| Underground reservoir to help California utility balance water demands
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Underground reservoir to help California utility balance water demands

San Diego County water officials will be better able to regulate untreated water flows to retail member agencies with a massive underground reservoir now under construction. And once it’s finished, outdoor enthusiasts who traverse the area will never know it’s there.

Construction of San Diego Water Authority's underground water storage facilityThe San Diego Water Authority (SDWA) is building a 4.88-million-gallon flow regulatory structure in Mission Trails Regional Park, a vast open space popular with hikers and mountain bikers. The $42 million Mission Trails Flow Regulatory Structure II, financed by bonds and cash, has been in the works for almost 20 years as a way to increase reliable water delivery to treatment plants serving the central and south sections of San Diego County. (Construction photo at left courtesy of SDWA)

“This is purely operational, like the shock absorbers on your car,” said Brent Fountain, principal engineer at SDWA. “Without it, we run the risk of spilling over or not meeting demands temporarily. This project improves our ability to reliably serve ratepayers.”

When there’s an uptick in demand, SDWA can use the structure to immediately release more water, backfilling the tank with water from upstream. And if there’s a sudden decrease in demand, such as a major treatment plant outage, SDWA has a place to store excess water.

Brent Fountain“Right now, it’s a balancing act with our operational folks,” said Fountain (pictured right). “We’re able to make do, but there have been a few times that we were running thin in the aqueduct. Or if they have a sudden drop in demand, we’ve had a spill at one of our control structures. And we don’t want that happening.”

SDWA has other flow regulatory structures for untreated and treated water, but at nearly 5 million gallons — enough to fill seven Olympic-sized swimming pools — this is one of the largest. The new concrete tank was originally planned to hold 18 million gallons, but as per capita potable water use in the region decreased nearly 50% between fiscal years 1990 and 2020, planners re-evaluated the need for such a large tank.  

“As demands went down, we right-sized the tank to match today’s demands,” Fountain said. 

The project has been a massive effort, with giant concrete pillars, pipes and valves stretching above the rugged terrain of the regional park. Fountain said SDWA regularly consults the American Water Works Association’s (AWWA) standards for design and construction specifications, making sure the project adheres to the most current best practices.   

Construction sight for SDCWA underground storageThe new tank will be near the Mission Trails Regulatory Flow Structure I, an underground tank built in the 1990s that does the same job for treated water. SDWA has worked in the craggy hills and valleys of the 8,000-acre Mission Trails several times in the past, on pipe rehabilitation projects and now this major effort, and has spent a lot of time building goodwill and public trust as courteous neighbors. (Construction photo at left courtesy of SDCWA)

“We’ve worked extensively with the community around this big regional park to make sure that once it’s finished, it’s out of sight, out of mind,” Fountain said.

SDWA created an online interactive map of trail closures and has maintained close communication with the Mission Trails community task force during planning and construction. Once the project is complete in 2023, the area will be leveled to its previous contours and revegetated with native plants, many seeded from plants within the park itself. 

Ultimately, the tank will help balance flows in the water system, helping SDWA better serve its customers. 

“Our current focus is maintaining the reliability of our system,” Fountain said. “We want to make sure that when people turn on their taps to run water in their sinks, they’ll have the water they need.”