3.3. Slope processes and development

Question 1

Define slopes

Answer 1

Refers to an inclined surface or hill slope. Generally, it is the area between a watershed (or drainage basin divide) and the base of the slope

Question 2

Define slope form

Answer 2

The shape of the slope in cross section

Question 3

Define slope evolution

Answer 3

Development of slopes over time

Question 4

Define slope processes

Answer 4

Activities acting on the slope

Question 5

Define regolith

Answer 5

The superficial and unconsolidated material found at the Earth’s surface. It includes soil, scree, weathered bedrock and deposited material

Question 6

Why is slope an open system?

Answer 6

• Inputs to the system = Energy from the sun, mass (water and sediment)
• Outputs from the system = Energy (re-radiated heat), and mass (water and regolith)
• Some factors occur on the outside of the slope (exogenetic)
• Some factors occur on the inside of the slope (endogenetic)

Question 7

Factors affecting slopes

Answer 7

• Climate
• Aspect, the direction it faces
• Vegetation
• Geology
• Regolith and soil

Question 8

How does climate affect slope?

Answer 8

• Humid areas - slopes are rounder due to chemical weathering
• Arid areas - slopes are more jagged due to more mechanical weathering

Question 9

How does geology affect slope?

Answer 9

• Faults create lines of weaknesses and therefore instability in a slope can create steep valley sides
• Rock types can affect slopes vulnerability to weathering
• Vulcanicity can cause igneous intrusions (batholiths) which when exposed can create steep slopes

Question 10

How does regolith and soil affect slope?

Answer 10

• Deep regolith can mean slope instability
• Regolith with high % of sand can cause slope failures

Question 11

How does aspect affect slope?

Answer 11

• In northern hemisphere: North facing slopes receive very little heat from the sun in mid winter. Conversely, south facing slopes receive much more heat. Therefore, a north facing slopes will usually develop a dramatically different snowpack than a south facing slope.
• In southern hemisphere: South facing slopes tend to be warmer and often develop thin ice crusts. Because these crusts tend to grow weak layers around them from near-surface faceting

Question 12

How does vegetation affect slope?

Answer 12

• Vegetation can decrease overland runoff and reduce erosion or binds the soil together
• However, biological weathering can occur, deepening regolith and increase slope failure

Question 13

Define mass movement

Answer 13

Any large scale movement of the earth’s surface that are not accompanied by a moving agent such as a river, glacier, or ocean wave

Question 14

Why does mass movement occur?

Answer 14

Due to slope failure, when gravity acts to move the material downwards and the resistance of the slope is not strong enough

Question 15

2 factors that cause slope failure

Answer 15

• A reduction in shear strength
• An increase in shear stress

Question 16

Factors increasing shear stress

Answer 16

• Removal of lateral support through undercutting or slope steepening - erosion by rivers, wave action, faulting
• Removal of underlying support - undercutting by waves
• Loading of slope - weight of water, vegetation and accumulation of debris
• Lateral pressure - water in cracks, pressure release
• Transient stress - earthquakes

Question 17

Factors reducing shear strength

Answer 17

• Weathering
• Change in pore water pressure - saturation, softening of material
• Changes of structure - creation of cracks
• Organic effects - burrowing of animals, decay of tree roots

Question 18

What are the types of mass movements?

Answer 18

• Flow
• Slide
• Falls
• Slumps
• Avalanches

Question 19

Define heave and how does it occur?

Answer 19

• Heaves/creeps are slow movements of materials up to the surface of the slope and down slopes
• Very slow and small scale process.
• Occurs mainly in winter
• Individual soil particles are pushed to the surface by wetting, heating or freezing of water

Question 20

Define flows and how does it occur?

Answer 20

• Are movements of a mass of a soil or rock that contain significant amount of water
• Slumps occur on weaker rocks , especially clay.
• This is often rotational along a curved slip plane
• Clay absorbs water and becomes saturated and exceeds its liquid limit
• It then flows along a slip plane.
• This can be due to the undercutting at the base of a cliff

Question 21

Define slides and how does it occur?

Answer 21

• Are an entire mass of material moving along the slip plane
• They occur where there is a combination of weak rocks, and steep slope and undercutting
• As the slide moves along the slip plane, it tends to retain its shape and structure until it hits the bottom of the slope
• Slip planes occur at fault lines, along a bedding plane and at the junction of two layers
• Rock slides are when a huge volume of rock moves together
• Landslides include rock, stones and soil

Question 22

Define falls and how does it occur?

Answer 22

• Are movements of rock/debris under the effect of gravity
• Falls occur on steep slopes (greater than 700)
• On bare rock faces where joints are exposed they are more common
• Normally caused by weathering, and once the rocks are detached they fall under the influence of gravity.
• If the fall is short it produces straight scree, if it is long it forms a concave slope

Question 23

Define avalanch and how does it occur?

Answer 23

• Avalanches are rapid movements of snow down a slope
• They are common in mountainous areas
• New snow can fall of older snow ( a dry avalanche)
• Or partially melted snow moves ( a wet avalanche)
• They occur most frequently on slopes over 22o
• They also occur on north facing slopes where the lack of snow limits snow stability (doesn’t have time to cohese with partial melting)