County of Orange Watershed Planning

Orange County, CA

PACE performed a watershed planning analysis and prepared a Watershed Infiltration and Hydromodification Management Plan (WIHMP) for the regional watersheds within the County of Orange, including those within both the Santa Ana and San Diego Regional Water Quality Control Board (RWQCB) areas, to illustrate implementation of new control measure requirements. These measures include low-impact development (LID), hydromodification conditions of concern (HCOC), and tools to support long-term watershed master planning goals. The planning process involved multiple steps of characterizing and analyzing the watershed and sub-surface conditions utilizing sophisticated Geographic Information System (GIS) mapping tools. This information was used to develop watershed plans for each of the 11 regional watersheds in the County to identify opportunities for sub-regional centralized Best Management Practices (BMPs) focusing on integrated water resource benefits.

The project resulted in a dynamic planning tool accessible by County staff and property owners. The planning tool provides watershed-specific support to integrate water quality, hydromodification, water supply, and habitat protection issues when selecting and designing BMPs to meet the requirements of the County’s Model Water Quality Management Plan (WQMP). Specifically, infiltration feasibility and hydromodification susceptibility at the watershed and subwatershed scale will help inform BMP selection and design for proposed projects. These are fundamental factors in the selection and sizing of BMPs to meet the requirements of the WQMP and Technical Guidance Document (TGD).

Following this effort, PACE supported the North Orange County (NOC) Municipal Separate Storm Sewer System (MS4) permittees through a partnership consisting of the County of Orange/OC Public Works, the Orange County Flood Control District (OCFCD), and multiple incorporated cities with the Watershed Improvement Projects Subcommittee (WIPS) program since its formation in 2018. This program is in place to identify opportunities to improve water quality and watershed health and achieve multiple benefits, such as augmenting water supply through the capture and use of stormwater and dry weather runoff within the four principal watersheds of NOC. As part of this effort, PACE developed a flexible Geographic Information Systems (GIS) tool to prioritize potential Best Management Practice (BMP) opportunities and rank these opportunities based on a number of parameters. These parameters were established for four different types of BMP solutions: Infiltration Basins (Open), Flow-through BMPs (Surface Basin), Infiltration Galleries (Underground), and Storage BMPs (Underground). Potential projects have been identified with concept layouts, and these projects have been evaluated based on land use, land ownership, distance to channel, dry weather volume, wet weather volume, BMP capacity, and BMP to 85th percentile storm volume ratio.

Key Features:

• Inventory of existing drainage facilities
• GIS-based tool to identify, analyze, and prioritize potential locations for regional BMP structures for runoff abatement
• Watershed connectivity for intelligent watershed mapping
• Evaluation criteria for scoring and ranking
• Automated watershed facility calculation tools
• Subwatershed plans and strategies planning / process
• Detailed fact sheet showing the scored values and ranking for each BMP opportunity