Friday, April 22, 2016
GF TRibune story, 5-31-2014
The half-century-old, earthen Swift Dam probably collapsed in a matter of minutes June 8, 1964, following a perfect storm of accelerated snowmelt and heavy rains that swelled Birch Creek to unbearable proportions. Coupled with the collapse of Lower Two Medicine Dam on Two Medicine Creek soon after on the same day, the deluge claimed 30 lives and caused an estimated half-billion-dollars of damage in today's dollars.
Both dams are rebuilt. And they're much more sturdy.
"The dam that's there now is a completely different structure," said Vernon Stokes, manager of Pondera County Canal and Reservoir Co. "The spillway is enlarged and is all concrete now. It also has an emergency spillway on top of the dam."
He said the Bureau of Reclamation designed and supervised the construction of Swift Dam and turned it over to the PCCR at completion in 1967. PCCR owns and operates the dam.
The current concrete-arch structure is 205 feet high with a length at its crest of 573 feet.
The old Swift Dam had only a little concrete in it. The rebuilt structure "is nothing like it." The all-concrete spillway now "doesn't erode away," and the capacity of the spillway was increased on the new dam, Stokes said.
Swift Reservoir has a storage capacity of 30,000 acre feet, which is the same as the original dam. An acre-foot is 43,560 cubic feet or about 325,853 gallons.
"We've got a lot better gates than what was on the old original Swift Dam," Stokes said.
PCCR workers constantly monitor both dam and water conditions to avert disaster.
The Swift Dam tender visually inspects the structure every day, also checking elevations and inflows. The structure gets yearly maintenance and is inspected every five years by a licensed engineer, he said.
This spring, "we have tremendous snowpack up there," on the nearby Rocky Mountain Front, Stokes said, so "inflows are checked about every two hours.
"We open the dam and release water according to what's coming in," he said. While Swift Reservoir has a maximum capacity of about 35,000 acre-feet, "once it gets up to 30,000 and we've had a rain event, we increase flow," through the dam. He said if the primary spillway fills up, there's an emergency spillway on top of the dam for excess flow, plus two operating gates. "We can open those up as needed."
Swift Dam was built to supply irrigation, not for flood control. He said with its small capacity, the reservoir "can fill up in five or six days if the flow is high enough."
The new structure is "a good strong dam with proven design," Stokes said. "It could withstand just about anything other than a pretty good-sized earthquake."
For any level of possible problem, the canal company has an emergency action plan. "If anything happens to go wrong or there's a big increase in flow, we have a call list," to warn residents to expect high water on Birch Creek, Stokes said. "I don't believe that was in place" in 1964.
There are also a lot more tools now, with stream-flow forecasts put out by USDA's Natural Resources Conservation Service, which includes snowpack and snowmelt. He said the Bureau of Reclamation has put Swift Dam on its Hydromet data-collection system, which provides "readings on inflow every two hours and a lot more tools to work with."
Lower Two Medicine Dam was 50 years old when it collapsed in the 1964 flood. The dam was rebuilt in 1967 and is under the jurisdiction of the Bureau of Indian Affairs today. The safety of dams officer/environmental engineer for the U.S. Bureau of Indian Affairs, Rocky Mountain Regional Office in Billings was unable to respond to the Tribune's questions about new dam and today's safety procedures for this story because he was unable to obtain permission from the BIA's public affairs officer in Washington D.C. in time to meet the publication deadline.
How about Great Falls' 5 dams?
Carrie Harris, manager of engineering and projects for PPL Montana, noted the hydropower producer's five Great Falls-area dams were all in place before 1964 and "withstood the flood without problems. They are designed to withstand a flood exceeding, by quite a bit, the volumes that were produce in the 1964 flood."
She said the PPL dams, although built long-before the Allenton, Pa.-based company acquired them in 1999, "were designed so they can 'overtop' and the dam still stands."
After a big nationwide campaign in the 1970s and 1980s, operators of some older dams installed post-tension anchors, which use thick, stranded wire cables tied into an anchor that is drilled and grouted into bedrock, she said. The anchors are tensioned and act to tie the dam to the riverbed.
"Black Eagle's spillway has rock anchors to bedrock," Harris said, and "portions of Rainbow and Ryan have post-tensioned anchors."
