3.3. Slope Processes Flashcards
Mass movement
describe all the down slope movement of weathered material (regolith) due to the force of gravity.
Regoliith
Weathered material
Shear Stress
a force that causes layers or parts to slide upon each other in opposite directions
Shear Strength
a material’s ability to resist forces that can cause the internal structure of the material to slide
How Shear Stress and Strength relate to mass movement
Mass movements occur when the force of gravity (meaning the shear stress) exceeds the shear strength (meaning friction) of the slope materials.
Classification of mass movements
1) Type of material
soil / regolith or rock / bedrock
2) Type of motion
Fall - free-fall of material
Slide - material slides as one mass along flat surface
Flow - material flows as a fluid
3) Speed of movement
Fastest - rock avalanche:
floats (flows) on trapped air
Slowest - creep: inches per year
(or less)Slowest to fastest mass movements (increasing in speed going downwards)
- Soil creep
- Solifluction
- Debris flow
- Mud flow
- Rock slides
- Slumps
- Rock fall
- Rock avalanche
Flows
involve the internal movement of soil and rocks down slope.
Characteristics of flows
- Flows occur under wet conditions than slides.
- Flows range in speed from very slow to fast.
- They occur on relatively gentle slopes. Slope gradients are commonly from 10% to as steep as 30%.
- Flows involve a lot of mixing of soil and rock particles within the moving mass.
- Flows usually consist of unconsolidated material.
Soil Creep
very slow downhill movement of material.
Cause of Soil Creep
caused by heaving - alternate expansion/contraction of material from freezing/thawing or wetting/drying
Heave
- Heave occurs when soil expands and contracts due to freezing and thawing, or wetting and drying.
- When soils become saturated or frozen, they expand, pushing rocks and boulders upwards perpendicular to the ground surface.
- When the soil dries or thaws, the boulders move down vertically resulting in a net down slope movement.
- forms terracettes
Solifluction
slightly quicker type of flow than creep, although it is still a slow flow process
Characteristics and causes of Solifluction
- It happens at a rate of 5cm-1metres per year
- This type of slow flow occurs in wetter soil than creep and usually occurs in areas of permafrost. Permafrost refers to the layer of groundwater that fills in the pore spaces of soil and rock that is permanently frozen.
- In times of warm weather, the ground will begin to thaw from the surface downward. All of the freshly melted water cannot absorb into or move through the permafrost layer. This causes the upper layer of soil and regolith to become saturated and flow down the slightest of slopes as it slips over the frozen ground underneath.
- “Solifluction Lobes” are a sign of this happening
Debris Flows
made up of loose rocks, mud and water. Debris flows also transport large objects such as trees.
Characteristics and Causes of Debris flow
- Debris flows commonly occur on moderately steep slopes. Slope gradients are commonly from 10% to as steep as 30%.
- This type of flow happens when the regolith is saturated (very wet).
- It happens at 1-15km per year (faster than creep or solifluction)
- Happens in humid (wet) areas on hillsides following heavy rain or melting snow, and where the regolith consists of fine-grained materials (clay).
Water and sediment movement on hillslopes
1) Rain splash
2) Surface run off (Sheetwash and Rills)
Rainsplash
- Raindrops impact break up clumps of soil.
- The lighter materials such as sand and silt are then carried off by surface runoff, leaving behind pebbles and gravel.
- The fine particles can mix with water and clog up pores, making it impermeable and forming puddles on flat land.
- On a slope, unabsorbed water flows downhill and carries away loosened soil particles.
- The effect is greater on steep slopes, due to gravity, as more soil is splashed downhill than uphill
Surface Wash
- Occurs when loose or detached soil loosened by rainsplash is transported downhill in an uniform layer, over the land rather than through the channels
- it occurs on gentle slopes, when the rate of precipitation exceeds the rate which water can infiltrate the soil
- This commonly occurs in winter as water drains across saturated or frozen ground, following heavy rainfall
Rills
Rills may be formed as a result of Surface wash. They are shallow channels that carry sediment for a short amount of time.
Mudflows
similar to debris flows but they are wetter and the soil / regolith particles are smaller.
Causes and Characteristics of Mudflows
- They have a higher water content and are made up mostly of clay.
- They are very rapid (the fastest flow!), flowing at 1km/hour.
- They are likely to occur in mountain areas after heavy rain or in permafrost areas at the time of maximum thaw.
- Mudflows are a serious volcanic hazard – melted snow and ice from a volcano top can combine with ash to form lahars.
Slides
Slides do not experience internal movement.
- The material moves as one mass along a planar surface - with the material moving at the same rate as at depth.
Characteristics of Slides
- Slides occur under drier rock conditions than flows.
- They occur on steep slopes. Slope gradients are commonly from 35% to as steep as 70%.
- The sliding surface may be curved or flat in shape.
Slides consist of consolidated materials.
Rockslides
- A Rockslide is a rapid movement of newly detached large pieces of rock along a flat (planar)surface.
- Rockslides take place under slower and drier conditions than slumps.
Slumps
- A mass of weathered rock is moved along a curved surface.
- It is likely to happen when weathered rock becomes saturated with water and gains weight.
- Most of the groundwater lubricates the curved surface underground.
- Slumps involve a mass of soil or rock sliding along a curved, rotational surface (shaped like a spoon.)
- At the bottom (or toe) of the slump, flow of soil occurs.
- Typically a steep scarp is developed where the moving material has pulled away from the upper slope. A pile of material collects at the front, or toe, of the slump.
Cause of Slumps
- Erosion along the base of a slope, which removes support for the overlying material.
- This may be caused naturally by stream erosion along its banks or by wave action at the base of a coastal cliff.
- It can also be caused by human activity, such as the construction of roads and in quarry walls.
Rock Falls
Rockfalls occur when a piece of rock or rocks on a steep slope (more than 40°) breaks off and falls down the slope but not along any particular type of surface.
Characteristics and causes of rock falls
- The conditions are dry.
- The individual rocks move at very rapid speeds, speeds ranging from 1m/sec to more than 100 m/sec.
- At the base of most cliffs is an accumulation of fallen material termed talus / scree
- Can be caused by Pressure release weathering and Earthquakes.
