3.3 Slope processes Flashcards
1
Q
What causes water and sediment movemnt?
A
rainsplash
surface runoff ( sheetwash
rills and gullies )
2
Q
What are the three different processes that can occur on slopes?
A
- Erosion and weathering on slopes that change the landscape
- the movement of water and sediment down a slope
- the mass movement of material
3
Q
What is rainsplash?
A
- occurs when rain falls with sufficient intensity
- if it does, then as the raindrops hit bare soil. it is able to detach and move soil particles a short distance by impact of the falling raindrop
- as the water hits the slope, Gravity causes the water and displaced soil granules to move downhill
4
Q
How does surface runoff in form of sheetwash cause water and sediment movement on hillslopes?
A
SHEETWASH:
- surface runoff moves downwards as a thin sheet of water
- it will move only slowly, have low energy and generally incapable of detaching/transporting soil particles
- however, on steeper slopes it will have more energy and the loose dislodged soil particles may be moved downwards as sheetwash
- more or less uniform layer of fine soil particles may be removed from the entire surface of an area
- commonly occurs on recently ploughed fields/areas with poorly consolidated soil material with little/no vegetation cover
5
Q
What are rills?
A
- where sheetwash takes place the soils surface will be lowered slightly
- over time, these preferential flow paths will be eroded to form small channels - rills
- these may develop/enlarge to form gullies
- rills form efficient pathways for the removal of both water/sediment from hillslopes
6
Q
What is mass movement?
A
- movement down a slope of weathered rock/soil, responding to the pull of gravity
- when pull of gravity greater than force of friction/resistance, slope will fail and material will move downwards
7
Q
What does slope failure depend on?
A
SHEAR STRENGTH: internal resistance to Stop sliding
SHEAR STRESS: forces truing to pull part of the slope down
8
Q
What factors contribute to increased shear stress?
A
- removal of lateral support through undercutting or slope steepening (e.g wave action)
- removal of underlying support
- loading of the slope e.g by the weight of water/vegetation
- transient stresses e.g earthquakes or vibrations from vehicle
- lateral pressure, for example by water freezing in cracks
9
Q
What factors contribute to reduced shear strength?
A
- Weathering effects – Disintegration of granular rocks or hydration of clay materials
- Changes in pore water pressure – e.g when slope becomes saturated. can also reduce cohesion by reducing frictional strength
- Changes in structure – e.g creation of small cracks in clays
- Organic effects – Burrowing of animals and decaying tree roots.
10
Q
What is soil creep? (type of heave)
A
- most common
- slow movement of particles down a slope under the influence of gravity
- as movement very slow, limited impact on overall shape
- main impact is to smooth and round the slope
- leads to accumulation of soil on the upslope side of fences, walls
- soil creep may result in formation of *small pressure ridges: terracettes on a hillside
- soil creep involves mechanism of heave
- where soil particles rise towards the surface due to wetting/freezing, only to drop back vertically to the slop when drying/thawing occurs
11
Q
What is solifluction? (type of heave)
A
- very similar to soil creep, but is normally a slightly faster down slope movement of materials that have a high water content
- occurs in the cold periglacial regions of the world and in cold, high mountainous areas
- takes place in summer when the surface ice melts to form a saturated active layer
- this slowly slips and flows downhill on top of permafrost below
12
Q
What are flows?
A
- faster than heaves, not as fast as slides and falls
- occur on slopes made from cohesive materials e.g silts and clays
- controlled by moisture within the slope, when soil becomes oversaturated, the cohesive bonds within the soil break, causing soil to move downhill like a viscous liquid
- may take place as earthflows at relatively slow speeds - 1 to 15km per year when material is transported on slopes of 5-15* with a high water content
- on steeper slopes mudflows may take place where speeds increase to between 1 to 40km per hour especially after heavy rainfall, adding both volume/weight to soil
- the heavy rain increases the pore water pressure which forces the particles into a rapidly flowing mass of material
13
Q
What are slides?
A
- fastest movement
- occur where a complete mass of material detaches itself from a slope and slides downhill
Two types: - Planar: where the mass movement leaves behind a flat side plane, such as along bedding planes
- Rotational slides/slumps: where the material slides out from a slope in a curved motion
- landslides most active in areas of high relief and unstable slopes
- can be triggered by seismic waves or gravitational stress
- large scale movement
14
Q
What are falls?
A
- occur on slopes made from rocks
- happen very suddenly and effects usually dramatic
- rockfalls occur on steep, almost vertical jointed and fractured rock faces, where the cohesion between masses of rock is overcome and the shear strength of the material is exceeded
Triggers:
- freeze-thaw processes, seismic waves or gravitational stress
15
Q
How does water effect slope processes?
A
- addition of weight to slope materials increasing downslope stress (the
gravity effect) - reduces internal strength by reducing cohesion by increased pore water
pressures creating a greater fluidity such as in mudflows - lubricating shear planes possibly leading to landslides
- water-induced weathering on rock faces leading to rockfalls
- freeze-thaw and wetting and drying in soils leading to heaves and
downslope movement - overland flow leading to sheetwash or rill/gully action
