Omaha, Neb. – The U.S. Army Corps of Engineers, Omaha District, is currently designing interim repairs to the spillway structure of Fort Peck Dam in Montana. A contract for the repairs is anticipated to be awarded by the end of September.
During the Flood of 2011, high releases from Fort Peck Lake pummeled the unprotected Bearpaw Shale of the plunge pool, enlarging it, and scoured along the spillway structure’s wing walls and downstream of the cutoff wall in the tailrace area.
"The power of the water was just immense. The force behind it tore away the shale at the cutoff wall on the spillway’s exit," said John Daggett, the operations project manager for Fort Peck Dam. "We need to repair that damage if we want to ensure the spillway operates well when faced with high releases again."
On Feb. 29, the Omaha District awarded a contract to AECOM to analyze the damage done to the spillway cutoff structure at Fort Peck Dam as well as assess the potential effects of future erosion along the wing walls that could be expected in the event of high releases. The contractor was tasked with crafting multiple alternatives to repair current damage as well as reduce the probability of similar damage developing under high flow conditions in the future.
At the height of flooding last year, releases from Fort Peck Dam reached nearly 66,000 cubic feet per second (cfs), of which 52,000 cfs passed through the spillway. The previous record release had been 35,000 cfs.
The primary components that the engineering firm examined were reinforcing the existing cutoff wall at the end of the spillway, reshaping the tailrace that extends out from the spillway, armoring impacted areas and installing a conventional stilling basin. A conventional basin would help to dissipate the energy behind the water in a more controlled manner as it exited the structure.
Each of the alternatives used two or more of these components and would create a system that would experience minimal damage at different spillway flows. One alternative would repair existing damage and accommodate spillway flows up to 65,000 cfs. The most comprehensive alternative would repair existing damage and accommodate spillway flows up to 265,000 cfs. The cost estimates of these alternatives vary greatly from $44 to $225 million.
Realizing that the cost of permanent repairs might outpace current allocations and budgets, the Corps requested the development of an interim repair alternative. This would provide the best and most critical repairs using available funding while additional funding for a more permanent solution is being pursued. U.S. Army Corps of Engineers – Omaha District 1616 Capitol Ave., Omaha, Neb. 68102 http://www.nwo.usace.army.mil/ Find us on Facebook at facebook.com/OmahaUSACE and on Twitter at twitter.com/OmahaUSACE
"At this point, what we can afford is the interim option," Daggett said. "Those repairs will be done in such a fashion that allows us to move seamlessly into larger repairs if we can secure additional funding. The important thing for us is to address the immediate need, and we can go forward from there."
The interim repairs would repair existing chute concrete damage and areas along the wing walls, which will improve the overall stability of the cutoff structure. It would also help to reduce some downstream scour action under high releases.
Also, the Omaha District will further assess the long-term performance of the spillway chute and conduct a flow test in early September. Interim repairs to the gate structure and downstream chute at Fort Peck are underway to repair damage from last year’s flood.
For regular updates on the repair efforts to flood control structures in the Missouri River Basin, visit the Omaha District’s Flood 2011 Repairs web page at http://www.nwo.usace.army.mil/op-e/srt.html.