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Dworshak Flow Changes Not So Easy With Coming Rain, Generators Out, And Rising Dissolved Gas
Posted on Friday, February 02, 2018 (PST)

More water is being released at Dworshak Dam on the North Fork Clearwater River to help maintain a low reservoir level as officials expect up to two inches of warm rain that could cause low elevation snow to melt in the river basin.

 

The rise in outflows from about 4,000 cubic feet per second to 5.8 kcfs at the dam was made Wednesday morning, Jan. 31, just as the reservoir’s elevation was rising and as a spike in the dam’s inflow was expected, increasing from 6 kcfs to 44 kcfs Sunday and Monday.

 

The U.S. Army Corps of Engineers, which operates the dam and had made the change, wants the reservoir level to be at a 1,523 foot elevation by the end of February and 1,520 feet by the end of March, annual measures the federal agency takes for flood control. The elevation Wednesday was 1,529.2 feet.

 

However, flow changes are made more difficult with the absence of two of the dam’s three generators. With just one of the smallest generators online (Unit 1), passing the 5.8 kcfs of water requires spilling 3.5 kcfs. That is creating a dissolved gas issue in the river and at the Dworshak National Fish Hatchery downstream. Although the second small generator is expected to be back on line Feb.8, somewhat alleviating the spill and TDG problem, the dam’s largest generator that had been scheduled to be back July 1 may be delayed.

 

With the rise in flow this week, total dissolved gas at the dam rose from about 99 percent to 114.8 percent at 10 am Wednesday, which is over the allowed limit of 110 percent set by the state of Idaho. That level of TDG has caused gas saturation levels at the hatchery to rise to 101.6 percent.

 

Elevated gas levels stirred up by water plunging into dam tailraces can negatively affect fish and other aquatic life. During the 2017 spring runoff, high flows caused TDG levels to rise to as much as 130 percent at the dam, with resulting levels at the hatchery rising to as high as 105 percent, even with new de-gassing equipment installed at the hatchery. To ensure their safety, hatchery operators released some juvenile fish early.

 

The Corps took Unit 2 offline Jan. 9 to be serviced. It will return Feb. 8, according to the Corps’ Steve Hall (Walla Walla District) who briefed federal dam operating agencies and federal and state salmon and steelhead managers at Wednesday’s interagency Technical Management Team meeting. Operating at full load, Unit 1 can pass just 2.4 kcfs. Once Units 1 and 2 are both online, maximum flow through the turbines will rise to 4.6 kcfs, which still requires some spill.

 

Hall said that to reach the lower elevation at the end of March to meet the Corps’ flood control curves requires Dworshak Dam flow to increase to 10.75 kcfs as soon as both of the small generators are back on line. That will result in even more spill and, he hopes, a TDG level no higher than 115 percent (http://pweb.crohms.org/tmt/agendas/2018/0131_31JAN2018_JAN-MAR_OPS_Outlook.pdf).

 

The dam’s largest generator, Unit 3, accounts for half of the dam’s generation and has been out of service for repair for more than a year. It won’t resume service until July at the earliest.

 

In fact, according to Hall, the likelihood that repairs to Unit 3 would be completed on time took a turn for the worse this week.

 

More than 300 of the 500 new bars that have been pressed into place with a hydraulic ram onto the generator’s rotor were installed backwards. “That’s a major problem,” Hall said. “They need to be removed and turned around.”

 

In that process, some may be destroyed, he continued, and there are only 15 spare bars, so the process has to be nearly perfect. “If they destroy more than the 15, they will need an additional four months” while the parts are on order.

 

Also see:

--CBB, January 19, 2018, “Outflow Increased At Dworshak To Make Way For More Rain, Higher Flows Into Reservoir,” http://www.cbbulletin.com/440102.aspx

 

-- CBB, January 12, 2018, “Uncertain Water Supply Forecast Results In Cautious Dworshak Operation Changes,” http://www.cbbulletin.com/440068.aspx

 

--CBB, August 11, 2017, “Temperatures To Cool In Lower Snake River, Riverboat Needs Higher Pool At Port Of Clarkston,” http://www.cbbulletin.com/439409.aspx

 

-- CBB, July 21, 2017, “Dworshak’s Largest Turbine Out Another Year; Poses Challenges For Salmon Management,” http://www.cbbulletin.com/439312.aspx

 

--CBB, March 17, 2017, “Precipitation, Snowmelt Has River Operators Working To Control Water Flow Through Mainstem Dams,” http://www.cbbulletin.com/438533.aspx

 

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