Mass Wasting and Its Factors

Study Notes

Mass wasting is the downslope movement of rock, regolith (unconsolidated material), and soil due to gravity.
Some mass-wasting processes are slow, while others are sudden and disastrous.

Slope: Material on a flat surface doesn't move. On a slope, gravity's force is divided into components normal and tangential to the surface. If the tangential component (downslope component) is stronger than the resisting force (component that keeps it in place), mass wasting occurs.
Water: Steepest angle of repose for a pile of unconsolidated grains. Dry/unsaturated soil is more stable than saturated soil.
Soil cover: More unconsolidated material, with percolating water reaching the bedrock interface, can create a sliding plane. Thicker soil cover means a greater volume of unconsolidated material.
Geologic features: Type of rock (presence of joints, fractures, bedding planes), direction of slope matching planar features (daylighting slope).

Type of movement: Fall (no fluidity), topple (blocks falling as a unit), slides (translational: along a surface; rotational: along a curved surface), slump, spread, and subsidence, flow.
Water content and rate of movement: Materials flow mixed with water/air. Examples include: slurry flows (solifluction, debris flows, mudflows), granular flows (creep, earthflows, grain flows, debris avalanches).
Type of material: Rock, solid, or debris.

Creep: Slow, continuous movement of regolith down a slope.
Solifluction: Flowage of regolith containing water. Produces distinctive lobes in saturated soil.
Earthflows: Fine-grained materials form a slurry and have a fluid motion. Associated with heavy rains, oozing rather than rushing, forming lobes (not long streams).
Debris flows: High velocity flows of rock and regolith. Result from heavy rains causing saturation. Move down slopes forming lobes with ridges and furrows.
Grain flows: Relatively dry material, such as a sand dune. A disturbance causes dry grains to rapidly move downhill.
Mudflows: Highly fluid, high-velocity mixtures of sediment and water. Move along valley floors at greater than 1 km/hr, ranging from soup-like to wet concrete.

Hazard maps: Identify areas prone to mass wasting. Proper land use planning is crucial.
Engineering measures: "Hard" measures such as retaining walls, concrete cover, and wire fences. "Soft" measures are monitoring slopes, rainfall conditions, using early warning systems, establishing hazard preparedness, along with rehabilitation and coping mechanisms.