By Steve Roades
The recently completed Upper Chiquita Reservoir in Southern California was built to provide the region with substantial new water reserves to meet customer demand during disruptions of water deliveries. These interruptions can be unanticipated—for example, the 1999 break in a primary supply pipeline—or planned, such as a shutdown of the filtration plant in Yorba Linda.
One of the hallmarks of the Upper Chiquita Reservoir’s design is its depth—almost 160ft. It is one of the deepest reservoirs in the United States.
The site’s steep slopes angle about 150ft to an 8ft-wide bench, then taper another 150ft down to what is actually a small floor. This diamond-shaped footprint is set into the western slope of California’s Chiquita Canyon near Rancho Santa Margarita in southeastern Orange County.
The terrain has created a natural seat for a reservoir. Furthermore, the geology of the location is stable and in excellent proximity to regional pipelines.
For the Santa Margarita Water District (SMWD), Chiquita Canyon proved to be an ideal location for the district’s newest reservoir, which is focused on emergency supply. It is the first large-scale emergency potable water reservoir approved and constructed in many decades in this part of the country.
The reservoir’s goal is to provide more than 240 million gallons (750 acre-feet) to serve approximately 170,000 families at 200gal/day for a week. While the district deemed this particular site to be the safest and most viable, true site security and conservation would come only through the use of a geosynthetic base and floating cover barrier system.
Site security and water conservation were critical to project approval. To maximize performance, a geomembrane base liner and floating cover were designed into the facility with an emphasis on longevity placed on the cover membrane. The Upper Chiquita Reservoir is a key infrastructure piece of California’s Orange County, the third most-populous county in the state. Despite the reservoir’s relatively small floor, with its steep slopes and 17.8-acre surface, it is not a small project in either capacity or need.
The funnel shape of the reservoir has been described as a huge martini glass. The bottom foot of the reservoir holds 150,000 gallons of water. The top foot—160 feet from the bottom—holds 5 million gallons.
The design, in-depth
A major pipeline break in 1999 may be what ultimately led to this project’s development. That break caused a disruption of potable water supply in the SMWD and highlighted the need for an alternative point of support.
More than 18 acres of liner were installed to provide the essential containment and cover security. For the slope and base liner, the project team used a 60-mil polypropylene geomembrane. Polypropylene geomembranes have shown good longevity as reservoir liners, particularly when covered. Protected in the Upper Chiquita design by the floating cover, the polypropylene liner’s service life is extended by the absence of UV degradation.
The floating cover is a 60-mil, three-ply CSPE geomembrane (two-ply rubber with a 10 × 10 scrim reinforcement) that capitalizes on the long-life performance and protection record of this geomembrane in exposed installations. The cover alone uses more than 900,000sf of geomembrane.
The lining materials were manufactured in a site-requested custom color (“pebble tan”), a color originally developed for an irrigation reservoir for the world-renowned Pebble Beach-area golf courses on the Monterey Peninsula in Northern California.
A key to the project’s economics: both the polypropylene base liner and CSPE floating cover were prefabricated into wide rolls at a controlled facility prior to site delivery.
In this instance, the geomembrane was manufactured in 750 lineal-ft rolls and in widths of approximately 5ft. So in the prefabrication facility, seven rolls were joined.
Once delivered to the site, these larger panels were unrolled into place and welded together. The base liner and cover were both prefabricated into approximately 35ft-wide rolls.
In addition to the geomembrane barrier layer, a tri-planar geocomposite for drainage and venting control was installed. Piezometers were installed for continuous monitoring of subsoil moisture content and integrity.
The Upper Chiquita Reservoir construction proceeded through several significant weather delays. The installer even pulled off the site for two months while the earthworks contractor repaired damage to access roads caused by the unseasonably heavy precipitation.
That erosion may have best underscored why the geosynthetic lining system was the right choice for the reservoir facility: to maximize protection of the soil integrity and strength within and around the site.
Some local residents expressed concern with the reservoir’s siting from an aesthetics standpoint. They feared the reservoir, when covered, would be an eyesore. But the unique shape of the canyon footprint largely resolved those concerns. At an elevation of 865ft at its highest point, the reservoir is mostly hidden from nearby roadway and subdivision views. (Pump station walls may be visible from some angles.)
The designers also took care to disguise the reservoir in the hill, and the custom pebble-tan colored geomembrane cover has also helped considerably in this regard.
More than 1.6 million cubic yards of earth was moved at the Upper Chiquita site. General site construction concluded in November 2010, and liner and cover work was completed during the first half of 2011. Full service for this reservoir is scheduled for fourth quarter 2011.
The $53 million project was facilitated through a partnership among the SMWD and four other south Orange County water agencies, each contributing a percentage of the cost per water amounts reserved.
Revegetation for the surrounding environment, including the planting of coastal sage scrub and native grasses, will commence before the end of 2011. It will be conducted in coordination with the California Department of Fish & Game and the U.S. Fish & Wildlife Service.