Geoscience Reference
In-Depth Information
Observation of the summit area of Nevados Huascarán suggests that there is still consider-
able potential for further destructive avalanches. Slopes in the source area remain oversteepened
and show signs of recent cracks parallel to the avalanche scar. Therefore the potential for further
disasters remains. Although the settlements are nearly 23 km from the source of the debris flows,
given the steepness of the mountain slopes, travel times are less than four minutes (Plafker
& Eriksen 1978). Even if a warning could be given evacuation to safe ground in such a short
time period is not a viable option. Therefore a more viable option would be the relocation of
settlements in the Santa valley to positions outside the maximum extent of the Pre-Columbian
avalanche. This, however, would require a major initiative in development planning.
Relevant reading
Plafker, G. & Eriksen, G.E. (1978) Nevados Huascaran avalanches, Peru. In
Rockslides and Avalanches
,
Vol. 1.
Natural Disasters
(Ed. B. Voight), pp. 48-55. Elsevier, Amsterdam.
Smith, K. (2001)
Environmental Hazards - Assessing Risk and Reducing Disaster
, 3rd edn. Routledge, London.
Whittow, J. (1980) Landslides and avalanches - avalanches. In
Disasters: the Anatomy of Environmental
Hazards
, pp. 163-70. Penguin, Harmondsworth.
went. Because of the high speed of this flow and
the mechanical fluidization through jostling of
the clasts, flow was very plastic (high-viscosity-
low-yield stress). High viscosity prevented turbu-
lence and the flow was almost laminar. Because
little energy was used up in internal deformation
most energy was lost to friction at the base,
thereby eroding the substrate.
Another example is the 1980 Mount St Helens
debris avalanche in Washington. This was caused
by the eruption of 18 May (Case Study 2.2). It
began catastrophically with a large lateral blast
Case study 2.2 Impact of an extreme tectonic/volcanic event on a mountain sediment system -
Mount St Helens eruption 1980
The 18 May 1980 eruption of Mount St Helens in south-west Washington was a major geo-
logical disaster claiming 57 lives and affecting several hundred thousand people. A magnitude
5
earthquake triggered a lateral blast, which sent a huge mass failure down the northern flank
of the mountain eventually exploding in a cloud of ash, rock and hot gas, sending ash up to
18 km vertically into the atmosphere. The avalanche of rock, ice and mud released in the earth-
quake surged 28 km down the North Fork Toutle River valley (Case Fig. 2.2a & b). A second
part of the slide flowed into South Coldwater Canyon and Spirit Lake raising the lake level by
70 m and damming the outlet with debris over 100 m deep. Downstream mudflows choked the
Cowlitz and Toutle rivers bringing shipping to a halt on the Columbia River (Case Fig. 2.2a).
The immediate engineering response to the Mount St Helens disaster was undertaken by the
US Army Corps of Engineers, who were responsible for navigation and flood control on the
Columbia River. The worst affected area was the Cowlitz River on highway I5 where efforts
were concentrated on raising river levees and roads and clearing channel constrictions (Case
Fig. 2.2a). Overnight the navigation channel on the Columbia River was reduced in depth
from 12 to only 4.2 m. Using large dredges by 23 May the channel was partially cleared and by
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