Orange County Branch Newsletter
ASCE OC EWRI
Gobernadora Basin – An Innovative Multi-Objective Basin for Stormwater Treatment and Reuse
By: Jenny Robinet, Design Engineer- PACE Stormwater Management Division
It’s rare to find a single flood control design facility which can provide numerous functions and meet multiple objectives. The Gobernadora Detention Basin, located in Southern Orange County, California, accomplishes just that. The innovative stormwater management multi-purpose basin is the very definition of integrated watershed planning with the intent to maximize water resources beneficial uses.
On Thursday, December 4, 2014, attendees of the Orange County Environmental and Water Resources Institute (OC EWRI) Technical Luncheon were treated to an in depth and personal exploration of the proposed 30-acre Gobernadora Detention Basin. The presentation was jointly given by Don Bunts, Chief Engineer with the Santa Margarita Water District (SMWD), and Bruce Phillips, Senior Vice President in the Stormwater Management Department at Pacific Advanced Civil Engineering, Inc. (PACE).
Mr. Bunts began by discussing the conditions in the area necessitating the multipurpose flood control facility. The basin is situated adjacent to a natural alluvial creek in an unincorporated area of Orange County, south of the Coto de Caza community development. The creek drains a 7.8 square mile watershed that has been completely urbanized over the past two decades with the development, increasing the need for flood mitigation and stormwater treatment in the area. Calling on his over 30 years of environmental engineering master planning experience, Mr. Bunts then explained the complex regional planning process for the design of the basin, as well as the multiple watershed stakeholders and water resource agencies involved in the plan formation. Although the Santa Margarita Water District, in partnership with Rancho Mission Viejo and the County of Orange, is proposing the detention basin, this project is truly a collaborative effort of a variety of companies and agencies, receiving numerous grant funds because of its unique benefits and multiple purposes.
The project presentation continued as Mr. Phillips described the innovative design of the Gobernadora Detention Basin, of which he was very involved. Attendees of the luncheon benefitted from Mr. Phillips’ over 30 years of stormwater management experience and vast knowledge of urban flood control facilities as he explained the interesting configuration of the detention basin. The large facility is actually made up of two distinct basins, an upper basin of 13.3 acres and a lower basin of 17.4 acres, which are each slightly incised and incorporate perimeter levees, interconnected by a spillway system. A specialized side weir located in the upper basin and adjacent to the creek allows peak storm inflows into the basin while bypassing sediment downstream to the natural alluvial channel. Mr. Phillips showed attendees various images of the upper basin low-flow diversion from the creek and the basin’s intricate system of interconnected water quality treatment cells to improve urban nuisance flow water quality. He also explained the storage capacity of the basin during large storm events, and the function of the two innovative inflatable rubber dams to be employed for hydraulic control – one for dry-weather nuisance flow capture, and another for flood operation to develop variable water levels for the side weir inlet.
To achieve maximum benefits from the site, the Gobernadora Detention Basin project meets numerous physical and regulatory constraints, including existing wildlife habitat, an active alluvial creek system, high groundwater, varied topography, existing utilities, adjacent public parks, and residential areas. The constraints made for an extensive planning process, with the incredible resulting facility set to meet maximum potential and provide multiple functions: (1) flood mitigation, (2) urban stormwater treatment, (3) groundwater recharge, (4) groundwater recovery, (5) non-potable water reclamation, and (6) stream stabilization and habitat restoration. The technical luncheon presentation not only provided insight into the process and design of the multi-objective basin, it also left attendees with the impression that this project is at the forefront of a new and improved multi-functional approach to flood control and stormwater management.