An open body of water is vulnerable to all types of contamination. Surface water runoff, bird and animal wastes, human activity, algal growth and airborne debris can be hazardous to all bodies of water, but uncovered reservoirs used to store finished drinking water are especially vulnerable to contamination.
Although many uncovered reservoirs have been taken out of service in recent years, a number are still in use across the U.S. To overcome the influx of organic pollutants that enter these reservoirs, municipalities often use additional chemicals for disinfection. These chemicals create carcinogenic disinfection byproducts, and their formation is exacerbated by direct sunlight.
The U.S. Environmental Protection Agency continues to enact new regulations to protect drinking water quality, and uncovered reservoirs have recently come under fire with the signing of the Long Term 2 Enhanced Surface Water Treatment Rule on Dec. 15, 2005. Under this new rule, systems that store treated water in open reservoirs must either cover the reservoir or treat the reservoir discharge to inactivate 4-log virus, 3-log Giardia lamblia and 2-log Cryptosporidium.
The second option of additional treatment may prove difficult for municipalities due to the recent adoption of the Stage 2 Disinfection Byproducts Rule. Under this rule, water systems are required to find monitoring sites where higher levels of disinfection byproducts are likely to occur and use these new locations for compliance monitoring.
With these regulatory challenges, constructing covers for open reservoirs is often the most feasible option. Covers can be constructed to suit many requirements, with various designs and materials available.
One district’s solution
The Carpinteria Reservoir in Carpinteria, Calif., is a concrete-lined basin with a storage capacity of 14 million gal. The reservoir was constructed in the mid-1950s as part of a gravity-fed irrigation water supply system, but today, the reservoir supplies drinking water to approximately 19,000 people in the Carpinteria Valley Water District (CVWD). Encompassing about 2.5 acres, the reservoir stores water treated at the city of Santa Barbara’s Cater Treatment Plant.
Although it met health standards when it was built, the Carpinteria Reservoir is vulnerable to contamination from airborne pollutants and bacteria from animals and humans. According to Charles Hamilton, general manager of the CVWD, even with fences to deter intruders, people and animals have been found swimming in the reservoir.
These ongoing issues became a primary concern when the California Department of Health Services issued a policy in 1995 that addressed the vulnerabilities of open reservoirs used to store potable water. According to Hamilton, the policy stated that uncovered reservoirs holding treated water must be taken out of service, covered, or the water must be treated again to reduce pollutants.
The CVWD was already using additional chlorine in the water for disinfection, and the district knew it would be increasingly difficult to meet disinfection byproduct regulations. After much discussion, the CVWD decided to cover the Carpinteria Reservoir at a cost of about $6.1 million.
Design challenges
The district faced many design challenges with this project.
“These reservoirs were designed and built in the early 1950s with no thought that they would need to be covered,” Hamilton said. “The structure that holds the roof was not designed for that, so we had to reconfigure that structure at a large expense.” Additional design issues included:
- Long span (the basin is 336 ft in diameter);
- Low height requirement to limit visual impacts;
- Maintenance of an existing underdrain system under the reservoir;
- Very high seismic loads;
- Design of a retrofit waterstop connection that would provide a redundant watertight seal; and
- System design that met the requirements of federal, state and local stakeholders.
Cover selection
The district then had to find a cover that met their design requirements. They examined various options and chose an aluminum truss-supported roof because it was the lightest roof structural system available.
“Since construction of the roof involved retrofitting the 50-year-old concrete structure of Carpinteria Reservoir, there was a need to use a roof system with the lowest structural loads possible,” said Bob McDonald, district engineer with the CVWD. “Structural steel and concrete roof systems were considered in the early planning process but were rejected due to the heavy structural loads from those systems.”
Conservatek Industries was chosen to provide the cover for the project. The company designs, manufactures and installs aluminum clear span covers used in the water, wastewater, petroleum and bulk storage markets.
“The truss-supported roof does not require columns for support,” said Nita Bailey, vice president of North American sales for Conservatek. “The aluminum provides a low-maintenance system with a long life cycle.”
According to Bailey, the final system design for the roof consists of 12 trusses that are 17-ft, 6-in. deep at the center. Eight-in.-deep aluminum roof beams are attached to the top chord of the trusses, forming a triangular grid pattern. The triangular openings are filled using 0.05-in.-thick aluminum panels, which are clamped to the roof beams, providing a watertight system.
Construction
Before the roof could be installed, initial retrofit work had to be done on the reservoir structure. This work occurred primarily during the rainy season from November 2002 to June 2003, so the tank could be taken out of service.
The retrofit included pouring new foundation walls around the tank to support the trusses; installing stainless steel embed plates to provide additional support and anchor points for the roof system; and creating a support structure along a concrete wall that already existed in the middle of the reservoir.
Roof construction took place during the dry season from June to December 2003. One of the most challenging aspects of construction was to build the roof structure in a very restricted area. The area around the reservoir was limited, and since the reservoir had to remain in service to meet the district’s water needs, the bottom of the tank could not be used as a construction platform. This eliminated traditional methods of construction, such as use of scaffolding and temporary supports.
The solution was a unique process of installing the roof supports with hand-operated pulleys and tracks. The individual trusses were built at the end of the tank over land, attached to a structure and rolled down the walls into position. Purlins were added to the roof before each section was rolled into place, and when the trusses were in their final location, aluminum panels were installed to complete the reservoir cover.
Satisfactory results
Since the completion of the project, the CVWD has been satisfied with the results.
“With the uncovered reservoir, we had a lot of maintenance,” Hamilton said. “We would have to drain it every year to remove all the sediment that would build up from the airborne pollution, plus what would be created by the chlorination. There’s very little maintenance associated with this cover because it’s a surface that doesn’t need to be treated.”
According to Conservatek representatives, the ability to assemble the components with relatively light equipment due to a restricted construction site, coupled with aluminum’s high strength to weight ratio, its resistance to corrosion and its lack of maintenance, simplified the design process and proved that aluminum was the ideal choice for this roof.
