GEOS 110
NATURAL DISASTERS
 
Landslides-I
1. "Landslides" or Mass Wasting
 Includes all processes that involve the movement of rock and/or soil downslope.
 - Important erosional and transport mechanism particularly in tectonically active, mountainous   areas and near steep shorelines.
 - Important as a significant engineering problem and human hazard
 
2. Slope Stability
 - Balance between driving forces (gravity) and resisting forces (friction).
  1) Driving force- the downslope component of weight (gravity driven).
    Fd = wsin? (downslope force = weight times the sine of the slope angle)
  2) Resisting force- related to the shear strength of the slope material and      the length of a potential slip surface.
 
 - Factors affecting slope stability
  1) Slope and topography- increase ? will increase Fd.
  2) Amount of water in the materials
   - surface tension increases shear strength if material is undersaturated.
   - decreases shear strength if material is saturated.
   - Related to climate and vegetation conditions
 
  3) Nature of the material and geological factors
   - variation in shear strength of rocks and soils.
   - local bedrock structure (ie. sloping layers)
 
3. Mass Wasting "Triggers"
 -Anything that causes the driving force to become > the resisting force and the shear strength   of the material is exceeded.
 1. Undercutting of the base of a slope (natural erosion or by humans)
 2. Loading of the upper part of the slope (usually human)
 3. Vibration due to earthquake
 4. Increase in moisture content (rainstorm or irrigation)
 5. Change in vegetation or climate
 
 
4. Slope Processes
 - Terminology based on:
  1) nature of material   (rock or unconsolidated)
  2) nature of the movement  (flowing, sliding, falling)
  3) speed of movement  (slow or fast)
 
 - Types of Slope Processes
  1) Creep- very slow (cm/yr) of unconsolidated material.
  2) Flow - unconsolidated material that mixes during movement
    (earthflow, debris flows, mudflows, debris and rock avalanches)
  3) Sliding- movement of a coherent mass of soil or rock.
    (slumps, debris slides, rocks slides)
  4) Falling- materials free fall through the air (rock falls)
 
Prof. Gorring         Apr. 9, 1998
GEOS 110
NATURAL DISASTERS
 
Landslides-II
Notable Landslide Events
 
 1. Vaiont Dam, Italy, October 9, 1963- World's worst dam disaster, 2600 deaths.
 - Coherent mass of rock termed a "translational slide". Involved 1.8 km (~1 mi)   wide, 2 km (~1.5 mi) long, and 200 m (~600 ft.) thick slab of limestone.   Speeds of ~100 km/hr (60 mph).
 - Filled in one end of the reservoir to over 150 m above the water level and    caused 100 m waves that overtopped the dam. Created 70 m wall of water   that raced downstream for several miles, killing 2,000+ people.
 
 - Factors contributing to the event:
  1) Local geology- "bowl-shaped" fold structure of sedimentary rocks.
  2) Saturation of the toe of the slide- filling of dam
  3) Heavy rainfall-
 
 - Precursor events: (engineers were monitoring slopes during reservoir filling)
  1) Smaller landslide event in 1960 caused only 2 m high waves.
  2) Various kinds of creep and earthflows occurred from 1960 to 1963.
   rate of motion was 1 - 30 cm per week (0.5" - ~1.2 ft).
  3) By late September, 1963 slide was moving at 25 cm/day (~1 ft/day).     Animals grazing on the slopes left on October 1.
  4) On October 8 and 9, engineers realized that the whole mass was     moving as unit. Started to lower reservoir level, but it didn't     work. At 10:40 PM, the slide occurred and reservoir water     overtopped the dam.
 
 2. Nevado Huascaran Event, Peru, 1970- Huge debris flows, ~25,000 deaths.
 - Earthquake-triggered, rock fall-debris flows that started from Nevado    Huascaran (~21,000 ft high) totally destroyed towns Yungay and     Ranrahirca. Involved large volume (100 million m3) of rock and ice    moving at speeds of ~300 km/hr (200 mph). Traveled ~15 km from    source, devastating 25 km2 area.
 
 - Factors contributing to the event:
  1) Local geology- Nevado Huascaran is composed of vertically fractured    granite, numerous EQ's along the Andes. Glaciers as a lubricant.
  2) Earthquake shaking- Mw = 7.7 subduction zone EQ only 135 km away.    shaking lasted 45 to 60 seconds.
  3) Extreme topography- vertical drop from summit to town is ~4,000 m    (~14,000 ft) occurs in only 12 miles horizontal.