SEATTLE, Wash. - The difficult and timely decisions that must be made to keep water from overtopping a dam during flood conditions were highlighted during last week's record flooding on the Wynoochee River, southwest of Olympia, here in Grays Harbor County.
"They're not easy choices to make," conceded Corps hydraulic engineer and reservoir regulator Marian Valentine, who came on duty the afternoon of March 18, at the peak of the flooding on the Wynoochee River. "But if we hadn't made them, we could have lost control of the water, and the dam would have filled to a point to where we would have had to release what was coming in. The inflow of water into the dam peaked at 14,100 cubic feet per second (cfs), and if that had been released, it would have been devastating to the lower valley."
By the time Valentine got to work at the Reservoir Control Center in Seattle at 2 p.m. that day, things were already looking ominous. It had been raining in Western Washington since Saturday, and on Monday, the Corps had received forecasts from the National Weather Service that flooding on the Wynoochee River was likely.
Meanwhile, Tacoma City Light, which operates the Wynoochee Dam for hydropower, was releasing 700 cfs of water. But once the Corps realized that rainfall was exceeding forecasts, engineers directed the utility company to increase the flow to 1,300 cfs.
Valentine explained that when flooding looks imminent, the Corps assumes control over dams in an area. At times like that, Corps engineers will often call for an increase in release rates in order to make more room in the dam.
"It gives us extra flood control space," Valentine explained, "and in cases like this every little bit helps."
And though the rain had begun in earnest Sunday evening - coming down at one-quarter to one-third inch an hour - by early Tuesday morning, it had increased to an average of one-half inch an hour. That was when the Corps made the decision to increase the flow to 2,000 cfs for two hours. But when the heavy rains continued to fall, doubts about whether that rate of flow should continue began to surface.
When Valentine came on duty in the early afternoon, she decided to decrease the flow to 250 cfs, the minimum allowable release.
"I wanted to keep the river flow down," she said. "At that time, the Wynoochee River at Black Creek (at Montesano) was rising higher and higher, and flowing at 13,000 cfs."
But even with the dam closed down to that low release rate, the river continued to rise. And the water level in the dam began to go up. "I was getting concerned," said Valentine, explaining that the inflow to the dam had been hanging under 10,000 cfs for five hours - as compared to normal levels of 1,000 cfs.
Downriver, the water was now flowing at 19,000 cfs - dangerous flooding conditions. And the dam was still filling up. In just one day, it had risen by 10 feet.
"We started thinking we might have to release some more water to keep the water in the dam from going up any higher," said Valentine. "It was rising 1.1 feet per hour."
But it wasn't an easy decision. And Corps engineers examined their options for the next several hours, trying all the while to get up-to-date accurate forecasts from the National Weather Service and the Corps' own meteorologist. It didn't help that the two sources had conflicting projections.
"I'll never forget his parting words," said Valentine about the Corps' prediction on March 18 just before he left. "He said we'd get 8.9 inches of rain that night."
Yet the National Weather Service was predicting only about half of that. "We knew the dam wouldn't fill too fast if the storm was over," said Valentine. "But we didn't know what the weather was going to do." It was time to go into emergency mode: Call in the supervisors, "the big guns," and start following a complex set of engineering computations designed to make sure a dam doesn't overtop.
At about 8 p.m., Valentine also contacted Ed McGowan at the county's Emergency Management Center and informed him that a major flood was occurring - that people in the the Wynoochee Valley who had never seen water in their homes would be seeing water in their homes for the first time. McGowan responded by immediately sending deputies and fire department volunteers up and down the river roads warning residents there to evacuate. But Valentine emphasized that the flood warning wasn't based on the flow from the dam, but rather on the lower river's natural flow due to the record rainfall. For example, it takes about 10 hours for water from the dam to reach Montesano, and by the time the dam releases had reached Montesano, the river had already seen its highest water levels there. "That we can't control," said Valentine about the river's natural flow below the dam during heavy rains or snow melt.
Meanwhile, the rain kept coming, and the water in the dam kept rising. At about 9 p.m., the Corps called Tacoma City Light and directed regulators there to increase the flow to 2,000 cfs.
The heavy rains continued, and the dam was kept filling. The situation was looking more ominous yet. At 11 p.m., the flow was increased to 2,800 cfs, and then at 1 a.m., to 3,900 cfs.
"We were reluctant to make those increases," said Valentine. "No one wants to do this. It's a difficult decision to make."
With rains still falling at one-half inch an hour, the flow was upped to an unprecedented 4,500 cfs from 3 to 4 a.m.
"We have never been forced to release that much water during a storm," said Valentine.
For the Wynoochee River, this was definitely a record-setting flood. Between 3 a.m., Tuesday morning, and 3 a.m., Wednesday morning, rainfall levels at the dam had reached 11.6 inches. For that same location, rainfall for that period of time would only have needed to be 10.6 inches to qualify as a 100-year flood.
Fortunately, by 7 a.m., March 19, the river level at Montesano was beginning to go down, and Corps engineers were beginning to sigh in relief. The dam hadn't filled too early, and flooding downriver had been controlled as best as possible.
"Without a dam, the flow in Montesano would have been as high as 37,000 cfs," said Valentine. "The highest regulated flow they saw this time was 25,600 cfs. That's because 12,000 cfs were held back by the dam. Without the dam, there would have been about three more feet of flooding." And those complex computations regulating flow rates over the dam during flood conditions proved their worth. According to the way they were designed, regulated river flow rates at Montesano would be 25,400 cfs during a 100-year flood.
"We had 25,600 cfs," said Valentine. "It was textbook perfect. We did as well as you can expect."
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