Los Vaqueros dam is one of the few major dams and water supply projects to be built in the US in the past 20 years. It is an important part of a massive water project that brings a reliable supply of water to 430,000 residents and 28 industrial customers in the Contra Costa Water District (CCWD), about 64km east of San Francisco.

The 58m high, zoned earthfill dam was designed by URS Greiner Woodward Clyde and is part of the US$450M Los Vaqueros project. The scheme surmounted an array of technical and environmental hurdles on its way to completion in 1999 and is the only major water project to be built in northern California in more than a decade.

For nearly a century, saltwater intrusion from San Francisco Bay into the Sacramento-San Joaquin Delta had been a problem for Contra Costa County residents, who draw their drinking water from the delta. The Los Vaqueros project allows water to be pumped from a new intake located further upstream from the bay, during months when the delta salinity is lowest. The water is stored in a new, 100,000 acre-foot, off-stream reservoir and blended with delta water when salinity rises.

The reservoir provides a three to six-month emergency supply of water, a significant increase over the previous three-day supply. This difference is important because Contra Costa County is subject to drought and the delta is prone to levee failures, earthquakes, chemical spills and other disasters that can disrupt the water supply. The reservoir supply also allows the intake pumps to be shut down, for example during the spawning season of endangered chinook salmon. Special provisions were made during construction for the relocation of all threatened local plant and animal life.

When planning began on the Los Vaqueros project in the mid-1980s, CCWD was aware that other water projects in northern California had failed to gain permits due to adverse environmental impacts. The water district decided early on to adopt a new approach to the complex permitting procedure. Rather than fight the environmental regulations, CCWD decided it would learn from failed water projects, engage the best scientific minds available, and work closely and collaboratively with state and federal regulators.

The water district identified 126 ways to achieve its dual objective of improving water quality and reliability. The goal was to select the most practical and least environmentally intrusive alternative. Following a rigorous feasibility analysis, the alternatives were narrowed to five. Some alternatives failed to achieve both quality and reliability; others, such as desalination, were not economically feasible; and some were rejected because they would have caused detrimental impacts on the environment.

To ensure that the permitting procedure progressed as smoothly as possible, the CCWD entered into agreements with federal, state and local agencies to pay staff for their time to process permits. The water district’s staff also implemented a communication plan with all levels of regulatory agencies.

Labour problems were averted by the development of a project labour agreement with the local trade unions. The agreement, the first of its kind developed by a public water agency in California, called for the exclusive use of local trade union labour. In the end, a total of 100 permits were secured within 10 years, including the 404 wetlands filling permit which is difficult to obtain. Extensive feasibility studies were carried out before the site of the dam was selected and its size determined. Alternative types of outlet works and spillways were also considered. Flood routing was performed to optimise the size of the spillway in relation to the height of the dam embankment. In addition, a comprehensive seismic study of the dam site was prepared, including evaluations of local faults and development of seismic parameters for use in the analysis of the dam.

Final design investigations included foundation studies for the dam, spillway and outlet works. This involved rock core and soil borings, seismic refraction and downhole surveys, in situ tests, water pressure tests and drilling 91cm diameter holes to allow visual exploration of the dam foundation. A test-fill programme was conducted on excavating and com-pacting local bedrock materials proposed for the core and shell of the dam.

As the dam is located in a high seismic environment, a comprehensive static and seismic stability analysis was a critical element in the final design work. A deterministic-statistical approach was used to select the seismic design parameters in accordance with criteria established by the California Division of Safety of Dams (DSOD). Geological investigations were carried out to date the activity of local faults and information on existing earthquake epicentres was refined, based on revising the crustal model of the region. The maximum credible earthquake (MCE) was used as the basis for the dynamic analysis of the dam. Studies covered both normal and earthquake loading conditions, including state-of-the-art finite element method analysis of the dam under earthquake loading conditions. The final design also included instrumentation to monitor the behaviour of the dam during reservoir filling and for long term monitoring.

