3.3 Rocks and weathering: slope processes Flashcards
Define mass movement
Mass movement is the movement down a slope of weathered rock and soil, responding to the pull of gravity.
When the pull of gravity is greater than the force of friction and resistance, a slope will fail and material will start to move downwards.
What factors does slope failure depend on?
Shear strength: how much internal resistance there is within a rock, or slope, to stop part of the slope falling, slipping, or sliding down the slope.
Shear stress: the forces that are trying to pull part of the slope down the slope. These include extra weight being added by water being absorbed into the slope or weight of added material being put on the slope, or the weight of extra vegetation growing on a slope.
Therefore resistant, hard rocks, such as granite, will be able to create steeper slopes than weaker, less resistant rocks such as sandstones and clays.
What are the types of heave
Soil creep
Solifluction
What is soil creep
Soil creep is probably the most common and widespread of mass movements. It is a slow and almost imperceptible movement of particles down a slope under the influence of gravity. As the movement is very slow, it has limited impact on overall shape. Its main impact is to smooth and round the slope. It leads to the accumulation of soil on the upslope side of fences, walls, and hedges, and causes trees and telegraph poles to become out of vertical alignnnet. Soil creep may result in the formation of small pressure ridges (terracettes) on a hill side. Soil creep involves the mechanism of heave, where soil particles rise towards the surface due to wetting or freezing, only to drop back vertically to the slope when drying or thawing occurs. In this way, over many cycles, particles slowly move downhill.
What is solifluction
Soliflluction is very similar to soil creep, but is normally a slightly faster down slope movement (5cm-1m per year) of materials that have a high water content. It occurs in the cold periglacial regions of the world and in cold, high mountainous areas. Solifluction takes place in the summer when the surface ice melts to form a saturated active layer. This slowly slips and flows downhill on top of the frozen permafrost below.
What are the types of flows
Earthflows
Mudflows
What are earthflows
Flows are much faster forms of mass movement. These may take place as earthflows at relatively slow speeds of 1-15km per year when material is transported on slopes of 5-15 degrees with a high water content.
What are mudflows
Mudflows take place on steeper slopes than earthflows where speeds increase to between 1-40km per hour, especially after heavy rainfall, adding both volume and weight to the soil. The heavy rain (or excess supplies of water from springs, etc.) increase the pore water pressure which forces the particles into a rapidly flowing mass of material.
What are slides
Slides can be extremely rapid processes. They occur where a complete mass of material detaches itself from a slope and slides downhill. There are two types of slides – planar – where the mass movement leaves behind a flat slide plane, such as along bedding planes or fault lines, and rotational slides or slumps – where the material slides out from a slope in a curved motion.
Landslides take place at speeds of between 1-100 metres per second on slopes that are often greater than 40 degrees and have a low water content. Here the material slides down a steep slope and forms a mass of broken fragments at the bottom of a slope.
Landslides are most active in areas of high relief and unstable slopes. In rock slides, the effects of bedding and joint planes are important in allowing the rock to fragment and they also provide slide planes. Landslides are the result of sudden and massive slope failure.
(Downslope movement of material ‘en masse’ along a slide plane. The mass keeps its integrity – no internal derangement relatively fast – landslides occur at speeds between 1 and 100m/sec on slopes that are often greater than 40°)
What are falls
detached movement of material, often very fast from a vertical/steep slope or cliff often due to weathering directly to the ground leading to an accumulation of a large volume of coarse material of mixed sizes; smaller boulders moved further and collected at bottom in cone/triangular shape
Occur on slopes of 40°or more
How does water and sediment move on hillslopes?
Rainsplash
Surface runoff (sheetwash and rills)
How does rainsplash move water and sediment on hill slopes
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 the impact of the falling raindrop. Over 200 tonnes of soil particles per hectare may be dislodged during a single rainfall event and then moved downslope!
However, as the soil particles can only be moved a few centimetres at most by this process, it is all merely redistributed back over the surface of the soil. As rainsplash requires high rainfall intensities, it tends to be the most effective under heavy convective rainstorms in the world’s tropical regions.
How does surface runoff move water and sediment
Sheetwash: as the surface runoff moves downslopes as a thin sheet of water, it will move only slowly, have low energy, and will be generally incapable of detaching or transporting soil particles. However, on steeper slopes it will have more energy and the loose dislodged soil particles may be moved downslope by water flowing overland as a sheet, sheetwash. A more or less uniform layer of fine soil particles may be removed from the entire surface of an area, sometimes resulting in the extensive loss of fertile topsoil from a field. Sheetwash commonly occurs on recently ploughed fields or in areas with poorly consolidated soil material with little or no protective vegetative cover.
Rills: where sheetwash takes place the soil’s surface will be lowered slightly. In time,these preferential flow paths will be eroded to form small, well-defined channels, called rills. These may quickly develop and enlarge into gullies. Rills form efficient pathways for the removal of both water and sediment from hillslopes.
How does the slope angle impact slope stability
Impact of slope angle = it affects gravity:
Rocks resting on a slope at 30° experience a force that is equivalent to 50% of gravity whereas rocks resting on a slope at 60° experience a force that is 87% of gravity so movement is more likely.
- Gravity acts to move material down the slope. The downslope movement is relative to the weight of the particle and the slope angle so adding weight to a slope can trigger mass movement.
- Gravity acts to stick the particles to the slope – this stick component gives slopes a natural resistance to movement. Slope angle influences gravity and the ‘stick component’ which will be less on steeper slopes and so increase the risk of mass movement. Steeper slopes are, therefore, more at risk of mass movement and steepening or undercutting of the toe of slopes by humans upsets the slope equilibrium and is a major factor triggering mass movement.
HOWEVER, if water is present, movement may occur on even gentle slopes eg solifluction may occur on slopes that are as gentle as 3° and water content will allow mud and earthflows to occur a much more gentle slope as the flows will be more fluid.
How does water impact slope stability
adds weight ie loading of slope which increases shear stress so that if it exceeds shear strength , the slope equilibrium is reduced and movement is triggered.
· Water reduces cohesion between rocks and soil particles by providing lubrication making them more slippery – this reduces the slope’s natural resistance to movement so movement is triggered
· In a non-saturated soil, the surface tension of the water tends to draw the particles together. This increases cohesion and reduces soil movement. In saturated soil, the pore water pressure forces the particles apart, reducing friction and causing soil movement – increased pore water pressure*
*Increased Pore Water Pressure –. The gaps between particles of rock and soil may fill with water which forces them apart under pressure ie increased pore pressure which reduces the shear strength of the material and the slope becomes weakened as the safety factor changes so that shear strength is overcome by shear stress. which increases the ability of the material to move downslope. This is especially significant in wet materials on low angles slopes which would otherwise be more stable.
Eg Clay is very cohesive (we know this from the Hjulstrom Curve!) – however, when saturated, the cohesion and shear strength may be reduced so that it starts to liquefy and slopes fail. Clay is vulnerable to this as it is a porous (not permeable soil) – 50% porosity in clay.
NB Vegetation may bind soil together and reduce slope movement – however, the presences of vegetation may allow more soil moisture to build up and make landslides more likely
· Heavy rainfall and meltwater add weight to slopes, heavy rain can increase the erosive power of any river at the base of the slope and if material is undercut, the slope will be less stable. Climate may lead to freeze-thaw or hydration weathering which cause expansion and contraction and creation of weathered material on the slope.
