In early 2017, Californians watched first with glee, and then horror, as drought-busting winter storms filled and then overfilled reservoirs, threatening 770-foot-tall Oroville Dam. It’s the nation’s tallest dam and impounds Lake Oroville, California’s second largest manmade lake, capable of storing more than 3.5 million acre-feet. As the dam reached and then exceeded capacity, the main and emergency spillways were damaged, leading to the evacuation of 188,000 downstream residents. It was a sobering reminder that infrastructure matters and that infrastructure can fail.
About 100 miles away, the Pardee Dam, along with Camanche Main Dam and Dikes 1 through 6, impound the Pardee and Camanche reservoirs. The two reservoirs have a total capacity of approximately 620,000 acre-feet and are the centerpiece of the East Bay Municipal Utility District’s (EBMUD) water supply system, which serves 1.4 million customers in the Bay Area’s Alameda and Contra Costa counties. (About 90 percent of EBMUD’s water supply comes from Pardee Reservoir.) These reservoirs also filled from the 2017 storms, reaching 103 percent of capacity in March, but the dams were not overtopped and releases were sustainable within the waterways.
Thanks to the installation of one of the nation’s most advanced automated GPS-based dam monitoring systems at these two facilities, along with other instrumentation improvements, EBMUD had the technology in place to monitor crest elevations at these dams and dikes remotely with improved temporal resolution. Having this type of data available is one more tool in an infrastructure owner’s tool belt for monitoring the condition and performance of critical facilities.
“The original survey monitoring system developed in the 1960s at Camanche Dam and Dikes involved sight-lining from reference monuments adjacent to the structures along with differential leveling,” said EBMUD Survey Supervisor Steven J. Martin, LS. “After the advent of real time kinematic (RTK) GPS surveying in the 1990s, staff had taken to running the sight lines with RTK; however, with the differential leveling, it was still a labor-intensive approach that took over a week to complete.”
The old monitoring scheme technically met dam safety requirements but only provided semiannual data on dam crest displacements, leaving significant room for improvement in spatial and temporal resolution. “EBMUD is committed to automating processes where it makes sense,” Martin said. “In this case, due to the remote location and critical nature of the infrastructure, it made a lot of sense.”
Designing for better data
Consulting with Cory Baldwin, president of sensemetrics, a San Diego firm specializing in networked sensor applications, EBMUD designed and proposed a sophisticated monitoring system based on a total of 31 Leica GMX901+ GPS sensors, four Leica GM10 GNSS reference stations, a radio network consisting of 900 Mhz mesh radios, 2.4 Ghz repeaters, and two radio towers, feeding into the Leica GNSS Spider and GeoMoS software solutions.
The use of Leica Geosystems receivers is important, according to Baldwin. “The GMX901+s are purpose-built for remote monitoring applications, with non-exposed, built-in antennas,” he said. “They were my first choice here because other vendors don’t really have a good option for monitoring in this environment.”
Three of the GNSS reference station receivers are solar powered and one is powered by a 120v AC feed. All are securely fastened inside enclosures installed near the Leica AR20 antennas, which are mounted on concrete pedestals. The network is largely autonomous, needing only occasional attention. Data flow is through 900 Mhz and 2.4 Ghz spread spectrum radios into an existing microwave telemetry link to EBMUD’s business intranet at its Oakland headquarters, where a server runs the Leica Spider and GeoMos software necessary to process the GPS data and results. The results are then presented through software customized by sensemetrics.
Five of the Leica GMX901+ sensors as well as four seismographs are installed on the Pardee Dam, connected via fiber optic cable directly to the microwave business intranet. These instruments continuously monitor dam movement and report remotely. Two of the Leica GM10 reference stations were installed near and on either side of Pardee Dam and are connected to the fiber optic line by 2.4 Ghz radio connections.
Downstream from Pardee Dam, the Camanche Reservoir site consists of one large earth-filled dam and six dikes. Camanche Reservoir is primarily used to control releases to downstream agencies and maintain flows for the salmon. Twenty-six Leica GMX901+ sensors are installed around the reservoir, and these also monitor and report continuously.
The State of California Division of Safety of Dams (DSOD) requires semiannual monitoring surveys. The new system provides more accurate information more rapidly, reduces staff time spent on monitoring, and is capable of being tied into statewide emergency and seismic monitoring systems as they emerge. After almost two years of service, DSOD monitoring requirements have been met with a more complete picture of overall performance.
“EBMUD’s infrastructure is spread out over a vast area and covers multiple counties,” said Baldwin. “In particular, the Pardee and Camanche sites are several hours away from main offices, and the semiannual surveys took over a week to complete. This new system provides more accurate data, more or less constantly, and of course reduces the time survey crews spend on this task. It’s a big improvement, and it is performing beyond expectations.”
