A system installed at Somerset Dam in Vermont continually monitors the dam for seepage and sudden changes in water levels. The system provides instantaneous detection, verification and reliable satellite communication with TransCanada’s control center.
By Jud Donaghy, Jerry Cross and Yogi Sookhu
In August 2011, just after Hurricane Irene was downgraded to a tropical storm, the storm hit the U.S. eastern seaboard and swept through the Deerfield River watershed in Vermont, where it produced nearly a foot of rain in 12 to 16 hours. TransCanada’s 77-MW Deerfield River Project was remotely operated from the company’s Deerfield River control center, which at that time was located about 30 miles away in Monroe Bridge, Mass.
The Deerfield project consists of eight dams from Somerset Reservoir in Windham County, Vt., to just below Gardners Falls in Franklin County, Mass. One storage and two hydropower facilities are located in Vermont: Somerset Dam, 4-MW Searsburg and 33-MW Harriman. Five hydropower facilities are located in Massachusetts: 7-MW Sherman, 17-MW Deerfield No. 5, 5-MW Deerfield No.4,5 -MW Deerfield No. 3 and 5-MW Deerfield No. 2.
The storm intermittently knocked out each development’s communications (via telephone lines and cell phone) and power, and all of the supervisory control and data acquisition (SCADA) systems were incapacitated. Other than voice communication updates, which were being provided regularly by radio, control room operators were blind to the conditions at the hydro sites because telephone lines and cell tower service were intermittent. The hydro sites are in remote locations, spread out and far from major population centers.
Prior to the height of the storm, personnel were able to gain access to each site in order to monitor them – with one exception, Somerset Dam.
The 9-mile-long road leading to Somerset Dam was already washed out when operators attempted to travel to the dam. Fortunately, all of the installed remote monitoring equipment continued to send valuable information to TransCanada’s control center and to the dam safety engineer. Throughout the storm, water level data was available, verifying that Somerset Dam was safe.
The nearest population center to Somerset Dam is Wilmington, Vt., a small town about 15 miles southeast of the dam. Greenfield, Mass., is the largest city near the dam and it is about 45 miles away.
Runoff from the heavy rainfall produced record floods approaching one-half of the probable maximum flood (PMF) event across the basin. This PMF equates to 22,600 cfs at the Somerset Dam and about 199,000 cfs at Deerfield No. 2 farther downstream. Destruction from flooding was extensive, washing out roads and bridges, damaging homes and businesses, and eroding dam and canal embankments.
Somerset Dam is on the East Branch of the Deerfield River (see Figure 1) and is the furthest upstream dam on the river. The project was built in 1912 as a storage facility and consists of a reservoir, dam, outlet works and spillway.
Somerset Reservoir is about 5 miles long and 1 mile across at its widest point, with a surface area of 1,500 acres and gross storage of 57,000 acre-feet. The semi-hydraulic earthfill dam has a crest elevation of 2,147 feet and is 2,100 feet long, with a maximum height of about 110 feet. The normal operating range is from elevation 2,113 feet to 2,128 feet.
The emergency spillway is a side channel spillway that has 3-foot-high flashboards located on the northwestern (right) end of the dam. It has a permanent crest elevation of 2,133.6 feet and is about 192 feet long and 7 feet high. The spillway discharges into a manmade channel that is about 700 feet long and empties back into the river downstream of the dam.
|Pole-mounted, thermal imaging cameras and photovoltaic cells line the access road to Somerset Dam at the toe of the dam’s embankment.|
Downstream hazard potential
Dam breach analyses determined the inflow design flood (IDF) for Somerset Dam is the PMF, about 22,600 cfs. Inundation maps for a dam breach under both sunny day and PMF conditions are included in the emergency action plan (EAP). A significant number of residential structures will potentially be impacted by a dam failure under both conditions.
Due to these potential downstream impacts in the event of a dam breach, the Somerset Dam is classified as a High Hazard dam by the Federal Energy Regulatory Commission (FERC).
Dam failure during the PMF will result in an outflow of about 600,000 cfs and a rise in river stage of 36 feet immediately downstream of the dam. Failure of Somerset Dam during the PMF causes the overtopping and potential failure of all downstream dams, from Searsburg to Deerfield No. 2.
