Mass Movements: Flows and Avalanches

1. Slides Turnagain Heights, Anchorage, Alaska, 1964 • Magnitude 9.2 earthquake triggered numerous mass movements • Rocks composed of glacially ground, clay-rich sediments • Sliding began after 90 seconds of shaking liquefied deep clays • Rotational slides trapped deep clay layer so it deformed internally, moving block above Figure 10.28 Figure 10.27

2. Flows • Mass movements that behave like fluids – internal movements dominate, slip surfaces absent or short-lived • Range of: • All sizes of materials • Wet to dry • Barely moving to > 200 mph • Gradation from movement on slip surface, to no slip surface • Many names: loess flow, earthflow, mudflow, debris flow, debris avalanche

3. Flows Portuguese Bend, California, Earthflow • Rock layers dip seaward, contain bentonitic clay, and ocean waves erode toe and keep ancient earthflow moving seaward • Unstable land used for farming until residential development built in 1950s Figure 10.30

4. Flows Portuguese Bend, California, Earthflow Figure 10.31 Figure 10.32

5. Flows La Conchita, California Slump, Debris Flows, 1995, 2005 • Cliff behind La Conchita is ancient landslide • 1995: two slow landslides destroyed 14 houses, but no deaths • 2005: 15% of 1995 slide mass remobilized into highly fluid debris flow, at 10 m/sec, that went over retaining wall • Destroyed 13 houses, damaged 23 others, killed 10 people 1995 – Figure 10.33 2005 – Figure 10.34

6. Flows Long-Runout Debris Flows • Most spectacular, complex movement – massive rock falls that convert into highly fluid, rapid debris flows that travel far (also called sturzstroms) • Rock falls and small-volume avalanches flow horizontally less than twice vertical distance of fall • Very large rock falls (more than 1 million m3) travel up to 25 times vertical fall – have lower coefficient of friction

7. Flows Blackhawk Event, California, 17,000 years ago • Huge rock fall in San Bernardino Mountains flowed out into Mojave Desert – flowed 7.5 times farther than fell, at speeds estimated up to 120 km/hr Figure 10.35

8. Flows Elm Event, Switzerland, 1881 • Farmers quarried slate from base of mountain until cracks opened up in hillside above • Fall, jump, surge: • Mass of mountain began to disintegrate as it fell • Hit floor of quarry and disintegrated completely • Shot out from mountainside ledge, flowed 2,230 m into valley Figure 10.36

9. Flows Turtle Mountain, Alberta, Canada, 1903 • 90 million ton mass of dipping limestone slid down daylighted bedding surface 3,000 feet into valley • Shattered, flowed 3 km across valley, 130 m up opposite side • Buried southern end of town, killing about 70 people Figure 10.37

10. Flows Nevados Huascaran Events, Peru • 1962: No perceptible trigger • Mass of glacial ice and rock fell  13 million m3 debris flow • Debris flowed up to 170 km/hr down river valleys, killing 4,000 people • 1970: magnitude 7.7 subduction-zone earthquake 135 km away • Portion of same peak collapsed and fell vertically 400 to 900 m • Mass landed on glacier and slid • Raced up side of glacial-sediment hill, launching debris into air • Rain of rocks and boulders for 4 km downslope • Flow buried town and 18,000 people beneath more than 30 m

11. Flows Nevados Huascaran Events, Peru • 1970 Flow buried town and 18,000 people beneath more than 30 m Figure 10.38

12. Flows Movement of Highly Fluidized Rock Flows (Sturzstroms) Hypotheses for fast and far movement: • Water provides lubrication and fluidlike flow • Some observed flows were dry • Steam liquefies and fluidizes moving mass • Frictional melting fluidizes moving mass • Some deposits containblocks of ice, lichen  no significant heat or friction • Falling mass traps air beneath and rides trapped air • Elm sturzstrom was in contact with ground • Identical flow features on ocean floor, Moon, Mars (no atmosphere)

13. Flows Movement of Highly Fluidized Rock Flows (Sturzstroms) Most likely hypothesis for fast and far movement: • Blocks in moving mass hit blocks in front of them, imparting kinetic energy  vibrational or acoustical energy propagates as internal waves, fluidizing rock debris (acoustic fluidization)