In addition to design and construction changes, she said PPL has a rigorous routine of workers looking for any changes in the dam structure.
• Daily observances by workers, with more thorough monthly examinations.
• Annual inspection done by engineers and the Federal Energy Regulatory Commission.
• Every five years an independent consultant is hired to inspect the dam to ensure the structures are meeting current dam stability criteria.
This is coupled with ongoing maintenance, she said.
Oldest dams rehabilitated
Black Eagle and Rainbow dams, the oldest in the Great Falls area, have been rehabilitated. Originally constructed as timber-crib dams, they had a timber framework filled with rocks and concrete to create a dam.
Black Eagle began in operation 1891. During a 1908 flood, workers had to blow a hole in the dam to ease the water flow. It generated no power for almost 20 years, until it was completely rebuilt as a concrete dam and went back online in 1927.
Rainbow first went online in in 1910. Harris said "a major rehabilitation of the spillway portion of the dam was completed in the early 1990s. The rehabilitation consisted of full and partial mass concrete replacement of the original rock-fill timber crib."
When water rises to a level of concern, "we have standard operating procedures for the spillway," and rules for opening "this gate, and then this gate," to let more water flow downstream.
She notes that the Great Falls dams — Black Eagle, Rainbow, Cochrane, Ryan and Morony — are hydroelectric dams, so their main function is power generation, not flood control. They operate on a run-of-river basis, meaning their normal status is to let through the natural river flow. "But Canyon Ferry is our friend upstream. If things get crazy on the Sun River (Canyon Ferry operators) can help us out by holding back water," in the Missouri to mitigate an abnormal influx of the Sun where it meets the Missouri in Great Falls.
The Bureau of Reclamation, owner of Canyon Ferry Dam, "is a big part of our emergency planning."
She said "for the Missouri River at Great Falls, downstream of the confluence with the Sun River, the largest estimated historic peak flood was pre-Canyon Ferry construction and occurred in 1908 with a flow of approximately 140,000 cubic feet per second. All the Great Falls dams are designed to withstand a probably maximum flood of 298,000 cfs."
Reach Tribune Staff Writer Marc Stergionis at 406-791-6585 or firstname.lastname@example.org. Twitter: @GFTrib_MarcS
Editor's note: Lower Two Medicine Dam is under the jurisdiction of the Bureau of Indian Affairs.
PPL Action Plan
PPL Montana has emergency action plans in place for all its facilities, said Carrie Harris, manager of engineering and projects. The dam operator provides safety training and conducts drills annually.
When a situation arises where flooding could become a problem, PPL has three different warning levels for the public.
• Non-failure flood warning.
A situation where waterways are at or near flood stage and PPL Montana operationally does something that may aggravate downstream flooding. This would be "actions that we might have to take to deal with the situation that would additionally increase outflows thus add flows to already flooding rivers," she said.
• Potentially hazardous situation is developing.
A failure may develop, but preplanned actions taken during certain events (major flood, earthquake, evidence of piping, etc.) may prevent or mitigate failure. Time permits a qualified engineer to inspect the dam and assess the potentially hazardous situation.
• Failure is imminent or has occurred.
Failure (uncontrolled release of the reservoir) caused by terrorism, sabotage, major earthquake, etc.
Harris said "all three levels have some contact with outside agencies who then may make additional notifications. The flood-flow warnings have the least number of external/agency notifications, while the dam failure has the most external/agency notifications.
"We also have functional exercises every five years for all people affected by a dam break," including emergency services personnel in the community where the exercise is being staged. An exercise was held in Great Falls last year, she said.
» May 25: 30 of the 31 people who died in the floods perished on the Blackfeet Reservation
» Monday: Most of west Great Falls, Sun River valley under water, plus military response
» Tuesday: Those on the scene will never forget the drama at the head gates
» Wednesday: Choteau evacuated
» Thursday: The flood had long-lasting effects on area streams, rivers and landscape
» Today: Columbia Falls, Kalispell also experienced devastating floods, Page 1A
» Historical pages: Keep your eye out for full-page reproductions of Tribune pages from 1964 during the floods
Sun River overrun, in the words of a meteorologist who was there in 1964, and a family who survived it
Historic photos of the Sun River flooding