Studied by 5 people
what is mass wasting?
gravitational downslope transport of rock, regolith (unconsolidated material), snow or ice
also called mass movement
factors that affect slope shape?
material strength
ability to avoid failure
material strength depends on if it is coherent or unconsolidated or has planes of weakness
roundedness plays a part too
climate
affects which agents or erosion are at work (ice, water, wind) and how fast; also affects soil and vegetation formation on slopes
more veg → more stability
process of formation
ex. glacially carved valley have a ‘U’ shape
river erosion causes a ‘V’ shaped valleys
factors controlling downslope movement of material?
slope angle
critical angle of repose
cohesion & water Saturation
Material Strength
slope angle stress?
Shear Stress (σd): stress resulting from application of force parallel to a surface (force pulling the boulder/grain downslope)
Normal Stress σn): component of stress perpendicular to the Earth’s planar surface (force keeping the boulder/grain from moving)
when is a slope stable?
when σd<σr
when resistance stress (friction and cohesion) is bigger than downslope shear stress
when is slope unstable?
when σd>σr
shear stress bigger than resistance
what is the critical angle of repose?
max angle at which sediment particles can stand w/o falling depends on:
grain size
grain angularity
rounded grains (more friction) < angular grains
moisture content
dry sediments typic is less stable than damp sediment
oversat sediment becomes less stable
what is cohesion?
attraction between small soil particles that is provided by the surface tension of water between the particles
friction and surface tension of water can provide some resistance to sliding
material strength categories?
loose, poorly cemented, dry material → weak
solid, well-cemented material → stronger
weak between sedimentary layers & along well dev factures
diff types of rocks layer up → change between one type of rock to other can be failure
what are the diff types of slope failure?
creep and solifluction
slumps
debris flows & mud flows
rockslides and rockfalls
sturzstroms (rock avalanches)
what is a creep?
extremely slow downslope flow of sediment on the surface
caused by cycles of freeze-thaw, wetting-drying and/or warming-cooling
when water freeze, expands ground (rises) n continues
so winter slope is higher than summer slope
how do creep occur?
slow, so loss is mostly from damage to buildings and infrastruct, rather than casualties
what is solifluction?
downslope creep driven by sequential freezing and thawing in tundra regions
results in soggy layer of ground above frozen permafrost that then moves downhill
this water can’t go back down
what are translational slump?
landslide that moves along a regular sloping planar surface
typic occurs when a block of material is underlain by a weak surface that is more or less parallel to the slope
planes between sedimentary beds
old faults and fractures
debris or cohesive mud over underlying bedrocks
block can travel down slope as a cohesive unit or lose internal cohesion and break up as it travels downslope
if it breaks, turns into diff landslide
what are rotational slumps?
landslide in which the mass rotates on a concave failure surface
examples of slumps?
Slumps dev along the South Saskatchewan River near Medicine Hat, Alberta
Slump destroyed Holbeck Hall in Scarborough, England, 1993
what are flows?
slope failures involving material that has lost internal cohesion
what is mud flow?
flow of mud, rock, and water dominated by clay-sized particles
includes lahars
what is debris flow?
slurry of rock, sand, and water flowing downslope; water usually makes up less than half of the flow vol
what does speed of flow depend on?
steepness of the slope
water content
example of debris flow?
Andres of Northwest Argentina that was rich in cobble and boulders
example mudflow?
Afghanistan, 2014
heavy rain turned a slump into a mudflow
buried part of a village in Afghanistan
Rio De Janeiro and landslides
highly weathered Sugarloaf mountains surround the city
poor communities (favelas) built on material where landslides are common
8 million mostly poor Brazilians face mudflow risk
wealthy communities build homes on stable ground
Vargas Tragedy - Venezuela 1999
several debris and mudflows over a 2-day period caused by heave rainfall
towns were built on debris from past flows
30,000 deaths
75,000 people displaces
dutch helped out (Netherlands)
Some buried to death
mudline on clothes suggest that the landslide was much higher than what videographer saw next day
what is quick clay?
water-sat mud deposited in salty water composed of clay flakes with large pore spaces between the flakes
highly unstable
example of quick clay landslides?
Led Clay in the ST. Lawrence, Saguenay, and Ottawa Valleys
Lemieux Landslide 1993 →
The headscarp retrogressed 680 m into level
ground above the riverbank. About 2.8 million tons of clay and silt liquefied and flowed \n into the South Nation River valley, damming the river
Alta, Norway 2020
what is a rockslide?
sudden movement of rock and debris down a non-vertical slope
Vaiont Slide
October 9, 1963; Southern alps in northwest Italy
dam changed water tables (raised it) → caused surface above to get wet, making it unstable → cause slide + river was blocked
slide displaced all the water in downstream half of the reservoir
125m high wave crashed over the dam
tsunami rushed upstream
2500 lives lost due to water spilling over dam
what is a rockfall?
slope failure that occurs when a mass free-falls down a cliff or a steep slope; slope is often vertical
rock fall examples?
Swiss Alps
India
what is frost wedging?
splitting of rock thru pressure exerted when water freezes
freezing water expands by 9.2%
coastal areas prone to frost wedging where temperature oscillates around freezing pnt
example: cliff in Torbay, Newfoundland and Labrador
what is Sturzstrom?
extremely rapid downslope movement of large vols or rocks and debris
largest and most destructive
typic being as a rock slide, but breaks up, entraining air n water
travel velocities as high as 100-300 km/hr
can travel distances up to 20x their vertical fall
mecha that causes sturzstroms to have such hig velocity and long flow paths is still under debate
sturzstrom may flow as a fluid composed of rock fragment suspended in air (fluidization)
examples of struzstroms?
1903 Frank, Alberta
1965 Hope, BC
Frank, Alberta
started as a translational slide
grained speed and dev into sturzstrom
event lasted less than 100 secs
slope failure occurred along fractured limestone planes
based on the speed and distance of the sturzstrom, it is thought to have traveled on a cushion of compressed air
The rock moved from Turtle Mountain (lower \n left), buried the town of Frank, crossed the river and surged 120m up the other slope.
The river eventually cut through and opened the channel again
hazard mapping & assessment
GIS software can help identify areas of past and potential failures by mapping attributes that contribute to slope failures
slope
bedrock type (lithology)
sediment texture and depth
vegetation
drainage
historic slope failures
reducing or mitigating risk?
roots of plants
reservoir lowering
river undercutting flow
retaining wall
pipes
metal screening
debris basin
rock bolt
how to reduce risk from slope failures?
installation of detection devices, such as acoustic flow monitors
only gives a couple of mins of warning
not enough time to evacuate peep downslope
prohibit building at the base of mountains and is dangerous areas along steep slopes
zone hazardous area as open space for parks, golf courses and agriculture
construct walls and dams to deflect or slow debris flows
construct debris basins large enough to channel and contain debris
Note 
Flashcard (24)