Chapter 13: Mass Wasting (
)
angle of repose
colluvium
creep
debris avalanche
debris fall
debris flow
debris slide
frost heaving
gelifluction
landslide
liquefaction
mass-wasting
mudflow
rockfall
rock glacier
rockslide
shear strength
shear stress
slump
slurry flow
solifluction
talus
Mass Wasting
Downslope movement of regolith and rock under
the pull of gravity.
A stable slope condition (short-term steady-state).
Hillslope may change with time (transient-state).
Mass wasting occurs on land and under water.
Role of Gravity and Slope Angle
Shear stress (gt): the tangential component of
gravity acts down a slope.
Normal stress(gp): the perpendicular component
of gravity acts down a slope.
How to Determine Slope Stability?
• Shear Stress
The downslope tangential component of gravity causes
objects to move downhill.
Driving force = Wsin
• Shear Strength
Cohesion
Frictional resistance
Resisting force = C A + Wcos
Binding action
tan
• Safety Factor
Fs = shear strength/shear stress
Slope Failure occurs when Fs
shear stress).
1 ( or shear strength <
Role of Water
Capillary attraction of water in the unsaturated zone holds
the wet sand as a cohesive mass.
When water pressure rises above a critical limit, the
saturated soil will loose its strength and begin to flow.
Increased water pressure buoys up a rock mass, and thus
reduces its normal effective stress and frictional resistance.
High water pressure will reduce the shear strength (friction
at the bass of a rock mass and natural cohesion between
grains), and thereby promoting mass movement.
Mass-Wasting Processes
Slope Failure
The sudden failure of a slope that results in the
downslope transfer of relatively coherent masses of soil
or rock debris by slumping falling or sliding.
Sediment Flow
The downslope flow of mixtures of solid material, water,
and air which are distinguished on the basis of velocity
and the concentration of particles in the flowing mixture.
86 8
18
8
30
200
20
28
80
50
Rock Fall is the free
falling of detached bodies
of rock
Debris Fall is similar to a
rockfall, but it consists of a
mixture of rock and weathered
regolith, as well as vegetation
Rockslide and Debris Slide involve the rapid
displacement of masses of rock or sediment along an
inclined surface, such as a bedding plane.
A talus at the base of
a steep mountain slope.
The debris movement
from cliff to talus is
chiefly by falling,
sliding, bounding, and
rolling.
Sediment Flows
olid particles move in a flowing motion.
Two classes based on sediment concentration:
–
Slurry flow: a moving mass of water-saturated sediment.
–
Granular flow: a mixture of sediment, air, and water.
Factors controlling sediment flow:
–
The relative proportion of solids, water, and air
–
The physical and chemical properties of the sediment
Water helps promote flow, but the gravity remains the
primary reason for their movement.
Slurry Flow
The poorly sorted sediment mixture in slurry flows is
often so dense that large boulders can be suspended in it.
There are three types of slurry flows:
1. Solifluction
2. Debris Flow
3. Mudflow
Solifluction
• The very slow downslope movement of saturated regolith on
hill slopes.
• Rates of movement are less than 30 cm/yr.
• Distinctive surface features: lobes and sheets of debris.
Solifluction lobes,
1-2 m thick, in
Italian Alps
Debris flow: The downslope movement of satuated regolith,
the greater part being sand and gravel
–
–
–
–
–
–
Rates of movement range from 1m/yr to 100 km/h.
Travel distance is limited to a few kilometers.
Flows from an area with a slump or debris slide.
Deposits commonly have a tongue-like front.
Frequently associated with intervals of extremely
heavy rainfall that lead to saturation of regolith.
Often occurs in creek valleys of steep slope.
Mudflow: The downslope rapid movement of
saturated clay, silt and sand.
After heavy rain in a mountain canyon, a
mudflow can start as a muddy stream that
becomes a moving dam of mud and rubble.
Most mudflows are highly mobile.
