Glide horizon: The surface along which a mass movements (landslides) move or slip.

Glide horizon: The surface along which a mass movements (landslides) move or slip.

Avalanches: A turbulent mixture of debris and air that quickly moves down a steep slope at high velocity; the debris can be rock and/or snow.

Types of avalanches: Slab: consolidated material Loose: unconsolidated material Movement: only through the air, on the ground, or both. Dry vs. Wet: may or may not be mixed with water. Sliding surface: can slide on like materials, or on different materials (i.e., bedrock). **NOTE** Rock and soil can be subsisted in for snow in all of the diagrams to the left.

Avalanche chutes in Alaska and Utah. These can be the result of snow and/or rock avalanches. These must have had at least some rock avalanches because of the cones of debris at the bases. From Geohazards by N. Coch. Avalanche chutes are common on the sides of steep sided (U-shaped) glaciated canyons. Sometimes condominiums From Geohazards at ski by resorts N. Coch are built at the base of avalanche chutes that have been inactive long enough for vegetation to grow over them.

Avalanches are very destructive. These downed trees mark the path of an avalanche.

Rock avalanche in the Alps. Note houses for scale. So again, avalanches can be snow, rocks/soil, or both.

These pictures show a skier triggering an avalanche. Human activity can induce an avalanche, as it often does.

This bar graph displays the avalanche fatalities by anthropocentri c category over a 19 year period.

Above right, rescue workers use long poles to search for people buried by an avalanche. Above left, A large gun (Howitzer) is use to induce a manmade avalanche. This is a type of avalanche mitigation. Below right, people in CO watch a manmade avalanche at a ski resort.

Rock falls: Free falling rock caused by wedging, undercutting, or gravity. Debris falls: same as above only consisting of rock and soil.

Geologists look for certain features to recognize mass movements (slumpearthflow here), either active ones (this one is moving slowly) or old ones that might be reactivated, especially by construction. From Geohazards by N. Coch.

Recognizing landslides (continued), including inactive slides. Back tilting occurs in slumps, which can result in formation of a swamp/marsh or shallow pond. So clues to recognize old slumps are flattish areas on a hillside which may be swampy or have a shallow pond, as well as back-tilted trees (trees are tilted downhill in creep or earthflows). Also, look for evidence for formation of a new scarp (e.g. cracks) Center of rotation From Environmental Geology by Pipkin

Incipient slumping -- new scarp is forming. At this stage, the curved slide plane has formed and movement will continue at some point in the future unless stabilized. Slump has occurred! This one was triggered by the 1989 earthquake near San Francisco.

Commonly, landslide and other mass movements can be triggered by vibration of the ground. This can occur through earthquakes, impacts, other large landslides, or can be anthropogenic (e.g., construction). These dust clouds are from landslides that were triggered by earthquakes in CA. Shaking of the earth, such as earthquakes, construction, or even firing of munitions can cause landslides.

An earthquake shook off an 800-m wide ice slab from a glacier which fell over 3.7 km. The ice disintegrated into an avalanche traveling >300 km/hr! scar Friction melted the ice and it mixed with rock to form a slurry (debris flow) viscous enough to carry house-size rocks. Some of the flow flew over a ridge and buried 18,000 people! Portrait of a Planet by S. Marshak

Portrait of a Planet by S. Marshak Quick clay. Such clays can liquefy during shaking. Geology and the Environment by W. Pipkin Earthflow into river at Nicolet, Quebec resulting from liquefaction of a quick-clay layer. Vibration from firing of nearby artillery triggered the liquefaction.

Geology and the Environment by W. Pipkin Large landslide caused by quick clay that liquefied during earthquake

Portrait of a Planet by S. Marshak Over steepening of a slope from (a) erosion, and (b) cut and fill. This may cause landslides, depending on the strength of the material. On some of the previous slides it was evident that roads were cut into the hillsides that failed -- the building of the roads were probably an important factor in triggering the landslides.

Once a landslide is present, what are the best ways to make it move again?! Add weight to top Cut away toe Oversteepen slope (e.g., roadcuts, cuts for houses) Add water (e.g., water lawns) Vibrations (trucks, trains, artillery) Remove vegetation (increase infiltration for groundwater) How can mass wasting hazards be prevented or mitigated? Landslide hazard maps (identify current & potential areas of mass movement) Don t over-steepen slopes (too steep depends on material and pre-existing surfaces) Drain water (and/or decrease groundwater with vegetation or cease watering lawns) Decrease slope angle Remove upper part of slide mass (reduce driving force by reducing weight) Retaining walls Rock bolts Diversion walls or levees (for mudflows/debris flows) Rock sheds covering roads (see text) Deep foundations (soil creep)

Simple remedial action: remove much of the slide mass and cover remainder with coarse rock. More extensive remedial measures -- more effective but more costly. Drains and pipes remove water, and fill acts as buttress wall. From Environmental Geology by Pipkin

From Portrait of a Planet by Marshak

Shotcrete covers surface as a retraining wall. This also prevents water from infiltrating. From Environmental Geology by Pipkin Retaining wall with drainage. From Environmental Geology by Pipkin

This home owner in CA is trying to stop this cliff from failing by covering it with plastic.

Lateral spreading includes flows resulting from liquefaction of quick clays From Environmental Geology by Pipkin