ASCE OC EWRI: Rancho Mission Viejo DMA-C Flood Basin Project
ASCE Orange County Branch members recently attended a presentation on the Rancho Mission Viejo DMA-C Flood Basin Project, hosted by the Environmental & Water Resources Institute. The project is an important regional flood-control facility being developed as part of the ongoing Rancho Mission Viejo community and infrastructure program.
The presenter, Tyler Holst, PE, Senior Vice President of Infrastructure Engineering with Rancho Mission Viejo, provided an overview of the planning, design, permitting, and regional coordination involved in developing the flood basin and its role in providing long-term flood protection for communities in South Orange County. He not only spoke on the future of the development; he also provided attendees with an understanding of the background and historical operations of the thriving ranch.
The presentation covered details of the DMA-C Flood Basin project, which is located along the southern edge of Rancho Mission Viejo's Planning Area 3 development. At ultimate buildout, the facility will encompass approximately 21 acres and feature an open-bottom basin with natural side slopes. The basin is being constructed in phases to correspond with the development of the surrounding watershed.
The initial phase of the basin, which is already receiving runoff from portions of completed phases, provides approximately 70 acre-feet of storage capacity. With the grading and construction associated with Phase 6, the basin will reach its ultimate capacity of approximately 170 acre-feet.
Upon completion of the planned watershed development, the basin's tributary drainage area will encompass approximately 1,275 acres. The completed DMA-C Flood Basin is planned to detain runoff from a 100-year storm event, helping preserve the flow capacity of San Juan Creek while providing additional flood protection for downstream communities, including the Cities of San Juan Capistrano and Dana Point.
Tyler described the multi-benefit nature of the project, and how it seeks to integrate flood-control infrastructure with large-scale community development. Rather than treating drainage improvements as a separate component of development, the DMA-C Flood Basin is being constructed as part of a broader regional infrastructure system designed to accommodate future watershed conditions and provide long-term flood protection.
Tyler praised the project and its collaboration among public agencies, private developers, and engineering consultants. Rancho Mission Viejo partnered with the Orange County Flood Control District, Orange County Public Works, and consultants, including PACE Advanced Water Engineering and Huitt-Zollars, throughout the design and permitting process. This coordination was critical to advancing the project that will ultimately serve both the developing Rancho Mission Viejo community and downstream areas of the regional watershed.
With approximately 6,000 acres, 14,000 planned homes, and more than five million square feet of non-residential uses—including schools, parks, recreational facilities, community farms, retail, and office centers—Rancho Mission Viejo represents a significant long-term development effort in South Orange County. The DMA-C Flood Basin is an integral part of the infrastructure supporting that growth while addressing regional water and flood-management needs.
The presentation provided ASCE members with an opportunity to learn more about the engineering design and interagency coordination behind the major flood-control facility and the role that civil engineers play in planning resilient infrastructure at both the community and regional watershed scale.
About the Author:
Jenny Robinet is a Project Manager at PACE Advanced Water Engineering in Orange County, CA. She loves working on various stormwater projects and being involved in the civil engineering industry, especially ASCE. Jenny can be contacted at jrobinet@pacewater.com or https://www.linkedin.com/in/jenny-robinet.
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