Soil Conservation An Assessment of the National Resources Inventory, Volume 2 (1986) / Chapter Skim
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4. Understanding Ephemeral Gully Erosion
4. Understanding Ephemeral Gully Erosion
Pages 90-128
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From page 90... ...
_ ~ ~ The classical forms of erosion by water that occur within farm fields are sheet, rill, and gully erosion (Hutchinson and Pritchard, 1976)
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From page 91... ...
. Among other terms, it has been called ephemeral gully erosion, concentrated flow erosion, and megarill erosion.
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From page 92... ...
In principle, erosion in each of these eroded channels is by concentrated flow, and therefore several of the erosional processes are the same for each type. USLE estimates include rill erosion, but the equation clearly does not encompass ephemeral gully erosion.
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From page 94... ...
Furthermore, even though ephemeral gully erosion is related to some USLE factors, no attempt should be made to estimate ephemeral gully erosion from NRI data. Since future NRIs should take an inventory of this type of erosion, the remainder of this paper describes processes, physical impacts, policy implications, and inventory methods associated with ephemeral gully erosion to provide background to assist policymakers, NRI users, and USDA personnel.
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From page 95... ...
When deposition occurs, sediment yield from the channel is largely controlled by the flow's transport capacity near the outlet of the channel rather than by the amount of upstream erosion (Foster, 1982a)
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From page 96... ...
holds that detachment rate depends on the fraction of the transport capacity filled by the sediment load, according to: Df = DC (1 - G/TC) , where Of - detachment rate along the channel boundary [mass/(area · time)
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From page 97... ...
: Dc = Kc(~ ~ ~c) ' where Kc = a soil erodibility factor for erosion by flow, ~ = shear stress of the flow acting on the channel boundary at a point in time and space, and TO = critical shear stress required to detach soil at a point in time and space.
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From page 98... ...
Equation 7 identifies the major variables that should be considered in developing an empirical procedure to estimate ephemeral gully erosion. It represents potential sediment production at a particular ephemeral gully location during a storm.
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From page 99... ...
Factors Affecting Ephemeral Gully Erosion Runoff The maximum flow rate must exceed a critical level with a given channel grade and cover if the shear stress of flow in an ephemeral gully is to exceed the critical shear stress of the soil. The parts of the drainage network where this does not occur, which varies within and from storm to storm, will experience no erosion.
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From page 100... ...
One reason that grassed waterways control ephemeral gully erosion is that grass significantly reduces the shear stress of the flow acting on the soil (Temple, 1980)
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From page 101... ...
Nonerodible Layer and Previous Erosion Ephemeral gully erosion in the Midwest is most obvious in the spring, when erosive rains occur on freshly prepared seedbeds. The surface-tilled soil has a low critical shear stress and is highly erodible.
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From page 102... ...
Whereas a single storm may cause most of the annual ephemeral gully erosion when the underneath, untilled soil acts as a nonerodible layer, each storm causes erosion in Proportion to its erosivity ~ . on solos In one absence or a nonerodible layer.
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From page 103... ...
Unfortunately, the mechanics of both flow and erosion at headcuts are not well understood. Uniform erosion is usually assumed in analyses of ephemeral gully erosion, which smooths erosion rates over some distance on either side of the headcut.
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From page 104... ...
If a later storm occurs after significant canopy has developed over the field that reduces sediment production from rill and interrill erosion without lowering the transport capacity of flow in the ephemeral gully areas, the sediment available for transport in those gullies is reduced relative to transport capacity. The result is that the location where deposition begins moves downstream, and previously deposited sediment may be eroded.
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From page 105... ...
suggest that sediment produced by ephemeral gully erosion can equal that produced by rill and interrill erosion. If it leaves fields, this sediment can cause more offsite sedimentation damage than would be expected by considering just rill and interrill erosion.
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From page 106... ...
Chemical Yield Another offsite water quality issue associated with sediment is the concentrations of chemicals on the sediment yield. Such concentrations from ephemeral gully erosion are likely to be less than that from rill and interrill erosion because sediment from gullies is usually from deeper within the soil profile.
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From page 107... ...
The long-term productivity loss from ephemeral gully erosion extends, therefore, over an area larger than the immediate channels and over a long time. The productivity issue must also consider whether a unit of ephemeral gully erosion averaged over a field has the same impact as a unit of rill and interrill erosion so averaged.
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From page 108... ...
