3.3. Slope processes and development Flashcards
Define slopes
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
Define slope form
The shape of the slope in cross section
Define slope evolution
Development of slopes over time
Define slope processes
Activities acting on the slope
Define regolith
The superficial and unconsolidated material found at the Earth’s surface. It includes soil, scree, weathered bedrock and deposited material
Why is slope an open system?
- 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)
Factors affecting slopes
- Climate
- Aspect, the direction it faces
- Vegetation
- Geology
- Regolith and soil
How does climate affect slope?
- Humid areas - slopes are rounder due to chemical weathering
- Arid areas - slopes are more jagged due to more mechanical weathering
How does geology affect slope?
- 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
How does regolith and soil affect slope?
- Deep regolith can mean slope instability
- Regolith with high % of sand can cause slope failures
How does aspect affect slope?
- 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
How does vegetation affect slope?
- Vegetation can decrease overland runoff and reduce erosion or binds the soil together
- However, biological weathering can occur, deepening regolith and increase slope failure
Define mass movement
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
Why does mass movement occur?
Due to slope failure, when gravity acts to move the material downwards and the resistance of the slope is not strong enough
2 factors that cause slope failure
- A reduction in shear strength
- An increase in shear stress
Factors increasing shear stress
- 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
Factors reducing shear strength
- 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
What are the types of mass movements?
- Flow
- Slide
- Falls
- Slumps
- Avalanches
Define heave and how does it occur?
- 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
Define flows and how does it occur?
- 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
Define slides and how does it occur?
- 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
Define falls and how does it occur?
- 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
Define avalanch and how does it occur?
- 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)
