Downhill creep
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Downhill creep, or commonly just creep, is the slow downward progression of rock and soil down a low grade slope; it can also refer to slow deformation of such materials as a result of prolonged pressure and stress. Creep may appear to an observer to be continuous, but it really is the sum of numerous minute, descrete movements of slope material caused by the force of gravity. Friction being the primary force to resist gravity is produced when one body of material slides past another offering a mechanical resistance between the two which acts on holding objects (or slopes) in place. As slope on a hill increases, the gravitational force that is perpendicular to the slope decreases and results in less friction between the material that could cause the slope to slide.
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[edit] Overview
The rate of soil creep down a slope depends on the steepness (gradient) of the slope, water absorption and content, type of sediment and material, and lastly vegetation. The rate of creep will take into account all of these factors to decide whether or not the hillside will progress downward. Creep is what is responsible for the rounded shape of hillsides.
Water is a very important factor when discussing soil deformation and movement. When building a sand castle at the beach you would notice that the presence of water making damp sand aids in keeping your castle standing up. The water will offer cohesion to the sand which will bind the sand particles together. However, when you pour a bucket of water over your sand castle or it gets hit by a wave, it destroys it. This is because the presence of too much water fills all the pores between the grains with water creating a slip plain between the particles and offering no cohesion causing them to slip and slide away. This holds true not only for sand castles but for hillsides and creep as well. The presence of water may help the hillside stay put and give it that cohesion but in a very moist/wet environment or during/after a large amount of precipitation the pours between the grains could become saturated with water and cause the ground to slide along the slip plain it creates.
Creep can be caused by the expansion of materials such as clay when they are exposed to water. Clay expands when wet, then contracts after drying. The expansion portion pushes downhill, then the contraction results in consolidation at the new offset.
Vegetation also can play a role with slope stability and creep. When a hillside contains many trees, ferns, and shrubs their roots can create an interlocking network that can strengthen unconsolidated material. They also aid in absorbing the access water in the soil to help keep the slope stable. They also however, add to the weight of the slope giving gravity that much more of a driving force to act on in pushing the slope downward. Slopes with the absence of vegetation have a greater chance of movement.
Design engineers sometimes need to guard against downhill creep during their planning to prevent building foundations from being undermined. Pilings are planted sufficiently deep into the surface material to guard against this behavior.
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[edit] Modeling Regolith Diffusion
For shallow to moderate slopes, diffusional sediment flux is modeled linearly as (Culling, 1960; McKean et al., 1993)
where 
is the diffusion constant, and 
is slope. For steep slopes, diffusional sediment flux is more appropriately modeled as a non-linear function of slope (Roering et al., 1999)
where 
is the critical gradient for sliding of dry soil.
[edit] Bibliography
- Culling, 1960.
- McKean et al., 1993.
- Roering, Kirchner and Dietrich, 1999. Evidence for nonlinear diffusive sediment transport on hilslopes and implications for landscape morphology. Water Resour. Res., 35:853-887.
- Easterbrook, Don J, 1999. Surface Processes and Landforms. "Prentice-Hall, Inc."
