# Downhill creep

Downhill creep, also known as soil 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, discrete 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 to hold 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.

Trees showing the presence of creep.

## 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.

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.

## Modeling regolith diffusion

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.

## References

1. ^ a b Roering, Joshua J., James W. Kirchner, and William E. Dietrich. "Evidence for nonlinear, diffusive sediment transport on hillslopes and implications for landscape morphology." Water Resources Research 35.3 (1999): 853–70.
2. ^ Rosenbloom, N. A., and Robert S. Anderson. "Hillslope and channel evolution in a marine terraced landscape, Santa Cruz." California: Journal of Geophysical Research 99.B7 (1994): 14-013.

## Bibliography

• 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.

Being Serbia's primate city, Belgrade has special administrative status within Serbia. It is the seat of the central government, administrative bodies, and government ministries, as well as home of almost all of the largest Serbian companies, media, and scientific institutions. Belgrade is classified as a Beta-Global City.

Creep (deformation)

In materials science, creep (sometimes called cold flow) is the tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stresses. It can occur as a result of long-term exposure to high levels of stress that are still below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods and generally increases as they near their melting point.

The rate of deformation is a function of the material's properties, exposure time, exposure temperature and the applied structural load. Depending on the magnitude of the applied stress and its duration, the deformation may become so large that a component can no longer perform its function — for example creep of a turbine blade will cause the blade to contact the casing, resulting in the failure of the blade. Creep is usually of concern to engineers and metallurgists when evaluating components that operate under high stresses or high temperatures. Creep is a deformation mechanism that may or may not constitute a failure mode. For example, moderate creep in concrete is sometimes welcomed because it relieves tensile stresses that might otherwise lead to cracking.

Unlike brittle fracture, creep deformation does not occur suddenly upon the application of stress. Instead, strain accumulates as a result of long-term stress. Therefore, creep is a "time-dependent" deformation.

It works on the principle of Hooke's law (stress is directly proportional to strain).

Earthflow

An earthflow (earth flow) is a downslope viscous flow of fine-grained materials that have been saturated with water and moves under the pull of gravity. It is an intermediate type of mass wasting that is between downhill creep and mudflow. The types of materials that are susceptible to earthflows are clay, fine sand and silt, and fine-grained pyroclastic material.When the ground materials become saturated with enough water, they will start flowing (soil liquefaction). Its speed can range from being barely noticeable to rapid movement. The velocity of the flow is dictated by water content: the higher the water content is, the higher the velocity will be. Because of the dependency on water content for the velocity of the flow, it can take minutes or years for the materials to move down the slope.

Gravity spreading is a phenomenon in which a geological body laterally extends and vertically contracts to reduce its gravitational potential energy. It has been observed on many different scales, and at numerous locations on Earth, from rhyolite lava flows to passive margins. Additionally, gravity spreading is likely to have occurred on both Mars and Venus.

A headstone, tombstone, or gravestone is a stele or marker, usually stone, that is placed over a grave. They are traditional for burials in the Christian, Jewish and Muslim religions, among others. In most cases they have the deceased's name, date of birth, and date of death inscribed on them, along with a personal message, or prayer, but they may contain pieces of funerary art, especially details in stone relief. In many parts of Europe insetting a photograph of the deceased in a frame is very common.

Hill

A hill is a landform that extends above the surrounding terrain. It often has a distinct summit, although in areas with scarp/dip topography a hill may refer to the particular section of flat terrain without a massive summit (e.g., Box Hill, Surrey).

Index of soil-related articles

This is an index of articles relating to soil.

Terrain

Terrain or relief (also topographical relief) involves the vertical and horizontal dimensions of land surface. The term bathymetry is used to describe underwater relief, while hypsometry studies terrain relative to sea level. The Latin word terra (the root of terrain) means "earth."

In physical geography, terrain is the lay of the land. This is usually expressed in terms of the elevation, slope, and orientation of terrain features. Terrain affects surface water flow and distribution. Over a large area, it can affect weather and climate patterns.

Vitello (crater)

Vitello is a lunar impact crater that lies along the southern edge of the small Mare Humorum, in the southwest part of the Moon's near side. It was named after 13th century Polish theologian and physicist Vitello. It lies just to the east of the lava-flooded crater Lee. To the northeast along the edge of the lunar mare is the Rupes Kelvin, an irregular fault line.

Water
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