If all the talk about lake levels and cubic feet per second have you wondering what’s really going on at Keystone Lake and in the Arkansas River, here are some answers courtesy of U.S. Army Corps of Engineers reference materials and Tulsa District spokesman Preston Chasteen and Dave Williams, chief of the Tulsa District’s hydrology and hydraulics engineering section.
Q. Did the release rise to 215,000 cubic feet on Wednesday?
A. The U.S. Army Corps of Engineers announced Tuesday that Keystone Dam would begin releases of 207,000 cubic feet per second on Wednesday afternoon because the lake was nearing the top of its flood pool and inflows were heavy. At 7 p.m. the release was at about 207,000, but it was increasing slightly every hour.
Q. What is the deal with cubic feet per second?
A. Cubic feet per second, or cfs, is the measurement of flow passing through a dam or down a river. A flow of 100,000 cubic feet per second of water is roughly equivalent to water flowing across a football field 2 feet deep every second. The release from Keystone Dam that flooded Tulsa in 1986 was 307,000 cubic feet per second. The release rate at the end of the day Wednesday was 207,000 cfs.
Q. Why is knowing inflow and release important?
A. The U.S. Army Corps of Engineers tracks lake inflows and releases in cubic feet per second. The public can see the charts and graphs online. If more is flowing in than going out, that means the lake level will rise. A reservoir can only hold so much. Keystone Lake tops out at 757 feet (above sea level). The lake level on Wednesday was at 752 feet.
Q. Why is Keystone Dam releasing so much right now?
A. Because it is near the top of its flood pool, the releases need to be at or near the amount flowing in. Keystone Lake is fed by area runoff as well as flows from the Cimarron and Arkansas rivers and their tributaries. The combined inflow going into Keystone Lake on Wednesday averaged roughly 235,000 cfs and the lake level from midnight to 7 p.m. rose from 750.57 feet to 752.16.
Q. How high could the flow get?
A. Williams said the release is expected to go no higher than 215,000 cfs given what parameters were in play on Wednesday, but additional rainfall could make a difference and the engineers and hydrologists would be discussing possible changes early Thursday. The Corps is authorized to release as much as is needed to safely operate, Chasteen said.
Q. Why not just let the dam fill up?
A. Williams explained that Tainter gates regulate water flow through the dam. If the water is allowed to go over the gates, they can become difficult or unsafe to operate.
Q. Is a rush of water coming down on us?
A. Not really. It takes about an hour for all 18 Tainter gates across the dam to be adjusted up so the increase is gradual. They are adjusted up manually, one by one. The river immediately below the spillway rises pretty quickly, but the result is a steady rise, not a wave. Effects from an increase at the dam take about 4 to 6 hours to reach the Sand Springs area and 8 to 10 hours to reach the south side of Tulsa, Williams said.
Q. What drives the decision to increase flow?
A. Chasteen explained that the Corps of Engineers’ responsibility is to minimize flooding, which unfortunately doesn’t always mean they can prevent it. All of the dams in the Tulsa District, which encompasses Oklahoma and much of Kansas, are part of a multi-river system that flows into the Arkansas and Red rivers. Opening a dam upstream one day could mean flooding hundreds of miles downstream days later. One dam may have to be closed and a lake made to rise to allow another to empty. Computer modeling that was not available when Tulsa flooded in the 1980s now is relied upon heavily in making these decisions, Chasteen said.
“The hydrologists and engineers continue to evaluate by the hour,” he said. “We literally update the models on an hourly basis and check and double-check the lake projects. Also, people are physically monitoring the dam operations 24-7.”