By minimizing peak rain-water runoff, utilizing regional water quality control board mitigation measures locally, and/or utilizing other mitigation methods described above, these laws aim to promote low-impact development. Site that has grayfield or infill. In places with impermeable surfaces, rainfall needs to be managed at a slower rate than in undeveloped areas.
CALGreen Code Section A5.106.3: Low impact development (LID).
All newly constructed projects shall mitigate (infiltrate, filter or treat) storm water runoff from the 85th percentile 24-hour runoff event (for volume-based BMP’s) or the runoff produced by a rain event equal to two times the 85th percentile hourly intensity (for flow-based BMP’s) through the application of LID strategies. Employ at least two of the following methods or other best management practices to allow rainwater to soak into the ground, evaporate into the air or collect in storage receptacles for irrigation or other beneficial uses. LID strategies include, but are not limited to:
1. Bioretention (rain gardens) filtration planters;
2. Precipitation capture (cisterns and rain barrels);
3. Green roof meeting the structural requirements of the building code;
4. Roof leader or impervious area disconnection;
5. Permeable and porous paving;
6. Vegetative swales and filter strips; tree preservation;
7. Tree preservation and tree plantings;
8. Landscaping soil quality;
9. Stream buffer; and
10. Volume retention suitable for previously developed sites.
CALGreen Code Section A5.106.3.1: Implementation.
If applicable, coordinate LID projects with the local Regional Water Quality Control Board, which may issue a permit or otherwise require LID.
Note: Further information on design of specific control measures may be found on the U.S. EPA, or the California State Water Resources Control Board (SWRCB) websites, and from local boards that require LID.
CALGreen Code Section A5.106.3.2: Greyfield or infill site.
Manage 40 percent of the average annual rainfall on the site’s impervious surfaces through infiltration, reuse or evapotranspiration.
Intent:
Implementation (A5.106.3.1). The intent of these provisions is to encourage low-impact development by reducing peak rain-water runoff, utilizing local Regional Water Quality Control Board mitigation measures and/or additional mitigation measures listed above.
Greyfield or infill site. (A5.106.3.2) The intent is to manage rainfall at a lower rate for areas of impervious surfaces than that for undeveloped sites.
Verify the existence of any local Regional Water Quality Control Board mitigation measures required for LID.
Compliance Method:
1. Design specific control measures in accordance with the Environmental Protection Agency (EPA) requirements and/or local Regional Water Control Board requirements for implementation of LID.
2. Show site design documents that indicate control measures for rainfall on undeveloped sites, using mitigation measures listed above or from other referenced sources.
3. For greyfield or infill sites with impervious surfaces, show how at least 40 percent of annual rainfall is to be managed on site.
Enforcement:
Plan intake: The plan reviewer should examine the construction documents to confirm compliance measures have been incorporated into the site design.
On-site enforcement: The inspector should verify that on-site control measures comply with the construction documents.
(Excerpted from ‘Guide to the 2022 California Green Building Standards Code Nonresidential’ – Appendix A5)
