Environmental Geology lecture outline
- Mass wasting and associated concerns.
Definition of mass wasting = down slope
movement of earth material (solid masses, dry aggregates and slurries)
under the influence of gravity.
Extent of concern - low grade chronic problem:
- average of 25-50 fatalities and $1.5 billion
damage per year in U.S..
- numbers much higher elsewhere.
- California and other west coast states
of struggled with in particular.
" In geology, it would be known
as a debris flow. Debris flows amass in stream valleys and more
or less resemble fresh concrete. They consist of water mixed with
a good deal of solid material, most of which is above sand size.
Some of it is Chevrolet size. Boulders bigger than cars ride long
distances in debris flows. Boulders grouped like fish eggs pour
downhill in debris flows. The dark material coming toward the
Genofiles was not only full of boulders: it ws so full of automobiles
it was like bread dough mixed with raisins. On its way down Pine
Cone Road, it plucked up cars from driveways and the street. When
it crashed into the Genofile's house, the shattering of glass
made terrific explosive sounds. A door burst open. Mud and boulders
poured into the hall. We're going to go, Jackie thought. Oh, my
God, what a hell of a way for the four of us to die together."
From John McPhee, 1989, Los Angeles Against the Mountains,
in The Control of Nature, Publisher -Farrar, Straus, Giroux.
Questions to play with:
are the various types of mass wasting events (classification)?
Important variables used to classify:
- type of material: rock vs. sediment, grain
size distribution (sorted vs. unsorted), clay content and character.
- movement mechanism: parting across a plane,
slip along a plane, fluid flow, free falling.
- movement speed: slow creep to catastrophic
Examples of some types:
- downhill soil creep.
- rock topple, fall or glide.
- earth slumps.
- earth/rock slides.
- debris and mud flows.
What is the
largest mass wasting event possible on earth?
- Scale variant or invariant behavior, what's
the difference? Over what range?
- Human history vs. geologic record?
- Are smaller or larger events of greater
risk, greater concern?
- The wave that paved Lanai: single gravel
layer with basalt and coral fragments extends up to an elevation
of 326 meters up the slope of Lanai. How did it form? There is
promote slope stability versus slope instability?
- Profile steepening: toe removal (undercutting),
magmatic inflation, angle of repose sedimentation.
- Change in material strength - > weathering.
- Water saturation: acts as suspension medium
to create slurries (debris flows), promotes weathering, reduces
viscosities, reduces friction on slip surface.
- Criteria for slip on a plane and the influence
sites of slope failure be predicted?
The best predictor of future behavior is
Recognition of past occurrences:
- geomorphic signature
- tree records of movement
- out of place rock
- potential slip planes:
- bedding planes.
- joint/fracture surfaces.
- soil-bedrock contact.
- active layer in permafrost.
Human activity can induce mass wasting in
an area that has been historically stable.
Slope stability analysis - what is strength
and what are forces acting on slope material.
Cretaceous Pierre shale: low strength and
very prone to mass wasting, widespread in upper midwest.
Can mass wasting
be prevented and/or mitigated?
- drainage of slope interior.
- rock bolts.
- berm barriers.
- retaining walls (better be drained and
- rock profiling.
- slope design.
- sometimes yes, sometimes no.
- drainage of lakes to prevent breakout
floods once it has occurred.
A few more spectacular case histories:
- Huascaran, Peru:
- occurred in 1970.
- triggered by earthquake, with source a
glacial cirque wall in Mount Huascaran.
- debris avalanche with 16 km runout distance
a source to toe relief of circa 4 km, and speeds in excess of
- movement mechanism?
- Covered portions of towns of Yungay and
Ranrahirca, circa 20,000 fatalities.
- somewhat smaller avalanche had reached
margin of Ranrahirca before in 1962, killing about 3000 people..
- Vaiont, Italy:
- occurred in Oct. 1963.
- triggered by reservoir construction and
filling in previous months.
- large slide on mountain side slid into
reservoir at speed possibly at 90 km/hour, sending a wall of
water over the dam down into the town of Longarone below. Wall
of water up to 70 meters high.
- circa 3,000 people killed.
- had detected creep in mountain side before,
but actions were too late.
- Blackhawk, California:
- occurred in prehistoric time, 135 km east
- rock avalanche of interest because of
8 km runout over 8 km distance on a slope of 2.5 degrees
- modeling suggests it had to move at 120
- acoustic lubrication is one model for
this anomalous behavior.
Some interesting links:
USGS mass wasting site.
Hungr, O., Evans, S. G., Bovis, M. J. & Hutchinson, J.
N., 2001, A review of the Classification of Landslide of the Flow
Type; Environmental and Engineering Geoscience, v. VII, p. 221-238.
Return to Environmental
Geology course index.
Return to Harmon's