Dam Monitoring Survey data from the Omaha District Surveys, Mapping and Geographic Information Systems Section Field Survey team is one element of a larger Dam Safety inspection and monitoring program.
“Our Dam Safety program and supporting deformation monitoring surveys help to ensure each of the District’s 27 dams is ready to capture floodwater from the next storm,” said John Bertino, Omaha District Engineering Division Chief.
The field survey team consists of five surveyors whose services include boundary, topographic, and hydrographic surveys using the latest high tech surveying equipment and tools.
In addition to conducting surveys at military installations, for civil works water resource projects, and supporting other U.S. Army Corps of Engineers Districts, Field Survey crews perform surveys at USACE dams as part of a regular inspection and monitoring program.
The Omaha District is charged with operating and maintaining 27 dams within the Missouri River Basin. Among them are six large hydropower dams on the main stem of the Missouri River including Fort Peck, Garrison, Oahe, Big Bend, Fort Randall and Gavins Point, as well as several smaller tributary dams located in Colorado, North Dakota, South Dakota and Nebraska.
There are 14 dams in Nebraska; 10 Salt Creek dams in the Lincoln area and four Papillion Creek dams in the Omaha area.
The dam safety program at each dam includes annual and five-year periodic inspection requirements. As part of the periodic inspection, Field Survey teams perform a deformation monitoring survey. Each dam has an assigned Dam Safety Engineer who is responsible for plotting the field survey results and evaluating the data against design assumptions and historical trends to ensure the continued safe operation of each project.
This process, including collecting survey data, is called a dam deformation study. Part of the dam monitoring process also requires that, whenever dam safety replacement instruments or additional instruments are installed at a dam, the team conducts a survey to provide information and document these modifications for comparison in future deformation studies.
Approximately five of the Omaha District’s 27 dams undergo periodic inspections each year. One of the dams that recently underwent a periodic inspection was Salt Creek Dam Site 8 at Wagon Train Lake near Hickman, Nebraska.
The dam safety engineer for Salt Creek Dam Site 8 provided a survey data task list, which requires the team to survey and record five different types of data points at the dam. The data point types include vertical or horizontal movement for 15 piezometers, four slope or crest movement markers, intake structure movement markers, outlet works conduit movement points, and a centerline profile of the dam.
Each dam has a minimum of two static (non-moving) project control points. The points, placed during construction, are designed to avoid shifting or displacement. A GPS base station is set up on each of the two fixed control points, then GPS data for instrumentation, such as a piezometer, is collected. This data is compared against previous surveys to determine if there are changes in the position. The project control points are ultra-precise and have a 1/1000-inch accuracy variance vertically and approximately a 0.02 tenths of a foot accuracy horizontally.
Any indication of movement from these data points can provide an early indication of a possible issue at a dam.
“Established points could be affected by a mower or other heavy equipment so redundant measuring of each data point ensures measurements are accurate,” said Survey Crew Liaison, Danielle Campbell. “If movement or shifting is occurring, we’ll see a change when we compare multiple data points with previous years’ surveys. Movement of a single point could indicate a data collection error or damage to the point itself. But, to eliminate the possibility of a collection error, redundant measurements are taken to virtually eliminate systematic errors and make our measurements very reliable and prove, or disprove, that there is shift or damage to each respective movement point.”
“On any given day, we have crews in the field,” said Campbell. “Our annual dam safety monitoring includes surveying and analyzing established and new dam safety instruments, as well as verifying existing data points (the control points). For instance, the inspection at Wagon Train Lake required shooting 150 historically documented data points. Some of our larger projects can require collecting more than 1,000 data points.”
Measuring movement for outlet works requires confined space training and using personal protective equipment to enter the conduit and measure for movement at each joint within the conduit. “These measurements determine if the elevation between the joints of each section of conduit is higher or lower than historical measurements. In other words, they determine a comparative elevation to evaluate against historical data. With our survey methods and equipment, we can measure movement in each section of conduit and plot the movement on an X, Y, Z axis,” said Campbell.
Once all the data is collected, it is compared to previously collected data as a way to look for errors and then the survey deformation data is provided to the dam safety engineer.
“We use the survey data to analyze vertical movement (settlement) and horizontal movement of the dam and outlet works. Additionally, it allows us to understand the vertical location (elevation) of the piezometers to more accurately define groundwater levels that are recorded quarterly and during any high water event,” said Carlie Mander, the dam safety engineer who leads dam safety efforts for the Salt Creek Dams.