Not time to get complacent about flood risk
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June 18, 2011 |
Columbia-Snake river water supply forecasts, and
reservoirs, continue to rise as a result of
continued above-average precipitation and
enormous mountain snowpacks that continue their
slow meltdown.
A “mid-month” forecast released Thursday by the
National Weather Service’s Northwest River
Forecast Center estimates that a total of 135
million acre feet of water will flush down
through the system from April-August and through
the lower Columbia’s The Dalles Dam.
That would be 137 percent of the 1971-2000
average and the third largest volume on the
NWRFC’s 41-year record. The largest was 140.91
MAF in 1997 and the second largest runoff volume
was 139.68 MAF in 1974.
The new forecast is for a total of 81.4 MAF (127
percent of average) past the mid-Columbia’s
Grand Coulee Dam in central Washington and 37.3
MAF (155 percent) past the lower Snake’s Lower
Granite Dam in southeast Washington. The Snake
joins the Columbia near Kennewick, Wash.
According to data compiled by the NWRFC,
precipitation in the Columbia-Snake basin
upstream of The Dalles from June 1-13 was 170
percent of average.
The flows past Bonneville Dam, which is the
lowermost hydro project in the system, have been
at or near 500,000 cubic feet per minute since
May 27. It is a goal of hydro operators to
manage flow during spring runoff to no more than
500 kcfs in order to prevent extensive flooding
in the Portland-Vancouver area. The Columbia’s
elevation has been slightly above minimum flood
stage since late May but is forecast to drop
below that 16-foot level within the next few
days.
The chief of U.S. Army Corps of Engineers’
Reservoir Control Center, Steve Barton said
Wednesday that estimates are that about
one-third to one-half of the snowpack has
melted.
“The moral of the story is we still have quite a
bit of the snowpack to come off,” Barton told
the Technical Management Team, a group of
fishery and hydro officials that convenes to
discuss dam operations that might be implemented
to benefit migrating juvenile and adult salmon
and steelhead.
Unseasonably cool weather this spring has
resulted in a changed runoff timeframe.
“It slid the hydrograph over by about three
weeks,” Barton said.
The Federal Columbia River Power System’s water
storage is about 55 percent full, with projects
such as Grand Coulee and Libby dams having been
held lower than normal to preserve flood control
space in anticipation of the late spring-early
summer runoff. Barton said that flows at
Portland-Vancouver would have been eight feet
higher if not for the system flood control
management. Grand Coulee is on the Columbia in
central Washington and Libby in on the Kootenai
River, a Columbia tributary, is in northwest
Montana.
Despite the flood control, inflows to Libby’s
reservoir “have been pretty impressive” in
recent days and it is “filling at a fairly steep
clip,” Barton said. Like Libby, the reservoir
backed up by Dworshak Dam in west-central Idaho
is rising at a rate of about two feet per day.
At the end of the day Thursday Dworshak’s
reservoir was 26 feet below full pool.
The Corps’ Seattle District has flood teams out
in three basins in western Montana, northern
Idaho and central Washington, and the Corps'
Emergency Operations Center remains in 24-hour
operation as rivers remain above flood stage.
The Pend Oreille basin team is assisting Bonner
County with emergency work on a 300-foot section
of the River Road Levee in Cusick, Wash., and on
a segment at Dufort Road upstream of Albeni
Falls Dam in Idaho. The team completed work to
protect the Johnson Creek Levee, strengthened
the Lightning Creek levee and provided sandbags
and sandbag training there.
In the Clark Fork basin, the Corps has closed a
breach in a levee in Sanders County in Plains,
Mont. The County will continue to monitor and
work on the levee as the high water persists.
The Okanogan team is continuing to monitor sites
in the basin on the Okanogan, Similkameen, and
Methow Rivers in Washington. The flood team
finished a protective berm along Salmon Creek to
protect the town of Conconully and completed
repairs to a 100-foot section of the Okanogan
City levee in Okanogan, and a 1,800-foot segment
of the Twisp levee.
The Corps has distributed more than one million
sandbags to date in western Montana, central
Washington and northern Idaho, and provided
pumps to two local communities. In addition, the
Seattle District has distributed hundreds of
thousands of sandbags to southern Idaho and
eastern Montana.
The Corps continues to operate its reservoirs to
reduce flood risk. Outflows from Libby Dam are
around 25,000 cubic feet per second, and inflows
are about 54,000 cfs. The Libby Dam outflow rate
should help to keep Bonners Ferry below flood
stage.
This morning inflows into Lake Pend Oreille were
124,000 cubic feet per second, and Albeni Falls
Dam continues its free flow operation, releasing
more than 117,000 cfs. Flood stage at Newport is
100,000 cfs. The lake elevation is about 2,064.2
feet and rising. Flood stage on the lake is
2,063.5 feet.
Although upper Snake River flood waters may
appear to be receding over the past few days,
Corps hydrologists at the Walla Walla District
warn residents there is still plenty of snow up
in the mountains yet to melt.
“Now is not the time to become complacent about
flood risk -- it’s not over, yet,” said Steve
Hall, a hydrologist in the Walla Walla
District’s Reservoir Regulation and Hydrology
Section. “The snowpack is still really high in
the upper basin. There’s a lot more water volume
headed for the rivers in the next few weeks.”
Most high-elevation SNOTEL gauge stations are
showing at least 10 inches more water than
previously recorded maximums for this time of
year, according to the National Resource
Conservation Service snow survey data.
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