Lecture 5-2 mass wasting Flashcards
What is a mass movement?
Downslope movement of soil or rock material under the influence of gravity without direct aid of water, wind or ice. Spontaneous
When do mass movements happen?
When the driving forces are larger than the resisting forces. Driving forces= gravity
resisting forces = mass, friction and cohesion
driving forces increase as slope angle increases
What is the Angle of Repose?
Maximum angle that loose material forming a slope can maintain without slipping
- it is a balance between gravity and friction
- solid rock could be 90 degrees
- loose material and soil 13-24 degrees
- coarse sand 30-33 degrees
What criteria is used for distinguishing mass movement types?
Material (soil/rock)
Velocity (slow/fast)
Water content (dry/wet/fluid)
Shape of failure
What is soil creep?
Slow downslope movement of soil or rock debris.
common rates of movement range between 0.1 and 15 mm/year
-shallow, limited to upper part of soil
-depends on slope angle and soil type
-causes: alternate freeze/thaw or wet/dry cycles, needle ice, water pressure, animals, earthquakes
What is solifluction?
a form of soil creep, soil and rock are saturated and flow downslope
-common in subpolar regions, drainage is inhibited
What are mudflow and earth flow?
Mudflow: downslope flow of water and soil, mostly fine sediment
Earth flow: moderately rapid downslope movement of masses of saturated soil.
- forms stepped (terraced) surface
-more rapid than solifluction
-flow is lobe shape
What is debris flow?
Flow of muddy water with large amounts of coarse material
-coarse material
-les water than mud flow
-poorly sorted
Occurs when: Large amounts of regolith are available, with large amounts of water, on steep slope
Are debris flows common in BC coastal mountains?
yes, because of heavy rains on steep slope mantled by loose sediments
What are slides?
Rapid sliding of large masses of rock or soil on steep slopes after heavy rain.
Water reduces the resistance, increases the weight and pressure.
depends on litology, rock structure, vegetation, earthquakes
What is Rock and Soil Fall?
Free fall of soil and rock on steep slopes or cliffs that accumulate at the base of the slope
-forms talus cone or scree slope. sits at the angle of repose and depends on particle size, lithology and sorting
What are the Major players of mass movements?
- Climate and meteorological events
- Lithology, layering, structural contols, gradient and surface materials
- VEgetation and Land use effects
WHy is logging and logging roads so important to consider?
Dying roots, roads, disturbed surface and surface water all contribute to soil instability
