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Idaho’s Dworshak Reservoir Held At Lower Elevation As Hedge Against High Winter Inflows
Posted on Friday, December 01, 2017 (PST)

Fisheries and dam managers agreed last week to maintain a lower than normal reservoir elevation at Dworshak Dam in order to avoid the high emergency outflows from the dam experienced last winter due to heavy snow and rain.

 

The U.S. Army Corps of Engineers, which operates the dam on the North Fork of the Clearwater River, agreed to target a reservoir elevation of 1,520 feet at the end of December and to limit outflows to the powerhouse maximum of 4,700 cubic feet per second with no spill.

 

Fisheries managers at the interagency Technical Management Team meeting Wednesday, November 22, asked for the change of operations protocol in order to avoid the potential of emergency spill and the total dissolved gas the spill creates if precipitation in the basin increases.

 

Outflow from Dworshak Dam is typically kept at low levels in the fall and winter as the reservoir is refilling, according to the Draft 2018 Water Management Plan (http://pweb.crohms.org/tmt/documents/wmp/2018/Nov_14_Draft/20171114_WMP_Draft_NWD.pdf).

 

“After summer fish operations are completed (including the NPT Agreement operations in September), flows from Dworshak will be limited to minimum discharge (one small turbine operating above the cavitation zone and within 110 percent TDG, approximately 1600 cfs, unless higher flows are required for flood risk management, emergencies, or other project uses,” the WMP says. “The purpose of these actions is to manage the filling of Dworshak reservoir while operating the project for multiple uses. Flows from Dworshak also may be maintained above minimum flow if Corps analysis determines there is flexibility to release a volume of water above minimum flow and still maintain a high reliability of meeting spring refill objectives.”

 

With the dam’s largest generating unit still out of service, the Corps is limited to passing just 4.7 kcfs through two smaller turbines. Additional outflow is possible, but only by spilling water and that creates a higher level of total dissolved gas in the Clearwater River downstream than is allowed by state of Idaho and Environmental Protection Agency clean water limits, which are set at 110 percent. The higher TDG also impacts hatcheries downstream.

 

Unit-3, which accounts for about half of the dam’s power output, has been out of service since September 2016 for a complete overhaul, but the Corps’ contractor immediately got behind schedule when it spilled oil while disassembling the generator. The Corps revised its expectation that the generator would be back in service from May 15 to July 15, 2017 after the oil leak and, and out to early August to include field tests and validation testing.

 

In July the Corps refined its estimate, saying that the turbine will be out of service for nearly another year, until July 1, 2018.

 

This past summer TMT found a balance between spill and the right amount of TDG (under 121 percent with a waiver). However, saturated gas levels this last winter exceeded water quality limits when higher than normal flows in the North Fork of the Clearwater River basin forced more spill and that forced hatcheries to release some juvenile steelhead and salmon early.

 

“We recognize that this has the slight potential to impact June refill,” fisheries managers said in System Operational Request 2017-1, November 21, 2017 (http://pweb.crohms.org/tmt/sor/2017/1122_SOR_2017_1.pdf). “However, with current forecasts, the probability of impacting June refill is extremely low.”

 

The SOR goes on to say that “Due to extended outage of Unit 3 and planned Unit 1 outage in January, we are concerned that reservoir volume may be such that spill in excess of 110 percent TDG may be necessary for flood risk management. In order to minimize the risk of this, we are recommending precautionary measures be taken in advance. Current predictions for La Niña water year increases the likelihood that 2018 will be above average.”

 

The Corps was releasing just 1.8 kcfs as of the TMT meeting, but ramped up to 2.4 kcfs Saturday, November 25, and will ramp the second generating unit up or down in order to hit the 1,520 foot elevation at the end of December, said Steve Hall of the Corps’ Walla Walla office.

 

“If inflows are really high,” he added, “we’ll ramp up to full load on two units and talk later.”

 

The reservoir’s elevation was near 1,525 feet, Monday, November 27 and outflow was 4.7 kcfs, the maximum for the two small units without spill.

 

Fisheries managers agreed that short-term fluctuations are inherently natural and need to be accommodated.

 

Also see:

 

--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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