14. Flows • Behave like earth mass movements – creep, fall, slide, flow • Small to large, barely moving to 370 km/hr, few meters to several kilometers • Small avalanches typically fail at one steep point, in loose, powdery snow, which triggers more and more snow moving downhill • Usually begin when snow reaches 0.5 to 1.5 m deep • Snow depth can reach 2 to 5 m before big avalanches occur, if snowflakes become rounded and packed

15. Flows • Loose-powder avalanches • Low cohesion with up to 95% volume as pore space • Slab avalanches • Slabs of snow that break free from base like translational slides, turning into flows on way down • Snow mass composed of layers with different ice, snow characteristics  different strength • Numerous potential failure surfaces • Dry snow forms faster avalanches than wet snow • Avalanches may flow for many miles, up and over ridges

16. Mitigation • Controlling rotational and translational landslides • Unloading head, reinforcing body and supporting toe • Controlling flows of earth or snow • Steering flow by building walls (gabions) and digging channels • Controlling rock falls • Removing rock and decreasing slope angle • Holding rocks in place with wire mesh nets • Using fence along road to catch fallen rocks Figure 10.44

17. Mitigation Submarine Mass Movements • Same mass movements occur below sea: rotational slumps in delta deposits; complex failures at subduction zones; debris flows down submarine volcano slopes

18. Subsidence • Ground surface sags gently or drops catastrophically as voids in rocks close • Slow compaction of loose, water-saturated sediments or rapid collapse into caves Slow Subsidence • Ground surface slowly sinks as fluids (water or oil) are removed below surface (squeezed out or pumped) • Removal of fluid volume and decrease in pore-fluid pressure compacts rock, lowering ground above

19. Subsidence Delta Compaction, Mississippi River, Louisiana • Delta: loose pile of water-saturated sand and mud  compacts and sinks down • Mississippi River delta underlain by 6 km thick sediments deposited in last 20 million years • Current river position constant for last 20,000 years, but shifts frequently and held in place now by human action Figure 10.47

20. Subsidence Delta Compaction, Mississippi River, Louisiana • New Orleans and region sinking by sediment compaction, dewatering, isostatic adjustment – about 45% of city below sea level, prone to high-water surges in hurricanes Figure 10.46

21. Subsidence Groundwater Withdrawal, Mexico City • Aztecs built aqueducts to bring water from mountains • Extraction of groundwater through wells began in 1846 • Withdrew water faster than it is replenished, causing land subsidence of 10 m • Groundwater withdrawal is now banned, but subsidence can not be reversed

22. Subsidence Oil Withdrawal, Houston-Galveston Region, Texas • Pumping of water, gas, oil began in 1917 • Houston-Galveston relies on groundwater withdrawals • Area has sunk up to 2.7 m, renewing movement on old faults that act as landslide surfaces Figure 10.48

23. Subsidence Long-Term Subsidence, Venice, Italy • Venice is built on soft sediments that compact under weight of city itself, as global sea level rises • Venetians have been building up islands with imported sand for centuries • 20th century pumping of groundwater rate of sea level rise in Venice doubled • Sea level projected to rise 50 cm in 21st century

24. Subsidence Long-Term Subsidence, Venice, Italy • Movable floodgates across entrances to lagoon • Would disrupt shipping, prevent outward flow of contaminants • More sediment to raise ground level • Pump seawater or carbon dioxide into sand below city to ‘pump up’ region Figure 10.49

25. Side Note: How to Create a Cave • Caves usually occur in limestone • Equilibrium equation to create or dissolve limestone: • Ca++ + 2HCO3CaCO3 + H2CO3 • Ca is calcium ion • HCO3is bicarbonate ion • CaCO3is calcite limestone • H2CO3is carbonic acid • Left to right – limestone is precipitated • Right to left – limestone is dissolved • Controlled by amount of carbonic acid, which is controlled by amount of carbon dioxide

26. Subsidence Catastrophic Subsidence: Limestone Sinkholes, Southeastern U.S. • Limestone forms from CaCO3shells of marine organisms, dissolves in naturally acidic groundwater flowing through  forms extensive water-filled caverns • When groundwater levels drop, caverns are empty and buoyant support of water holding up cavern roofs is removed  roofs collapse, forming sinkholes Figure 10.50 Figure 10.51

27. End of Chapter 10