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 four 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.
Note: EPA’s online rainfall erosivity factor calculator is currently under construction. In the meantime, please use the
Construction Rainfall Erosivity Waiver Fact Sheet (PDF) (12 pp, 1.39MB) to assist in determining the R Factor for a particular small construction site. Appendix C (PDF) (4 pp, 99K) of the 2012 CGP also provides information on small construction waivers.