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This online calculator will assist you in estimating the rainfall erosivity factor (R factor) for your construction project.
The NPDES Stormwater Phase II Rule allows NPDES permitting authorities to accept "low erosivity waivers" for small construction sites. The waiver process exempts small construction sites (disturbing under five acres) from NPDES permitting requirements when the construction activity takes place during a relatively short time in arid or semi-arid areas.
EPA accepts low erosivity waivers for the five states and other areas where EPA is the permitting authority. Several states that are authorized to implement the NPDES permitting program also accept low erosivity waivers. Check with your state NPDES permitting authority for more information.
This online calculator provides an easy way to estimate the erosivity factor for a given construction project. Simply enter some basic information about your project into the calculator and it will provide you with an "R factor." If the R factor is less than 5, you may submit a Low Erosivity Waiver (if accepted by your permitting authority). If the R factor is greater than 5, you will need to submit a Notice of Intent to obtain coverage under an appropriate NPDES Construction General Permit. (Please note: if your construction project disturbs five acres or more, or is part of a larger common plan that disturbs five acres or more, you are not eligible for a low erosivity waiver.)
To use the Low Erosivity Calculator, you will need to enter the following information:
- Name of your project
- Estimated start and completion dates
- Location of your project, either:
- latitude and longitude
or
- mailing address
If your project is located in an area where EPA is the permitting authority, your project disturbs less than five acres, and your R factor is less than 5, you may submit a Low Erosivity Waiver Certification via EPA's eNOI system or complete and mail a paper Low Erosivity Waiver Certification form [PDF - 402 KB - 4 pp].
If EPA is not your NPDES permitting authority, check with your state NPDES permitting authority to determine availability of and procedures for obtaining this waiver.
For more information, see the small construction activity waivers page.
NOTE: This calculator is only valid for construction sites within the continental U.S. Additional information about the R factor is available from the following USDA publication:
Renard, K.C., G.R. Foster, G.A. Weesies, D.K. McCool, and D.C. Yoder. 1997. Predicting soil erosion by water: A guide to conservation planning with the Revised Universal Soil Loss Equation (RUSLE), Agricultural Handbook 703, USDA-ARS, U.S. Government Printing Office, Washington, D.C.
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