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Mass Wasting and Hillslopes

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1 Mass Wasting and Hillslopes
Gravity overcomes Friction Steep slope Gd > F W = mg F0 = Gd F Boulder moves F downslope Moderate slope Gd= F Gd Gp Gd W Gp Boulder on verge of moving W F Gentle slope Gd< F Gd Gp W Boulder is stable

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 the downhill force = resisting force pointing uphill Downhill force = mass x gravity x sine of dip F0 = mg sin (dip) (1) a is the same as the dip F0 = mg sin(α) dip mg h α

3 Your book uses mg = weight "w"
Downhill force = mass x gravity x sine of dip F0 = w sin (dip) (1) a is the same as the dip Shear Force = F0 = w sin(α) dip Aside: Bloom confuses shear force with shear stress. Stress = Force / unit area Stress units are, e.g. Newtons/m2 or pounds force/ inch2 aka psi That said, we will skip the issue by staying with Forces w h α

4 Role of water for slabs Friction 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 dip mg N h α

5 Friction Coefficient c?
Ff uphill = N x constant “c” Notice N = mg cos a When it slips, F0 = Ff = N x constant Then F0 = mg sine a = mg cos a x c so c = sine a / cos a

6 Example Suppose the rock slips at a = 30o sine 30o = 0.5
Cosine 30o = 0.866 c = Sine 30o /cosine 30o c = 0.5/0.866 = 0.577 dip mg N h α

7 Water's role for slabs: Before Fall

8 Water's role for slabs: After Fall
Of course, in our area, winter freezing causes frost wedging, breaks loose any remaining bonds

9 Classification of slope movements
Slides Flows Slumps Falls (note rotation)

10 Slow mass movement indicators
Example: Soil Slump

11 Soil Creep CD Scarp Lobe DF

12 Signs of Soil Creep Vertical features exposed in new roadcut
Vertical features (if available) curved near surface Vertical features exposed in new roadcut

13 Creep Typical Features
“Drunken forest”

14 Solifluction Soil saturated with water, soggy mass flows downhill
When soil moisture cannot flow deeper, trapped in soil

15 Gelifluction: Freezing lifts particles, thaw drops them further downhill

16 Gelifluction: Thaw

17 Rapid Mass Movement Flows: mixture moves downslope as a viscous fluid
Slumps: move downslope along a concave slip surface Slides: move downslope along preexisting plane of weakness as a single, intact mass Falls: rock drops from steep slope

18 Rapid Mass Movement

19 Flows Mixture moves downslope as a viscous fluid 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

20 Debris Avalanche Yungay Avalanche May 31, 1970 Ancash Earthquake
Town in Peru Earthquake dislodged Slab ice => landslide 25000 killed Source: Lloyd S. Cluff


22 Lahar

23 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.

24 Mudflow in Sarno, Italy, 1998

25 Slumgullion Earthflow
Earthflows dry masses of clayey regolith 1-2 meters per hour San Juan Mtns, CO Volcanics Dams 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 mass Slumps: special case, weakness is curved Mudslides Rock Slides Avalanche and Debris Slides

27 Slump Slumping with visible Scarps in Dorset, England These are rotational

28 Little Hat Mountain Slump, CA
scarp Toe, no veg.

29 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

30 Snow Avalanche slump scar

31 Turtle Mountain Debris Slide

32 East limb limestones at steep angle
Locals mining coal seam under thrust fault April 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 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

35 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.

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 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.

38 Natural Triggers Natural triggers such as:
torrential rainstorms 1967 central Brazil Earthquakes 1812 New Madrid, Missouri volcanic eruptions 1980 Mount St. Helens produce damaging mass movements

39 Human Triggers excessive irrigation clear-cutting of steep slopes
slope oversteepening or overloading mining practices can also cause mass movement.

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