Mass Wasting Flashcards
what is mass wasting?
downslope movement of rock or regolith (loose weathered materials + soil driven by force of gravity)
What should we care about mass wasting?
The cost of recovery from mass wasting (all forms combined) is more each year than earthquakes + volcanoes, tornadoes combined
What are the controls of mass wasting?
Properties of material involved (rigid, cohesive or loose)
Slope angle
Water content of material
Removal of vegetation
Mass movement triggers
what are the properties of material involved in mass wasting?
Rigid, brittle material (e.g. rock)
Soft, cohesive material (e.g. clay-rich soil or rock in which particles stick tgt + form a coherent mass)
Loose particles (e.g. gravel, sand, etc.)
Phys + chem weathering can further weaken geologic materials + make them prone to failure (esp in case of rock)
What is the angle of repose?
maximum angle that a given material will sustain
Diff in angle of repose according to grain size, grain shape (round vs angular) + water content
What is the slope angle?
Gravity has greater effect on potential downslope movement of material on steeper slopes
Increase the sloped, increase the driving force (mass wasting events tend to be most spectacular on steep mountain slopes)
What are the natural slopes prone to mass wasting?
River banks
Mountain slopes
Fault surfaces
Wave-cut shorelines
Glacial valleys
What are the Man-made slopes prone to mass wasting?
Road-cuts
Excavation projects
Gravel pits
Quarries
Drainage ditches
what can water do to particles
Presence of some water can increase cohesion of loose particles (due to surface tension)
But addition of lots of water can destabilize material
Many cases:
Water along fractures + btwn particles can act as a lubricant (Promoting blocks to slide)
Water can cause particles to remain separated + to move freely (Material behaves like fluid)
What can plants do for material?
Roots of plants anchor loose material such as soil in place on a slope
Removal of vegetation removes this stabilizing factor + can promote downslope movement
What can trigger mass movement?
vibrations , from shaking + liquefaction (lubrication of particles by invading water)
Weather events
Slightest added stress on a material can trigger mass movement
Types of mass wasting classified according to what 3 criteria?
type of movement
type of material moved
rate of movement (velocity)
what are the types of movement?
Falling (freefall)
Sliding (along planar or curved surface)
Flowing (material internally deforms like a fluid)
what are the types of material moved?
Brittle solid material (most rock)
Cohesive solid material (e.g. clay-rich soil or rock)
Loose particles (e.g. particles of soil, mud, debris of assorted sizes)
what are the rates of movement
rapid or slow
What is a rockslide?
Blocks of solid, brittle rock slide down a planar surface
Generally very fast + destructive
what is a rock avalanche
After a mass of rock is dislodged (often in a rockslide), it can break into many smaller pieces + move downslope as a rock avalanche
Fragments of rock travel downslope on a cushion of air within + beneath the moving mass of rock fragments
what is a slump
Material moves as coherent blocks along curved surfaces (little or no internal deformation within blocks)
Commonly occurs in cohesive clay-rich sedimentary rocks (e.g. shales) + soils (clay helps particles stick together)
At surface, slump scarps mark these curved surfaces of sliding
Rate of movement depends on material involved (but usually slow)
what is an earthflow
When clay-rich soil becomes saturated w/ water, it behaves like a viscous fluid (deforming internally)
Commonly produced at the foot of a slump (where water is leaking from curved slump surface)
Typically move slowly, but can move quickly if lots of water has soaked into the material and/or the slope above is steep
what is a debris flow
if assortment of surface materials, both fine- and coarse-grained
what is a mudflow
if predominantly fine grained materials
both debris flow + mudflow consist of what?
loose material mixed w/ large amt of water
- less viscous than earthflows + can move extremely quickly (extremely destructive)
- movement of this liquefied material is often confined to channels
what are lahars
debris flows + mudflows specifically associated w/ volcanoes
how are lahars formed
Formed when large amts of water (e.g. from heavy rainfall, or from melting snow at the summit of an active volcano) mixes w/ loose pyroclastic materials (e.g. ash) + flows downslope
what is a creep
Expansion + contraction of surface material leads to very slow + gradual downhill movement of soil
Its effects can be observed in the tilting fences + headstones + curvature of tree trunks + damage to building foundations
Unspectacular but very costly
what are the human influences on mass wasting
Natural Processes + increased population density + increased human activity = increased hazard potential
list the human influence on mass wasting hazards
Weakening of slope material by saturation by water
-Enhanced due to land use changes such clear cutting + agriculture
Steepening of the slope by artificial or natural undercutting of toe of slope
-For buildings and roads
Addition of weight at top of slope
-Dams, buildings, urbanization, roads
Humans are sculpting the surface of Earth faster than natural processes – no slope is too rugged for development
what are some strategies to reduce damage from slope movements
Grading (add material to reduce slope angle)
Surface + subsurface drainage control (devices to permit drainage of excess water from soil + underlying weathered rock)
Retaining walls, and slope supports (stabilize slope + support surfaces of potential weakness such as fault planes + rupture surfaces under potential slump blocks
Anchoring into bedrock (to reduce load + prevent slippage of man-made structures due to deformation of loose materials)
