Downhill creep

A group of Victorian headstones, England. Soil creep has led to the headstones being tilted at an angle over time.

Water is a very important factor when discussing soil deformation and movement. For instance, a sandcastle will only stand up when it is made with damp sand. The water offers cohesion to the sand which binds the sand particles together. However, pouring water over the sandcastle destroys it. This is because the presence of too much water fills the pores between the grains with water creating a slip plane between the particles and offering no cohesion causing them to slip and slide away. This holds true for hillsides and creep as well. The presence of water may help the hillside stay put and give it cohesion, but in a very wet environment or during or after a large amount of precipitation the pores between the grains can become saturated with water and cause the ground to slide along the slip plane it creates.

Creep has caused the soil to spread over this pavement.

Creep can also 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.

Objects resting on top of the soil are carried by it as it descends down the slope. This can be seen in churchyards, where older headstones are often situated at an angle and several metres away from where they were originally erected.

Vegetation plays a role with slope stability and creep. When a hillside contains many trees, ferns, and shrubs their roots create an interlocking network that can strengthen unconsolidated material. They also aid in absorbing the excess water in the soil to help keep the slope stable. However, they do add to the weight of the slope giving gravity that much more of a driving force to act on in pushing the slope downward. In general, though, slopes without 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.

For shallow to moderate slopes, diffusional sediment flux is modeled linearly as (Culling, 1960; McKean et al., 1993)

${\displaystyle q_{s}=k_{d}S\,\!}$

where ${\displaystyle k_{d}\,\!}$ is the diffusion constant, and ${\displaystyle S\,\!}$ is slope. For steep slopes, diffusional sediment flux is more appropriately modeled as a non-linear function of slope[1]

${\displaystyle q_{s}={\frac {k_{d}S}{1-(S/S_{c})^{2}}}\,\!}$

where ${\displaystyle S_{c}\,\!}$ is the critical gradient for sliding of dry soil.

On long timescales, diffusive creep in hillslope soils leads to a characteristic rounding of ridges in the landscape.[1][2]

Convex hillslopes near Lebec, southern California, USA. Creep in the underlying soils has produced the rounded form of the hilltops.

• Colluvium
• Mass wasting
• Gravitational erosion
• Sediment transport
• Solifluction

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• Culling, 1960.
• McKean et al., 1993.
• Monkhouse, F. J. (University of Southampton). A Dictionary of Geography. London: Edward Arnold (Publishers) Ltd. 1978.
• Roering, Kirchner and Dietrich, 1999. Evidence for nonlinear diffusive sediment transport on hilslopes and implications for landscape morphology. Water Resour. Res., 35:853–87.
• Strahler, Arthur N. Physical Geography. New York: John Wiley & Sons, 1960, 2nd edition, 7th printing, pp. 318–19
• Easterbrook, Don J., 1999, Surface Processes and Landforms, Prentice-Hall, Inc.