Geoscience Reference
In-Depth Information
A major part of the problem is assessing the
representativeness of the available data on rates
of natural and human-induced erosion in space
and time, especially the relative and absolute
importance of rare high-magnitude events com-
pared with continuous slow mass movements
(Ives & Messerli 1989). In general there is no sub-
stantive evidence that human activities such as
deforestation, subsistence farming, irrigation and
road construction have had a major impact on
soil erosion on the large scale. Locally such pro-
cesses may be a problem but at the large scale
the overwhelming importance of geophysical and
climatic controls on the delivery of sediment from
the rapidly uplifting and eroding mountains to
the Ganges-Brahmaputra plains is paramount
(Ives & Messerli 1989). Incorrect diagnosis of
the cause of the large-scale problem could result
in costly but unproductive changes in land man-
agement, the longer term effects of which would
be hard to predict.
2.4.2 Road construction in mountain
environments - the Karakoram Highway
Increased access to mountain areas usually
involves the construction of roads and highways
for the passage of motor vehicles. This brings
with it a suite of special engineering problems
(Fookes et al. 1985). Gerrard (1990) outlines
five stages in mountain road construction which
usually involve: feasibility and project planning;
reconnaissance of road corridors; site invest-
igation of the road alignment; construction; and
post-construction maintenance and observation.
Routing a road through a steep, unstable moun-
tain environment presents major challenges to
the highway engineer at each of these stages. For
example, if a road is planned across a talus slope
then the alignment should be across the lower
slope because these areas have the lower slope
angles and larger sediment sizes, which provide
greater strength during excavation. Further-
more rock-fall hazard reduces towards the toe
of the slope. Alternatively if a road has to cross a
mudflow zone the road should be as high on the
slope as possible where mudflows are thinnest
(Gerrard 1990).
Fig. 2.16 The Karakoram highway in the Hunza Valley
Pakistan. Frequent road maintenance is required to keep the road
open and free of debris. (Photograph C. Warburton.)
Perhaps the best known and most widely
studied mountain highway is the Karakoram
Highway in the Hunza valley, Pakistan (Fig. 2.16).
The geographical setting makes road construc-
tion and maintenance a continuing problem
owing to:
1 an overdeepened glaciated valley with very
steep unconsolidated valley slopes;
2 an unstable valley floor with very varied terrain
consisting of moraine ridges, outwash fans, gorge
sections and sedimentation zones of till, out-
wash gravel and debris flow deposits;
3 a highly variable hydrological regime with
runoff in excess of 900 mm yr −1
(peak flow
>
2000 m 3 s −1 ) and flood history caused by
outburst floods from adjacent glaciers or dam-
break floods from landslide and debris-flow
valley dams;
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