Core Physical Rocks &weathering 3.3 Slope processes

• The term 'slope' refers to an inclined surface (hillslope) or an angle of inclination (slope angle)
• Slopes can be sub-aerial, sub-marine, aggradational, degradational, or transportational
• Geographers study the hillslope, the area between the watershed and the base

Slope Processes

• Slopes vary with climate; humid areas have rounder slopes due to chemical weathering and soil creep, while arid regions have jagged slopes due to mechanical weathering and sheetwash
• Climatic geomorphology studies how different processes in different climatic zones produce different slope forms
• Geological structure, including faults, dip angles, and vulcanicity, influences rock strength and potential weaknesses, affecting weathering and downslope movement
• Rock type and character influence vulnerability to weathering and resistance to downslope movement
• Slopes with diverse rock types are more prone to landslides due to differential erosion
• Soil structure and texture dictate water retention
• Clay soils hold more water than sandy soils
• Removing vegetation on a clay slope increases the risk of mass movement
• Aspect, the direction a slope faces, influences climatic conditions and slope processes
• Vegetation cover reduces overland runoff through interception and storage
• Deforestation exposes slopes to erosion and gullying
• Vegetation can also increase the risk of landslips by accumulating soil and deepening the regolith

Mass Movements

• Mass movements are large-scale movements of the Earth's surface not driven by a moving agent like a river or glacier
• Examples: very slow movements (soil creep), fast movements (avalanches), dry movements (rockfalls), and very fluid movements (mudflows)
• Processes vary in magnitude, frequency, and scale, ranging from infrequent rockfalls to continuous soil creep
• Classification of processes is based on speed of movement, water content, type of movement (flows, slides, slumps), and material

Causes of Mass Movements

• Slope failure likelihood is expressed by a safety factor, comparing slope strength/resistance to moving forces
• Key factors: gravity, slope angle, and pore pressure
• Gravity acts to move material downslope and to stick particles to the slope
• Water lubricates particles, and pore pressure increases the material's ability to move
• Slope failure occurs due to reduced internal resistance (shear strength) or increased forces attempting to pull a mass downslope (shear stress)
• Weathering reduces cohesion and resistance
• Water weakens slopes by increasing shear stress and decreasing shear resistance
• The weight of the potentially mobile mass can be increased by an increase in the volume of water, heavy or prolonged rain, a rising water table and saturated surface layers
• Water reduces cohesion through saturation, whilst the water pressure in saturated soils decreases the frictional strength of the solid material to weaken the slope
• Downslope movement is opposed by friction, cohesive forces, pivoting and vegetation

Types of Mass Movement

• Heave or creep is a slow process mainly in winter, more common in environments without many flows or slides
• Talus creep is the slow movement of fragments on a scree slope.
• Soil particles are pushed to the surface.
• Moisture changes induce most soil-creep movement
• Freeze-thaw and normal temperature-controlled expansion and contraction are important in periglacial and tropical climates
• Soil Creep rates are slow but form terracettes, are more important in well-vegetated humid temperate areas than slope wash, and observation of soil creep is difficult.

Slumps and Flows

• Slumps occur on weaker rocks (especially clay), involving rotational movement along a curved slip plane.
• Clay absorbs water, saturates, and exceeds its liquid limit, causing flow along a slip plane.
• The base of a cliff is weakened by erosion thus reducing its strength.
• Flows are more continuous and may contort the mass into a new form.
• Particle size involved in flows is small, for example sand-sized and smaller.
• Mudflows are faster and more fluid than earthflows, enabling flow across gentle angles.

Slides

• Slides occur when an entire mass of material moves along a slip plane, including rockslides/landslides and rotational slides.
• Common where weak rocks, steep slopes, and active undercutting combine.
• Caused by changes in water content or very cold conditions.
• Material retains shape/structure until it hits the bottom, and range from small to large-scale.

Slip Planes

• Slip planes occur at the junction of 2 layers, at a fault line, where there is a joint, along a bedding plane, and shear stress becomes greater than the shear strength.
• Weak rocks are vulnerable to slip plane development.

Rockslides

• In 1959, near the epicentre of the earthquake, in the Madison River valley, a slope of schists and gneiss with slippery mica and clay was supported by a base of dolomite.
• The earthquake cleanly broke the dolomite.
• A huge volume of rock, 400 meters high and 1000 meters long, slid into the valley.

Landslides

• Loose rock, stones and soil move downslope when the downward force exceeds resistance.
• Landslides are sensitive to water content through reducing the material strength by increasing the water pressure, and through added weight.

Case Study: The Abbotsford landslide, Dunedin, New Zealand

• From 1978, several families in Abbotsford noticed hairline cracks, leading geologists to discover water had made layers of clay on the hill soft, and the sandstone was sliding on this slippery surface.
• A civil defence emergency was declared and a section of Abbotsford started down the hill with people/houses aboard.
• There were over 7 million in costs from the destruction of homes, however there were additional non-monetary costs incurred.
• Landslide was a block slide of sandstone on a bed of weaker clay.
• A small rift was left at the head of the slope.
• Impermeable hard rock rests on an impermeable soft rock.
• Water collected in the impermeable clay, caused sandstone to slip.
• Dangerous landslides can occur on relatively gentle slopes, and human activity can destabilise slopes.
• However, there was a lack of landslide insurance and mapping and dating of old hazards.

Falls

• In October 2010, mud buried part of a town in the southern Mexico state of Oaxaca because a large chunk of a nearby mountain collapsed after relentless rain.
• Mountain road rescues were hampered by smaller landslides.
• Landslides are a danger in mountainous parts of the country and those that have long suffered deforestation.

Falls (slope)

• Falls occur on steep slopes (greater than 40 degrees), especially on bare rock faces where joints are exposed.
• The initial cause is weathering or erosion.
• If the fall is short, it produces a relatively straight scree, and if is short, it forms a concave scree.
• Falls are significant for the retreat of steep rock faces and supplying debris for scree slopes.

Water and sediment movement on hillslopes

• surface wash occurs when the soil's infiltration capacity is exceeded, this is more common during winter months, or in arid and semi arid environments
• sheetwash is the unchannelled flow of water over a soil surface over areas of high velocities
• sheetwash erodes soil through raindrop impact, resulting in a uniform layer being eroded.
• rills are shallow channels, common in agricultural areas, following the remvoal of vegetation during the harvest season
• throughflow refers to water moving down through the soil, it is channelled which causes it to transport material
• raindrops have an erosive effect on hillslopes of a 5 degree slope (60 percent of the movement), and a greater movement on slopes of 25 degrees
• most impactful during the start of a rainfall event and slopes of 33 - 45 degrees.