Oroville Dam auxiliary spillway suffers erosion, area evacuated
In mid-February, the auxiliary spillway at Oroville Dam suffered erosion during high flows, leading to the evacuation of a reported 188,000 nearby residents.
According to the California Department of Water Resources, evacuation orders were delivered to residents surrounding Lake Oroville at about 4:30 pm on Sunday, Feb. 12. The concern was that erosion at the head of the auxiliary spillway threatened to undermine the concrete weir and allow large, uncontrolled releases of water from Lake Oroville that potentially could exceed the capacity of downstream channels.
Lake Oroville is the largest reservoir in the State Water Project, with a capacity of 3.5 million acre-feet. The auxiliary (emergency) spillway has not been used since the dam was finished in 1968.
DWR said it was monitoring conditions at the main and auxiliary spillways around the clock for signs of erosion that could threaten the integrity of the emergency spillway and allow large, uncontrolled flows to the Feather River.
Water from the auxiliary spillway eroded the roadway below the spillway.
To lower the lake level, thus reducing flows and the potential for erosion at the top of the emergency spillway, DWR increased flows down the main spillway to 100,000 cubic feet per second from 55,000 cfs. This was damaged Feb. 10, when a cavity opened on the concrete due to erosion. DWR said releases remained within the capacity of downstream channels.
For comparison, DWR said the morning of Feb. 11 that inflow to the lake was about 95,000 cfs. However, on Feb. 9, DWR reported inflow to the reservoir was about 192,000 cfs, thanks to “a storm that stalled over the watershed.”
Hampered by the erosion to the main spillway, Lake Oroville exceeded the elevation of 901 feet above sea level shortly after 8 am on Saturday, Feb. 11, at which point water began to flow over the concrete weir of the auxiliary spillway.
Also on Feb. 11, DWR reported it was focusing on ways to get its 645-MW Hyatt Power Plant, at Oroville Dam, back in operation because 140,000 cfs can be discharged from the plant when it is operating. Power generation was halted when the water levels in the channel that leads from the power plant became high enough to compromise operation.
Oroville Dam is a separate structure from the emergency spillway, and DWR says it remains sound. Oroville is the tallest dam in the U.S. at 770 feet and is a zoned earthfill embankment.
Ameren announces Bagnell Dam safety upgrade program
Ameren Missouri has announced that a major construction project will begin at Bagnell Dam and its associated Osage Energy Center this spring, to install a series of new anchors and concrete on the downstream side of the dam.
This work is anticipated to cost $52 million and take about 18 months.
“This project is about keeping this vital asset providing clean energy in the long term, using the best possible engineering available today,” says Warren Witt, director of hydro operations at Ameren Missouri.
The 242.59-MW Osage Energy Center, on the Osage River, has operated for 85 years.
The last major structural upgrade at Bagnell Dam was completed in the early 1980s with the installation of 277 post-tensioned anchors to hold the dam into the bedrock, Ameren Missouri reports.
This latest upgrade will be conducted in three parts: installing 68 new post-tensioned anchors to hold the dam to the underlying bedrock, placing concrete between the highway piers to add weight to the dam, and installing a new concrete overlay to replace worn and cracked concrete on the east and west sections.
In total more than 66 million pounds of new concrete will be added to the structure, Witt said.
Documents hint at federal dam safety measures
The Trump Administration has been tight-lipped about specific infrastructure projects it sees as being federal priorities, but a document distributed to the National Governors Association might provide some clues.
The report, titled “Emergency & National Security Projects,” was compiled while President Donald Trump was still President-elect Trump.
Among other measures, the document describes an $850 million federal allocation to be made to the U.S. Army Corps of Engineers that would be used for repair and safety improvement work at more than 600 dams in South Carolina.
These dams, identified as either high or significant hazards, are amongst those damaged during a thousand-year rainfall event in 2015 and again in 2016 during Hurricane Matthew.
According to the report, the undertaking could create up to 2,200 direct jobs.