Update (6:30 a.m. Friday): The Keystone Dam output has been reduced to 200,000 cubic feet per second as of 6 a.m., with an inflow estimate of 182,000 cfs. The inflow has dropped about 13.5% since midnight, according to the Army Corps of Engineers' website.
When the U.S. Army Corps of Engineers informed local officials gathered in the Emergency Operations Center on Wednesday morning that it would begin reducing the flow of water into the Arkansas River from Keystone Dam, the room filled with applause.
It didn’t last long. With the good news came a word of caution that was shared with the public at a news conference later in the day: The release from the dam must be done slowly, or the levees could be damaged.
A day later, with the sun shining brightly and no rain in the forecast for days, the condition of the levees remains the foremost concern for local officials.
“With the levee that we have, which is primarily sand materials, with the river being up on it for as long as it’s been … the levee has been loaded,” said City Engineer Paul Zachary. “When those soils become wet, they become heavier, and the river is actually providing lateral support, or side support, to help contain that additional weight of the levee.”
Remove that support too quickly, Zachary said, and the wet, heavy soil that is no longer supported by the flow of the river could begin to slough, or slip, down the face of the levees into the water below.
“You want to allow the levee to drain as you lower the (river) water surface level elevation,” he said. “Because if you do a rapid draw-down, you will increase the chances of a slide of soils along the face of that levee, and that is not what we want.”
After several days of releasing 275,000 cubic feet of water per second from Keystone Dam, the Corps announced late Wednesday that the outflow had been reduced to 265,000 cfs at 4 p.m. and that thereafter, it would be reduced by 10,000 cfs every six hours, or 40,000 cfs a day.
As of 6 p.m. Thursday, the outflow was 220,000 cfs, according to the Corps’ website.
The elevation of the reservoir was at 755.42 feet, about a foot and a half below the top of the flood pool.
The elevation of the river, meanwhile, peaked at 23.41 feet Wednesday, more than 5 feet above flood level.
At 5:30 p.m. Thursday, it was at 20.98 feet, according to the National Weather Service.
Another potential problem keeping public-safety officials up at night is the extensive number of “boils” that have popped up on the land side of the levee system.
They are just what they sound like, small piles — or tiny volcanoes — of soil from which water is escaping. Where that water comes from is key to understanding why the boils are so dangerous.
“The water coming out of it is river water that has found its way underneath or through the levee, and it is coming up on the land side of the levee,” Zachary said.
As water passes under and through the levees in search of a place to escape on the land side of the levees, the pathways could widen, increasing the speed at which the water travels, and making it more likely that the current will pick up soil along the way.
That’s a big problem.
“When the water coming up through the boil is clear, it indicates it’s not moving any soil,” Zachary said. “If the soil becomes cloudy or the color of the river water, it is eroding the soils underneath the levees.”
That is why inspection and observation of the levees is so crucial and why National Guard troops along with Corps personnel are monitoring the levees 24 hours a day, Zachary said.
“The biggest part of a levee operation and loading, is your constant and vigilant inspection and watching the levee, and getting the observers out there and catching these issues in their infancy,” he said. “Just the presence of water on the land side of the levee, that is anticipated, especially with these kinds of soils and this type of levee.”
The Corps has been flooding areas where boils have appeared. The weight of the water effectively plugs them, and mitigates the underground water pressure on the land side of the levee.
Zachary said one point on the levee system has been found to be sloughing, and that the Corps and National Guard were out Thursday to perform maintenance and address any concerns.
City officials have not indicated when people who have evacuated their homes will be allowed to return, but Zachary said the Corps has reported that generally speaking, within the city of Tulsa, the river can hold approximately 160,000 cfs within its banks.