Stormwater Treatment Requirements
The San Mateo Water Pollution Prevention Program (SMCWPPP) is a partnership of the City/County Association of Governments (C/CAG), each incorporated city and town in the county, and the County of San Mateo, which share a common National Pollutant Discharge Elimination System (NPDES) permit, also referred to as the Municipal Regional Permit (MRP).
The SMCWPPP program provides extensive information for C.3 Regulated Projects, which are applicable to new development on a county-wide level, on its Flows to Bay website, including:
- C.3 Regulated Projects Guide
- Hydromodification Management (HM) Requirements
- Biotreatment Soil Mix Supplier List
- C.3 Sizing Worksheets
- Operation and Maintenance Requirements
What is a C.3 Regulated Project?
The goal of Provision C.3 of the MRP is for the municipalities regulated by the permit, including the County of San Mateo, to use their permitting authority to include appropriate source control, site design, and stormwater treatment measures in new development and redevelopment projects to address both soluble and insoluble stormwater runoff pollutant discharges and prevent increases in runoff flows from these projects. This goal is primarily accomplished through the implementation of low impact development (LID) techniques. Not withstanding exceptions listed in Provisions C.3.d and C.3.e of the NPDES Permit, all Regulated Projects are required to implement LID source control, site design, and stormwater treatment onsite. Regulated Projects are defined in the following categories:
- Special Land Use Categories: New or redeveloped Auto Service Facilities, Retail Gasoline Outlets, Restaurants, or Uncovered Parking Lots, as defined in Provision C.3.b., that are stand-alone or part of any other development project, that create and/or replace 5,000 square feet or more of impervious surface.
- New development or redevelopment projects that create or replace 10,000 square feet or more of impervious surface (collectively over the entire project site) including commercial, industrial, residential housing subdivisions (i.e., detached single-family home subdivisions, multi-family attached subdivisions (town homes), condominiums, and apartments), mixed-use, and public projects. Detached single-family home projects that are not part of a larger plan of development are specifically excluded.
- Road Projects: Types of road and trail projects as specified in Provision C.3.b. that create 10,000 square feet or more of newly constructed contiguous impervious surface.
Is My C.3 Project a Regulated Project?
The C.3 and C.6 Development Review Checklist captures a formal calculation of existing, new and replaced impervious surface at the project site. At the time of permit application, a completed checklist is required for all projects requiring a Planning or Building permit that would result in greater than 150 square feet of new and/or replaced area(s) of impervious surface. The checklist which will be used to determine if your project is a C.3 Regulated Project. If the project involves the development if more than one property at a time within properties owned by a common property owner, one form shall be completed for all properties involved in development.
Alternative Compliance: Provision C.3.e of the Municipal Regional Stormwater Permit (MRP) allows municipalities to grant “alternative compliance” to new development or redevelopment projects in lieu of requiring full onsite treatment of the Provision C.3.d amount of stormwater runoff and pollutant loads with low-impact development (LID) measures. Projects that receive alternative compliance must still provide LID treatment in full, but the treatment does not have to take place onsite. More information on alternative compliance is provided in Chapter 9 of the C.3 Regulated Projects Guide. Options for implementing green infrastructure in the right-of-way in the vicinity of the project parcel may be available as alternative compliance solutions based upon the project location. Contact the Planning and Building Department for more information.
Special Projects: The MRP allows LID treatment reduction credits for three categories of smart growth, high density and transit-oriented development project, called Special Projects. Projects that receive LID treatment reduction credits may be allowed to use specific types of non-LID treatment, if the use of LID treatment is first evaluated and determined to be infeasible by the permitting jurisdiction. More information on Special Projects is provided in Appendix J of the C.3 Regulated Projects Guide.
C3.i Small Projects: Projects that do not meet C.3 Regulated Project criteria but create and/or replace greater than 2,500 square feet of impervious surface may be subject to C3.i requirements.
- Instructions for Electronic Checklist Submittal
- Instructions for Completing Checklist: Single-Family homes and Small Projects
Not a C.3 Regulated Project but still interested in stormwater treatment? Also see our Green Infrastructure information.
What is an LID Treatment Measure?
LID treatment measures treat stormwater using harvesting and re-use, infiltration, evapotranspiration, or biotreatment methods. The MRP requires each Regulated Project to treat the amount of runoff identified in Provision C.3.d for the Regulated Project’s drainage area(s) with LID treatment measures onsite or with LID treatment measures at a joint stormwater treatment facility.
The C.3 Regulated Projects Guide (Link) includes technical guidance for Specific Treatment Measures, such as:
- Flow-through Planter
- Tree Well Filter
- Infiltration Trench
- Extended Detention Basin
- Pervious Pavement
- Grid Pavements
- Green Roof
- Rainwater Harvesting and Use
- Media Filter
- Subsurface Infiltration System
Maintenance of Constructed LID Treatment Measures
The MRP also requires the on-going maintenance of constructed LID measures, as well as inspection of these measures by the County at least every 5 years. The responsibility for the maintenance of stormwater treatment measures belongs to the property owner. Maintenance duties are described in an Operation and Maintenance Agreement. A County-approved Operation and Maintenance Agreement will be required prior final approval of the building permit. Please use the agreement template provided, turn on tracked changes, and submit your draft electronically to the contacts provided.
- Chapter 8 of the C3 Regulated Projects Guide provides a summary of O&M requirements, instructions for preparing maintenance-related documents, and describes specific maintenance activities by treatment measure.
- Maintenance Templates for Specific LID Treatment Measures are available in Appendix G of the C3 Regulated Projects Guide.
- Operation & Maintenance (O&M) Template Agreement (County of San Mateo only)
- Website for Property Owners: Operation & Maintenance (O&M) of Private Stormwater Treatment Measures