Motion on the dam can be detected relative to the five local reference stations. The utility adds data from the broader array of statewide receivers into both the real-time and post-processing-engine computations for the regional monitoring work. It's an impressive array of information and tools for analysis, and Schrock's team uses it to determine if motion is due to the receivers on the dam rather than movement of the local reference stations.

The links to the statewide network add a high level of performance to the monitoring system at no additional cost: “The regional network infrastructure was already there, so new monitoring sites could be added for just the cost of receivers, mounting, Integrity Management software modules, power, and communications,” explains Schrock.

On July 29, 2008, an earthquake showed up in Schrock's rapid-motion-engine data a few minutes after the event occurred. By Los Angeles standards, it was moderate: magnitude 5.4.

The quake's epicenter was more than 950 miles south of Tolt Dam. A quick check against local seismographs confirmed that the change in dam data coincided with the quake. The next day, a magnitude 3.0 quake occurred about 50 miles northwest of the dam and was again detected by the receivers.

These two events signify an important — if unexpected — bonus. The Pacific Northwest often experiences slow, deep earthquakes, and the system proved itself capable of understanding the surface effects of such events. GNSS provides actual measurements of displacement that aren't obtainable from seismic or other sensors, providing a more accurate picture of what takes place when the shock of an earthquake passes through the dam.

In January, a series of storms presented an opportunity to analyze positional data from the monitoring system under extraordinary conditions. Record levels of snowmelt and rainfall prompted the utility to discharge high volumes of water from the dam. Within minutes of a call to the survey team, there was good news: less than 3 cm (1 inch) of movement, with no permanent displacement, was registered.

The Tolt Dam project includes calibrating the receivers and determining thresholds for alerts. Installation and commissioning took place over seven months, and the monitoring system was fully operational by June 2008. The project was funded by Seattle Public Utilities' dam safety budget and an academic partner. The utility is developing protocols for sharing GNSS information with data from other sensors already connected to the dam's emergency warning system.

The utility also plans to install GNSS monitors on a series of concrete dams in northeast Washington that will tie to the statewide network, and is considering other sites and structures as well.

“GNSS monitoring is an important advance for public works,” says Schrock. “It's a good fit for bridges, skyscrapers, landslides, critical slopes, seawalls, and even buoys.”

— Stenmark, a licensed surveyor (LS), is a freelance writer and consultant working in the AEC and technical industries. He has more than 20 years experience in applying advanced technology to surveying and related disciplines.