C.3 Technical Guidance Manual Update
The San Francisco Bay Regional Water Board Quality Control reissued the Municipal Regional Stormwater Permit (MRP 3), and it went into effect July 1, 2022. Like the past stormwater permits, MRP 3 mandates stormwater control measures for specified development projects. MRP 3 included several significant changes including changes to the types and sizes of projects regulated. Most of these changes go into effect July 1, 2023. ACCWP is in the process of updating the Technical Guidance Manual to incorporate these changes.
One change is effective as of July 1, 2022. Tree Interceptor Credits, as described in the Section 4.5 of the Alameda Countywide Clean Water Program’s C.3 Stormwater Technical Guidance are no longer permitted be used to reduce the amount of stormwater treated in the numeric sizing calculations for stormwater treatment systems. This change is reflected in the redlines in Version 7.1 of the Manual.
Look for the release of the fully updated Manual in early 2023.
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 Stormwater Discharge Permit.
The C.3 Technical Guidance Manual gives directions on how to implement the provision C.3 requirements for development sites.
Typical pollution prevention measures in development include:
These structural elements remove pollutants from stormwater before it reaches the stormdrain system or a waterway like a creek. Typical measures are bio-retention areas, vegetated swales and infiltration trenches. See Green Infrastructure for details and examples.
These features are designed to keep pollution away from stormwater. Examples include trash areas that are enclosed, covered and plumbed to drain to the sanitary sewer system. Most business establishments and certain residential developments are required to install source control features.
Development projects sites can be designed to increasing pervious areas, for example by utilizing a landscaped area in between impervious areas as a storm drainage treatment feature, so that stormwater can be absorbed into the ground and naturally cleansed by soil and plants. All development projects are required to consider site design features appropriate to the development.
Hydromodification Management (HM)
Also called Hydromodification Management or HM, this measure 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 current (2013) version of the Bay Area Hydrology Model (BAHM), a software tool specifically designed for this purpose.
- Bay Area Hydrology Model User Manual