3.3 Slope processes Flashcards
What are slopes?
Inclined surfaces or hillslopes, known to be an open system with inputs and outputs.
What factors cause processes to vary?
Exogenic factors occur outside a slope and endogenic factors work within the slope
Climate: humid temperatures cause round surfaces due to chemical weathering. Arid areas are jagged due to mechanical weathering.
Geological structure - faults, angle, vulcanity and rock type can all affect slopes
Soil structure - porosity, permeability, strength, cohesiveness etc.
Aspect - steeper more dangerous
Vegetation - holds slope together, soil structures
What are mass movements? How are they categorised?
Mass movements are large scale movements of the earth’s surface, not accompanied by a moving agent. They can be classified by speed, water content and material.
Gravity moves material downslope, proportional to the angle and the weight. Water also lubricates the surface, adding pore pressure which causes movements.
They are caused when the stress is greater than the strain.
They can be categorised using a triangle with speed on the x axis and heave, dry and wet on the points.
Creeps are slow heaves, flows are middle speed wet movements, rock and landslides are dry and fast.
What reduces shear strength?
Change of rock structure e.g. fissures and cracks
Organic effects (burrowing, decay)
Weathering e.g. granular disintegration leads to less cohesion
Removal of vegetation
Change in pore water pressure of soft material
What increases shear stress?
Removal of lateral support e.g. erosion
Removal of underlying support e.g. wave action
Loading of slope e.g. water, trees, debris, waste - adds friction and weight
Lateral pressure - freezing cracks, water in cracks
Transient stresses e.g. earthquakes.
What is a creep?
A slow, small scale process. Talus creep is the movement of fragments on the scree slope which is coarser material. The regolith and soil move very slowly under gravity due to plastic deformation - it is common in winter.
Soil particles expand and contract to the surface by heave - due to heating, wetting or freezing of water. Expansion then pushes them up to the surface and contraction moves it downslope.
This creates terracettes due to soil contracting downslope, forming little ridges.
What factors influence/accelerate creep?
Since it is heavily dependent on water content, hydration, freeze/thaw and thermal expansion. Water acts as a lubricant.
The material is usually clay rich and so subject to plastic flow and accelerated.
Waterlogged permafrost areas cause solifluction causing accelerated creep as the soil flows through surface runoff, washing it away.
Slope angle and soil type is important - rates are high in tropical areas.
What is a flow?
The saturation of clays lead to a fast flow along a slip plane due to the turbulent mixture of sediment and water.
steep valleys, narrow flows, low vegetation, thick regoliths, rainfall and earthquakes are common causes.
Water content adds weight and lubricates material which is fine and small.
Mudflows can occur on the end of a landslide.
Where the material flows, it leaves a scar and a toe.
What are slides?
One off, spontaneous movements of rock down a slip plane. There are two types: Transitional landslides/rockslides of material or rock Rotational landslides (concave, downward and outward movement due to curved slip planes)
Slope failure occurs when stress exceeds strain, possibly due to steep angles, fractures, weak rocks (clay), undercutting, permeability/porosity, removal of toe, lubricant/weight, loading, EQs.
Where the rock moves it creates a scar, which slides down the slip plane and creates a detached block. At the bottom there may be debris from previous slides.
What are falls?
Similar causes to landslides - often due to undercutting by sea.
Involves the vertical movement of rock on a bare face, usually from mechanical weathering or erosion on lines of weakness. Once detached they fall under gravity.
Short falls produce straight scree and long falls make concave scree. Size and shape also contribute to the rock fragments.
Concave scree slopes (talus) can accumulate and are concave due to the long falls.
The free face is 80 degrees which straightens to about 30 degrees. The concave scree is roughly 20 degrees as scree collects at the bottom.
What are slumps?
Like a flow, the rotational movement of a curve slip plane leads to rock being detached from hillsides and move along the plane. It is made out of clay liable to slumping as they absorb water easily and may be undercut.
This leaves a scar at the top, then the slope tilts backwards creating a terrace. There is then a mudflow at the toe.
How does water movement affect slopes?
Rain splash erosion occurs when soil particles detach and raindrops erode initially and further with the splash
Surface wash is when infiltration is exceeded so surface runoff occurs. It leads to sheet wash erosion which is when unchanneled water flows near the surface at high velocity, which can transport rain splash material, and on steep slopes can create deep gullies inland when this becomes concentrated into gully erosion as it moves in uniform ‘streams’.
Sheet erosion is the opposite - when uniform sheet wash is spread and so creates a flat surface, a uniform sheet is eroded in between the two rills, creating shallow channels on intermediate slopes.
