All new development and significant redevelopment projects that create or replace more than 10,000 SF of impervious surface must install stormwater treatment facilities on-site. This requirement applies to certain projects (auto service facilities, gas stations, restaurants and uncovered parking lots) that create or replace over 5,000 SF of impervious surface. This “C.3" requirements are separate from, and in addition to, requirements for erosion and sediment control and for pollution prevention measures during construction.
Project applicants are encourage to minimize the area of new roofs and paving. Where feasible, pervious surfaces should be used instead of paving so that runoff can percolate to the underlying soil. Runoff from impervious areas must be captured and treated. Projects may also be required to detain or infiltrate runoff so that peak flows and durations match pre-project conditions. In addition, project applicants must prepare plans and execute agreements to ensure the stormwater treatment and flow-control facilities are maintained in perpetuity.
Stormwater Treatment Facilities and Maintenance
These are typically landscape-based features such as rain gardens, swales or infiltration basins that treat runoff before it discharges to the storm drain. Property owners are required to maintain these facilities in good condition. City inspectors will periodically inspect them for proper operation and maintenance.
- C.3 Process Flowchart [PDF]
- Stormwater Management Exhibit Notes [PDF]
- Walnut Creek's Small Projects and Single-Family Homes requirements [PDF].
- How to prepare a Stormwater Control Plan for small projects [PDF].
- Stormwater C.3 Guidebook [PDF] 7th Edition (May 17, 2017). See Appendix B of the Guidebook for Soils specification.
- Click here for Contra Costa County C.3 Webpage
- Click here for the Municipal NPDES Regional Permit (MRP) from the San Francisco Bay Regional Water Quality Control Board's website.
- Special Projects Guidance (January 17, 2012)
- Model Stormwater Facilities Construction Inspection Checklist [PDF] (November 15, 2018)
- Model Stormwater Facility Operation and Maintenance Verification Report [PDF] form (April 24, 2012)