|Revised Universal Soil Loss Equation 1.06 - Definitions|
RUSLE1.06c uses a particular set of definitions, partly because the disciplines involved in soil erosion have not developed a common set of definitions. Observance of RUSLE definitions is critical to getting accurate results.
RUSLE1.06c estimates average annual soil loss, expressed as mass per unit area per year, which is defined as the amount of sediment delivered from the slope length assumed in the RUSLE1.06c computation. RUSLE1.06c uses US, customary units and computes soil loss in units of tons/acre/year, which is the sediment load at the end of the slope length divided by the slope length. In that context, RUSLE1.06c is a sediment yield equation that describes sediment yield at the end of the RUSLE1.06c slope length.
The RUSLE1.06c slope length is defined according to the problem being addressed. The typical application for RUSLE1.06c is development of an erosion control plan to protect the eroding portion of a landscape from being excessively degraded by soil erosion. In this application, slope length is defined as the distance from the origin of overland flow along the flow path to the point where deposition begins to occur on concave slopes or to a concentrated flow channel. In some cases, the slope can flatten to cause deposition and then steepen so that erosion occurs on the lower portion of the slope. Slope length passes through the depositional area when soil loss is being estimated on the lower portion of this slope.
Another application of RUSLE1.06c is to estimate the amount of sediment leaving a landscape that may cause off-site damages such as sedimentation in a road ditch. In this case, the slope length is the distance from the origin of overland flow through depositional overland flow areas to the first “concentrated flow” area that collects the overland flow or to the point that the runoff can no longer be considered overland flow. Analysis outside of RUSLE1.06c must be given to deposition that occurs in concentrated flow areas, except terrace and diversion channels that are considered by RUSLE1.06c, to fully estimate sediment yield from a landscape area.
RUSLE1.06c can also compute LS values by slope segment so that soil loss can be manually be computed for individual slope segments. These soil loss values represent net sediment production for those segments.
Detachment is the separation of soil particles from the soil mass, which adds sediment to the sediment load being transported downslope. Deposition is the transfer of sediment from the sediment load back to the soil mass. Local deposition is the deposition of sediment very near to the point where the sediment was detached. Deposition of Sediment eroded from soil clods and deposited in nearby depressions formed by the clods is an example of local deposition. Remote deposition is the deposition of sediment distant from its point of origin such as deposition in a terrace channel or on the toe of a concave slope.
Sediment load is a measure of the amount of sediment being transported downslope. Sediment yield, as used by RUSLE1.06c, is the sediment load at the end of the slope length, at the outlet of terrace diversion channels, or at the discharge point from sediment basins that are considered in a RUSLE1.06c computation.