Upper Chiquita Reservoir update - Geosynthetics Magazine

Features | October 20, 2024 | By: Brian Fraser

This article is a review of the Upper Chiquita Reservoir project originally constructed in 2011 and today remains one of the largest and most challenging potable-water liner and geomembrane floating cover projects installed in the United States. The reservoir liner and floating cover have been in operation approaching 15 years and a 2024 project inspection confirmed the geosynthetic cover system performing very well as designed.

The Upper Chiquita Reservoir is a large reservoir that is part of the Santa Margarita Water District (SMWD) in Southern California. The Santa Margarita Water District is the second largest water district in Orange County, covering 52,000 acres of land just east of Mission Viejo and San Juan Capistrano.

Today, the SMWD provides water to approximately 200,000 residents and businesses in Mission Viejo, Rancho Santa Margarita, Coto de Caza, Las Flores, Ladera Ranch and Talega. All domestic water supplied to the SMWD has to be purchased from outside sources as there are no local water sources. Most of the region’s water is piped from the Robert B. Diemer Treatment Plant in Yorba Linda. The district had experienced major supply disruptions with a previous unplanned break in the district supply pipeline and the additional planned outages of the Diemer Plant in previous years. Any disruption in water supply required immediate and severe water conservation measures in the district.

The main purpose of the Upper Chiquita Reservoir was to increase reserve storage capacity to ensure water security to the customers the SMWD served. In 2010, the total installed cost for the investigation, design and construction of the reservoir was $53 million and was a shared project between a number of local agencies. These agencies included the SMWD, the City of San Clemente, San Juan Capistrano, Moulton Niguel Water District (Laguna Niguel) and South Coast Water District (Dana Point). The SMWD operates the reservoir.

The Upper Chiquita Reservoir is one of the first large emergency potable-water reservoirs to be built in Orange County in decades. It has a design capacity of 244 million gallons (923.6 million liters) and has a surface area of approximately 18 acres (7.3 hectares). The reservoir was of new construction, being built into the side slopes of the Chiquita Canyon.

The reservoir was formed by constructing an earthen dam across the mouth of a section of the canyon to create a roughly triangular containment. At its deepest point, it is 120 feet (36.6 m) with side slopes ranging from 3 to 1 up to 2 to 1. The construction of the reservoir began June 2009 and was put into service October 2011.

The reservoir has a number of interesting challenges and details. The first is the depth and steepness of the slopes. This led to safety concerns during construction and to problems in removing rainwater during an exceptionally rainy season. The second issue was the highly irregular shape of the containment. Designing a defined-sump floating cover on an unusual shape requires some different techniques. In this case, an earth bench was designed and built into the side slopes at the halfway point. This earthen bench provided a flat surface on which to construct the floats and weights that tensioned the cover. The bench also acted as an intermediate anchor for the lengthy slopes and contained a drain system to monitor leakage for dam safety purposes.

A geosynthetic underdrain was the first layer of material underneath the entire reservoir. This underdrain was placed to monitor the performance of the lining system and to provide drainage during construction. The underdrain also provided drainage for the earthen dam section. The underdrain consisted of a tri-planar double-sided geocomposite. The core of the geocomposite was 300 mil (7.6 mm) thick and sandwiched between two layers of 8 oz/yd2 (270 g/m2) nonwoven polypropylene geotextile.

The original specification for the primary geosynthetic barrier system (geomembrane) called for the use of a three-ply 60 mil (1.5 mm) chlorosulfonated polyethylene (CSPE) geomembrane. During the time of construction, there were challenges in the supply of CSPE materials, and the lining was changed to a flexible polypropylene material to get both the lining and the cover materials delivered on time. While fRPP as a geomembrane material had encountered performance problems in potable-water applications as a floating cover, as a liner it would not be exposed to UV light and was determined to be acceptable. The final liner material selected was a three-ply fabric-reinforced flexible polypropylene geomembrane (fRPP) 60 mil (1.5 mm) thick. The fRPP materials for the liner were prefabricated prior to installation according to a detailed panel layout. All seams were welded using a 3-inch-wide (7.6 cm) wedge welder that bonded the materials edge to edge without leaving a flap of material at the seams.

