Stormwater regulations require that new and redevelopment projects incorporate features that control and treat stormwater runoff to reduce pollutants flowing to the storm drain system and our waterways. These features are designed to filter rain water, removing pollutants that have been collected, slow the rate of stormwater flow, and allow the water to infiltrate into the ground.
Provision C.3 of the County's NPDES permit sets specific standards requiring projects that trigger certain thresholds to install Stormwater Management Facilities (SMFs) that are sized according to specific criteria.
In general, thresholds for C.3 regulated projects include*:
Facilities, roads, sidewalks, and trails that create or replace 5,000 square feet of impervious surface
Detached single family homes that create or replace 10,000 square feet of impervious surface
Road maintenance, lane widening, and utility trenching projects contiguous over one acre
Additionally, 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 the Stormwater C.3 Guidebook.
Individual SMFs (treatment and/or flow-control) are also referred to as Integrated Management Practices (IMPs). 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.