[env-trinity] Trinity Journal- Trinity Dam safety concerns discussed

[Tom Stokely]

http://www.trinityjournal.com/news/local/article_e86e135e-50a3-11e7-a0d4-2b4aa8fb8200.html



Trinity Dam safety concerns discussed
   
   - By Sally Morris The Trinity Journal
    
   - Jun 14, 2017
   
   - Seeing almost 200,000 Yuba, Sutter and Butte County residents ordered to evacuate after Oroville Dam sustained a nearly catastrophic spillway failure in February prompted many in Trinity County to question their own fate living below Trinity Dam.A response came last week from the U.S. Bureau of Reclamation’s Northern California Area Manager Don Bader who offered reassurance that Trinity Dam is well-maintained, monitored 24/7 and is in excellent condition during a dam safety presentation requested by the Trinity County Board of Supervisors.The board also heard from retired Trinity County Natural Resources Planner Tom Stokely who is not so sure of the dam’s long-term safety and believes adding a spillway to Trinity Dam warrants serious consideration. Board members agreed to pursue that possibility in a letter to be drafted for future action.Completed in the mid-1960s, Trinity is an earthen dam, 538 feet high and 2,450 feet across. It is operated by the Bureau of Reclamation under safety criteria established in 1976. Unlike Oroville Dam, Trinity does not have a spillway. Releases of water are made either through the power plant, two jet gates at the base, or a glory hole at 2,370 feet of lake elevation.The glory hole is 25 feet below the crest of the dam, and the reservoir has spilled water into the glory hole nine times in its history. The greatest spill was in 1974 when the lake level rose eight feet above the glory hole inlet, resulting in an uncontrolled discharge of up to 14,000 cubic feet per second of water for almost a month.After that, the dam safety plan was established to keep the lake level at prescribed elevations through the winter. Exceeding those levels triggers releases.“We won’t wait. We need to ensure adequate storage during the winter months when we get those major storm events,” said Bader. He added that if the same safety measures had been in place at the time, the 1974 spillage would not have occurred.He said flood releases are managed not to exceed the channel capacity of the Trinity River, considered to be 11,000 cubic feet per second “which is as high as we dare to go. If we do get into flood control, it could necessitate releasing more than 11,000 cfs, up to 25,000 or 30,000 cfs out of the dam and we don’t want to ever see that. We’d preempt that with earlier releases if need be.”Bader said Trinity Dam is visited daily and operations are monitored 24/7, both at the area office in Redding and in Sacramento. Dam instrumentation is analyzed monthly and annual reviews are also conducted. A more comprehensive facility review is performed every three or four years, and the upcoming review is expected to evaluate whether there is a need to add a spillway.He also spoke of the landslide below Trinity Dam’s east abutment that was identified in 1959 and stabilized over the years by the installation of 182 horizontal drains. A 2006 Trinity Dam landslide risk analysis concluded that the slide has remained stable, even during extremely wet years, and that if it were to fail, it would not likely reach the glory hole chute to damage or block it.A comprehensive formal review of the dam in 2011 confirmed that the dam has a very low risk of failure or overtopping: a one in 60,000-year chance. Bader said a 60,000-year event has a low probability of occurrence. The emergency activation plan for the dam is up-to-date and exercised annually. Operating guidelines were last evaluated in 2016 and confirmed.Representing the Downriver area of Trinity County, District 4 Sup. Bill Burton said the problems at Oroville Dam in February resulted from infrastructure “that wasn’t working right. Then there was Plan B and Plan C, so all the safety is predicated on everything working right. If it doesn’t, it gets real scary. We should be funded for a Plan C here in case the glory hole doesn’t work right.”Retired Natural Resources Planner Tom Stokely spent 20 years working on Trinity River environmental issues for the county and is currently employed as a policy analyst for the Pacific Coast Federation of Fishermen’s Associations and Institute for Fisheries Resources. He said that when he first heard of the Oroville Dam spillway failure, he immediately thought of Trinity Dam “because at least Oroville has a spillway. Trinity does not have one. I’m not here to contradict Don Bader. I think the Bureau is doing a good job, but there are some long-term considerations.”He said that although the likelihood of dam failure is slight, the consequences would be catastrophic, taking out the Lewiston Dam and sending a 250-foot wall of water down the Trinity River valley. Citing a previous technical study produced by the Bureau in 2000, he said about 3,500 people would be placed at risk and lost project benefits would amount to approximately $3.5 billion in damaged infrastructure.Stokely said the maximum probable flood, though unlikely, could reach 400,000 cfs with 86 percent of that going over the dam.“The dam cannot safely pass the maximum probable flood. Then there’s the landslide. It is quite stable. There’s been a lot of work done on that by the Bureau, but if the glory hole tunnel got blocked, it would cause major damage to the dam. It’s probably not likely, but there would be problems,” he said, sharing a photo of the dam construction and glory hole before the lake filled.“Look how small the glory hole is in scale with the rest of the construction,” he said.Furthermore, he said eventually the lake will fill in with sediment “and what happens then? None of us will be here in 500 years, but when it fills in, there will be no storage space for the floods. Will the Bureau dredge the sediment, or breach the dam?”Stokely urged the board to seek a full investigation into the need for an emergency spillway on Trinity Dam, saying “it sounds like they’ll be looking at it, but Congress would have to approve a feasibility study, then an environmental document and, if feasible, figure out a way to pay for it. Reality is CVP customers will be pulling out of contracts in 2024 and I don’t know if there’s anybody to pay for it.”From the audience, a resident downstream from Trinity Dam said he has friends    below Oroville Dam who received only one hour’s notice to evacuate.“It was terrifying and chaotic. There is no warning system or plan in place here for a catastrophic event. You assume people would know what to do if the dam breaks, but no one has been through that. What would the evacuation route be? It seems like there needs to be some sort of a plan,” he said, adding that Oroville also had monitoring and inspections, “and people were told everything was fine until it wasn’t.”  Board members said the Trinity County Sheriff’s Office of Emergency Services does have emergency action plans in place, but they are not very familiar with them and would follow-up with a review. Sup. Burton offered to draft a letter for future consideration seeking funding for a spillway evaluation.“If we have a dam failure, that’s a once in a lifetime event for any of us. If we have a fear of that, we need a siren or other warning device, too — something more than a plan,” he said.


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