Fluvial slope processes

• slopes are eroded by running water where vegetation is lacking (e.g. badlands)

1. raindrop impact and rainsplash
   
   
   • the force of raindrops on bare soil causing disaggregation of surface soil
   • when the soil is wet, the impact of raindrops creates small craters, which represent the displacement (erosion of the soil); on a slope these craters will be elongated downslope

   
   

2. rainwash (sheetwash)
   
   
   • erosion by layered (laminar) overland flow
   • soil particles are dragged along in the overland flow but not suspended because the flow is thin and laminar
   
   

   overland flow

   movement of water over slopes when precipitation intensity exceeds the soil infiltration capacity according to soil porosity and permeability, vegetation, slope gradient, antecedent moisture and seasonal factors (e.g. ice)

    

3. rill erosion
   
   
   • overland flow deepens downslope, reaching a critical depth where laminar flow cannot be maintain and turbulence begins to develop
   • shallow ephemeral channels called rills form where soil has been eroded from the slope
   • because water flows directly down slopes (doesn't meander like streams), rills tend to straight and extend only to the base of the slope
   
   
4. gullying
   
   
   • the erosion of deep narrow channels, commonly by piping, the formation of natural tunnels by underground water erosion, and and sapping, when the roof of a tunnel collapses
   • whereas rills are confined to slopes, gullies are part of the drainage network, that is, they shed and water and sediment form slopes and into higher order streams

    

Mass wasting

• the downslope movement of earth materials in response to gravitational stress, where stress is force over area (kg m^-2)

shear stress

the component of gravitational stress that is parallel to the slope and causes earth materials to move downslope

normal stress

the component of gravitational stress that is directed into (perpendicular to) the slope, and gives earth materials strength and resistance to erosion and mass wasting 

shear strength

the resistance to shear stress as a function of the normal stress , cohesion, inter-particle friction, and porewater pressure

factors that cause increased shear stress and potential for mass wasting

• undercutting of slopes: increases the slope angle
• loading of slopes (e.g. water, snow or sediment): increases the weight of earth materials
• earthquakes or man-made vibrations: transient and instantaneous increase in stress
• regional tilting: increases slope angles over large areas

factors that cause decreased shear strength 

• excess soil moisture (porewater pressure): forces particles apart and thereby reduces friction
• weathering: reduces cohesion among grains and crystals
• inherent weakness: poorly consolidated soil and rock 
• structural changes: progressive changes to soil and rock structure (e.g., the breaking apart of soil aggregates)

types of mass wasting

1. creep
   
   
   • the slow imperceptible plastic deformation of soil and weak 
   • it is caused by the expansion and contraction of soil in response to wetting and drying or freeze and thawing, thus creep tends to be seasonal
   • the soil expands away from the surface, but during the contraction phase it moves slightly downslope in response to gravity (shear stress)
   • creep is common on grassland slopes where there are no tree roots to bind the soil together
   • with a low sun angle, early or late in the day, terracettes (small terraces) are visible on prairie hillsides
   
   
2. flow
   
   
   • the internal shearing (fluid deformation) of earth materials
   • the most rapid movements of soil and rock
   • caused by excess porewater which lifts the earth materials and initiates the flow
   • an earth flow extend across hillsides; a debris flow or mud flow occurs in a stream channel but gravity forces the material downstream not the stream water
   
   
3. slide
   
   
   • the movement (shearing) of earth or rock over a well-defined surface
   • rockslide: sliding in rock over a pre-existing surface like a bedding plane in sedimentary rocks
   • a rotational landslide (slump) occurs in poorly consolidated material, like glacial deposits or weak rock, where a curved sliding plane develops in response to the shear stress, cross-cutting any existing discontinuities and causing the material above the sliding plane to rotate (moves out and down)
   
   
4. fall
   
   
   • the free fall of blocks of rock from weathering cliffs
   • produces talus, slopes in coarse loose rock debris at the base of cliffs