
loose.
9.2.9
Debris Flows
General
Debris flows are similar to debris avalanches, except that the quantity of water in the
debris-flow mass causes it to flow as a slurry; in fact, differentiation between the two
forms can be difficult. The major causes are very heavy rains, high runoff, and loose sur-
face materials.
Recognition
Occurrence is similar to debris avalanches, but debris flows are more common in steep
gullies in arid climates during cloudbursts, and the failing mass can move far from its
source (see
Figure 9.4c)
.
9.2.10
Rock-Fragment Flows
General
A rock mass can suddenly break loose and move downslope at high velocities as a result
of the sudden failure of a weak bed or zone on the lower slopes causing loss of support to
the upper mass. Weakening can be from weathering, frost wedging, or excavation. Failure
is sudden, unpredictable, and can be disastrous.
Recognition
High, steep slopes in jointed rock masses offer the most susceptible conditions. The ava-
lanche illustrated in
Figure 9.47
could also be classified as a dry rock flow because of its
velocity and lack of water. In the initial stages tension cracks may develop; after the final
stage a scarred surface remains over a large area, and a mass of failed debris may extend
far from the toe of the slope.
Example of Major Failure
Event
The Turtle Mountain slide of the spring of 1903 destroyed part of the town of Frank,
Alberta, Canada. More than 30 million m
3
of rock debris moved downslope and out onto
the valley floor for a distance of over 1 km in less than 2 min.
