Two revised municipal stormwater permits with the Santa Ana and San Diego Regional Water Quality Control Board, adopted in 2009 in Orange County, include specific implementation requirements for low impact development (LID) and hydromodification control for new development and significant redevelopment projects. These permits require projects to “retain” (no surface discharge) stormwater on-site using infiltration, evapotranspiration, and/or harvest and use BMPs to the maximum extent practicable (MEP).  Stormwater that is not retained on-site must be biotreated if feasible, before a project may consider off-site regional solutions, conventional treatment and release, or alternative programs such as fee-in-lieu or mitigation. The permits also require watershed-based planning efforts for hydromodification control.

The County of Orange and municipal co-permittees, with the assistance of Geosyntec Consultants and CDM, have updated the Model Water Quality Management Plan (WQMP) that provides project proponents with the requirements for preparing a Project-Specific WQMP as required in the Municipal Stormwater NPDES permit.  With these updates, a detailed Technical Guidance Document (TGD) has been developed for the preparation of WQMPs to more effectively ensure that water quality protection, including LID principles, is considered in the earliest phases of a project.  The TGD addresses both technical issues and principles and the process of how these will be incorporated into the existing WQMP review and approval process. 

The Technical Guidance Document contains technically-based criteria for project evaluation to determine the feasibility of implementing LID BMPs and retaining runoff on site. These criteria address both on-site approaches and the technical potential for off-site mitigation opportunities using infiltration, evapotranspiration, harvest and use, and biotreatment. This document is one of this first of its kind in providing detailed guidance for evaluating the feasibility of on-site LID and is expected to set precedent for other municipal stormwater programs in California with similar renewed permit requirements. 

This presentation will cover both technical and process-related aspects of the updated technical guidance, as well as the process by which the guidance was developed. Multiple presenters will address topics ranging from the stakeholder process, to the technical aspects of the BMP selection, sizing, and feasibility framework, to the process by which Orange County developers will be required to integrate LID into their WQMPs.  This presentation is intended to be at least a long format (1 hour), but could fill a 1.5 to 2 hour session.

Other supporting authors not listed in submission form: Aaron Poresky (Geosyntec), Daniel Bounds (CDM)