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.

Stormwater runoff is part of a natural hydrologic process. However, land development that increases impervious area can significantly alter natural drainage patterns, resulting runoff leaving developed areas that is significantly greater in runoff volume, velocity, and peak flow rate and duration than pre-development runoff from the same area.

Impervious area includes pavement; roof tops; walkways; patios; driveways; parking lots; storage areas; impervious concrete and asphalt; and any other continuous watertight pavement or covering.

Development projects are required to implement permanent controls to mitigate the impacts of the creation or replacement of impervious areas. These controls are referred to as Low Impact Development (LID), or post construction Best Management Practices (BMPs), or Green Infrastructure.  

LID reduces water quality impacts by preserving and re-creating
natural landscape features, minimizing imperviousness, and then infiltrating,
storing, detaining, evapotranspiring (evaporating stormwater into the air
directly or through plant transpiration), and/or biotreating stormwater runoff
close to its source, or onsite.

The Municipal Regional Stormwater Permit (MRP) mandates LID for specified development projects. If your proposed project creates or replaces impervious areas, you may need to comply with the MRP Provision C.3. The C.3 Technical Guidance Manual provides guidance for project proponents on complying with the C.3 mandates.

If you would like to learn more about LID and Green Infrastructure in Alameda County, visit the Green Infrastructure page.

MRP Provision C.3 and the C.3 Technical Guidance Manual

The San Francisco Bay Regional Quality Control Water Board reissued MRP in 2022 and amended it in October 2023. Like the past stormwater permits, the reissued MRP, referred to as 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. 

ACCWP updated the C.3 Technical Guidance Manual to be consistent with the MRP 3 including the October 2023 amendment.  

Additionally, the following factsheets explain some of the major changes.

The ACCWP C. 3 Technical Guidance Manual provides directions on how to comply with the MRP Provision C.3 requirements for development sites. Required LID measures include Site Design, Source Controls, Stormwater Treatment Controls, and Hydromodification Management.
Site Design

Site planning techniques that help reduce stormwater pollutants and prevent increases in the peak runoff flow and duration by protecting existing natural resources and reducing impervious surfaces of development projects. 

Source Controls

Structural controls or operational “good housekeeping” practices that prevent pollutant discharge and runoff at the source and keep pollutants from coming into contact with 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.

Stormwater Treatment Measures

Engineered systems designed to remove pollutants from stormwater runoff before it reaches the storm drain system or waterways by settling, filtration, biological degradation, plant uptake, media absorption/adsorption or other physical, biological, or chemical processes. This includes landscape-based systems such as bioretention areas as well as proprietary systems.

Hydromodification Management (HM)

Engineered systems designed to minimize changes to the hydrograph (hydromodification) resulting from development by matching the flow durations (long-term temporal patterns of volume and rate) of the pre-project runoff. Stormwater Treatment Measures provide some HM control, but larger projects need to implement the HM requirements of MRP Provision C.3.g. 

Bay Area Hydrology Model (BAHM)

Projects subject to the MRP Provision C.3.g need to use the latest version of the Bay Area Hydrology Model — BAHM2023 to size hydromodification management or flow control facilities to mitigate the effects of increased runoff (peak discharge, duration, and volume) from proposed land use changes that impact natural streams, wetlands, and other water courses. The BAHM Download page provides the model, user’s manual, and training on the use of the model.

 Suggestions for the C.3 Technical Guidance Manual

If you have suggestions to improve the C.3 Technical Guidance Manual or identify errata, please use this form to submit your suggestions for consideration during the next update: C3TG Suggestion Form

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