<H1>Geography 111 - Mass wasting</H1>

Mass wasting (chapter 15)

This chapter in the textbook provides descriptions of different mass-wasting processes, but is not as strong on explanation of the physical principles that control slope stability and the occurrence of landslides or other forms of mass wasting. Therefore I suggest that you follow these notes, together with your class notes, rather than relying strictly on the textbook. Read the chapter, but please make note of my comments. I also recommend that you look over some of the links to other sites with information on mass wasting in order to supplement the material in the book.

The book omits a basic discussion of the balance of forces on a slope, which ultimately is what controls the likelihood of mass wasting. We will discuss this in class and the basic elements of the discussion are presented below.

Here are the topics that I am emphasizing:

Definition of mass wasting:

downslope transport of rock, soil, or sediment under the influence of gravity
may involve water as lubricating agent, or through liquefaction and formation of slurry, but water is not the actual transporting agent

Balance of forces on a slope:

shear stress (motivating force) vs. shear strength (resisting force)

For loose material on a slope, frictional properties of surface determine relative magnitude of resisting force that opposes downslope component of weight

In engineering calculations, factor of safety = ratio of shear strength/shear stress

when f.s. > 1, slope is stable
when f.s. = 1, slope is just barely stable
when f.s.< 1, slope is unstable

What affects this balance?

increased slope angle (steepening by basal erosion or excavation)
role of water
increased weight of materials on slope (may be caused by construction or by saturation during rain, snowmelt)
weathering and loss of cohesion
loss of vegetation cover causing loss of cohesion
episodes of ground shaking causing loss of cohesion
steepening of slopes by natural erosion or human interference (e.g. as in road cuts for highway construction)

Role of water particularly important

in loose sand, small amounts of water help sand grains stick together (surface tension)
may cause liquefaction, loss of cohesion, loss of shear strength when enough water is present
in other cases, acts as lubricant
adds to weight of slope materials
also excess pore water infiltrated through soil cover is under pressure, supports the weight of the surface layer, and allows it to slide laterally downslope

Definitions of talus, angle of repose (also known as angle of internal friction)

Types of mechanisms involved in mass wasting:

fall
slide

planar displacement
rotational displacement (slump)

loss of cohesion, liquefaction, slurry transport (flow)

earthflow (slow)
mudflow (fast)
debris flow (fast)

slow, incremental movement of surface layer (creep)

alternating cycles of expansion and contraction due to freeze-thaw processes at surface

on slopes with permafrost, freeze/thaw of surface layer above frozen subsurface, slow downslope transport (solifluction)

Note the range of rates, from mm/yr to hundreds of km/hr; some of these mechanisms may occur across a wide range of velocities (e.g. relatively slow earthflow vs. rapid debris flow and mudflow)

Some prominent examples illustrating effects (we may not get to look at all of these in class)

Madison Canyon landslide damming valley to form lake following earthquake near Hebgen Lake, Montana, 1959
Gros Ventre slide, Wyoming, 1925
Nevado de Huascaran debris avalanche following earthquake, 1970
Vaiont Dam disaster following earthquake, 1962
Blackhawk slide (prehistoric?) - rock debris riding cushion of air?
Lahar at Nevado del Ruiz, Colombia, 1985
Debris flows caused by Hurricane Camille, Nelson County, Virginia, 1969
Debris flows caused by local intense thunderstorm, Madison County, Virginia, 1995
Smaller landslides over low-permeability bedrock, West Virginia, November 1985