To reduce creases in the lining material, the fabricator on this project used a 36-foot-wide (11 m) winder to create rolls of fabricated materials that did not require any folds in the material. There is some thought that creases in fRPP materials can lead to stress concentrations that can initiate failures. This is especially problematic in thicker materials such as a 60 mil (1.5 mm) material. By fabricating without folds, this particular fabricator was eliminating this area of potential problems.

The liner was anchored to the perimeter of the containment using a concrete stub wall and steel battens. This concrete wall went around the entire perimeter. Once the lining system was in place, the entire area was surveyed using an electrical leak location method. Weather problems prevented the reservoir from being installed in a straight-forward fashion and earthworks, lining installation and cover installation all took place at the same time in different parts of the reservoir. To test areas that couldn’t be filled with water, the leak location survey was performed via the water puddle method (ASTM D7002).

The Upper Chiquita Reservoir remains one of the larger covered reservoirs in California and also one of the most irregular. The irregular shape is a challenge when designing the method in which the cover will rise and fall in response to water level changes. The design of this cover used the “defined sump” method of construction as first described in US Patent 3,991,900 (1976). A defined-sump floating cover uses a series of weighted sumps and floats to maintain tension on the cover material as it changes elevation with water movement. The design of the cover layout needs to maintain continuous tension on the cover to prevent wind damage.

The material used for the cover was a chlorosulfonated polyethylene (CSPE) three-ply fabric-reinforced 60 mil (1.5 mm). CSPE is a synthetic rubber material noted for its chemical resistance including chlorine, temperature extremes and ultraviolet light. CSPE as a geomembrane was first developed more than 50 years ago in the United States as pond liner material. It also has been used extensively in California and various regions of the world as a geomembrane liner and floating cover material for municipal potable-water storage and industrial process water containment. The CSPE for this project was issued with a 30-year weathering warranty. The floating cover system was factory prefabricated to match a detailed panel layout prior to deployment in the field.

The normal sequence of operations for cover construction is to complete the liner installation, then start placing the cover. The cover usually is installed over the entire area of the containment before floats, sump weights and other details are added. In this project, excessive rain forced changes to this sequence. Since the base of the containment could not be kept free of water, the liner and then the cover were advanced down the side slopes with the base remaining open. Only when the base was finally dried out could the liner, then the cover, be completed. Layout and location of floats and sump weights are usually done on-site once the cover panels are installed and tested. A special earth bench was included in the earthworks of this project to provide a flat area about halfway down the slopes where the main floats and sump weights for the cover could be constructed. Much of the detail work on the cover took place on this bench while the base of the containment was still too wet for construction.

Another adaptation to the weather was to transfer the construction of appurtenances back to the factory. This moved a significant amount of the field labor inside when the weather was poor. The final cover design covered an area of 900,000 ft2 (83,600 m2) and included 18 hatches, 71 air vents and 40 rainwater removal pumps. Once the cover was completed, it was inflated with air using fans on the access hatches. Because of the size of this floating cover, it was inspected in sections. Sandbags isolated sections so that each area was inflated in turn. This reduced the risk of damage in the event of unexpected winds. The floating cover was 100% inspected from the inside for any defects (defects show as pinholes light). Once the cover inspection was completed, the air was released and the cover was ready for service.

The Upper Chiquita Reservoir project actually had been started nearly 20 years earlier as part of the long-range plans for the water district. There were a number of sites deemed suitable for a large reservoir and environmental studies began. These environmental and planning studies started in the 1980s and took many years to work their way through the system until the site was selected and approved for a reservoir.

