Surface 10: Hillslopes

Catenas

• formed by hillslopes

• sequences of linked slope units running from drainage divide to valley floor

Soil-mantled hillslopes

• hillslopes mantled with soil or regolith

• dominant type

rock slopes

• formed in bare rock

rock slope formation (3 cases)

• where either uplift or deep incision means that they sit at too high an elevation for debris to accumulate and bury them

• where active processes at their bases remove debris preventing accumulation

• where the terrain is too steep or climate is too cold/dry for chemical weathering and vegetation to create and sustain a regolith

• where slope angles exceed about 45 degrees

2 extreme cases of rock properties

• hard rocks → very high internal strength, fail along partings in the rock joints and fractures

• soft rocks → lower intact strength, intense fracturing that behave more like soils

  • generally bare rock slopes form on hard rocks

Stress & Strain

• stress → any force that tends to move materials downslope (driven by gravity)

• strain → the effect of stress upon a soil body

• friction → force that acts against gravity and resists movement

• cohesion → the tendency of particles within the soil body to stick together → generates internal friction

hillslopes (define + significance)

• inclined surfaces of land that connect higher elevations to lower ones

• they are sites of erosion, transport, and deposition processes that shape landscapes

Primary processes that shape hillslopes

• weathering → breakdown of rocks

• erosion → removal of soil and rock

• mass movement → downhill movement of material due to gravity

• deposition → accumulation of material at the base of the slope

Types of mass movement processes

• falls → sudden detachment of material

• slides → downhill movement of coherent material

• flows → movement of unconsolidated material

• creep → slow, gradual movement of soil

Factors affecting stability of hillslopes

• slope angle → steeper slopes are more prone to failure

• material type → loose or weak materials are more unstable

• water content → excess water can reduce stability

• vegetation → roots can stabilize soil

Role of soil creep in hillslope evolution

• soil creep is the slow, downhill movement of soil due to repeated cycles of wetting and drying or freezing and thawing

• contributes to the gradual shaping of hillslopes over time

Landslides (occurence + impacts)

• when the force of gravity overcomes the strength of slope materials, leading to rapid downhill movement

• can cause significant damage to infrastructure and landscapes

USLE (universal soil loss equation)

• a model used to estimate soil erosion on hillslopes

• based on rainfall, soil type, slope length, & landuse

Human impact on hillslope stability

• deforestation, construction, agriculture → increase erosion rates & reduce slope stability → landslides + soil degradation

Role of vegetation in hillslope stability

• reduces erosion → roots hold soil in place

• intercepts rainfall → reduces runoff & erosion

• improves soil structure → enhances soil strength