Pipestem dam risk tops $450m

U.S. Army Corps of Engineers officials used the May 19 ribbon-cutting at Pipestem Dam to make a point that went beyond celebrating a finished construction job. At the event near Jamestown, North Dakota, Corps officials said the project was driven by the scale of damage a failure could cause downstream and by what engineers learned from a similar spillway erosion event at Cottonwood Creek Dam in 2009. The completed work cost about $250 million and focused on hardening the spillway against erosion before it is ever needed. ### Why did officials keep citing a $450 million damage figure? Makenzie Leonard of the Corps’ Omaha District said at the ceremony that “potential property damage resulting from a failure of Pipestem Dam is estimated at more than $450 million.” Leonard tied that figure directly to the reason the project was pursued, saying the Cottonwood Creek flood in 2009 showed that a dam with a similar spillway could suffer severe erosion during a flood. (ksjbam.com) Jamestown sits about 4 miles downstream of Pipestem Dam, according to KSJB and Corps materials, which is why local officials framed the project as protection for lives and property in the city. Mayor Dwaine Heinrich said the spillway modification was like an insurance policy that residents hope never to use. ### What exactly did Cottonwood Creek change? The 2009 flood at Cottonwood Creek Dam, about 50 miles southeast of Pipestem, became the comparison point that reshaped the Corps’ risk assessment. (ksjbam.com) The Omaha District said severe spillway erosion at Cottonwood Creek highlighted the possibility that Pipestem could face similar erosion because of comparable underlying geology. North Dakota National Guard and other responders were deployed during that 2009 event after officials observed the Cottonwood Creek spillway eroding rapidly. Later project materials from the Corps said that episode prompted engineers to reexamine the geologic conditions beneath and around Pipestem’s spillway. (nwo.usace.army.mil) ### What was the engineering problem at Pipestem? Andrew Barry, chief of the Dam and Levee Safety branch at the Corps, said the original 1970s design made an assumption about erosion rates that proved inaccurate. Barry said officials concluded they needed to prevent erosion similar to Cottonwood Creek from happening at Pipestem because it could send a surge of water through Jamestown. The Corps said a multidisciplinary team found highly erodible soils more than 100 feet below the Pipestem spillway area. (119wg.ang.af.mil) Engineers determined that under extreme flood conditions the spillway could progressively erode, raising the risk of a breach and an uncontrolled release from the reservoir. ### How does the dam normally operate if the spillway has never been used? (ksjbam.com) Pipestem Dam was completed in 1973 and includes an earthen embankment, a gated outlet structure and an earthen spillway, according to the Corps. Under normal operations, water is released through the outlet structure, which is designed to pass up to 2,300 cubic feet per second. The largest flood-related outlet release on record was about 1,420 cubic feet per second in April 2009. (nwo.usace.army.mil) The spillway is the emergency path for water if reservoir levels rise beyond the outlet’s capacity during extreme storms or snowmelt. Corps materials say the spillway is designed to pass up to 110,000 cubic feet per second, but they also say extreme flow over an erodible spillway could threaten the dam if the feature were not reinforced. ### Who built it and what comes next? (nwo.usace.army.mil) The Omaha District and Barnard Construction completed the project after about three years of work, following a groundbreaking held on May 25, 2023. Col. Robert Newbauer, commander of the Omaha District, said at the ribbon-cutting that the project showed the Corps had delivered on commitments to reduce flood risk in Jamestown. Corps project pages say Pipestem Dam remains owned, operated and maintained by the U.S. (nwo.usace.army.mil) Army Corps of Engineers, and the agency has posted public background materials and operating information for the site online. Those materials describe the next phase as continued operation of the dam’s outlet and spillway system under the Corps’ dam safety program. (ksjbam.com)