The reservoir design has a number of interesting features, including the large earth-fill dam that encloses the canyon to create the containment. This dam is designed for stability in California’s strong earthquake environment. The other noticeable detail is the earth bench approximately halfway down the slopes. This bench and road access to the bench was a design feature to improve access for maintenance. The bench lets operators reach much of the reservoir surface without having to drain the reservoir completely. Part of the design was a concrete trough more than 400 feet (122 m) long from the top of the slope to the toe of the slope. This trough held the pneumatic lines used to operate the valves in the bottom of the reservoir. A final feature of the project was the construction of the berms so that the reservoir is not normally visible from the nearby busy highway.

The construction of the reservoir started in the summer of 2009 and progressed well until the winter of that year. The winter of 2009/2010 was very rainy and there were many project delays. The large volumes of water kept the base of the reservoir very wet so that the earthworks could not be completed and the lining could not cover the base of the reservoir. The winter of 2010/2011 was even wetter, with one particular week seeing more than 10 inches of rain. “The rain gauges overflowed three days in a row,” said Bart Lantz, project manager for SMWD. This rain event washed out a portion of the subgrade underneath the liner, and a large section of liner was removed and the subgrade repaired. More than 200,000 ft2 (18,600 m2) of liner was pulled back to access this repair area. Based on the 120’ reservoir depth, steep slopes and large volumes of rainwater, dewatering of the reservoir floor was a major challenge with up to 10 large capacity pumps required. Safety was also a major concern for the field crews based on the steep slopes involved and slippery, wet site conditions. It took longer than a month for the earthworks contractor to dry out the site after one of the larger rain events. Finally, after many weather delays and significant effort in repairs, the liner and cover were complete. The reservoir went into service October 2011.

The Upper Chiquita Reservoir was the culmination of a long-range plan to ensure water security to Santa Margarita Water District and area users. The reservoir is an excellent example of a large, geosynthetic covered potable-water reservoir typical of the California area. Fifteen years after completion, a recently performed site inspection confirmed the reservoir and its geomembrane liner and floating cover continues to function without issue, protecting the drinking water for residences in Southern California. Today, the reservoir remains one of the largest and most challenging municipal potable-water floating cover projects in the world.

Geomembrane floating cover systems are used to protect water from outside contamination and for evaporation control. They are the most economical method of constructing and storing large volumes of potable water.

Mills, A., and Falk, S. (2013). “Upper Chiquita Reservoir floating cover and liners.” Geosynthetics Conference, Long Beach, California, USA.

ASTM D7002-22 American Society for Testing and Materials. “Standard Practice for Electrical Leak Location on Exposed Geomembranes Using the Water Puddle Method”

Volzke, J. (2011). “Securing the water supply.” The San Clemente Times, July 7, 2011.

Fraser, B., Roades, S., Neal, M., and Gersch, A. (2019). “CSPE performance and history in long term potable water storage applications.” Geosynthetics Conference, Houston, Texas.

Brian Fraser is the vice president of Layfield Geosynthetics and has been active in the geomembrane industry for over 40 years including manufacturing, construction and product development working in the United States, Canada and internationally. He has authored and co-authored numerous technical papers and case studies on new innovation and best practices of geomembranes and geosynthetic floating covers. He is currently on the board of advisors of the Fabricated Geomembrane Institute and recently completed a three-year term on the board of the Geosynthetic Institute. Brian has an MBA from Royal Roads University in Victoria, British Columbia and is based in Yuma, Ariz.

All photos courtesy of Layfield Geosynthetics USA.

Upper Chiquita Reservoir update

Location: Las Flores, Calif.

Owner: Santa Margarita Water District

Contractor: Layfield Geosynthetics USA

Construction company: Sukut Construction

Engineers: AECOM

Geo product: CSPE 60 mil, fRPP 60 mil, Geocomposite

Geo manufacturer: Burke Industries, Carlisle Syn-tec, Layfield

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