This webinar, hosted in February by Stormwater Solutions on Stormwater University, provided an introduction to the Revised Universal Soil Loss Equation (RUSLE), one of the most widely used methods for calculating erosion in the U.S. RUSLE is an evolution of the Universal Soil Loss Equation (USLE), developed as a predictive tool to estimate soil erosion caused by rainfall and surface runoff. The most current version, released by the Natural Resources Conservation Service, is RUSLE 2.
The webinar explained the factors that go into calculation of erosion using RUSLE: A = R • K • LS • C • P., where A is the average annual soil loss, R is the rainfall-runoff erosivity factor, K is the soil erodibility factor, LS is the slope length factor (topographic factor), C is the cover factor, and P is the practice factor. The instructors discussed each of these factors and where users can find data for calculations. The instructors will explain differences between erosion and sediment yield and will discuss strengths and limitations of RUSLE.
Below, the speakers, Andrew Earles, executive vice president, and Lily Montesano, project manager, both from Wright Water Engineers, respond to audience questions. The full webinar can be found here.
Audience: Do the exponent and coefficient change between watershed or does it remain constant?
Andrew Earles & Lily Montesano: In MUSLE, the coefficient and exponent remain the same, but the rainfall and peak flow rate will vary from one watershed to another.
Audience: Can you review how you determine a weighted average, slope length, and slope?
AE& LM: For an area-weighted average, the value being evaluated (R for example), is determined for each sub-watershed. For each sub-watershed, multiply the sub-watershed area by the R value, and then add all of these up. Finally, divide by the total watershed area, and that will give you an area-weighted R value. For the LS factor, we usually measure at a number of different locations and the average the measurements.
Audience: How do you apply the LS table if your slope Length is greater than 1000 LF?
AE & LM: It is very uncommon to have a slope length longer than 1,000 feet. Most slope lengths are in the range of a few hundred feet. Sheet flow does not usually persist for more than a few hundred feet.
Audience: Is it possible to implement the erosion rates as a design parameter (e.g., such as BMPs, stormwater utilities (e.g., RCPs, RCBs, etc.), channel design)? Are there any existing regulations, guidelines, etc. for providing general values for the erosion rate through the Revised Universal Soil Loss Equation, and is it possible to incorporate the soil loss/erosion rate calculations into a hydrodynamic model, particularly HEC-RAS 2D?
AE & LM: RUSLE can be used to design BMPs. It provides estimates of erosion volumes that may need to be managed by a BMP, and it also can be used to assess effects of erosion and sediment control practices on erosion through the C and P factors. Some states require estimates of erosion using the RUSLE as a part of their stormwater permitting process. To incorporate with a hydraulic model such as HEC-RAS 2D, the sediment volume calculated using RUSLE can be combined with the volume of the hydrograph to estimate a volumetric sediment concentration to apply to the flow in HEC-RAS 2D.
Audience: When using RUSLE for an arid Southwest high-desert chaparral, steep terrain with nearly 100% burn, I estimated C and P factors that were off the charts. I arrived at what seemed to be realistic numbers...was this appropriate?
AE & LM: C and P factors can be very high following a fire. It is always important to do a reasonable check on results. As stated in the presentation, I trust RUSLE more in a comparative sense than an absolute one.
