Urban development traditionally involves replacing natural landscapes with impervious areas, such as roofs, pavements, streets, and storm drain systems, causing greater amounts of polluted stormwater runoff to flow directly into creeks.
The impact of urban development on waterways can be reduced by using Low Impact Development (LID) techniques. LID techniques reduce runoff and mimic a site’s natural (predevelopment) hydrology by minimizing disturbed areas and impervious surfaces, and retaining and treating stormwater runoff using infiltration, evapotranspiration, rainwater harvesting and use, or biotreatment.
Private and public new and redevelopment projects that create or replace amounts of impervious surface that exceed certain thresholds are required to include appropriate site design, pollutant source control, treatment measures, and where appropriate, hydromodification management measures that improve, protect and treat urban runoff. Local municipalities have developed Green Stormwater Infrastructure (GSI) Plans for incorporating LID features into public lands, such as streets, buildings, parking lots, and parks.
Key Guidance Documents
- SCVURPPP C.3 Stormwater Handbook
- Provision C.3 Data Form (download only). Please download and complete this form. Completing it online in the web browser may lead to calculation errors.
- Special Projects Worksheet
- C.3 Stormwater Treatment Measure Sizing Worksheets
- Fact Sheets on New Stormwater Quality Control Requirements
Site Design Measures
Site design measures are site planning techniques that can help reduce the impacts of increased runoff and pollution from land development on water quality. Examples include preserving existing vegetation, reducing impervious surfaces, directing runoff to vegetated areas, using pervious pavement, and directing roof downspouts to landscaped areas. See the SCVURPPP C.3. Stormwater Handbook for more information on site design measures.
The MRP requires that some small projects implement specific site design measures. The following four fact sheets provide information on how to select and design appropriate site design measures for these project sites:
Pollutant Source Control Measures
Source controls prevent potential pollutant sources from coming in contact with rainfall and runoff. Examples include: roofed trash enclosures, covered outdoor materials handling and storage areas, storm drain labeling, and sanitary sewer drains for vehicle wash areas (with sewer agency approval).See the SCVURPPP C.3. Stormwater Handbook for more information on source control measures.
Treatment Control Measures
Stormwater treatment measures are systems designed to remove pollutants from stormwater before it reaches the storm drain system. Examples of allowable treatment measures for most projects include: bioretention areas, flow-through planters, infiltration facilities (e.g., trenches, pervious pavements, or subsurface systems), and rainwater harvesting systems. Vault-based treatment systems are only allowed at certain high-density projects that meet the Special Projects criteria described in Appendix J of SCVURPPP’s C.3 Stormwater Handbook.
Stormwater treatment measures are designed to treat a specified volume or flow of stormwater, depending on how they function. Volume-based treatment measures detain stormwater for a certain period and treat primarily through storage and infiltration. Flow-based treatment measures treat pollutants from a moving stream of water through filtration, infiltration, and/or biological processes.
To learn more about selecting, designing and sizing stormwater treatment controls, see the C.3. Stormwater Handbook.
Hydromodification Controls
When land is covered with buildings and pavement, runoff enters creeks at higher rates and volumes, resulting in channel erosion, flooding and habitat loss. These changes in runoff characteristics are known as hydromodification. Hydromodification management measures are detention and/or infiltration facilities that are constructed with special discharge structures to match pre-project runoff patterns.
Hydromodification controls help prevent worsening of creek channel erosion problems from new development or redevelopment projects. Hydromodification management requirements are different from flood control requirements.
New development and redevelopment projects that create or replace at least one acre of impervious surface, increase impervious surface area above the pre-project condition, and are in a susceptible area, are required to implement hydromodification management requirements.
Resources:
- Santa Clara County Hydromodification Management Map (PDF)
- Santa Clara County Hydromodification Management Map (Opens in new window)
- Local Hydromodification Management Applicability Maps
- Hydromodification Management Plan
SCVURPPP, along with the San Mateo and Alameda County stormwater programs, helped fund the development of the Bay Area Hydrology Model (BAHM), a tool for simulating pre- and post-project hydrology and automatically sizing flow control facilities to better manage hydromodification. Clear Creek Solutions developed this model for the Bay Area, and SCVURPPP helped calibrate the model in Santa Clara Valley watersheds. Click below to download the BAHM2013 tool.
Operation & Maintenance
Maintenance is essential for assuring that stormwater treatment and hydromodification management measures continue to function effectively and do not cause flooding, provide habitat for mosquitoes, or otherwise become a nuisance. To ensure that property owners are maintaining treatment measures, Santa Clara Valley municipal agencies have inspection and reporting programs in-place.
Property owners are required to conduct inspections and maintain installed stormwater treatment measures. Inspection documentation templates are available in Appendix G of the SCVURPPP C.3. Stormwater Handbook.