Slope Systems – 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

Mass Wasting

Controls on Mass Wasting

• Requires weathering of rock or unconsolidated material

• Primary driving force is GRAVITYGRAVITY• Force mechanics: For the rock to

move downslope; force (A) (A) has to be greater than the resistance force (RR) holding it up, e.g. friction.

• Slope angle influences force (AA); the greater the angle the greater the force

AA

AA

RR

RR

Secondary Controls

Factors that either weaken cohesion forces or increase downslope force

1. Saturation of the material

2. Over-steepening of the slope

3. Slope Modification

4. Ground vibrations

5. Expansion/contraction cycles of soil/regolith

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)

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.

Angle of Repose

• The steepest angle material can create yet remain stable

• Function of– Particle size

– Particle shape

– Moisture Content

Slope Modification

• Removal of Vegetation– 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

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.

Expansion/Contraction Cycles• Movement of material due to

– Wetting and drying cycles– Freeze-thaw cycles

Daily Assignment

Identify five Identify five concepts/characteristics concepts/characteristics and place them and place them appropriately on the appropriately on the Venn Diagram.Venn Diagram.See next slide for list of See next slide for list of concepts/characteristicsconcepts/characteristics

Concepts/characteristics

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 Fall – free-fall of detached

particles, slope steep enough that material falls to base

– SlideSlide – material remains cohesive and moves along a well-defined surface

– FlowFlow – material moves downslope as a viscous flow (most are saturated with water)

• 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,

CreepCreep

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)

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

• 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

Land-Rock Slide

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

Subaqueous Slope Failure

• Mass failure in which the material movement occurs below water

Dynamics of a slope system