Mass Movements and Their Human Impacts
Mass
Movement is defined as the down slope movement of rock and regolith near the
Earth's surface mainly due to the force of gravity. Mass movements are an important part of the
erosional process, as it moves material from higher elevations to lower elevations
where transporting agents like streams and glaciers can then pick up the
material and move it to even lower elevations.
Mass movement processes are occurring continuously on all slopes; some
act very slowly, others occur very suddenly, often with disastrous
results. Any perceptible down slope
movement of rock or regolith is often referred to in general terms as a
landslide. Landslides, however, can be
classified in a much more detailed way that reflects the mechanisms responsible
for the movement and the velocity at which the movement occurs.
As human
populations expand and occupy more and more of the land surface, mass movement
processes become more likely to affect humans.
The table below shows the impact of mass movement processes on human life
over the last century.
In a typical
year in the United States, landslides cause over $2 billion in damages and 25
to 50 deaths. In other countries,
especially less developed countries, the loss is usually higher because of
higher population densities, lack of zoning laws, lack of information about
mass movement hazards, and lack of emergency preparedness. Between 1969 and 1993, worldwide, landslides
caused an average of about 1550 deaths per year.
Knowledge
about the relationships between local geology and mass movement processes can
lead to better planning that can reduce vulnerability to such hazards. Thus, we
will look at the various types of mass movement processes, their underlying
causes, factors that affect slope stability, and what humans can do to reduce
vulnerability and risk due to mass movement hazards.
Types of
Mass Movement Processes
The
down-slope movement of material, whether it be bedrock, regolith, or a mixture
of these, is commonly referred to as a landslide. All of these processes
generally grade into one another, so classification of such processes is
somewhat difficult. We will use a classification that divides mass movement
processes into two broad categories (note that this classification is somewhat
different than that used by your textbook).
Slope
Failures - a sudden failure of the slope resulting in transport of debris down
hill by sliding, rolling, falling, or slumping.
Sediment
Flows - debris flows down hill mixed with water or air.
Slope
Failures
Slumps (also
called Rotational Slides)- types of slides wherein downward rotation of rock or
regolith occurs along a concave-upward curved surface (rotational slides). The upper surface of each slump block remains
relatively undisturbed, as do the individual blocks. Slumps leave arcuate scars
or depressions on the hill slope. Slumps can be isolated or may occur in large
complexes covering thousands of square meters.
They often form as a result of human activities, and thus are common
along roads where slopes have been oversteepened during construction. They are also common along river banks and sea
coasts, where erosion has under-cut the slopes.
Heavy rains and earthquakes can also trigger slumps.
Falls - Rock falls occur when a piece of rock
on a steep slope becomes dislodged and falls down the slope. Debris falls are
similar, except they involve a mixture of soil, regolith, vegetation, and
rocks. A rock fall may be a single rock or a mass of rocks, and the falling
rocks can dislodge other rocks as they collide with the cliff. Because this
process involves the free fall of material, falls commonly occur where there
are steep cliffs. At the base of most
cliffs is an accumulation of fallen material termed talus.
Slides (also called Translational Slides) -
Rock slides and debris slides result when rocks or debris slide down a
pre-existing surface, such as a bedding plane, foliation surface, or joint surface (joints are regularly spaced
fractures in rock that result from expansion during cooling or uplift of the
rock mass). Piles of talus are common at the base of a rock slide or debris
slide. Slides differ from slumps in that
there is no rotation of the sliding rock mass along a curved surface.
Sediment
Flows
Sediment
flows occur when sufficient force is applied to rocks and regolith that they
begin to flow down slope. A sediment
flow is a mixture of rock, and/or regolith with some water or air. They can be
broken into two types depending on the amount of water present.
Slurry
Flows- are sediment flows that contain between about 20 and 40% water. As the
water content increases above about 40% slurry flows grade into streams. Slurry flows are considered water-saturated
flows.
