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TMT Continues Dworshak Combined Generation/Spill; Water Supply In North Fork Clearwater Downgraded
Posted on Friday, February 10, 2017 (PST)

Forecasting a slightly lower water supply in the North Fork Clearwater River basin this week, the U.S. Army Corps of Engineers opted to continue a combined generation and spill operation at Dworshak Dam, maintaining total dissolved gas levels at just under 110 percent, but still within clean water limits.

 

The Corps made the decision Wednesday, February 8, after conferring with fisheries managers at the interagency Technical Management Team meeting. The operation will continue until next Wednesday’s TMT meeting when river operators and fisheries managers will again review weather and water supply, inflows and outflows, as well as potential operations options at the dam.

 

This is the third week running that TMT has overseen operations at Dworshak Dam to ensure TDG in the river below the dam doesn’t exceed 110 percent while also ensuring the reservoir is drawn down to meet April flood control concerns. Total flow through the dam is 8,100 feet per second: 4.8 kcfs through two turbines and 3.3 kcfs spill.

 

The North Fork Clearwater runoff forecast is still higher than normal at 2.55 to 2.6 million acre feet, although that is down from last week’s estimate of 2.6 to 2.7 MAF (average is 2.45 MAF). According to operational graphs provided by Steve Hall of the Corps’ Walla Walla District Office, that should temporarily ease the dam’s operations and not require higher spill and, consequently, TDG in April, as he had previously predicted.

 

One thing the current operation does do, however, is drop the Dworshak reservoir level below the elevation required by April 15 – 1,520 feet -- under the flood control rule curve, and that worries Erik Van Dyke of the Oregon Department of Fish and Wildlife. He said Oregon would prefer to maintain the reservoir level at its current elevation (about 1,524 feet), rather than drop it further and risk refilling beginning at the end of April. He asked the Corps to stop spill but continue passing water through the two small powerhouses.

 

Flood control operation and maintaining TDG below 110 percent would be much simpler if all generation were available at the dam. However, just two smaller generators are working as the dam’s largest generator is out of service for an overhaul. That makes it difficult to drain enough water from the reservoir to meet April flood control targets and still meet water quality standards for TDG

 

Unit-3, the dam’s largest generator, has been out of service since September for a complete overhaul, but the Corps’ contractor is behind schedule by two months. The Corps expects the generator to be back in service July 15 – it was initially to have been in service May 15 – with field tests and validation testing into early August.

 

The updated schedule to bring unit-3 back on line is at http://www.nwd-wc.usace.army.mil/tmt/agendas/2017/0201_Dworshak_Unit_3_Rehab_milestones_Jan30_2017_2051.pdf.

 

The outage limits the powerhouse capacity at the dam to about 45 percent of normal, but it also limits the amount of discharge from the dam to about 4.7 kcfs per second through the two small turbines. All other water must be spilled and that is causing TDG to hover in the 110 percent range, a state of Idaho clean water limit. If TDG rises above 110 percent, the Corps must apply for a state waver.

 

Also worrisome is the impact of high TDG at the Clearwater National Hatchery downstream of the dam. The hatchery uses North Fork Clearwater River water and dissolved gas exceeding the clean water limit could impact juvenile salmon in the hatchery.

 

Elevated gas levels stirred up by water plunging into dam tailraces can negatively affect fish and other aquatic life.

 

Again this week, Hall provided a range of operating options in four scenarios, two at 2.55 MAF and two at 2.6 MAF (http://www.nwd-wc.usace.army.mil/tmt/agendas/2017/0208_6FEB17_2.55_and_2.6_MAF_Low_TDG_and_EOM_Scenerios.pdf).

 

All scenarios show a sudden spike in inflow Saturday, February 11, going from about 4 kcfs to almost 12 kcfs overnight due to snow and rain storms expected late this week. That will cause a nearly imperceptible rise in reservoir elevation for a few days, partially alleviating Van Dyke’s fears of a reservoir dropping below a recovery level for refill.

 

The first scenario (2.55 MAF) is favored by the Corps at this time. Although inflow rises considerably in April, TDG remains below 110 percent.

 

The second scenario (2.55 MAF) shows just outflow through the turbines through March 1, predicting a needed rise in outflow in April causing TDG to rise to over 120 percent.

 

The third scenario (2.6 MAF) holds outflow to about 110 percent TDG until April 1, when flows and TDG rise to about 115 percent.

 

The fourth scenario (2.6 MAF) is similar to scenario two, dropping outflow to that of the two small generators without spill and with TDG rising to more than 120 percent April 1.

 

A water quality report for TDG is at http://www.nwd-wc.usace.army.mil/ftppub/water_quality/dwqi.txt.

 

Also see:

 

--CBB, February 3, 2017, “With Dworshak Generation Down, River Managers Balance Runoff, Flood Control Targets, Dissolved Gas,” http://www.cbbulletin.com/438280.aspx

 

--CBB, September 23, 2016, “Dworshak Oil Spill Into North Fork Clearwater Slows Turbine Overhaul, Cleanup Continues,” http://www.cbbulletin.com/437598.aspx

 

--CBB, May 13, 2016, “Needed Work, Low Flows At Dworshak Dam Pose Challenge As Water Needed For Juvenile Sockeye Migration,” http://www.cbbulletin.com/436712.aspx

 

--CBB, April 8, 2016, “Agencies Set For Spill Tests At Dworshak To Judge Impacts To Hatchery Fish During Generator Overhaul,” http://www.cbbulletin.com/436412.aspx

 

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