Slopes Flashcards
What is Hillslope?
The elevated land between valley bottoms
What are hillslope processes?
Movement of rock and soil by mass wasting, rain splash and overland flow
Characteristics of bedrock slopes?
Soil production < erosion
Little to no soil cover with bedrock at surface
Sediment transport is weathering limited
Slope form controlled by rock properties
Characteristics of soil-mantled slopes?
Soil production > erosion
A surficial layer of soil covers the bedrock
Sediment transport is transported limited
Slope form controlled by soil properties, production & transport rates
What are bedrock hillslopes?
WEATHERING LIMITED
- With steep slopes
- Thin to absent soil mantle
Where are bedrock hillslopes found?
Arid climates
Common in humid areas with low weathering rates
What are the processes of bedrock hillslopes?
- Falls and slides
What are the physical characteristics of bedrock hillslopes?
Parallel retreat at threshold angles
What are soil-mantled hillslopes?
Transport-limited
With thick soil mantle
Where are soil-mantled hillslopes found?
Humid climates
What are processes of soil-mantled hillslopes?
Soil creep and slides
what are the physical characteristics of soil-mantled slopes?
Convex profiles
Typically parabolic
What does surface erosion impact?
Individual grains, detached separately and transported over the slope surface
What are some examples of surface erosion?
- Rain-splash
- Overland flow
- Rills
- Gully
- Reactivated ancient gullies
What are the types of overland flow?
Horton Overland flow
Slope foot saturation - termed saturation overland flow
What is the Horton Overland Flow?
Rainfall rate excess of infiltration capacity
What is overland flow?
Water flowing over the soil surface
What infiltration rate?
The rate that’s actually taking place
What is the infiltration capacity?
A hypothetical maximum rate
What does the Horton Overland Flow depend on?
Rainfall rate and surface
What does the saturation overland flow depend on?
Rainfall amount
What increases in both Horton Overland Flow and Saturation overland flow?
Damage increase with gradient
What are the four major styles of mass wasting?
1/ Fall
2/ Slide
3/ Flow
4/ Creep
Where are rock falls most common?
Arid and semi-arid lands
and along shorelines
What can trigger rock falls?
Earthquakes
Factors of a rock fall?
Rapid and observable
What happened on Monday 7th Jan 2020?
5.8 magnitude earthquake destroying Punta Ventana
Puerto Rico
What is rock fall?
Downward motion of rock or soil through the air
On what kind of slope does rock fall occur?
Very steep faces or cliffs
What is slide/ slumps?
Slipping of coherent roc or soil units often with a backwards rotation
What happened to the A625?
Large parts were removed during a large landslide in 1977 (following the dry summer of 1976 and heavy rain in the winter of 1976/77) the road developed large steps in the carriageway. The road finally closed in 1979
What are debris flows?
- Slow to the very rapid flow
- Saturated soil & rock debris down distinct channels
- Typically coarse-grained materials
- Lots of internal deformation
- Relatively high water content/ fluid-like flow
- Failure typically along a well-defined shear plane at the soil-bedrock interface.
Where does water-logged material moving slowly typically occur?
Fine-grained soil or rock
What can bedrock slides limit?
The relief of mountain ranges
What happened to Mt. Cook, New Zealand?
Top 10 meters of summit fell away in a massive debris avalanche on December 14, 1991
What does soil creep impact?
Whole soil mass rather than individual grains
What is soil creep?
Continuous gravity driven process with no discrete boundaries
What is soil creep?
The continuous gravity-driven process with no discrete boundaries
What is soil creep caused by?
- Seasonal processes such as thermal- or moisture-driven expansion/contraction
- Bioturbation
What is bioturbation?
The disturbance of the mixing of sediment by living organisms
What is solifluction?
A type of creep particularly associated with periglacial conditions and freeze-thaw cycles
- Faster than most other forms of creep
What is stress?
The force acting over a cross-sectional area within a solid or liquid
What is normal stress?
Force acting perpendicular to the principle plane within a mass
What is shear stress
The force acts perpendicular to normal stress = slippage
What happens if the two stress exceed the strength or resistance of the slope material?
slope failure
What factors influence the balance between stress and strength?
Angle of the slope, cohesion of the materials, presence of water, and geological characteristics
When do failures occur?
When the gravitational force exceeds the resistance offered by the slope materials, causing movement and deformation
What is the role of water?
Adds weight to the slope: air in pore space is replaced by water
How does water impact the angle of repose>
- Low amounts of water causes cohesion
- Saturation causes buoyancy, reduction N and thus friction
- Over pressuring can cause liquefaction
- Angle of repose maximal with least water
- Angle of repose strongly reduced
- Generally initiated by shock
What is the role of vegetation on a slope?
- Plant roots, trees in particular, substantially increase slope strength, reducing shallow slides and slumps
Planting is an effective tool in slope stabilisation
What are the triggers for failure
- Shocks eg earthquakes
- Slope modification (removal of vegetation)
- Undercutting
- Changes in hydrology
- Volcanic eruptions
How can earthquakes impact slopes?
- They have destructive potential
A secondary effect of earthquakes
What happened on 12th January 2001 in El Salvador?
Las Colinas slide claimed the lives of more than 500 people
What happened in the Regolith landslides?
The upper layer of soil slipped due to heavy rainfall
Striking Manawatu, New Zealand In Feb 2004
Compared to the area of steep pasture with the scrub-covered gullies (upper right), where protective vegetation reduced the number of slips
