[env-trinity] Closer to glory; Ins and outs of Trinity Dam differ from Oroville’s

[Tom Stokely]

Closer to glory

  
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Closer to glory
 By AMY GITTELSOHN The Trinity Journal While reservoirs around the North State have been releasing huge volumes of water to make room for storm inflow,...  |   |

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Closer to glory

Ins and outs of Trinity Dam differ from Oroville’s
   
   - By AMY GITTELSOHN The Trinity Journal
    
   - 38 min ago
    
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While reservoirs around the North State have been releasing huge volumes of water to make room for storm inflow, Trinity Dam in contrast was the scene of a complete non-spectacle during a recent visit.The federal Bureau of Reclamation is evaluating any need for potentially increasing flows and will put out an announcement if and when any increases are scheduled, said Don Bader, Reclamation’s Northern California Area manager.At midday Friday there was no water at all coming through any of Trinity Dam’s exits. That would be changing around 5 p.m. when water would be sent through the Trinity PowerPlant to meet the increased demand for electricity, said Robert Stewart, hydraulic engineer with the Trinity River Restoration Program.After years of drought the Trinity reservoir started the water year extremely low. With bountiful rain and snowfall it has come up considerably, to the delight of boaters and tourism businesses in the area.Not only did Trinity Lake have low carryover storage, but due to its placement in the watershed it tends to fill more slowly than other reservoirs such as Shasta reservoir, where high releases have caused flooding, and state-run Lake Oroville where damage to two spillways forced large-scale evacuations.Although the Trinity reservoir is smaller than Shasta and Oroville, it is larger in proportion to the watershed that feeds it, Stewart noted.In an average water year half of what it would take to fill the reservoir flows in.“Most of the other reservoirs in California, the average inflow they get is larger than the total volume they can hold,” Stewart said.As of Monday, the Trinity reservoir held 1,916,611 acre-feet of water and was 78 percent full. That’s fuller than average for this time of year – 106 percent of average to be exact. Last year at this time the reservoir held less than half what it holds now.Also on Monday the lake was at 2,335 feet elevation, 35 feet below the glory hole spillway.The agency will be looking at information on snowpack and incoming storms to determine if additional releases are needed, Bader said.During the winter the lake is kept down to certain maximum elevations, although the goal is to let it fill if possible in April and May, Bader said.There are several ways in which water is released through the 538-foot Trinity Dam, which was completed in 1962.It can flow into an intake that shepherds the water through the dam and into the power plant to generate electricity before entering Lewiston Lake. Or, water can be split off to bypass the generators and enter Lewiston via two hollow-jet valves that dissipate the force of the water to prevent erosion.If the lake rises to the lip of the glory hole at 2,370 feet elevation, it will automatically flow through a pipe in the dam and out the above-ground concrete spillway, ending with another energy dissipating structure known as a flip bucket that forms a “rooster tail” of water as it enters Lewiston Lake. Trinity Lake can also be drawn down if needed with an auxiliary valve located deep in the reservoir. This water is let out via the same spillway connected to the glory hole. When the lake is low and water from the other access points is too warm for fish, this bottom auxiliary valve has at times been used to release cold water down the spillway.With water going through the glory hole and auxiliary valve, the spillway can release upwards of 25,000 cubic feet per second from the Trinity reservoir on top of what is released through the power plant and hollow jets, Bader said.Lewiston Dam has the capacity to release whatever is coming through the Trinity Dam, he said.Bader said the CVP reservoir structures are checked annually by Reclamation staff, and by outside inspectors every three years. He said he’s confident the spillways and outlets at Trinity are functioning well.However, inflow to the reservoir is capable of outstripping even these very high releases, and the earthen Trinity Dam is not designed to be overtopped. Also, even in an extremely wet year the peak spring release from Lewiston Dam for fish is 11,000 cubic feet per second to avoid damage to roads and other structures.Holding the reservoir down using safety of dams criteria, Bader said, “That’s really your safeguard.”“You’re managing that to discharge water ahead of the major storm event,” Bader said. “You’re always trying to avoid the glory hole.”   
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