The design of the Los Vaqueros dam also presented a significant number of engineering and construction challenges. Construction activities were initially designed and sequenced around a major two-lane highway which passed through the construction area until construction of 13 miles of new highway was completed. In addition, geological studies during design showed that a large landslide existed on the left abutment of the dam. This landslide was removed during construction because it had the potential to affect dam safety. This excavation required the removal of 336,424m3 of rock.

Design considerations

The design of Los Vaqueros dam was determined by the foundation and seismic conditions at the site and the types of construction materials available locally. Rockfill, roller compacted concrete and earthfill dams were considered for the site. The roller compacted concrete could not be constructed because the weak sedi-mentary claystone which comprised the majority of the foundation conditions, was not suitable for a dam of that size. Rockfill was also rejected because the local sandstone rock was too weak for a rockfill structure, and other sources were not environmentally acceptable.

The earthfill dam, which has a volume of approximately 2.14M m3, includes a thick central clay core supported by sandstone and claystone shells. Any water which might seep through the core is removed by filter and drain zones imported from off-site. The materials for the core and shell were excavated from local borrow sites within the project boundaries. To provide a stable foundation, some 1.8M m3 of earth were excavated 12m to the bedrock. The foundation consists of a dipping sedimentary rock foundation composed of sandstone, siltstone and claystone. A double-row grout curtain was placed beneath the core of the dam to reduce potential seepage through existing formation fractures and to increase stability under all conditions.

Once construction had started, additional tech-niques were necessary to secure the foundation. During excavation a series of open, continuous, near-vertical joints were discovered in the left abutment sandstone that were almost perpendicular to the axis of the dam. Further excavation into the abutment under the core of the dam was performed to remove the jointed sandstone. Backfill concrete was used to rebuild the foundation so that it was suitable for core placement, and extensive grouting was performed. The work, along with all construction activities, was co-ordinated with the DSOD and an independent panel of experts in geology and dam construction.

The dam has a 4.5m wide concrete-lined chute spillway on the left abutment, which is designed to pass the probable maximum flood test while maintaining a satisfactory freeboard to prevent overtopping of the dam. The intake/outlet works consist of a sloping five-level intake structure located on the right abutment.

The intake structure discharges water into a 2m diameter, 390m long tunnel to the outlet control building at the downstream toe of the dam. Water is pumped through this system into the reservoir. When water is needed, it is released through the system into the water district’s distribution system.

Remote control

The Los Vaqueros project is remotely controlled and monitored from the CCWD’s Antioch operations centre, located about 19km northeast of the reservoir. The water district’s existing system control and data acquisition (SCADA) network was extended to the dam via a fibreoptic data transmission link. Much of the instrumentation is monitored by an automatic data acquisition system (ADAS). The key aspect of ADAS is that it allows real-time monitoring of the ADAS instruments and analysis of the data. If ADAS detects a reading above a pre-determined threshold limit, it automatically sends a message to the CCWD operations centre. This eliminates the delays that frequently occur when personnel must read instruments manually and then interpret the data.

Construction of the Los Vaqueros project began in 1995. The first surge of water into the reservoir occurred in February 1998. The benefits of the project were immediately apparent. In 1999, water quality in the delta at the CCWD locations was seriously affected by the closure of the Delta Cross Channel to protect chinook salmon. However, CCWD customers continued to receive low-sodium water directly from Los Vaqueros throughout the incident. In meeting the myriad of federal, state and local permitting requirements, the environment was not only protected but improved.

Environmental concerns

In addition to the advantages derived from storing an abundant supply of water in the off-stream reservoir, the project also incorporated: state-of-the-art fish screens; tunnels under relocated roads so that tiger salamanders can cross safely; new wetlands; special fencing to protect San Joaquin kit foxes; and wooden ‘frog hotels’ to shelter endangered red-legged frogs until willow trees grow around new marshes.

Above all, Los Vaqueros provides a consistent and reliable water supply to residents throughout the Contra Costa Water District.