Presentation on theme: "Mass Wasting and Hillslopes"— Presentation transcript:
1 Mass Wasting and Hillslopes Gravity overcomes FrictionSteep slopeGd > FW = mgF0 = GdFBouldermovesFdownslopeModerateslopeGd= FGdGpGdWGpBoulderon vergeof movingWFGentleslopeGd< FGdGpWBoulder isstable
2 Sliding Threshold when gravity component = friction component, both parallel to slope Shear Forces are parallel to 2 touching surfaces.If the slab is about to move, then thedownhill force = resisting force pointing uphillDownhill force = mass x gravity x sine of dipF0 = mg sin (dip) (1) a is the same as the dipF0 = mg sin(α)dipmghα
3 Your book uses mg = weight "w" Downhill force = mass x gravity x sine of dipF0 = w sin (dip) (1) a is the same as the dipShear Force = F0 = w sin(α)dipAside: Bloom confuses shear force with shear stress.Stress = Force / unit areaStress units are, e.g. Newtons/m2or pounds force/ inch2 aka psiThat said, we will skip the issue by staying with Forceswhα
4 Role of water for slabs Friction Force is proportional to Normal Force It is the amount of Force needed to lift the surfaces apartIncreased water pressure between the surfaces lifts the upper slab, and it will slip at a lower dip angle.Proportionality constant cdipmgNhα
5 Friction Coefficient c? Ff uphill = N x constant “c”Notice N = mg cos aWhen it slips, F0 = Ff = N x constantThenF0 = mg sine a = mg cos a x cso c = sine a / cos a
6 Example Suppose the rock slips at a = 30o sine 30o = 0.5 Cosine 30o = 0.866c = Sine 30o /cosine 30oc = 0.5/0.866 = 0.577dipmgNhα
17 Rapid Mass Movement Flows: mixture moves downslope as a viscous fluid Slumps: move downslope along a concave slip surfaceSlides: move downslope along preexisting plane of weakness as a single, intact massFalls: rock drops from steep slope
19 FlowsMixture moves downslope as a viscous fluidFlows with a high water content are less viscous, faster and more dangerousDebris avalanches- rain- regolith detaches 200 kilometers per hourLaharsLiquefaction- Quick Sand due earthquake - increased pore water pressure - grains separate - liquefies instantaneouslyMudflow swift slurry- heavy rainsEarthflows dry masses of clayey regolith1-2 meters per hour
20 Debris Avalanche Yungay Avalanche May 31, 1970 Ancash Earthquake Town in PeruEarthquake dislodgedSlab ice => landslide25000 killedSource: Lloyd S. Cluff
23 Liquefaction - Quick Clay or Sand Asphalt Parking LotCaused by EarthquakesSediment not compacted is like “pick-up-sticksSeismic waves increase fluid pressure, force grains apart, structures above resting on water, they sink in.
25 Slumgullion Earthflow Earthflows dry masses of clayey regolith1-2 meters per hourSan Juan Mtns, COVolcanicsDams Lake Fork of the Gunnison
26 Slides Slumps: special case, weakness is curved Mudslides Rock Slides Slides: move downslope along preexisting plane of weakness as a single, intact massSlumps: special case, weakness is curvedMudslidesRock SlidesAvalanche and Debris Slides
27 SlumpSlumping with visible Scarps in Dorset, England These are rotational
32 East limb limestones at steep angle Locals mining coal seamunder thrust faultApril 1903
33 Falls: Rockfall Frost heave, Yosemite NP. Glacier Point climbing area. 162,000-ton granite slab.160 mph speed.Killed several people.
34 Angle of ReposeFor loose materials, the angle of repose dictates the maximum steepness a material can be arranged before it will move downslopeBloom claims: p 189 lower right to 190 “The angle of the talus is a function of fragment size and angularity ….”Rockfall Talus Slope
35 An ExampleThese talus cones illustrate the characteristic steep slopes. Talus, due to its large grain size, has a steep angle of repose.Talus cones from Glacier National Park in Canada.
36 Angle of Repose depends on particle size and shape? Is this right? Should we believe this? Do an experiment.What is your null hypothesis?
37 Slope StabilitySlope characteristics such as composition, vegetation, and water content also influence slope stability.Haiti is plagued by slides after many trees were cut down.
38 Natural Triggers Natural triggers such as: torrential rainstorms 1967 central BrazilEarthquakes 1812 New Madrid, Missourivolcanic eruptions 1980 Mount St. Helensproduce damaging mass movements
39 Human Triggers excessive irrigation clear-cutting of steep slopes slope oversteepening or overloadingmining practicescan also cause mass movement.