Dam safety: Repair, decommission or mitigate risk?

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Should systems within aging hydroelectric facilities be repaired, decommissioned or something in between? Site-specific procedures mitigate dam safety issues at two hydroelectric power plant facilities in the Pacific Northwest, 23.2-MW Leaburg-Walterville on the McKenzie River in Eugene, Ore., and 175-MW Jordan River on Vancouver Island in British Columbia.

For the Jordan River facility, owned and operated by BC Hydro and Power Authority (BC Hydro), the dam’s age and site location compound the difficulty in addressing its functionality.

The 103-year-old Jordan River Diversion Dam is on the Cascadia Subduction Zone (CSZ), the most active seismic region in the Pacific Northwest.

According to the Pacific Northwest Seismic Network, “The CSZ ‘megathrust’ fault is a 1,000 kilometer long dipping fault that stretches from Northern Vancouver Island to Cape Mendocino, California.”

According to Tim Walsh, chief hazard geologist at the U.S. Department of Natural Resources, “It used to be thought that Cascadia was not an active fault. Not only has Cascadia been found to be an active fault, it has a 10% chance that it will cause an earthquake in the next 50 years.”

Walsh made the comments in October while discussing preventative measures he and other researches were putting in place to help structures withstand a tsunami should activity at the CSZ cause a tsunami to strike the Pacific Northwest coastline.

“It [an earthquake on the CSZ] is more than 10 times more likely than the chance you will be killed in a traffic accident,” Walsh said.

According to BC Hydro’s Seismic Study and Action Plan, released Dec. 5, “the expected ground motion at Jordan River in an extreme event -- an 8 to 9 magnitude earthquake -- is much greater than previously thought due to its proximity (about 40 km) to the CSZ.”

Fortifying or repairing the Jordan River facility to survive an 8 to 9 magnitude seismic event is neither possible nor economically feasible, according to BC Hydro. Additionally, decommissioning the dam or lowering the level of the impoundment reservoir are not viable options because the system produces about 35% of hydroelectricity generated on the island.

Age factors and location required BC Hydro to seek alternatives to repair or decommissioning; in this case, they chose risk mitigation.

BC Hydro is working with the province’s Capital Regional District “to improve emergency preparedness and awareness of seismic risks, including the potential to restrict future residential development and overnight camping” in the inundation zone that would be affected by a catastrophic dam failure.

The company is also advising permanent residents to develop plans to evacuate should a high-magnitude earthquake compromise the dam.

Leaburg-Walterville facility  

It is less catastrophic, but major circumstances opposite to dam failure are among the issues before the Eugene Water & Electric Board, which operates the 73-year-old Leaburg-Walterville facility on the McKenzie River.

Cylindrical steel gates that roll or rotate, up and down, regulate the flow of the river at Leaburg Dam. Three floodgates, each 16 feet tall and 100 feet wide, regulate how much water the releases, but two of the floodgates are inoperable.

On Dec. 23, a malfunction at one of the floodgates left the structure with one functioning roll gate. Prior to this year’s pre-Christmas floodgate failure, in January 2012, the middle roll gate experienced a failure that is being repaired.

EWEB has decided to repair both floodgates at the facility because if left uncontrolled, seasonal rains can cause problems.

The dam is a run-of-river project about 27 miles east of Eugene. Water is diverted at Leaburg Dam into a power canal that is used to generate electricity at a powerhouse 4 miles downstream. The amount of released water determines how much power is generated, but, more importantly, the facility reduces instances of flooding in the area downstream.

“Repairs to the middle roll gate should be completed by mid-January,” said EWEB in a press release.

In both instances, dam owners had to make decisions on what to do with regard to aged facilities. Government and private companies that own and operate hydropower facilities will continue to make similar tough decisions as equipment breaks or new information on the safety conditions at dam sites becomes available.
 

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