GEOG 2051 : GEOG 2051 EXAM 2
CH. 13: SLOPES AND MASS MOVEMENTS
Slopes
• A curve or inclined surface that represents the boundary of a land
• How steep a slope is determines its stability
• Several systems for talking about steepness of slopes
o Angles
▪ more or less than 90 degrees
o Tangent
▪ The rise (distance you go up) over the run (the horizontal distance)
o Gradient
▪ Tangent X 100% (ex. 0.1 X 100% = 10% gradient)
• Can think of the slope as a SYSTEM
o Overtime you end up developing a cliff and as soon as the block of rock rises
up it is exposed to the surface and begins weathering and bits and pieces of
rock will be pulled down by gravity and create a pile at the foot of the cliff.
o The pile of material has a slope and can be thought of as a system
▪ Inputs – bits of rock that come from weathering
▪ Outputs – material that is transported off of the pile
▪ Pile builds up to get steeper and steeper and eventually reach a point
where pile becomes unstable and material will tumble off b/c it is at its
maximum steepness called the angle of repose
▪ Angle of repose = equilibrium state for a slope system
The balance angle for our slope
▪ Lose material in form of transport off of pile which often occurs in
form of mass movements (ex. land slides)
• Whether or not a pile of material is stable depends on the balance of forces that are
acting on the slope
• 2 categories of force
o Resisting Force (FR)
▪ Acting to hold particles in place
▪ Acting perpendicular to the slope
o Driving Force (FD)
▪ Trying to pull materials down hill
▪ The driving force
▪
o If the resisting force > driving force = more stable FR > FD
▪ In some cases the Fd > Fr (as the slope becomes steeper)
• Gravity
o Main force that drives mass movements
o Both driving and resisting force
o Always acts directly downward to the center of the earth
o Driving force - Component acting parallel to the slope
o Resisting force - Acts perpendicular to the slope, acting to hold the material in
place
o Less steep slope FD < FR, particles are more stable
o Steeper slope FD > FR, particles are more unstable
• Friction
o Acts as a resisting force
o Force that is imparted by a surface on objects moving across that surface
o Nature of the material that makes up the slope
▪ If material is angular – particles will lock together and will have to
overcome a lot of friction to move (might interlock)
▪ If material is more rounded - Will roll relatively easy, less amounts of
friction
• Cohesion
o Cohesive forces cause particles to stick together
o Influences slope stability
o Modeling clay – high cohesive forces
o Sand - Very little cohesion forces
o Can increase cohesion with water
• Water
o Can cause cohesion, cause particles to stick together
▪ Need just the right amount of water to cause cohesion
▪ Reduces slope stability in mist cases
o Acts as a lubricant by reducing friction – more friction on land than in the
water
o Acts as Buoyancy force (tendency to float in water)
o Adds weight to the slopes
Slope Elements
• Waxing slope (means to increase)
o At the top of the slope, relatively flat surface
o Angle of the slope gets steeper and steeper
• Free Face/ Fall face
o Steepest slope
o Tends to the area that supplies material to the lower portions of the slope
• Waning Slope (means to decrease) AKA Debris slope
o Steepness starts to decrease
o composed of debris that is weathered out higher on the slope
Mass Movements
• We recognized different types of mass movements based on characteristics:
o Speed
▪ Some MM are extremely fast (avalanche) (60-80 mph)
▪ Some MM are very slow (Soil Creep - few mm or cm per year)
o Amount of Moisture
▪ Mud flow – require a lot of water
▪ Rock Fall – don’t require any water at all
o Type of Material involved
▪ Can have earth flows – where material is primarily earth (soil)
▪ Debris flow – mix of soil, chunks of rocks, trees, bush
• Categories of Mass movements
o FALLS/ Avalanches (10 meters/sec)
▪ Typified by being very fast and don’t require moisture
But could be involved
▪ Material completely leaves contact with the ground
▪ The only thing slowing it down is the friction of the air
▪ Avalanche – a bit slower than a true fall but still extremely fast
A little bit of contact on the slope (with the ground) – every
time it bounces off the slope it slows down a little
o Flows
▪ Can vary in speed (moderate to fast speeds)
▪ Tremendous amounts of water involved (material is saturated)
▪ Ex. Earth Flows and Mud Flows
o Slides
▪ Moderate speeds, much more friction involved (not as fast as an
avalanche)
▪ Moisture not necessarily involved but very frequently is
▪ The way that the material moves is what distinguishes a slide
Document Summary
If material is angular particles will lock together and will have to overcome a lot of friction to move (might interlock) But could be involved: material completely leaves contact with the ground, the only thing slowing it down is the friction of the air, avalanche a bit slower than a true fall but still extremely fast. Creep: slowest end of the scale (1 cm or mm per year, upper layers of soil are moving down hill slowly, can have freeze thaw where individual particles will rise up a little, freezing and thawing. Hydrologic cycle: movement of water from place to place around the earth"s surface, cycle components, changes in state, 3 phases: solid, liquid, gas, sublimation solid to water vapor. Water gets lifted by the wind: horizontal movements (one place to another) Surface runoff of water, channel wide flow in rivers and streams, the movement of ocean currents: advection.
