How Did Gavins Point Get To 160,000 cfs?

The Omaha World-Herald had some excellent flood coverage in its Sunday edition, including a lengthy interview with Corps officials. It provides a play-by-play of how releases got to where they are today.

Here is one of the most interesting exchanges. Jody Farhat is the chief of the Water Management Division in Omaha.

Farhat: This was the real kicker. About the second weekend in May, we got two or three inches of rain in eastern Montana. We had a good rise on the Yellowstone River. Again, still manageable.

We were going to have to increase our releases from Garrison Dam and the dams downstream. But the weekend of May 20 to 22, much of the eastern half of Montana got 5 to 8 inches of rain.

Down here we can get 5 or 8 inches of rain. It happens once in a while over a small area. A county or a community gets a heavy rainstorm. But this was over the whole eastern half of Montana. A tremendous volume of water came in. Then it was followed by more rain in the following weeks into June.

As a result, we had a tremendous volume of water come into the reservoir system. We’re estimating that this rainfall event was between 4 million and 5 million acre-feet of runoff — and it used up all the storage that we had remaining into the reservoirs that we were planning to use for the snow.

So now we’re full.

Q. The winter snowpack runoff wouldn’t exceed 4 million to 5 million acre-feet?

Farhat: The volume would have, but we would have releases at the same time. We had enough storage to manage that runoff. It usually takes two and a half months to get the snow out.

Q. Tell us more about the May 22 rains. It seemed that you were increasing the Gavins Point Dam releases daily that week.

Farhat: It took a couple of days to really get a good handle on the volumes that we were going to be looking at and also the potential timing for that snowmelt, because we were already late into the season. We were concerned it would melt off rapidly, so we ran different scenarios of our snowmelt and, of course, with additional rain in the forecast as well.

We thought we would be right at 110,000 cubic feet per second to 120,000 cfs from Gavins Point. That following weekend, they forecast three or four inches more rain over this same saturated area. We knew at that point that those releases would have to go up.

During that period we had some of the highest storage gains in the reservoirs. Over 400,000 acre-feet a day. I worked here a long time before I ever saw a storage change of 100,000 acre-feet a day.

It was just an unbelievable amount of water coming into the reservoir.

McMahon: So it might have looked like we weren’t on our game, but we’re monitoring it closely, making adjustments on the best available information and not wanting to incite panic, because now we were edging up into historic releases.

It looked like what it looked like, but it was the best we could do under the circumstances.

Each one of those bumps up of releases were “Oh, my God,” because we hadn’t been there before. Oh, my God, 100,000. Oh, my God, 120,000. Oh, my God, 150,000.

Q. A lot of people wonder why you didn’t start releasing in April when you saw the snowpack rise.

Farhat: On the 28th of January we had all of the flood-control capacity available.

The fact that 2010 was a big runoff year didn’t reduce our ability to provide flood control this year because we had evacuated all of last year’s water. The full flood-control capacity was available.

Read the whole interview here. It is worth your time.

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