ASHLAND — The dam that creates Howard Prairie Lake will get a seismic retrofit over the next year to boost its chances of surviving a large earthquake should one rock Oregon’s southern Cascades.
A Portland firm has won a $5.9 million contract to beef up a large portion of the 1,040-foot-long earthen dam that is not actually resting on bedrock like the rest of it does, according to its owner, the U.S. Bureau of Reclamation.
Those portions of the 100-foot-tall dam base sit on 10 to 14 feet of stream bed “that could liquify during an earthquake and all slide out,” said Chris Regilski, the bureau’s regional dam-safety coordinator.
“The overall risks are higher than we’d accept,” Regilski said.
During construction, the old stream bed under the dam will be drained, excavated and replaced, with all the work done on the dry side.
The reservoir’s unimproved boat ramp and parking lot adjacent to the dam closed Friday to the public for all types of recreation in preparation for work crews to begin moving in Monday.
The area will be closed until construction is done sometime next year, according to the bureau.
However, the construction will not impact the ability of irrigation districts to transfer water from Howard Prairie Lake, said Jim Pendleton, manager of the Talent Irrigation District.
TID as well as the Medford and Rogue River Valley irrigation districts draw water released from Emigrant Lake into Bear Creek for dispersal.
Also, the dam work will not impact operations of the Bonneville Power Administration’s Green Springs Power Plant, Regilski said.
That was not the case last year when the bureau conducted seismic and other improvements to nearby Hyatt Dam at Hyatt Lake.
Hyatt Dam was leaking, and the reservoir had to be drawn down to less than half full to expose the seepage areas, Regilski said.
There are no immediate plans to conduct any retrofitting of Emigrant or Keene Creek dams, but the bureau will continue regular monitoring of them, Regilski said.
The Howard Prairie Dam contractor is Natt McDougall Co. of Portland.
The bureau will pick up 85 percent of the cost, while BPA and the irrigation districts will pay the remainder as beneficiaries of the stored water.
The irrigation districts will pay their share off in no-interest loans, Pendleton said.
Completed in 1958, Howard Prairie Dam contained 416,000 cubic yards of material. It dams Beaver Creek to store up to 62,100 acre-feet of water, according to the bureau.
Reach Mail Tribune reporter Mark Freeman at 541-776-4470 or firstname.lastname@example.org. Follow him on Twitter at www.twitter.com/MTwriterFreeman.