U.S. Army Corps of Engineer officials are planning a $175 million project to prevent possible erosion at the emergency spillway of Pipestem Dam, according to Chris Fassero, project manager for the Corps of Engineers.
Fassero said the dam is structurally sound and possible problems would only occur if water flowed over the emergency spillway. The closest the lake level came to reaching that level was in the spring of 2009 when the lake level peaked 4 feet below the emergency spillway, according to Bob Martin, former manager of Pipestem Dam, who retired last fall.
The concern is erosion south of the emergency spillway could jeopardize the integrity of the dam's structure or lead to a failure.
The emergency spillway is located west of the paved road across the top of Pipestem Dam. Immediately south of the spillway is a broad relatively level area that had previously been used as a shooting range. From that area, there is a steeper slope with nearly a 90-foot vertical drop to Pipestem Creek below.
It is the area of the steeper drop that is at risk of eroding away because of layers of sand under the topsoil, Fassero said.
Fassero said in 2009, a dam on Cottonwood Creek near LaMoure did see erosion when high waters overflowed the emergency spillway. Studies showed that dam had a similar design and soil profile as Pipestem Dam.
The preliminary design of the project includes a concrete wall along the steep slope above Pipestem Creek with a concrete stilling basin below where the water would slow and calm before flowing into the creek.
The project also calls for "guiding walls" along the east and west side of the emergency spillway to prevent water from flowing away from the wall structure. A cement wall would be added to the top of the current earthen berm that makes up the emergency spillway to better control water flow.
Fassaro said very preliminary engineer's estimates place the cost of the project at $175 million and all costs would be covered by the federal government. Work would extend over three construction seasons and could possibly start in the spring of 2021.
Jerry Bergquist, Stutsman County emergency manager, said if water levels at Pipestem Dam reached the level of the emergency spillway, the city of Jamestown would be evacuated.
"The population at risk if there were a failure is 8,500," Fassero said. "There is a high enough risk to address this problem."
Fassero said there is no risk under normal or even most adverse conditions.
"The risk is only in a flood event of great magnitude," he said.