Development Related Issues
Background on Development Related Treatment Requirements
Urban development is a major contributor to stormwater-caused pollution. Requirements for new development and re-development projects fall in several categories, defined in section C.3 of the Municipal Regional Permit (2.8 MB).
NOTE: The new Municipal Regional Stormwater Permit (MRP) became effective Dec. 1, 2009. While many engineering aspects of C.3 requirements are unchanged, there are differences in site planning approaches and potential exemption status of proposed projects. Please contact the Clean Water Program or local new development representatives for status of updates to existing guidance. For more information, view the flyer on New Additional Low Impact Development Requirements.
Treatment controls are the facilities that are to designed and installed to remove pollutants from stormwater before the stormwater reaches the public drainage system or the creek. Typical measures are bio- retention areas, vegetated swales, and infiltration trenches. Projects creating or replacing 10,000 square feet of impervious surface are required to design the treatment control for a specific flow rate.
New, additional requirements for low impact development have gone into effect as of December 1, 2011, and may affect projects currently under review. Click here for more information.
Source control features are designed to keep pollution away from stormwater. They include trash areas that are enclosed, covered and plumbed to drain to the sanitary sewer system and designated wash areas that are plumbed to the sanitary sewer system, covered car wash areas that are plumbed to the sanitary sewer system. Most business establishments and certain residential developments are required to install source control features.
Site design features include reducing impervious areas, increasing pervious areas utilizing landscaped areas in between impervious areas as a storm drainage treatment feature. All projects are to consider implementing site design features appropriate to the development.
HYDROMODIFICATION MANAGEMENT (HM)
Hydrograph Modification Management, also called Hydromodification Management or HM, ensures that runoff from developed areas matches flow durations (long-term temporal patterns of volume and rate) of the pre-project runoff.
- All projects are to consider HM on the site through site design features. Projects creating or replacing one acre or more of impervious surface and located in HM control areas (Map) are also required to implement the HM Standard for flow duration control. (Hydromodification Fact Sheet)
- On-site flow duration control involves detaining increased runoff and discharging the detained volume through a special outlet structure, which may be sized using the Bay Area Hydrology Model (BAHM), a software tool specifically designed for this purpose.