One day during Oklahoma’s ongoing floods stands out as the most stressful moment for the man who has been the area’s public face for the U.S. Army Corps of Engineers.
David Williams, chief of the Tulsa District’s Hydrology and Hydraulics Engineering Section, said that day also offers one example of why he believes the Corps’ operational plan worked as it should and that the weeks-long event has been “managed about as well as it could be managed.”
“It was that Tuesday,” Williams said, “The short-term weather forecasts were not promising.”
May 27, the day before “that Tuesday,” the water elevation in Keystone Reservoir tickled the upper capacity of the dam at 757 feet above sea level.
About 2.7 times the combined flow of Niagara Falls rushed through the dam, a rate of roughly 275,000 cubic feet per second.
But the Arkansas and Cimarron rivers raged upstream and pushed an even higher combined flow into the lake. The level was rising, and the reservoir was almost out of space.
With heavy rainstorms forecast and 2-to-3 inch accumulations expected that Tuesday, every armchair hydrologist from Ralston to Vian expected Williams to announce a historic release on par or higher than the record 307,000 cfs that covered Tulsa in 1986. Niagara Falls times three, maybe more.
But Williams stuck to the Army Corps playbook. He said the release would remain at 275,000.
“Just give me two more days,” Williams said informally after that May 27 news conference.
A week later Williams calmly emphasized it was a stressful 24 hours but not a white-knuckle ride.
“We were always in control,” he said. “But we had exhausted the capacity of the (reservoir), and if it rained upstream from the dam, the release would have to be adjusted to meet the inflow.”
The operational plan has the Corps making decisions “based on what is on the ground,” Williams said.
“That’s not been a real popular phrase recently,” he said, “but this (example) illustrates exactly why we manage that way.”
The rain did not fall upstream from the dam that Tuesday. Instead, those 2 to 3 inches hit areas downstream. South Tulsa, Jenks and Bixby saw severe flash floods.
“Hypothetically, if in our eagerness to pre-release based on that weather forecast we had gone to 300,000 (cfs) before it rained, how much worse would that have been downstream?” he said.
Release through the reservoir’s gates never surpassed the 275,000 cfs setting, though gauge readings fluctuated higher and lower, and one hourly reading, at 3 p.m. May 29, shows a peak of 277,252 cfs.
A few hours later inflow to the lake decreased and the Corps started dialing back on Keystone’s releases.
Williams got his two days, and Tulsa dodged a big, wet bullet.
“Most people don’t fully appreciate why you don’t draw down the lake, even with what seems like perfect forecast information,” he said. “You don’t realize you’re in an historic flood until you’re into it, and you don’t realize it’s a drought until you’re into a drought. What if you draw it down and six months from now it still hasn’t rained? Those are always the challenges you face, and that’s why we have an operational plan.”
Plans do evolve, however. An “after-action report” will examine the chronology of this event, the steps taken and what went well and what didn’t.
“We’ll identify areas for improvement,” he said. “One of the outcomes of the 1986 flood was to make a big investment in upstream gauging. That improved our ability to forecast inflow, and that really paid off during this flood.”
Comments on social media and in public meetings have laid blame on the Corps for allowing Keystone Lake to remain elevated in the weeks before the big rains despite long-range forecasts and May being a notorious month for floods.
The Corps simply did not leave enough capacity to handle the extra water, and if they had drawn down the lake ahead of time, trouble could have been avoided, the theory goes.
Williams explained some math, geometry and legal hurdles that make that idea moot, however.
The reservoir is much narrower at lower elevations, so a few feet of water elevation at that point make little difference compared to levels reached during a huge flood, he said.
Besides, the Corps is not authorized to draw the lake down below its “normal” level, which is 723 feet above sea level.
“This business about a draw-down,” he said. “There is just not enough additional volume there.”
Volume of the reservoir is measured in acre-feet (imagine a one-acre property covered with 1 foot of water).
The maximum flood-control volume for Keystone, the water above that “normal” 723 feet up to the operational top of the flood gates at 757 feet, is about 1.8 million acre-feet, Williams said.
“It’s a multi-use reservoir, but its primary purpose is flood control,” Williams said. “That’s why the vast majority of the volume is dedicated to flood control.”
The other purpose for the lake, as initially designed by the Corps and approved by Congress in 1949 under the Flood Control Act, is power generation and water supply. The lake, at elevations from 723 feet down to 706 feet, is the “conservation pool,” sometimes called the “power pool.” The volume of that pool is roughly 550,000 acre-feet, Williams said.
The Tulsa District reports that Congress assigned roughly 87 percent of that volume to Southwestern Power Administration (a federal agency). Power Supply Company purchased another 11.5 percent, and 13 percent is dedicated to water supply agreements. The remaining 1.5 percent could still be sold.
Water below the 706-foot level is labeled “inactive pool” and is essentially 200,000 acre-feet of water to collect sedimentation.
The conservation pool is used at the direction of those who own it, Williams said. The Corps tries to maintain normal level for both energy storage purposes and for its role in conservation, outdoor recreation and tourism, he said.
Congress would have to propose lowering the normal lake level seasonally or permanently, he said. But it would have little impact on flood operations either way, he said.
“Several feet at that level really wouldn’t make a difference in flood control with the kind of event we just experienced,” he said. “Consider the surcharge (the top 3 feet available for flood control) is 300,000 acre-feet and compare that to the entire conservation pool (the lower 17 feet), which is only 550,000 acre-feet.”
To drive home the point, Williams offered “a fun fact.”
In 30 days, from May 4 to June 4, roughly 5 million acre-feet of water passed through Keystone Dam. Imagine water filling an acre lot, about 209 feet by 209 feet, stretching up into the sky 950 miles. That's the distance from Tulsa to Winnipeg, Canada.
It’s also an amount roughly equal to twice the full capacity, top of the gates to silty bottom, of the Keystone Reservoir.
“The project did what it was supposed to do: to mitigate impacts,” Williams said. “There were downstream impacts, and there was a large release for an extended period, but imagine what it would have been like if the dams were not there.”
Clarification: A comparison illustrating 5 million acre-feet has been edited.