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In hillslope geomorphology, a rill is a shallow channel (no more than a few / deep) cut into soil by the erosion action of overland flow surface water. Similar but smaller incised channels are known as microrills; larger incised channels are known as gully.
Artificial rills are channels constructed to carry a water supply from a distant water source. In landscape or garden design, constructed rills are an aesthetic water feature.
Rills are fairly easily visible when first incised, so they are often the first indication of an ongoing erosion problem. Unless soil conservation measures are put into place, rills on regularly eroding areas may eventually develop into larger features such as gully or even (in semi-arid regions) into badlands.
Rills cannot form on every surface and their formation is intrinsically connected to the steepness of the hillside slope. Gravity determines the force of the water, which provides the power required to start the erosional environment necessary to create rills. Therefore, the formation of rills is primarily controlled by the slope of the hillside. Slope controls the depth of the rills, while the length of the slope and the soil's permeability control the number of incisions in an area. Each type of soil has a threshold value, a slope angle below which water velocity cannot produce sufficient force to dislodge enough soil particles for rills to form.Planchon, O., E. Fritcsh & C. Valentin. 1987. Rill Development in a Wet Savannah Environment. Bryan, R.B. (ed). Rill Erosion: Processes and Significance. Catena Supplement 8. W. Germany:Catena Verlag. 55-70. For instance, on many non-cohesive slopes, this threshold value hovers around an angle of 2 degrees with a shear velocity between 3 and 3.5 cm/s.Rauws, G. 1987. The Initiation of Rills on Plane Beds of Non-Cohesive Sediments. Catena Supplement 8. W. Germany:Catena Verlag. 107-118.
After rills begin forming, they are subjected to variety of other erosional forces which may increase their size and output volume. Up to 37% of erosion in a rill-ridden area may derive from mass movement, or collapse, of rill sidewalls. As water flows through a rill, it will undercut into the walls, triggering collapse. Also, as water seeps into the soil of the walls, they weaken, amplifying the chance of wall collapse. The erosion created by these forces increases the size of the rill while also swelling its output volume.Govers, G. 1987. Spatial and Temporal Variability in Rill Development Processes at the Huldenberg Experimental Site. Catena Supplement 8. W. Germany:Catena Verlag. 17-33.
Less commonly, dissolution of limestone and other soluble rocks by slightly acidic rainfall and Surface runoff also results in the formation of rill-like features on the surface of the rock.Ford, D.C. & J. Lundberg. 1987. A Review of Dissolutional Rills in Limestone and Other Soluble Rocks. Bryan, R.B. (ed). Rill Erosion: Processes and Significance. Catena Supplement 8. W. Germany:Catena Verlag. 119-139
Unfortunately, rills' considerable effect on landscapes often negatively impacts human activity. Rills have been observed washing away archaeological sites. They are also very common in agricultural areas because sustained agriculture depletes the soil of much of its organic content, increasing the erodibility of the soil. Agricultural machines, such as tractors, compact the soil to the point where water flows over the surface rather than seeping into the soil. Tractor wheel impressions often channel water, providing a perfect environment for generating rills. These rills may erode considerable amounts of Arable land soil if left alone.Fullen, M.A. & A.H. Reed. 1987. Rill Erosion of Arable Loamy Sands in the West Midlands of England. Catena Supplement 8. W. Germany:Catena Verlag. 85-96.
Under proper field management, rills are small and are easily repaired by Contour plowing the soil. This will prevent, for a time at least, the rills from growing and eroding the landscape more rapidly with time.
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