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Corps Modeling Looks at Flood Control, Flow Targets by CBB Staff Columbia Basin Bulletin - February 20, 2004

Armed with a final February water supply forecast, the U.S. Army Corps of Engineers completed its first of the year model runs, known as QADJ, that helps the agency set flood control targets at storage dams and allows it to predict the chance of early year and spring flows in the Columbia and Snake rivers.

With water supply forecasts at near normal levels, the Corps is able to reset flood control targets at storage reservoirs and to operate reservoirs to that planning level. In addition, the final water supply forecast allows the Corps in its modeling process to predict the chance of meeting flow targets in the rivers on a month-by-month basis, although future monthly water supply forecasts could change how the Corps operates.

According to the Corps' Cindy Henriksen, the water supply forecast for Grand Coulee Dam is 95 percent of normal, which is down from the January final forecast. At Libby Dam in Montana, the forecast is for 97 percent normal water supply and the water supply for The Dalles is about 95 percent of normal, also down from January.

With that information, she gave the Technical Management Team (TMT) information this week that shows it is possible that the flow objective of 247,300 cubic feet per second at McNary Dam would be met April 15 through June 30, and that the objective of 200 kcfs would be met in July and August.

Of 59 years of flow information, the 200 kcfs target has never been met Aug. 16 through Aug. 30. The average flow during that period has been 161 kcfs. In addition, flows have averaged 200 kcfs only twice in 59 years, Aug. 1 to Aug. 15, when flows averaged 145 kcfs.

The 125 kcfs flow objective at Bonneville Dam, February through April, would easily be met. Statistically, the flow objective has been met nearly every year in the 59-year history used by the Corps.

Lower Granite, according to the modeling, will meet its flow objective of 100 kcfs in April and May, 84 kcfs in June and 52 kcfs in July and August. Again, statistics show no occurrences of meeting the August objective during that period over the 59-year history. The target is met in just 25 of 59 years in July, with flows during that period averaging 49 kcfs.

In addition, the Corps assumes in its model that refill will occur by June 30 at Libby, Hungry Horse, Grand Coulee and Dworshak dams. Three of the dams typically refilled much more often than not over the 59-year history. Libby showed the lowest occurrence of refilling in 29 of 59 years. That's due to special flows provided for bull trout and sturgeon. Grand Coulee refilled by June 30 in almost all 59 years.

When doing the study, the Corps assumed that Grand Coulee would help meet minimum flows of 70 kcfs February through March at Vernita Bar, downstream of Priest Rapids Dam. The dam is owned by Grant County Public Utility District. However, while meeting Vernita Bar flows, Grand Coulee would draft no lower than an elevation 1,259.2 feet in February and 1,261 feet in March. Then it would operate to help meet a McNary Dam flow of 247.3 kcfs through April 15, drafting no lower than 1,250.1 feet during that period.

The Corps is targeting an elevation at Dworshak Dam of 1,533.2 feet in March, 1,542.2 feet by April 15 and then to fill by June 30 at 1,590 feet. That assumes drafting the reservoir to 1,520 feet by Aug. 31, Henriksen said. The operation recommended by Idaho Department of Fish and Game and the Nez Perce Tribes calls for and elevation of 1,530 feet at the end of August and to continue drafting the reservoir to 1,520 feet by Sept. 15 in order to provide Dworshak's cool water benefits to the Snake River later into the migration year. With near normal water levels, that operation was possible in 2002 and 2003.

In addition, there has been a shift in flood control water from Dworshak, which now has a flood control target that is slightly higher than normal this spring, to Coulee, which will need to draft a little more. That shift, which increases the likelihood Dworshak Dam will refill, has the reservoir filling to 1,533.2 feet by the end of March, instead of 1,511 feet, Henriksen said. Coulee is scheduled to draft lower to 1,265.8 for the same period. The water supply forecast for Dworshak Dam is 98 percent of normal.

Higher water in the reservoir would be better, said Dave Statler of the Nez Perce Tribes. He said that in one year, the reservoir was 68 feet above flood control, but the result was still good.

The Corps increased flows from the dam this week to 8 kcfs in order to avoid spilling water later in case the spring freshet is greater than planned. That flow could be adjusted, as could all these numbers, with the next water supply forecast. "It's too soon to tell what the actual flow will be in May and June," Henriksen said.

Related Pages:
Workshop Looks at New Ways to Handle Flood Control, Columbia Basin Bulletin, 2/13/4

Related Sites:
TMT: www.nwd-wc.usace.army.mil/TMT/index.html
U.S. Army Corps of Engineers, Portland District: www.nwp.usace.army.mil
Bureau of Reclamation, Pacific Northwest Region: www.usbr.gov/pn
U of W Climate Impacts Group: www.hydro.washington.edu/Lettenmaier/Projects/fcst