The Flood of 2009 on the Cedar River in Renton wasn’t as bad as originally thought.
That’s why the U.S. Geological Survey, the federal agency that tracks and records the flow of rivers in the state, says its initial online figures are “provisional.”
What happened in Renton is a perfect example of the caution. The gage that measures the river’s flow just downriver of the Bronson Way bridge was reporting inaccurate readings as the river rose on Jan. 8.
“It (the gage) was reading high,” said Bob Kimbrough, the hydrologist who manages the U.S. Geological Survey stream-gage program in Washington state.
Based on initial gage measurements, the Cedar’s flow peaked 11,309 cubic feet per second on Thursday, Jan. 8, the height of the flood. That would have placed the flow at about 700 cubic feet per second above the flow recorded in the devastating flood in November 1990.
The relative height of the river, another way to measure the river’s flow, also was off, too. The Jan. 8 peak initially was reported at 16.6 feet; flood stage is considered at 12 feet. The revised figure placed that height at 16.03 feet or the third-highest flow since records have been kept.
The peak reading on Jan. 8 surprised agency hydrologists.
“We immediately suspected that we had something going on at that site,” said Kimbrough. “We didn’t think we had that much water.”
The agency was able to reconstruct the measurements. Now, the peak flow is estimated at 9,090 cubic feet per second.
Kimbrough said it’s possible the gage’s censor was damaged in the flood, perhaps by a log. The logjam under the Williams Avenue bridge slowed the flow of the river, creating an artificially high pool of water at the gage’s censor.
The need for accurate readings drives home the need for frequent “field trips” to recalibrate the gages to ensure the readings are accurate, Kimbrough said.
Mother Nature routinely changes her rivers.
Such high flows can play havoc with a river’s bottom, filling it with silt or gouging it out. Such activity will change the river’s bottom, which will change the height of the flow.
“All rivers are dynamic,” said Kimbrough.
The damage from the January storm – estimated at about $10.2 million – was not as severe as what occurred in 1990, when much of the Renton Municipal Airport was under water. Since 1990, the City of Renton has dredged the lower Cedar and the Boeing Co. has made improvements on its property at the airport.
TO SEE THE REAL-TIME FLOW OF THE CEDAR RIVER, GO TO:
