mass wasting 2

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

THANK YOU!!