MASS WASTINGRAMULONGO T.F
NQINA P
Mass wasting
Mass wasting refers to several processes that have the following in common:
1. Down slope movement of rock or weathered material
2. Movement is due to pull of gravity 3. There is no flowing medium (water, air or ice)
that carries the material *Generally, mass wasting occurs when gravitational
forces exceed frictional or shear forces (strength) of material.
Shear force (stress) – apparent force exerting a pdowns2lope ull on the object
Normal force – apparent force exerting a pull perpendicular (normal) to the slope
Shear strength (friction) – frictional resistance of the slope material to down slope motion
Gravity
Shear stressNormal force
Shear strength (friction)
If the shear stress actingon the slope exceeds theslope’s normal force plusshear strength, whatshould happen to the slope materials?
Mass Wasting Variables
Classification of Mass WastingType of Material(composition)•Bedrock – Rock•Unconsolidated material – DebrisSoilRegolithSediment
Rate of movement(speed)•mm/yr•km/hr
Water content (lubrication)•adds weight•increased pore pressure in saturated debris decreases shear strength•surface tension in unsaturated debris increases shear strength
Type of Motion (mechanism of movement)
Fall – free-fall of detached particles, slope steep enough that material falls to baseSlide – material remains cohesive and moves along a well-defined surfaceFlow – material moves downslope as a viscous flow (most are saturated with water)
ROLE OF WATER CONTENT
Controlling Factors
Slope angle- gentle vs steepLocal relief- low vs highThickness of debris over bedrock- slight vs great
Planes of weakness ( in bedrock)bedding planes; foliation; jointsplanes at right angle to slope vs parallel to slope most dangerous
TYPES OF MASS WASTING
CREEP
gentle slopes
vegetation slows movement
very slow flow (< 1 cm/year)
facilitated by water in soilor by freeze-thaw in colder climates
Indicators of creep
‘pistol butt’ treesleaning tombstones, walls, posts
EXAMPLES OF CREEP
Rock falls and Rockslides
Rock fallBedrock breaking loose on cliffsTalus at base of cliffs
RockslideBedrock involvedSliding along planes of weakness parallel to slope
Bedding planes; foliation planes; fractures in rock (joints)
Examples of rock falls and rock slides
Rotational slides and land slides
Slow to rapid movementMaterial moves as a coherent unit along a curved surface (spoon-shaped)Blocks of material rotateDebris flows commonly associated with sump
ROCK AVALANCHE
Sudden and rapid free fall movement of rockOccur on steep slopesForms talus pilesCan grade into a rockslide if material greater a slope greater than the angle of repose
MUD FLOW
Flow of watery debrisOccurs where lack of vegetation:
Dry climatesVolcanoesAfter forest fires
DEBRIS FLOW
Mixture of rocks, mud, & waterMoves as a viscous fluidCommon after heavy rainsRapid movement – up to 50 km/hr, the more water present the faster the rate of movementCommon in semi-arid regions and along volcanoes (lahars)
EXAMPLES OF MUD FLOW AND DEBRIS FLOW
Land-Rock SlideMovement of material along a defined slippage planeSudden and rapid movementLarge blocks of rock detach along bedding planes, joints, fractures, etc.Occur on steep slopesCan be triggered by rain falls or ground vibrationFastest and most destructive type of mass wasting
SOLIFLUCTION AND PERMAFROST
Solifluction:Flow of water saturated debris over impermeable material
Permafrost:Ground that remains frozen for many years
common to Arctic regions CIIII Arctic regions
Risk of building on permafrost even when the ground is level
Classification of mass wasting
Type
of M
ovem
ent
Classification Material VelocityCreep Debris Imperceptibly Slow
Earth Flow Debris Slope and Material Dependent <5 km/hrMudflow Saturated Debris
Avalanche Debris or Rock Very Fast 100 km/hr
Rotational Slide Debris Slow-mod. (short)
Rock Slide Bedrock Fast
Debris Fall Debris Fast
Flo
wS
lide
Fal
l
Rockfall Bedrock Fast
EXAMPLES OF MASS WASTING
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