Baldwin said one important design goal was interconnectivity with existing and future monitoring systems. “For obvious reasons, California, and especially the Bay Area, is really focused on being responsive to seismic events, and continuous monitoring and alerting is a big part of that,” he said. “This big network is exemplary when it comes to continuous, autonomous monitoring and alerting, and the software design makes it easy to add in more sensors if needed. Down the line, the information can be directly tied into statewide systems. The potential to automate and improve emergency responses to seismic events is a major advantage of monitoring networks.”
“While the goal is to eliminate the legacy manual-read monitoring systems, inevitably issues with a power supply or other minor components will crop up and require some attention,” Martin said. “Sensemetrics provided support and training through the first two years to help resolve most issues, and Leica Geosystems updated firmware based on feedback from the project. So while the labor of a manual-read system will go away, some more technical support and software skills are required in an automated system. The trade-off between having an automated monitoring system that gives you a survey every day (or every hour if you wish) versus a survey only twice a year is worth the investment.
“Once a communications backbone is built out to support an automated survey system, other automated sensors such as seismic recorders or vibrating wire piezometers (measures water pressure inside the dam) can easily be added to the system,” he said. “Installation of this network was a proactive system improvement. EBMUD now has more survey information immediately at its fingertips regarding structure performance than at any previous moment in the district’s history. By being proactive and automating monitoring systems, EBMUD has greatly improved dam safety monitoring.
“The GPS system at Pardee Dam now gives us a complete picture of the seasonal deformations due to thermal expansion and contraction of the concrete structure,” Martin said. “With the Camanche Reservoir portion of the GPS monitoring system, we are able to meet DSOD monitoring requirements without long trips out of town by the survey crew and to check for any possible deformations remotely in near real time.”
Angus W. Stocking, LS, has been writing about infrastructure since 2002. To learn more about monitoring solutions, visit www.leica-geosystems.com.
Pardee Dam is a fascinating structure, both historically and in engineering terms. When completed in 1929, it was the tallest concrete gravity arch dam in the world. It’s still impressive, one of America’s 300 tallest dams at 345 feet high, and quite long as well at 1,337 feet.
Constructed in less than two years, Pardee Dam serves as a monument to its designer and builder, Arthur Powell Davis. Davis, a nephew of Colorado River explorer John Wesley Powell, was famous in his time as a visionary engineer who traveled all over the world to consult on dams and irrigation projects. In fact, the idea for a dam on the Colorado River, which eventually came to be known as the Hoover Dam, is largely attributed to Arthur Powell Davis. He rose through the ranks of the then newly formed Bureau of Reclamation to become director from 1914 until 1923, when it is said that he resigned in the office of the Secretary of the Interior over a disagreement on how to run the Bureau.
So in 1923, when EBMUD was formed, the top dam building engineer in the nation, and possibly the world, came to EBMUD. Pardee Dam was his last major work. A month before he died in 1933, Davis was named as consulting engineer on the Hoover Dam project, one he had championed for many years. As a side note, Davis was also a co-founder of the National Geographic Society in 1888.
[divider]What makes a good monitoring GNSS receiver?[/divider]
The Leica Geosystems GMX901+ GPS sensor was a good solution for the EBMUD monitoring network for several reasons. A relatively low-cost L1 GPS-only solution purpose-built for monitoring, it features:
- low power consumption;
- integrated, non-exposed antenna;
- highly accurate and precise ability to monitor small movements;
- durable and robust housing designed to withstand extreme temperature variations, vibrations, and constant exposure; and
- seamless connection to dedicated monitoring network software, in this case Leica GeoMoS and Leica GNSS Spider.
[divider]Automated dam monitoring in action[/divider]
A recent example of the benefits of automating a survey monitoring scheme comes from another EBMUD automated dam monitoring project on San Pablo Dam. In 2008-2009, the dam underwent a seismic improvement to buttress the toe of the dam to bedrock using a cement deep soil mixing process. Increased monitoring requirements from the DSOD while working on an active dam were met via an automated motorized total station (AMTS) system, which has been running several times a day since that project was completed in 2009. EBMUD also has a program to visually inspect dams and reservoirs immediately after an earthquake to check for cracking or other visible damage as a quick ground truth.
After a 4.4 magnitude earthquake in January 2018 on the Hayward fault centered in Berkeley, less than 5 miles from the San Pablo dam site, EBMUD geotechnical engineers were able to log in to the AMTS automated monitoring system to review whether there had been any movement or slumping on the dam, and they confirmed that there was no significant movement. This ability to have information immediately and at their fingertips has proven to be a huge asset in managing dam safety at EBMUD.