Stormwater regulations require that new development projects incorporate features that control stormwater runoff to reduce the quantity of pollutants introduced into the storm drain system and our waterways. These features are designed to remove pollutants from stormwater runoff, and to slow the rate of stormwater flow.
Provision C.3 of the County's NPDES permit sets specific standards requiring projects that trigger certain thresholds to install Stormwater Management Facilities that are sized according to specific criteria.
Projects creating and/or replacing at least 10,000 square feet of impervious surface are required to implement Stormwater Management Facilities (SMFs) that are designed and sized to provide treatment to remove pollutants from stormwater runoff.*
Projects creating and/or replacing at least 1 acre (43,560 square feet) of impervious surface must design their stormwater management facilities to provide both stormwater treatment and flow control functions (in order to maintain pre-project runoff volumes and durations.)* In addition to treating stormwater runoff, these projects control the volume and rate at which runoff is released so that it does not contribute to erosion in waterways.
* Please note that these are simplified descriptions of the applicability of C.3. For more detail refer to Stormwater C.3 Guidebook.
Individual SMFs (treatment and/or flow-control) are also referred to as IMPs (Integrated Management Practices). The Contra Costa Clean Water Program has created a free computer program, the IMP Sizing Calculator, to facilitate preparers of Stormwater Control Plans in the design and sizing of IMPs.