Presentation on theme: "Mass Wasting and Hillslopes Steep slope G d > F Gentle slope G d < F GpGp GpGp GpGp Moderate slope G d = F Boulder on verge of moving Boulder is stable."— Presentation transcript:
Mass Wasting and Hillslopes Steep slope G d > F Gentle slope G d < F GpGp GpGp GpGp Moderate slope G d = F Boulder on verge of moving Boulder is stable Boulder moves downslope F F F GdGd GdGd GdGd W W W Gravity overcomes Friction W = mg F 0 = G d
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 the downhill force = resisting force pointing uphill Downhill force = mass x gravity x sine of dip F 0 = mg sin (dip) (1) F 0 = mg sin (dip) (1) is the same as the dip F 0 = mg sin(α) α mg h dip
Your book uses mg = weight "w" Downhill force = mass x gravity x sine of dip F 0 = w sin (dip) (1) F 0 = w sin (dip) (1) is the same as the dip Shear Force Shear Force = F 0 = w sin(α) α w h dip Aside: Bloom confuses shear force with shear stress. Stress = Force / unit area Stress units are, e.g. Newtons/m 2 or pounds force/ inch 2 aka psi That said, we will skip the issue by staying with Forces
Role of water for slabs Friction ForceNormal ForceFriction Force is proportional to Normal Force It is the amount of Force needed to lift the surfaces apart Increased water pressure between the surfaces lifts the upper slab, and it will slip at a lower dip angle. Proportionality constant c α mg h dip N
Friction Coefficient c? F f uphill = N x constant “c” Notice N = mg cos When it slips, F 0 = F f = N x constant Then F 0 = mg sine mg cos x c so c = sine cos
Example Suppose the rock slips at = 30 o sine 30 o = 0.5 Cosine 30 o = c = Sine 30 o /cosine 30 o c = 0.5/0.866 = α mg h dip N
Water's role for slabs: Before Fall
Water's role for slabs: After Fall Of course, in our area, winter freezing causes frost wedging, breaks loose any remaining bonds
Slow mass movement indicators Example: Soil Slump
Scarp CD Lobe DF Soil Creep
Signs of Soil Creep Vertical features exposed in new roadcut Vertical features (if available) curved near surface
Creep Typical Features “Drunken forest”
Solifluction Soil saturated with water, soggy mass flows downhill When soil moisture cannot flow deeper, trapped in soil
Gelifluction: Freezing lifts particles, thaw drops them further downhill
Rapid Mass Movement Flows: mixture moves downslope as a viscous fluidFlows: mixture moves downslope as a viscous fluid Slumps: move downslope along a concave slip surfaceSlumps: move downslope along a concave slip surface Slides: move downslope along preexisting plane of weakness as a single, intact massSlides: move downslope along preexisting plane of weakness as a single, intact mass Falls: rock drops from steep slopeFalls: rock drops from steep slope
Rapid Mass Movement
Flows Flows with a high water content are less viscous, faster and more dangerous –Debris avalanches- rain- regolith detaches 200 kilometers per hour –Lahars –Liquefaction- Quick Sand due earthquake - increased pore water pressure - grains separate - liquefies instantaneously –Mudflow swift slurry- heavy rains –Earthflows dry masses of clayey regolith 1-2 meters per hour Mixture moves downslope as a viscous fluid
Yungay Avalanche Yungay Avalanche Source: Lloyd S. Cluff Debris Avalanche Town in Peru Earthquake dislodged Slab ice => landslide killed May 31, 1970 Ancash Earthquake
Liquefaction - Quick Clay or Sand Asphalt Parking Lot Caused by Earthquakes Sediment not compacted is like “pick-up-sticks Seismic waves increase fluid pressure, force grains apart, structures above resting on water, they sink in.
Mudflow in Sarno, Italy, 1998
Slumgullion Earthflow San Juan Mtns, CO Volcanics Dams Lake Fork of the Gunnison Earthflows dry masses of clayey regolith 1-2 meters per hour
Slides Slumps: special case, weakness is curved Mudslides Rock Slides Avalanche and Debris Slides Slides: move downslope along preexisting plane of weakness as a single, intact mass
Slumping with visible Scarps in Dorset, England These are rotational Slump
Little Hat Mountain Slump, CA scarp Toe, no veg.
La Conchita Slump La Conchita Slump Typical urban landslide, after heavy rains –Preexisting slide masses –Development to the edge of existing –Lawsuits –9 houses destroyed –Property values down
slump scar Snow Avalanche
Turtle Mountain Debris Slide
infamous-frank-slide/ East limb limestones at steep angle Locals mining coal seam under thrust fault April 1903
Frost heave, Yosemite NP. Glacier Point climbing area Rockfall 162,000-ton granite slab. 160 mph speed. Killed several people. Falls: Rockfall
Angle of Repose For loose materials, the angle of repose dictates the maximum steepness a material can be arranged before it will move downslope Bloom claims: p 189 lower right to 190 “The angle of the talus is a function of fragment size and angularity ….” Rockfall Talus Slope
An Example These 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. erosion/talus-creep.htm
Angle of Repose depends on particle size and shape? Is this right? Should we believe this? Do an experiment. What is your null hypothesis?
Slope Stability Slope characteristics such as composition, vegetation, and water content also influence slope stability. Haiti is plagued by slides after many trees were cut down.
Natural Triggers Natural triggers such as:Natural triggers such as: –torrential rainstorms 1967 central Brazil –Earthquakes 1812 New Madrid, Missouri –volcanic eruptions 1980 Mount St. Helens produce damaging mass movements
Human Triggers excessive irrigationexcessive irrigation clear-cutting of steep slopesclear-cutting of steep slopes slope oversteepening or overloadingslope oversteepening or overloading mining practicesmining practices can also cause mass movement. can also cause mass movement.