Granular
Flows - are sediment flows that contain between 0 and 20% water. Note that
granular flows are possible with little or no water. Fluid-like behavior is
given these flows by mixing with air.
Granular flows are not saturated with water.
Each of
these classes of sediment flows can be further subdivided on the basis of the
velocity at which flowage occurs.
Slurry Flows
Solifluction
- flowage at rates measured on the order of centimeters per year of regolith
containing water. Solifluction produces distinctive lobes on hill slopes. These
occur in areas where the soil remains saturated with water for long periods of
time.
Debris Flows
- these occur at higher velocities than solifluction, with velocities between 1
meter/yr and 100 meters/hr and often result from heavy rains causing saturation
of the soil and regolith with water. They sometimes start with slumps and then
flow down hill forming lobes with an irregular surface consisting of ridges and
furrows.
Mudflows -
these are a highly fluid, high velocity mixture of sediment and water that has
a consistency ranging between soup-like and wet concrete. They move at
velocities greater than 1 km/hr and tend to travel along valley floors. These
usually result from heavy rains in areas where there is an abundance of
unconsolidated sediment that can be picked up by streams. Thus after a heavy
rain streams can turn into mudflows as they pick up more and more loose
sediment. Mudflows can travel for long distances over gently sloping stream
beds. Because of their high velocity and long distance of travel they are
potentially very dangerous. As we have seen, mudflows can also result from
volcanic eruptions that cause melting of snow or ice on the slopes of
volcanoes, or draining of crater lakes on volcanoes. Volcanic mudflows are
often referred to as lahars. Some lahars can be quite hot, if they are
generated as a result of eruptions of hot tephra.
Note that the media often refers to mudflows
(and sometimes debris flows) as mudslides. This is inaccurate because mud flows
rather than slides down a slope. Thus, in this course the word
"mudslide" is an illegal word - one that you should never use.
Granular
Flows
Creep - the
very slow, usually continuous movement of regolith down slope. Creep occurs on
almost all slopes, but the rates vary. Evidence for creep is often seen in bent
trees, offsets in roads and fences, and inclined utility poles.
Earthflows -
are usually associated with heavy rains and move at velocities between several
cm/yr and 100s of m/day. They usually remain active for long periods of time.
They generally tend to be narrow tongue-like features that begin at a scarp or
small cliff.
Grain Flows
- usually form in relatively dry material, such as a sand dune, on a steep
slope. A small disturbance sends the dry unconsolidated grains moving rapidly
down slope.
Debris
Avalanches - These are very high velocity flows of large volume mixtures of
rock and regolith that result from complete collapse of a mountainous slope.
They move down slope and then can travel for considerable distances along
relatively gentle slopes. They are often triggered by earthquakes and volcanic
eruptions.
Snow
Avalanches are similar to debris avalanches, but involve only snow, and are
much more common than debris avalanches. Snow avalanches usually cause hundreds
of deaths worldwide each year.
Mass Movements in Cold Climates
Mass movement in cold climates is governed by
the fact that water is frozen as ice during long periods of the year. Ice,
although it is solid, does have the ability to flow, and freezing and thawing
cycles can also contribute to movement.
Rock
Glaciers - a lobe of ice-cemented rock debris (mostly rocks with ice between
the blocks) that slowly moves downhill.
Frost
Heaving - this process is large contributor to creep in cold climates. When
water saturated soils freeze, they expand, pushing rocks and boulders on the
surface upward perpendicular to the slope. When the soil thaws, the boulders
move down vertically resulting in a net down slope movement.
Subaqueous
Mass Movements
Mass wasting
processes also occur on steep slopes in the ocean basins. A slope failure can
occur due to over-accumulation of sediment on slope or in a submarine canyon,
or could occur as a result of a shock like an earthquake.
3 types – of
mass movements are common, based on degree of disintegration of the material
during movcement:
Submarine
slumps – Coherent blocks break and slip.
Submarine debris flows – Moving material
breaks apart.