However, the reduction of erosion and associated benefits from practices like grassed waterways and other water disposal systems used to control ephemeral gully erosion have not been well quantified. To evaluate the total impact of erosion on farm fields, the amount of ephemeral gully erosion, the reduction in this erosion from installation of conservation practices, and the benefits from the reduction in this erosion must be estimated in addition to the common estimates of rill and interrill erosion.
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From page 109... ...
has proposed an objective method to identify ephemeral gully areas based on 2-foot interval contour maps, convexity of the contours, upslope contributing area, and local slope gradient. This method does not require evidence of ephemeral gully erosion to identify the drainage network.
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From page 110... ...
One method measures voided cross sections and reach lengths along the ephemeral gully network following erosive events. A difficulty with this approach is ensuring that the sample represents average ephemeral gully erosion over the field and average annual erosion.
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From page 111... ...
Cf. where E = ephemeral gully erosion, a = a coefficient, Ff = flow erosivity factor, Kf = soil erodibility factor for flow, ~ = an index that defines areas susceptible to ephemeral gully erosion (t = Ails, A = upstream area, s = channel grade, and ~ = contour convexity)
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From page 112... ...
to extensive. ~ · The amount of detail available ranges from little Some users are satisfied with very general t~gures -- tor example, that ephemeral gully erosion is about two-thirds of rill and interrill erosion in most fields.
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From page 113... ...
Unlike rills, these eroded channels are reformed each year in the same locations and gradually become incised in the landscape, a process that steepens adjacent overland flow slopes and accelerates rill and interrill erosion on them. Thus the impact of ephemeral gully erosion extends over a significantly larger field area than just the immediate eroded channel area.
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From page 114... ...
Inventories will probably be conducted by making field measurements for data to be put into an empirical prediction method. Multiplying USLE rill and interrill erosion estimates by an ephemeral gully erosion factor seems inappropriate.
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From page 115... ...
1981. Estimating sediment transport capacity in watershed modeling.
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From page 116... ...
In press. Seasonally ephemeral cropland gully erosion.
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From page 117... ...
1977. Use of the Universal Soil Loss Equation in the semiarid Southwest.
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From page 118... ...
1984. Prediction of Soil Loss Due to Ephemeral Gullies in Arable Fields.
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From page 119... ...
. The computations were made using the 1982 NRI procedure and a procedure specifically designed to compute the average soil loss for a slope segment (Foster and Wischmeier, 1974; Renard and Foster, 1983; Wischmeier and Smith, 1978)
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From page 120... ...
. in Table A-1, the NRI method underestimates average soil loss for the profile by 7 percent, which is not great considering other errors in USLE estimates.
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From page 121... ...
Furthermore, the calculated soil loss over the last 20 percent of a uniform slope is 40 percent in excess of soil loss tolerance when the average soil loss equals soil loss tolerance. This range of soil loss variation along a uniform slope is usually neglected because of imprecision in soil loss tolerance values.
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From page 122... ...
This soil loss value would be compared with a soil loss tolerance value to determine if erosion is a problem at the sample point. If such a procedure is followed, present soil loss tolerance values need adjustment to reflect permissible soil loss at a point rather than average soil loss over a uniform slope.
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From page 123... ...
2, (B-2) where DCt = total detachment capacity for the storm, ~ = a coefficient, Kc = a soil erodibility factor for detachment by flow, V = runoff volume expressed as an average depth over the upstream drainage area, A = upstream drainage area drained by the location on an ephemeral gully, s = grade of the channel, Cc = factor for cover conditions in the channel, arc = critical shear stress of the soil, and up = peak runoff rate expressed as average depth over the drainage area per unit time.
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From page 124... ...
: Tp BQpSCC, where B = a coefficient, and Qp = peak discharge rate. However, Qp can be approximated by: Qp = Acp.
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From page 125... ...
Discussion B.] Barfield Andre C McBurnie Foster's review of ephemeral gully erosion provides an excellent overview of the present state of our understanding of the physical processes involved in the movement of soil in the channelized flow areas.
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From page 126... ...
An independent estimate of ephemeral gully erosion is necessary since the Universal Soil Loss Equation (USLE) , which is the major tool for predicting soil erosion, was developed from a data base that did not include channelized flow.
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From page 127... ...
In fact, these nonuniformities may lead to the formation of the head-cut or knickpoint. · During the formation of ephemeral gullies, the nonuniformity of the channel properties results in a series of chutes and pools.
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From page 128... ...
1986. Understanding ephemeral gully erosion.
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