What causes water and sediment movemnt?
rainsplash
surface runoff ( sheetwash
rills and gullies )
What are the three different processes that can occur on slopes?
- Erosion and weathering on slopes that change the landscape
- the movement of water and sediment down a slope
- the mass movement of material
What is rainsplash?
- 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
How does surface runoff in form of sheetwash cause water and sediment movement on hillslopes?
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
What are rills?
- 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
What is mass movement?
- 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
What does slope failure depend on?
SHEAR STRENGTH: internal resistance to Stop sliding
SHEAR STRESS: forces truing to pull part of the slope down
What factors contribute to increased shear stress?
SWAVEL:
- slope angle: steeper slopes = stronger influence of gravity
- water saturation: weight or water and reduced soil cohesion
- Addition of weight: rainfall/snow or human acitivites
- Vibration: earthquakes
- Erosion: removal of support = less stable
- lateral pressure: causes internal stress along fractures or bedding planes
What factors contribute to reduced shear strength?
- Water saturation (reduces friction)
- Weathering (breaks down rock cohesion)
- Removal of vegetation (roots add strength)
- Weak rock structures (joints, bedding planes)
- Earthquakes (disrupt bonding)
- Clay-rich soils (weaken when wet)
What is soil creep? (type of heave)
- 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
What is solifluction? (type of heave)
- 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
What are flows?
- 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
What are slides?
- 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
What are falls?
- 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
How does water effect slope processes?
- 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
all the different mass movements and their characteristics
- soil creep:
- very slow
- low water content
- soil - solifluction:
- slow
- high water content (in periglacial regions)
- saturated soil over frozen ground - Earthflows:
- slow to moderate
- moderate to high water content
- fine grained soils - mudflow:
- fast
- very high water content
- saturated fine material - Rock fall:
- very fast
- very low water content
- solid rock - Landslide:
- fast
- low to moderate water content - slump (rotational slide):
- moderate speed
- moderate water content
- soil/weak rock
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how does saturation of soil contribute to mass movements?
- saturation of soil means water fills pore spaces in between particles
- = creates pore water pressure
- pushes particles apart
- reducing friction
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1.1 The drainage basin system30
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1.2 Discharge relationships within drainage basins28
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1.3 River channel processes and landforms40
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1.4 The Human Impact35
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3.1 Plate tectonics27
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3.2 Weathering20
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3.3 Slope processes17
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3.4 The human impact26
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2.1 Diurnal energy budgets19
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2.2 The global energy budget14
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2.3 Weather processes and phenomena26
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2.4 The human impact10
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Mock5
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2023 paper8
