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Construction BMPs Chapter 7
• Provide sediment basins or barriers (silt fence) at or near the toe of slopes to trap sediment or to
reduce slope lengths. When flows are concentrated and conveyed down a slope using a slope drain or
channel, energy dissipation measures will be required at the conveyance outlet at the toe of the slope.
See the Sediment Control BMP Fact Sheets for several options for controlling sediment at the base of
slopes.
5.3 Using the Revised Universal Soil Loss Equation
The Revised Universal Soil Loss Equation (RUSLE) is an erosion prediction method that has evolved
over time, resulting from data collection and analysis efforts extending from the 1930s through the 1970s,
ultimately published in Agriculture Handbook 282 (Wischmeier and Smith, 1965), then Agriculture
Handbook 537 (Wischmeier and Smith, 1978) and Agriculture Handbook 703 (Renard et al., 1997).
Although originally developed for agricultural land use, it is also a useful method for estimating erosion
potential on construction sites and adjusting BMPs to reduce the estimated erosion. The RUSLE is also
incorporated into several modern erosion prediction models. The Modified Universal Soil Loss Equation
(MUSLE) is similar to the RUSLE, but is differentiated by the fact that MUSLE is event-based while
RUSLE is an annual method (with the option to calculate monthly or seasonal erosion). This section
provides a brief overview of RUSLE and describes how it can be used to help select erosion control
practices at construction sites.
where:
A = RKLSCP Equation 7-4
A = Computed spatial average soil loss and temporal average soil loss per unit of area, expressed in
the units selected for K and for the period selected for R. Typically, A is expressed in tons per
acre per year.
R = Rainfall-runoff erosivity factor – the rainfall erosion index plus a factor for any significant
runoff from snowmelt.
K = Soil erodibility factor – the soil-loss rate per erosion index unit for a specified soil.
L = Slope length factor – the ratio of soil loss from the field slope length to soil loss from a 72.6 ft
length under identical conditions.
S = Slope steepness factor – the ratio of soil loss from the field slope gradient to soil loss from a 9
percent slope under otherwise identical conditions.
C = Cover-management factor – the ratio of soil loss from an area with specified cover and
management to soil loss from an identical area in a bare condition. Values range from 0.01 to 1.
P = Erosion control practice factor – the ratio of soil loss with a certain conservation practice
(erosion control BMP) to that of no practice. Values range from 0.8 to 1.2.
The slope length, L, and steepness factor, S, are commonly combined as one variable, LS. Values for LS
are quantified relative to a 72.6 ft slope length with a 9 percent slope. A slope with these two values will
have an LS factor of 1.
A detailed discussion of RUSLE factors is beyond the scope of this manual; however, Agriculture
Handbook 703 can be obtained at no charge from the USDA publications website and used to develop or
November 2010 Urban Drainage and Flood Control District 7-21
Urban Storm Drainage Criteria Manual Volume 3