Mass Movements Flashcards
What do mass movements cost Canada?
-1000s every year
-the most expensive natural disaster for us at 200-400 M$/year
-630 deaths since 1840
What makes the stability of the slope?
-stability is decided by two forces: driving force vs resistance force
driving force > resistance = movement
What is a driving force?
the force affected by gravity
-it causes the downslope movement; it operates 24/7
What is a resistance force?
-it is the cohesion of material
(chem bonds, cement, electrical charges, interlocking crystals, etc)
-friction
What is the angle of repose?
the maximum angle at which debris can accumulate (after which the resistance force is not strong enough to hold particles together)
Explain conditions of slope instability
-they are factors that increase the risk of slope failure
-they increase the weight or angle if the slope- affecting driving force
What are the 5 conditions of slope instability?
1) type of material
2) slope angle
3) climate
4) vegetation
5) water
Explain how the type of material affects slope stability
-the material can affect both type and frequency of MM
-main characteristics to consider include
1)mineral comp
2)cementation
3)planes of weakness
Explain mineral composition
-rock vs regolith
-clays (they expand and contract a lot due to air and water absorbed, liquification)
Explain Cementation
-the decrease in cohesion due to the dissolution of the cement that binds the grains together/ chem alteration of the rocks
Explain the Plains of weakness
a surface within rock where resistance is lower then surrounding material
*Fracture (e.g. frost/thaw, exfoliation)
*Bedding plane
*Metamorphic foliation
*Soft rock
*Old failure surface (landslide or fault)
—Especially if high angle
Explain how the slope and topography affects slope stability
Slope: the steeper the slope, the more parallel the larger component is to the slope
Topography: mountainous relief = high relief = steep slope
Explain how the climate affects slope stability
four main determinants and effects
-temperature
-quantity and type of precipitation
-the timing of water that infiltrates the ground
-the type and abundance of vegetation
Explain how vegetation affects slope stability
-creates cover against falling rain
-roots stabilize ground
-can be heavy
-roots can fracture rocks
Explain how water affects slope stability
water plays a key role in promoting MM
-erosion of the base (undercutting)
-weight (water is heavy)
-saturation (liquefaction)
-freezing
-thawing of the active layer
-chemical alteration
Explain the effects of water freezing
-when water freezes, it increases in volume and can cause fractures (resulting in rock falls)
Explain the effect of the thawing of the active layer
(active layer is the layer that freezes and thaws)
-sediment slides at the permafrost interface following the thaw of the active layer and the saturation that follows
Explain how water can chemically alter rocks
-when water moves on and through rocks, it alters the rock forming minerals into other minerals- therefore decreasing cohesion
What are triggers?
conditions of instability push slopes to the brink of failure, a trigger initiates MM
what are the most common triggers
-heavy rains
-snow melt
-inc. of the slope angle
-earthquakes/ sudden vibrations
-thawing of frozen ground
-construction projects
-‘bird on top of slope…”
How are mass movements classified?
1)type of movement
2)type of material
3)speed/ rate of movement
Explain falls and what type of movement classification they are
-free fall of rocks/ debris
-<300km/h
-can turn into an avalanche
-violent blast of wind upon impact (also can cause earthquake upon impact)
*classified as a fall
Explain slumps and what type of movement classification they are
-it is a slide along a curved failure surface
-driving mass moves downslope as resistance mass is pushes upwards, and the slump stops when a new equilibrium is reached
-head scarp is vertical
*classified as a slide
Explain rock/ debris slide and what type of movement classification they are
-slide along a planar failure surface corresponding to a plane of weakness
-<300k/h
-can turn into an avalanche
*classified as a slide
