Bend dam rated high hazard
七月 23, 2017
Bend BulletinA 105-year-old dam in Bend is on a list of 17 Oregon dams rated as a “high” hazard if they were to collapse, with no emergency plan for how to handle a catastrophic failure. A pending state law would force local officials to plan for the worst.
The North Canal Diversion Dam, built in 1912, is regarded as a key engineering achievement in the history of Central Oregon, feeding the canals that sustained development of Redmond and brought irrigation to Madras.
That very growth is what landed the dam on the hazard list. Built when Bend was home to 4,000 people, it sits on Division Street in the midst of a city of 87,000 people. If an earthquake ruptured the 33-foot-high retaining wall, more than 13 million gallons of water would crash down the Deschutes River.
Under legislation passed with large bipartisan majorities in the state House and Senate, the 17 dams on the list would be required to map areas that would likely be flooded by a collapse, create criteria for detecting a breach, establish an early-warning system, and determine who will give evacuation orders and command recovery efforts. Regular drills would be held.
Gov. Kate Brown is reviewing a backlog of bills passed in the final days of the 2017 session of the Legislature.
“The dam bill should land on her desk within two weeks,” said Bryan Hockaday, Brown’s spokesman.
The dam plan is among a number of infrastructure safety initiatives the state has launched since the July 2015 publication of “The Really Big One” in The New Yorker magazine. The story, which won a Pulitzer Prize, highlighted the little understood Cascadia Subduction Zone, a fault running from Cape Mendocino, California, to Vancouver Island, British Columbia. A 9.5-magnitude earthquake would devastate the Pacific Northwest. Though an expert in the story said everything west of Interstate 5 would “be toast,” government reaction has been to fund seismic improvements in roads, bridges, dams and other key power, water and transport routes.
One thing the article could not solve is the mash-up of federal, state, local and private authority and rights over waterways and dams. That limits the impact of the new legislation.
Many dams are small, private operations that don’t warrant inspection under current law. A dam must have a wall face at least 10 feet high and hold a minimum of 9.2 acre-feet of water. An acre-foot of water would cover an acre up to 1 foot depth; 1 acre-foot is just under 326,000 gallons. A dam would have to hold about 3 million gallons before it requires inspection by the Oregon Water Resources Department.
Of the 900 dams that meet the criteria, 75 are regulated and inspected by the federal government.
The remainder have to meet two criteria to land on the list:
First, they have to be rated as a “high” hazard under a system developed by the U.S. Army Corps of Engineers. The rating has nothing to do with the structural integrity of a dam. It only establishes the threat level to life and property that would be caused if the dam were to rupture. The ratings are low, significant and high. A “high” hazard rating indicates the likelihood of loss of life in the event of a dam failure.
Oregon has 75 state-regulated “high hazard” dams; 58 of those have emergency action plans.
Location helps explain why the North Canal Diversion Dam and its 13 million gallons is rated as a “high” hazard while the remote Big Three Creeks Dam in Deschutes County, holding 456 million gallons, is rated a “low” hazard.
There are 15 state-monitored dams in Deschutes County. Ten have a “low” hazard rating. The Mirror Pond dam on the Deschutes River and the McKenzie Canyon Dam on Squaw Creek are rated “significant,” the middle standard.
Three are considered “high” hazard, including the two largest dams.
To make the Legislature’s list, the dam also had to have no plan on how to deal with a catastrophe.
Wickiup Reservoir, on the Deschutes River, is rated a “high” hazard. It has a 100-foot-high wall face and holds 71 billion gallons of water. A rupture would be a massive catastrophe. But the dam has an emergency response plan in place. So does the “high hazard” Crane Prairie Dam on the West Fork of the Deschutes River, with a 32-foot-high face in front of 83,000 acre-feet of water.
North Canal Diversion does not.
Because of these many quirks and criteria, North Canal Diversion Dam — the second-smallest in Deschutes County — is the only dam in Deschutes County that falls on the list of 17 “high hazard” dams in the state. Now local irrigation managers, the city of Bend and the state will work together to plan and practice a disaster plan.
They will also get a little extra help. Currently one full-time state engineer and one part-time engineer are assigned to work with dam owners to draw up emergency plans. Lawmakers added $122,000 over the 2017-19 budget cycle to convert the second position to full time. The Federal Emergency Management Agency matched the funds.
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