Mudflows produce sediments fans at the base of
mountain slopes.
Andean Mudflow Hazard
The mudflow generated by an eruption of Andean volcano
overwhelmed the city of Armero, Colombia at 11 p.m. of
November 13, 1985, killing more than 20,000 people.
A history of volcanic mudflow back to 1595.
The dormant volcano became active in 1984.
The eruption of volcano was predicted by geologists
and evacuation of Armero was urged at 3 p.m. of
November 13, but the warning went unheeded.
Andean Mudflow
an eruption of Andean volcano overwhelmed the city of
Armero, Colombia at 11 p.m. of November 13, 1985,
killing more than 20,000 people.
Granular Flows
•
Often made up of weak regolith.
•
Occur on gentle to moderately steep slopes (2o to 35o)
•
Velocity ranges from less than 1 mm/day to hundreds m/h
•
There are four types of granular flows:
Debris avalanche
Grain flow
Earthflow
Creep
Creep: a very slow type of granular flow
Creep rate: millimeters to centimeters per year.
Rates tend to be higher on steep slopes than on
gentle slopes.
Loose, angular, poorly soted, incoherent deposits
on slopes that are moving mainly by creep are
called colluvium.
In contrast, Alluvium consists of rounded
particles, sorted and deposited in layers.
Earthflow
An earthflow is concave upward near the head and
convex upward near the toe.
liquefaction occurs in saturated sediment consisting
of silt to sand-size particles weakened by earthquakes.
– An abrupt shock increases shear stress and may
cause a momentary buildup of pore water pressure
which decreases the shear strength.
– A rapid fluidization of the sediment causes abrupt
failure.
liquefaction occurs in wet, highly porous sediment
consisting of silt to sand-size particles weakened by an
earthquake.
An abrupt shock increases shear stress and may cause
a momentary buildup of water pressure in pore spaces
which decreases the shear strength.
A rapid fluidization of the sediment causes abrupt
failure
Grain Flow
Grain flows are the movement of a dry or nearly
dry granular sediment with air filling the pore
spaces, such as sand flowing down the dune face.
Velocities of the moving sediments typically range
between 0.1 and 35 m/s.
Debris Avalanches
A huge mass of falling rock and debris that breaks
up, pulverizes on impact, and then continues to
travel downslope.
The flanks of steep stratovolcanoes are susceptible
to collapse that can lead to debris avalanches.
–
Such a collapse occurred 300,000 years ago at Mount
Shasta.
–
The volume of the landslide on Mount St. Helens
was about 10 times smaller than that of the Mount
Shasta event.
Causes of Landslides
Immediate causes
– Heavy rainfall, flood
– Earthquake
– Volcanic eruption
– …..
Real causes
– Geologic processes
– Poorly designed artificial slope
– Excavation or undercutting
– ……
Debris Flow in the Indus River Canyon
In January 1841, an earthquake triggered a landslide
in the Himalaya. The slide debris dammed the Indus
River, forming a lake. that steadily grew until 150m
deep and 30km long.
In June 1841, the rising water overflowed the
landslide dam and cut through unconsolidated
debris. The flood rushed 400 km downstream,
sweeping a Sikh army camp on the plains.
Mass Wasting Under Water
An extremely common and widespread
means of sediment transport on the seafloor
and in lakes.
A gravity-induced movement of rock and
sediment.
• Distribution of large blocky
landslides and debris flow
deposits on the continental
slope and rise off the eastern
coast of the United States
Vast areas of the seafloor are
disrupted by submarine slumps,
slides, and flows in the Western
North Atlantic.
Mass Wasting in Hawaii
Coral-bearing gravels found up to altitudes of 326 m
on Lanai and nearby islands.
A giant wave that deposited the coral fragments high
above sea level?
The wave is believed to have resulted from a huge
submarine landslide off the western coast of the
Island of Hawaii.
Based on dating of the corals on Lanai, the landslide
occurred about 105,000 years ago.