A piping failure of Somerset Dam, under normal operating conditions, will also cause the overtopping failure of Searsburg Dam, 8 miles downstream, and impact residential areas in the town of Searsburg in about 1 hour. But due to Somerset Dam’s remote location, without proper monitoring equipment a dam failure could go without detection until after the residential areas are impacted.
FERC requires detection, verification, notification and evacuation to be completed prior to impacts from a breach flood wave. Without some form of remote monitoring and rapid verification of a breach, this requirement could not be met for Somerset Dam.
In 2010, Gotham Analytics LLC provided equipment that would function in a remote location without the need for an external power source or telecommunication lines, and that could provide nearly instantaneous detection and verification of a dam failure.
Gotham Analytics installed its AquaEdgeTM product, which consists of a solar-powered control system that receives sensor information, processes the information and transmits the processed data via satellite communications to a dashboard that can be viewed in the control room that monitors the facility or on a smart phone.
|The Deerfield watershed in Vermont consists of eight dams from Somerset Reservoir in Windham County, Vt., to just below Gardners Falls in Franklin County, Mass.|
As part of this system, two pairs of off-the-shelf water level pressure sensors were installed at different locations at or near Somerset Dam (see Figure 2 on page 28). Having a pair of sensors provides the ability to determine if one sensor is having problems.
A dam failure would be detected immediately by a sudden rise in tailwater. The drop in headwater would also be detected, but to much less magnitude due to the size of the reservoir. Installation of the sensors was completed in spring 2011 and the system was in operation in August when the hurricane hit.
If the water level sensors at Somerset Dam detected an event exceeding threshold levels, an alarm would have sounded in the Monroe Bridge control center.
Throughout the storm event, despite the intermittent availability of telephone and cell tower service, all of the equipment continued to send valuable information to the hydro control center. The water level data was displayed on the dashboard monitored by the dam safety engineer, verifying that Somerset Dam was safe.
|This graphic depicts an aerial view of Somerset Dam with arrows delineating the location of AquaEdge water sensors and thermal imaging cameras.|
Modifications to the system
After Tropical Storm Irene, TransCanada opted to install seven thermal imaging infrared cameras. Six cameras were installed at the downstream toe of the earthen embankment and one was installed at its crest to provide a method for detecting or verifying a dam failure caused by piping. Piping would be detected by a change in temperature on the downstream face of the dam caused by water emanating from the embankment. Thermal imaging cameras can detect changes in temperature, such as cold water emanating from a warm embankment, even during a moonless night. Thus, the thermal cameras will pick up an increase in seepage long before a piping failure develops.
The cameras image the facility in four-hour increments and that data is displayed on the facility dashboard and is sent via satellite to Gotham Analytic servers where it is archived. In the event of an alarm, control operators can remotely task the cameras to immediately image the facility and view the data that is transmitted to the server. In addition to thermal imagery, Somerset Dam also has a standard optical camera that functions the same as the thermal cameras with regard to sending data and the ability for operators to task real time images.
The system also has the capability to initiate a reverse emergency 911 call or to send an audio message to a series of telephone numbers.
In the event of an incident (i.e., an alarm sounding in the hydro control room) operators switch on the cameras, record data and review the data to verify the failure. Once verified, control room personnel can immediately begin making notifications included in the EAP.
Tropical Storm Irene caused billions of dollars in damages across the region. For various periods during the storm event, all of the SCADA and automated communications systems for the Deerfield River facilities failed because of impacts to telephone and cell tower service resulting from the storm, with the exception of the remote monitoring equipment installed at Somerset Dam. After several years of operation, the system continues to allow success in monitoring the safety of Somerset Dam.
Future enhancements to this system may include installing the following: Pressure sensors in weir boxes to track seepage; temperature probes in the reservoir to correlate thermal images; and an optical camera at the tailwater gage location to verify high tailwater alarms.
Jud Donaghy is chief dam safety engineer for TransCanada. Jerry Cross is chief civil engineer and Yogi Sookhu is chief executive officer of Gotham Analytics LLC.