RIVERDALE It doesn't look like much, but it sure is important. What has the appearance of a small creek on the backside of Garrison Dam is actually a relief channel that was part of the original design of the massive earthen structure.
The relief channel sits at the "toe" of dam, the lowest point of the nearly two and one-half mile dam opposite the side that backs up Lake Sakakawea. There is usually a small amount of water in the channel. The source of the water is sometimes seepage that is common to any earthen dam.
"Water seepage can flow through pervious layers of soils such as sands, gravel, silts, or coal seams under the dam due to the high pressure of water related to the depth of the lake on the other side of the dam," explained Brandon Sailer, U.S. Army Corps of Engineers. "Relieving water pressure helps stabilize the dam."
This small channel on the backside of Garrison Dam is used for the important purpose of alleviating seepage pressure that can build up inside the earthen structure. Note the “boxes” on high ground to the immediate right of the channel. Those boxes are where riser pipes from relief wells are located.
Seepage that enters relief wells imbedded in Garrison Dam flows into culverts along the relief channel. From there the water flows into the Missouri River.
Imbedded in Garrison Dam, anywhere from 30 to 100 feet into the ground, are 53 relief wells that were installed when the dam was constructed. The relief wells are spaced from 90 to 150 feet apart along the bank of the relief channel.
"The relief wells are eight-inch pipes," said Sailer. "They provide water pressure relief for various layers of pervious soils under the foundation of the dam. The culverts exiting into the relief well channel are the outlets for the relief wells. It's where the water seepage can escape."
Seepage flowing out of the culverts passes through a "flap gate." The flap gates are designed to allow the exiting of water from the culvert but protect against return flows should the relief channel become filled with water. The channel is sloped so that water entering it flows into the Missouri River.
"All earthen dams have water penetrating through them," said Todd Lindquist, Corps project manager. "The culverts and relief wells are designed to move water, or seepage, within the embankment. We don't want to move material, just the water. Similar systems should be found in all earth dams."
While the amount of seepage entering the relief channel can be visually inspected, it is only one of several indicators used to determine the status of the dam. Inclinometers are used to detect any possible shift or movement in the embankment. Piezometers are other instruments regularly used by those tasked with constantly monitoring the safety of Garrison Dam.
"There's a series of piezometers to measure water pressure in the embankment," said Lindquist. "At higher pool levels we watch for corresponding flows in relief wells."
According to Sailer, the flow coming out of the relief wells is measured throughout each year as part of the dam's surveillance plan. Regular inspection of the relief wells is important as they can provide an early indication of any possible problems with seepage related to the dam, which could also be evident in the amount of water flowing in the relief channel.
"Toward the end of the channel we have a small structure called a Parshall Flume that is used to measure the amount of water flowing through the channel," said Sailer.