1. Slope Systems – Mass Wasting

2. Mass Wasting
The downslope transfer of material through the direct action of gravity
Component of erosion and transport of sediment
Follows weathering, which weakens and breaks the rock

3. Controls on Mass Wasting
Requires weathering of rock or unconsolidated material
Primary driving force is GRAVITY
Force mechanics: For the rock to move downslope; force (A) has to be greater than the resistance force (R) holding it up, e.g. friction.
Slope angle influences force (A); the greater the angle the greater the force R A

4. Secondary Controls
Factors that either weaken cohesion forces or increase downslope force
Saturation of the material
Over-steepening of the slope
Slope Modification
Ground vibrations
Expansion/contraction cycles of soil/regolith

5. Saturation of Material
Addition of water
Lubricates the material (decreases cohesion)
Adds weight (increases downslope force)
Increases pore pressure (increases downslope force and decreases cohesion)

6. Over-steepening of the Slope
Can be human-induced or by natural processes – increases the downslope force.
Stream undercutting a valley wall (headward erosion, bank erosion, etc.).
Waves cutting cliffs on a shoreline.
Construction of roads, buildings, homes etc.

7. Angle of Repose
The steepest angle material can create yet remain stable
Function of
Particle size
Particle shape
Moisture Content

8. Slope Modification Removal of Vegetation Building of structures
Roots of plants and trees hold regolith together
Plants and trees remove water from the soil
Removal decrease cohesive force
Building of structures
Depending upon the structure and the location, either a decrease in cohesive force &/or increase downslope force due to added weight will occur

9. Ground Vibrations Earthquakes – decrease cohesion forces
Initiates movement (it is easier for a particle to move than it is to begin to move)
Liquefaction induced slides
Human induced – blasting for construction, large equipment, etc.

10. Expansion/Contraction Cycles
Movement of material due to
Wetting and drying cycles
Freeze-thaw cycles

11. Daily Assignment
Rockslide
Slump
Debris flow
Identify five concepts/characteristics and place them appropriately on the Venn Diagram.
See next slide for list of concepts/characteristics

12. Concepts/characteristics
Bedrock
Downslope movement
Unconsolidated material
Gros Ventre is an example
Fast movement
Mechanical weathering is involved
Slow movement
Debris flow can be associated with
Motion-slide along a defined plane
Vibrations can trigger
Motion – flow, no defined plane
Type of mass wasting
Gravity induced
Component of erosion and transport
Water plays a role
Common after volcanoes – lahar
Curved slip surface

13. Classification of Mass Wasting
Type of Material
Bedrock - Rock
Unconsolidated material - Debris
Soil
Regolith
Sediment
Rate of movement
mm/yr
km/hr
Type of Motion
Fall – free-fall of detached particles, slope steep enough that material falls to base
Slide – material remains cohesive and moves along a well-defined surface
Flow – material moves downslope as a viscous flow (most are saturated with water)

14. Creep Slow downslope movement – mm/yr
Process so slow one cannot observe it in action
Expansion/contraction cycles play a key role
Enhanced by burrowing organisms, periods of prolonged rains or snow,

16. Solifaction Special type of creep
Occurs in regions underlain by permafrost (permanently frozen, water-bearing ground)
During warm periods top portion thaws and becomes saturated
Saturated zone flows over frozen layers

17. Debris Flow Mixture of rocks, mud, & water Moves as a viscous fluid
Common after heavy rains
Rapid movement – up to 50 km/hr, the more water present the faster the rate of movement
Common in semi-arid regions and along volcanoes (lahars)

19. Earthflow A type of debris flow, generally move slower
Found on hill sides
Can remain active over periods of years

20. Slump Slow to rapid movement
Material moves as a coherent unit along a curved surface (spoon-shaped)
Blocks of material rotate
Debris flows commonly associated with sump

22. Land-Rock Slide Movement of material along a defined slippage plane
Sudden and rapid movement
Large blocks of rock detach along bedding planes, joints, fractures, etc.
Occur on steep slopes
Can be triggered by rain falls or ground vibration
Fastest and most destructive type of mass wasting

24. Rock Fall (Rock Avalanche)
Sudden and rapid free fall movement of rock
Occur on steep slopes
Forms talus piles
Can grade into a rockslide if material greater a slope greater than the angle of repose

26. Subaqueous Slope Failure
Mass failure in which the material movement occurs below water

27. Dynamics of a slope system