MASS WASTING

Mass wasting

Downslope movement of rock, regolith, and soil under the direct influence of gravity.

Mass wasting (transport)

Does not require a transporting medium such as water, wind, or glacial ice.

Controls and triggers

Gravity, water, over-steepened slope, vegetation, earthquake.

Water (effects)

Filling of pores: destroys cohesion. Oversaturation: reduces internal resistance. Additional weight.

Cohesion

Act of particles sticking together.

Over-steepened slope

Moves unconsolidated granular materials. Produces unstable slopes.

Over-steepened slope (cause)

Caused by nature and human activities.

Angle of repose

Angle at which material remains stable.

Vegetation

Root systems bind soil and regolith together. Shields soil surface from erosion due to rain.

Earthquake

Dislodge enormous volumes of rock and unconsolidated material.

Mass wasting classification

Type of material. Type of motion. Rate of movement.

Soil

Loose materials covering the surface of the Earth.

Regolith

Unconsolidated rocky material covering bedrock.

Rock

Mass of bedrock.

Fall

Freefall of detached individual pieces of any size. Common in very steep slopes.

Slide

Presence of distinct zone of weakness separating the slide material form the more stable underlying material. Types: rotational and translational.

Flow

Material moves downslope as a viscous fluid. Usually saturated with water.

Slump

Downward sliding of a mass of rock or unconsolidated material along a curved surface. Distance: relatively near. Common in thick accumulations of cohesive materials (e.g. clay). Occurs due to an oversteepened slope.

Rockslide

Blocks of bedrock break loose and slide down a slope. Occurs at inclined strata or joints and fractures parallel to the slope. Debris slide: material is mostly unconsolidated soil and regolith. Fastest and most destructive. Trigger: earthquake, rain, melting snow.

Debris flow

Relatively rapid flow of soil and regolith containing large amount of water.

Mudflow

material is primarily fine-grained

Lahar

debris flow along steep slopes of some volcanoes. Occurs in semiarid mountainous regions along canyons and stream channels.

Earthflow

Viscous downslope movement of saturated clay and silt on hillsides in humid areas. Occurs faster during times of heavy precipitation or snowmelt. Tongue- or teardrop-shaped slope scarp with varying size and depth. Movement depends on slope steepness and material's consistency.

Creep

Gradual downhill movement of soil and regolith due to alternating expansion and contraction of surface. Factors: raindrop impact, plant roots, burrowing animals, saturated ground.

Solifluction

Occurs when soil is saturated with water and cannot escape. Common in regions underlain by permafrost.

Prevention and mitigation

Identify regions at risk. Revegetation. Regrading of over-steepened slopes. Reducing subsurface water. Preventing undercutting. Constructing safety structures. Controlled blasting of unstable slopes.

Revegetation (mitigation)

Revegetating a slope results in the growth of roots that can hold a slope together.

Regrading (mitigation)

Redistributing the mass on a slope can stabilize it. Terracing can help catch debris.

Reducing subsurface water (mitigation)

Lowering the level of the water table can strengthen a potential failure surface.

Preventing undercutting (mitigation)

Relocating a river channel can prevent undercutting.

Safety structures (mitigation)

Adding riprap can slow undercutting of coastal cliffs. A retaining wall can trap falling rock.