Maintain site water balance, protect water quality, and reduce negative impacts to aquatic ecosystems, channel morphology, and dry weather base flow by replicating natural hydrologic conditions and providing retention and treatment for precipitation on site.
Through on-site infiltration, evapotranspiration, and harvest use, retain or treat the maximum precipitation volume possible beyond the required baseline precipitation volume of the 60th percentile precipitation event described in Water P3.1: Manage precipitation on site.
These requirements pertain to the entire area within the SITES project boundary.
- Retain or treat precipitation volume for the following percentile precipitation events:
- 80th percentile precipitation event 4 points
- 90th percentile precipitation event 5 points
- 95th percentile precipitation event 6 points
- Retain or treat the maximum precipitation volume on site for the percentile precipitation event associated with the desired point total above, as defined by the U.S. EPA in the Technical Guidance on Implementing the Stormwater Runoff Requirements for Federal Projects under Section 438 of the Energy Independence and Security Act (or local equivalent for projects outside the United States). Use daily rainfall data and the methodology in the EPA document to determine the target percentile volume.
- For treatment of any precipitation volume not retained from the selected percentile precipitation event, listed above:
- Treat any volume of runoff using an appropriate technology.
- Ensure the entirety of the selected percentile precipitation event not retained is treated, and that remaining runoff will be discharged at rates consistent with the natural rate of erosion in the receiving water channel (i.e., discharge rate limit).
- BMPs must be scaled to account for the additional volume of runoff entering the BMP from areas off site.
- The treatment practices implemented must achieve, at a minimum, an average discharge concentration of less than or equal to 25 milligrams per liter total suspended solids for the volume treated.
- In cases where receiving waters are impaired or threatened by specific pollutants of concern, treatment must be provided for these pollutants.
- Ensure the sections of the site maintenance plan (see O+M P8.1: Plan for sustainable site maintenance) are complete and detail the proper maintenance activities used to ensure long-term effectiveness of stormwater features, including water quality treatment activities.
See Recommended strategies in Water P3.1: Manage precipitation on site.