1 Natural Disasters Mass Wasting: Landslides

2 What is a Mass Wasting? Downslope movement of soil or rock

3 Anatomy of a Landslide Scarp is crescent-shaped cliff at upslope end Bulge is usually at toe where soil accumulates Tensional crack at top

4 Mass Wasting Hazards

5 Mitigation - Identification

6 Factors that Influence Slope Stability Gravity – main force responsible for mass wasting –Pulls object toward the center of the Earth

7 Gravity Force of gravity can be broken into two components: 1.Force acting perpendicular (g p ) –Helps hold object in place on slope 2.Tangential component (g t ) –Shear stress pulling object down-slope

8 Shear Strength Shear Strength – forces resisting movement down slope Two components: 1.Frictional resistance 2.Cohesion When shear stress becomes greater than shear strength, slope failure will occur F s = shear strength/shear stress If F s < 1, slope failure is expected

9 Example Problem If a body of rock has a shear stress of 35 and a shear strength of 25, what is the force? Strength Stress F = = Will slope failure occur?Yes

10 Factors Controlling Mass Wasting Slope Angle Pore Water Material Orientation

11 Slope Angle Angle of Repose –The angle at which loose material sits at rest The Angle of Repose increases with size of grain

12 Pore Water Water held between pore spaces –Slightly wet may provide surface tension –When saturated, adds weight and provides lubrication – grains move freely

13 Material Clays are expansive and hydrocompacted –Water enters crystal structure and volume increases –Loss of water causes volume to decrease (shrink)

14 Material Salts that hold clay structure may dissolve when water is added causing clays to compact

15 Orientation When layered rocks dip in the same direction as slope, failure is likely –Most stable slopes are when rock layers dip upslope

16 Types of Mass Wasting Slow Movement Creep Rapid Movement Landslides Slumps Debris flows Rock falls

17 Creep Continuous gradual downhill movement of slope material

18 Landslides Blocks of rock and solid move down along a well- defined plane

19 Slumps A mass of material sliding along a curved, rotational surface (shaped like a spoon

21 Debris Flows Downslope movement of a viscous (thick) fluid

22 Rock Falls Free fall of detached pieces of material of any size

23 Triggering Events Shaking Modification of Slopes Undercutting of Slopes Changes in Hydrologic Cycles Volcanic Eruptions

24 Shaking Sudden movements may trigger instability Friction is lost when water gets between grains Solid ground acts like a fluid

25 Yungay, Peru Nevados de Huascarán - Peruvian Andes Mtns. 7.7M earthquake 45 seconds of shaking Large block failed producing a debris avalanche 18,000 people buried 600+ died on opposite side of slope

26 Modification of Slope By humans or natural causes, may change slope angle Mass wasting event can restore stability of slope

27 Elm, Switzerland Large demand of slate for blackboards Mining for slate began Mass of rock collapsed and bounced uphill 115 people died

28 Changes in Hydrologic Cycles Heavy rains, rapid snow melt, changes in groundwater can saturate grounds Increase fluid pressure

29 Vaiont Reservoir, Italy Dam was built with steeply dipping clay layers Heavy rain added weight to rocks above dam Large block fell into reservoir displacing large amounts of water Waves killed nearly 3000 people

30 Volcanic Eruptions Eruptions may cause rapid snow melt; thus enhancing the probability of a debris flow

31 Nevado del Ruiz, Colombia Town of Armero end of valley at base of volcano Small eruption triggered rapid snow melt 23,000 people died

32 Prediction & Hazard Assessment Past evidence may leave distinctive clues of previous landslides Hazard maps help locate areas prone to mass wasting Can be used to make decisions for land use policies

33 Mitigation - Prevention Steep slopes can be graded into gradual slopes Retaining walls can be built Drain pipes to reduce pore water