District releases Mirror Pond dam report

Consultant estimates replacement would cost millions

By Hillary Borrud / The Bulletin / @hborrud

Officials who want to keep Mirror Pond should consider replacing or completely rehabilitating the dam that created the water feature, according to an inspection report prepared by engineers hired by the Bend Park & Recreation District.

Phoenix, Ariz.-based contractor Gannett Fleming Inc. determined the dam will eventually fail and estimated it might cost $2 million to $3.7 million to replace or overhaul it. The total cost for the project would be much higher because it would also require engineering, permitting, dredging and possibly fish passage.

Local officials are discussing whether the park district should acquire the dam from PacifiCorp, the power company that owns it. PacifiCorp is searching for someone to take over the dam because the utility has said it no longer makes financial sense to maintain it.

“Due to the age and type of structure, there are no inexpensive short-term alternatives for repairing the dam that would provide assurance that the dam would not experience a sudden leak or failure,” engineers for Gannett Fleming wrote in the report. They concluded that “as the main structural features of the timber crib dam continue to deteriorate, a larger failure of the timber crib dam will eventually occur that will require more extensive and expensive repair methods.”

The park district hired Gannett Fleming to inspect the dam in March and write a report on the findings, for which the district will pay $23,500.

The park district released the inspection report on Monday afternoon in response to a public records request from The Bulletin. The district originally denied the request, but The Bulletin appealed to Deschutes County District Attorney Patrick Flaherty, who ordered the district to release much of the report. Park district officials ultimately decided to release the entire report.

City Councilor Mark Capell is a member of the Mirror Pond ad hoc committee, a group that also includes park board members and citizens and is tasked with deciding the future of this section of the Deschutes River. “The numbers are lower than I would have expected,” Capell said on Monday, of the cost estimates to replace or rehabilitate the dam. “Now we have to have a really solid conversation within the community about what our priorities are. … We didn’t have enough information when we were having these discussions before.” The ad hoc committee is scheduled to meet May 21.

Park District Executive Director Don Horton had a similar reaction to the cost estimates. “I think their estimates on replacing the dam were a little less than I’d expected it to be, but I really didn’t have anything to base that on,” Horton said Monday.

However, replacing or fixing the dam is just one part of the calculation, and the community actually will face a much higher bill if it wants to maintain a dam on this section of the Deschutes River. At the low end, the total cost to replace or rehabilitate the dam and dredge sediment might be $3.9 million , if the state does not require the owner to install fish passage and the owner can dredge the sediment behind the structure for the lowest estimated price. The high end of the cost range could reach $22.8 million, based on estimates included in the report. Neither the low or high estimate includes engineering design, permitting costs, or any riparian habitat restoration or other work along the riverbanks.

Gannett Fleming’s engineers estimated it might cost $2.1 million to $3.4 million to rehabilitate the dam with steel sheet pile, and it could cost $2 million to $3.7 million to replace it with a new concrete dam. In addition, officials have been discussing the need to dredge the sediment that has accumulated behind the dam and is creating mudflats. Previous studies estimated it could cost $1.8 million to $18.5 million to remove this sediment, the consultants wrote.

Under state law, any dam owner who alters 30 percent or more of the structure must install fish passage. The consultants extrapolated that a certain type of fish passage might cost $250,000 to $540,000, based on a 1990 report that they adjusted for inflation. “Based on our recent experience designing and constructing (one type of fish passage), we believe the cost to design and construct (that passage) at this site is on the higher side of this range, and possibly higher if a fish counting facility is required to confirm the performance of the fishway,” the consultants wrote.

A less expensive option would be to remove the dam. Gannett Fleming estimated it might cost $600,000 to $1.9 million to remove the dam, plus up to $13 million to remove sediment buildup and restore the river channel. According to the report, the dam is approximately 100 years old. “Almost all timber crib dams in the United States have been removed, replaced or modified with more permanent construction materials,” the engineers wrote.

Gannett Fleming also suggested the dam owner install warning signs and buoys immediately downstream of the dam, because of dangerous hydraulic conditions.

The engineers who evaluated the dam found that leakage caused the dam to act as a “strainer,” a condition in which “leakage through the dam creates higher flow velocities in the vicinity of a leak and can potentially suck or trap and drown a person near the dam against the upstream face of the dam.”

Horton said there is no access from the riverbank to this section of the river, and he never observed anyone on that section of river when the park district headquarters used to be located nearby.

PacifiCorp spokesman Bob Gravely said on Monday that the utility’s engineers read through the report and found much of it to be accurate. “There are some aspects of it where if we had been given a chance to review a draft or could comment on (it), we would take issue with some of it,” Gravely said. “But nothing that is real material to what the report was supposed to do, and that’s guide the local entities on the big picture questions.”

— Reporter: 541-617-7829, hborrud@bendbulletin.com