راهنمای فارسی مدل wepp 2011

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Erosion Equations

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RILL EROSION

The DuBoys channel scour equation as developed by sedimentation engineers (Vanoni, 1975) has been modified for rill erosion (Meyer, 1964). In the WEPP model (Lane et al., 1987) rill detachment is expressed as:

Both research (Graf, 1971) and field observations (Foster and Meyer, 1972b) have shown that the rate of sediment detachment is reduced as the concentration of sediment being carried by a channel increases. This effect was observed immediately downstream from major reservoirs, where relatively clear water caused accelerated erosion on the channel floor. The same phenomenon was also observed by the authors during erosion research in 1987 and 1988, when it was noted that more severe scouring occurred at the top of rills immediately below the point of adding clean water than further downrill. At some highly erodible sites, deposition was observed at the downstream end of some of the rill plots. Because of the effect of sediment load on rill erosion the rill detachment subroutine in the WEPP model includes algorithms to account for a decrease in detachment due to increased amounts of sediment load. Hence, it was necessary to examine the effect of sediment load on the rill erosion rates observed during this experiment.

According to Foster and Meyer (1972b), an intuitive expression for the effect of suspended sediment on rill detachment by scour is:

 

The above expression allows for the increase of sediment load due to rill detachment only. If the interrill contribution per unit length and width of rill (E), is added to the rill sediment load, then the following equation can be used to relate the change in rill load to sediment load, sediment transport capacity, and interrill sediment contribution:

Equation {8} cannot be solved directly, but an approximate solution (Elliot, 1998) which included dividing increases in sediment load between interrill and rill sources, and assumed a constant shear along a rill, was:

فرسايش شياري يا آبراهه اي

فرسايش شياري يا آبراهه اي

 

 

منشا اين تخريب اغلب باران است و در پيدايش آن عامل، شيب بسيار موثر مي باشد. فرسايش شياري يا آبراهه اي، در دامنه كوهها و حتي در سطح زمينهاي كم شيب نيز به سهولت ديده مي­شود. اين شكل فرسايش، پيشرفته تر از فرسايش سفره­اي بوده و ممكن است به صورت خطوط موازي نيز ظاهر شود كه ابتدا كم عمق است ولي به سرعت عميق تر مي­شود. اين شكل فرسايش، تا زماني كه سنگ مادر ظاهر نشده است، بنام فرسايش شياري خوانده مي­شود.

چند مقاله در رابطه با فرسایش شیاری

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Interrill and Rill Erosion on a Tropical Sandy Loam Soil Affected

CSIRO LAND and WATER Prediction of Sheet and Rill Erosion

Sheet and Rill Erosion and Sediment Delivery to Streams

RILL EROSION IN COHESIVE SOILS

SOIL EROSION ON AGRICULTURAL LAND IN SLOVENIA – MEASUREMENTS

 

فرسایش شیاری

Rill Erosion

Rill erosion is the removal of soil by concentrated water running through little streamlets, or headcuts. Detachment in a rill occurs if the sediment in the flow is below the amount the load can transport and if the flow exceeds the soil's resistance to detachment. As detachment continues or flow increases, rills will become wider and deeper.

 

rill erosion

description

Rill erosion often occurs with sheet erosion and is commonly seen in paddocks of recently cultivated soils following high-intensity rainfall. It is easily identified as a series of little channels or rills up to 30 cm deep.

process

If rainfall exceeds infiltration, a surface film of water forms (see sheet erosion). Rill erosion results from a concentration of this surface water into deeper, faster-flowing channels which follow depressions or low points through paddocks. The shearing power of the water can detach, pick up and remove soil particles making these channels the preferred routes for sediment transport. Rill erosion is often described as the intermediate stage between sheet and gully erosion.1

impacts

The loss of topsoil and nutrients reduces productivity greatly, as the remaining subsoils are often much less fertile. Also related soil deposition off-site causes sedimentation of streams, dams and reservoirs, resulting in water-quality deterioration and damage to aquatic habitats.

potential problem areas

Rill erosion is common on agricultural land devoid of vegetation and so is often seen in cropping areas after tillage. Following intense rainfall cultivated topsoils overlying denser cohesive subsoils often exhibit rill erosion. Texture-contrast (duplex) soils are susceptible, as are poorly managed pasture areas where overgrazing occurs.2

early signs

Watch for the development of surface water flows generated from moderate rainfall events. Beware in cropping areas of leaving soil bare during periods of high-intensity rainfall. Red-brown earths on slopes are particularly predisposed to erosion.3 Rills up to 30 cm deep will be easily visible in recently cultivated soils or areas cleared of vegetation. In tilled soils, rills will often extend to the depth of the tilled layer.4

what else to look for

Muddy run-off may indicate high sediment loads; look for evidence of off-site soil deposition. In some cropping situations, jolting of harvest machinery may indicate rills.4

action

Revise land use on potential problem areas, avoid extensive summer fallow. Modify cultivation and rotation of cropping land. Control grazing pressures, improve vegetation cover and soil management to increase organic matter and promote water infiltration and evapotranspiration by plants.1

key words

fallow, rill erosion, subsoil, tillage, tilled layer, topsoil.

references

1.McDonald Holmes (1946). 2.Office of the Commissioner for the Environment (1991).
3.Gibbons and Papst (1990). 4.Hunt and Gilkes (1992).