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Slope Systems – Mass Wasting

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Presentation on theme: "Slope Systems – Mass Wasting"— Presentation transcript:

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

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