Land Instability as a Hazard - Natural Hazards - page 259

in a particular zone, can cause the clay minerals to
realign themselves, reducing shear strength in this layer,
which subsequently becomes the location for develop-
ment of a shear plane. The cohesiveness of clay
decreases with increasing moisture content; so, contin-
ual creep in clays has its highest rate where the clay is
moist for the longest duration. Creep processes are not
restricted solely to clays or to locations with expansive
soils. Because creep operates under the effect of gravity,
any disturbance to a soil on a slope will result in downhill
movement of material. Animal burrowing, penetration
by plant roots, tree collapse, and simple trampling by
animals or people can be the cause of this disturbance.
While the short-term disturbance seems minor, aggre-
gated over time these effects can have significant
influence on near-surface creep rates.
The process of solifluction involves additional
factors. Solifluction is restricted to environments where
ground freezing takes place. The expansive force
initiating creep is due to moisture freezing in the soil,
because frozen water occupies a greater volume than
the equivalent weight in liquid form. Solifluction can
produce accelerated rates of downslope movement
of material on slopes as gentle as 1°. Freezing in soil
can also concentrate water at a freezing front. Large
volumes of water can be drawn to this front by capillary
action, forming ice lenses that can constitute up to
80 per cent by volume of the soil mass. The melting of
all or some of this ice can increase pore-water pressures
and form shear planes for slope failure. In areas under-
lain by permafrost, where seasonal melting prevails in
an active surface zone, solifluction can totally dominate
a landscape such that material on all slopes appears to
be in a continual state of rapid movement downslope.
The discussion of solifluction and its geomorphic
ramifications are beyond the scope of this text; but it
should be realized that ground ice and solifluction
processes represent a serious hazard to construction
and transport in Arctic regions of Canada, Alaska, and
Russia. Most mountain regions are also cool enough,
because of their high elevation, to be affected by
solifluction. About 20 per cent of the world's landmass
is susceptible to these deleterious agents.

controlling factor in its behavior. Flows can be sub-
divided into two categories, based upon the type of
material. If clays dominate, the flow is termed a
'mudflow' or 'earth flow' in the United States and
a 'mudslide' in the United Kingdom. Note that this
term incorporates lahars mentioned in the previous
chapter. The term 'lahar' is more restrictive in that it is
a mudflow originating in volcanic ash. If the range in
particle size is highly variable, then the flow is termed
a debris flow . Generally, the material in a flow is water-
saturated and unconsolidated. Not only can water
permeate through the material, but it can also be
absorbed by the sediment. For this reason, most flows
represent the further downslope movement of material
that has already undergone some sort of failure. For
instance, a rockfall that disintegrates into smaller
pieces while falling will produce an unconsolidated
debris mantle when it comes to rest. Over time, this
debris can chemically weather, breaking down into
smaller particles. When wetted, this material may
absorb water, increase in weight, and begin to fail again
at lower slope angles.
Flows occur where high water content increases
pore-water pressure in the deposit, a process that
decreases shearing strength. Flows usually begin
moving along a basal shear plane. Rates of movement
can be as low as 1-2 m yr -1 , or as high as 600 m or more
per year. Fastest movement occurs during the wettest
months. Mudflows do not necessarily move at con-
sistent rates; they commonly surge. In southern
California, surge rates of 3-4 m s -1 have been observed
in large mudflows. Typically, even with lahars,
velocities average less than 20 km hr -1 . Slow-moving
earth flows have caused property damage, but minimal
loss of life, in Czechoslovakia, England, Switzerland,
and Colorado. In periglacial environments dominated
by solifluction, mudflows are a common occurrence.
Coarse debris flows are much more difficult to
move. Fine material must be present to aid water
retention. The debris flow may consist of very low
volumes of sediment moving downslope at rates of
several kilometers per hour. Coarser debris tends to
get extruded to the surface and to the sides during
movement, such that coarse-sized levee banks are built
to the sides and front of the moving deposit. Typically,
debris flows require slopes of 30-40° for movement
to start, but flow continues over slopes of 12-20°
(Figure 12.10). Note that this type of flow does not
include large, catastrophic events moving coarse debris

Mud and debris flows

(Sharpe, 1968; Bolt et al., 1975; Cornell, 1976; Whittow,
1980; Innes, 1983; Wieczorek et al., 2001)

A flow is any slope failure where water constitutes
a major component of the material and a major