Landslides, Part 2 Beverly Hills slide, Feb 2005

Outline Slide examples Flows Avalanches

Block Slide The rapid movement of large blocks of detached bedrock sliding more or less as a unit.

Block Slide

Debris Slide Rock material and soil move largely as one or more units along planes of weakness.

Debris Slide

1925 Gros Ventre Slide, Wyoming

Gros Ventre Slide, Wyoming

Fig Vaiont Reservoir Slide Debris slide caused reservoir wall to collapse, sent water downstream

Lateral Spreading

Turnagain Heights, Alaska

Flows Mass movements behave like fluids Materials include boulders, sand, ice, mix Speed varies - barely moving to ~200 mph Various types of flow, depending on material, slip surface

Loess Flow Dry flows of fine silt deposits Example: 1920 China –160 by 275 km hill of loess flowed after earthquake –Killed ~200,000 people by burying villages very rapidly

Earth Flow A fluid movement of relatively fine-grained material (soils, clays)

Earthflow

Debris Flow A fluid mass movement of rock fragments supported by a muddy matrix. May move a speeds of up to 100 km/hr!

Mudflow A flowing mass of material (mostly finer than sand, along with some rock debris) containing a large amount of water. May travel large distances and high speeds Can carry particles as large as a house!

Debris Avalanche Fast (up to 280 km/hr) downhill movements of soil and rock, usually occurring in humid mountainous regions

Mt Huascaran, Peru (before 1970) Mountain region in Peru, before a Mw 7.7 occurred in the subduction zone offshore

Mt Huascaran, Peru (after 1970)

Combination of Processes: Mt. Huascaran

Snow Avalanches Snow has same pull from gravity –Can fail in creep, fall, slide, flow Snowfall, wind-blown snow load slope towards failure –Also skiers

Snow Avalanche Large events - slab of snow breaks away Occurs because each layer of snow has different properties –Layers deposited at different times, vary in strength, hardness, density, thickness

Snow! Can travel at very high speeds, trapping skiers

Ways to Reduce Losses Due to Landslides Include: Avoid construction in areas prone to mass movement Build in a way that does not make naturally stable slope unstable Engineer water drainage to prevent strata to become water saturated and prone to fail

Note dipping beds undercut by excavation for house

Box 12.1 Failure occurs when water- saturated strata slide along slippery clay unit, breaching thin retaining wall

Submarine Movements Same types of failures occur underwater Example: Hawaii has significant slumps, debris avalanches Volcanic flank collapse - whole side of volcano falls off –Leads to tsunami

Hawaii

Big Island, Hawaii Motion along blocks near Kilauea today!

Next Time Subsidence and review for midterm