Concerned Trinity Dam could suffer the same fate as Oroville Dam — which had a near catastrophic failure this past winter — the Trinity County Board of Supervisors on Tuesday agreed to continue to pursue getting an emergency spillway built on the dam.
Supervisor Bill Burton said residents in his district watched the Oroville Dam incident unfold in February and became concerned something similar could happen in Trinity County.
“I’m the downriver supervisor, so my constituents would be very concerned if we ever had these troubles,” Burton said during a presentation Tuesday on Trinity Dam safety.
Nearly 200,000 residents downstream of Oroville Dam were evacuated for several days in February when part of the main spillway eroded and broke apart. When dam operators tried to use an emergency spillway on the dam, that too began to fail.
Residents downstream of Trinity Dam, an earthen dam similar to Oroville, became concerned this winter as Trinity Lake filled for the first time in several years.
On Tuesday the lake was 93 percent full.
The county sent a letter to the bureau in March asking for someone with the agency to address the safety of the dam.
The board’s letter refers to a U.S. Bureau of Reclamation report written in 2000 that seems to support their worries.
“Trinity Dam cannot safely pass the probable maximum flood (PMF), which is a SOD (safety of dam) concern,” according to the report, titled “Trinity Dam Enhancement Technical Appraisal.”
“If the dam were to fail during an overtopping event, the resulting breach would discharge in excess of 1,000,000 (cubic feet per second). Therefore, there is a risk of dam failure due to a large flood event,” the report says.
Don Bader, the bureau’s area manager, gave a presentation to the board about the dam, said there is very little risk of filling the lake to the point where water would flow over the crest of the dam.
He said there is a 1 in 60,000 chance of having a series of storms strong enough to produce enough rain to fill the lake and send water over the top.
“Right now it doesn’t warrant it, based on the risk assessment,” Bader said of the need for an emergency spillway.
He said dam operators can release enough water to prevent the lake from spilling over the crest during a heavy storm.
The dam has three outlets to prevent water from going over the crest, Bader said. There is a “glory hole” at the surface of the lake and two other controlled outlets below the water surface.
The 28-foot diameter main outlet is 250 feet below the dam crest, and a 7-foot diameter auxiliary outlet 400 feet below the crest. Bureau officials can control how much water flows through both outlets.
Water flows into the glory hole when the lake level gets within 25 feet of the crest. Dam operators can’t control the water after it flows into the glory hole, a 54-foot diameter concrete pipe that diverts water under the dam.
Bader said water has gone over the glory hole only nine times since 1976.
Tom Stokely, a salmon and water policy analyst for the Pacific Coast Federation of Fishermen’s Associations and Institute for Fisheries Resources, told the board the dam posed a danger in the event water flowing into the lake reached 400,000 cubic-feet per second flowing into the lake.
“I believe Trinity Dam needs an emergency spillway because A, it can’t handle the probable maximum flood, which is over 10 times more than what the reservoir can spill” through the glory hole and its two other outlets, Stokely said.
The lake also is doomed to fill with sediment, which may be hundreds of years in the future. But when that happens, the reservoir won’t have the capacity to handle high flows into the lake, he said.
After Bader’s presentation, the board agreed to bring the issue up again at a future meeting.
The Trinity supervisors aren’t the only public officials concerned about the safety of Trinity Dam. U.S. Rep. Jared Huffman, who represents Trinity County, asked the bureau to do a new safety study of the dam and consider building an emergency spillway for the structure.
Bader said the bureau wrote back to Huffman explaining that the dam is safe and an emergency spillway is not warranted.
Article by: Damon Arthur
Published: June 6 2017, by Redding Record Searchlight