Nebraska Water Science Center Podcast Series

Episode 10: USGS Activities During Missouri River Flooding 2011, transcript: August 10, 2011

USGS Nebraska Water Science Center Associate Director Richard "Rick" Wilson describes the reasons for and extent of the flooding on the Missouri River in 2011, the risks associated with the flooding, and the work the USGS is doing on the river to characterize the amount of scour and other potential damage resulting from the flood waters.

Interviewer: Rachael Hoagland, Nebraska WSC

[Intro]

Rachael Hoagland: Welcome to Nebraskast where we talk to real USGS scientists about the important water-resources work they are doing all over Nebraska. My name is Rachael Hoagland. I'm here with Rick Wilson. He is the associate director for the USGS Nebraska Water Science Center. And we're here to talk a little bit about flooding that's been going on here this summer. The images that you're seeing are of our hydrologists and our hydrologic technicians working on the river. Rick, what can you tell me about the flood that's going on on the Missouri (River)?

Richard Wilson: This is a historic event in many ways, and the Missouri is a complicated system, and I'll describe that in a little bit, but the flood goes from the confluence at St. Louis up to Montana. That's over 2,000 miles. So it started primarily in snow melt. This has been going on primarily in June and July of 2011. Now it's actually extending into August. The Missouri River is a complicated river and the reason is there are a series of reservoirs on the mainstem channel, and that complicates the flow, that regulates the flow, and in a lot of ways it protects downstream. What happened was the reservoirs filled up. There was so much runoff and rainfall in the upper basin that filled the reservoirs and so that caused releases from the dams to go on for months on end to evacuate the extra water.

Because of the high quantity, high volume of water, releases have been high. It's flooded agricultural areas, along the way. Fortunately, in that stretch of the river, there are levees that have protected much of the infrastructure, but throughout the season we had breaks in levees. We had large areas inundated from flooding because of that. We talk about the duration and magnitude of this flood. In historic terms, we don't really know what this event is compared to prior to dam construction. We think that it's probably a 500-year event, which is a very extreme event. Releases out of Gavins Point Dam, the most downstream reservoir in the system, peaked at 160,000 cfs (cubic feet per second).

Typically in the winter releases from Gavins Point Dam can be about 20,000 cfs and in the summer months, it's approximately 40,000 cfs. So you can see we were four times the volume of water, and this was a long duration, over 10 weeks in length. A normal flood occurs quickly, the water rises, it floods the area. In this flood event, it's as I just said, it's over a 10-week period of time.

Rachael Hoagland: What can you tell us about the USGS work that's going on on the river in past weeks and coming into September?

Richard Wilson: Well we want to know the volume of water because 1) with that information we can estimate how high it is, and the stage, because that is how we predict flooding and how we respond to the flood event, and 2) the velocity of the water. Velocity is critical because it causes scour. Scour is a phenomenon that with high-velocity water, high-volume of water, it erodes critical structures, it erodes the bed of the river, but also it can undermine bridges, water-intake structures, levees, and cause very serious problems. We responded with a series of sonar device (surveys). One was called a multibeam echosounder that would actually allow us to determine scour holes in critical infrastructure, including pipelines and transmission towers.

Also, we used a tool called electromagnetic geophysics to do surveys of levees. Levees were a major concern because they protected key infrastructure. We used this tool to determine the geologic properties, the soil properties, and the underlying geology of levees that protected a coal power plant. We were able to use this tool to survey almost 3 ½ miles of levee to determine areas of potential seepage, where seepage could undermine the levee, or flow through the structure itself.

Rachael Hoagland: As this water is moving, it's also changing the bed of the river, is it not? Are you collecting data that looks at how the bed of the river is changing as the water is moving through over such a long period of time?

Richard Wilson: The Missouri River is a unique river. It's a sand-bed river. This is not like a mountain stream which is a hard rock surface which is stable. The Missouri changes, and the more water, the more scour we observe or bed degredation. We used the sonar devices to make measurements to determine how the bed evolution has occurred. So with that device, that will allow us to determine change. Also we used a technology where we would take samples not only of the suspended sediment in the water itself - the amount of sediment in the water column -- but also we were measuring bed load and that's the amount of material that movies along the bottom of the river bed. In these extreme events, in these high-flow events, that typically is a lot of material that is being moved by the river.

Rachael Hoagland: So flooding is probably going to continue for a handful more weeks so there's still more data to be collected, correct?

Richard Wilson: Correct. I think we've dodged the bullet. As of Aug. 1, flow rates are now being reduced out of Gavins Point Dam. By the end of September, they will be down to normal levels, 30,000 cfs. So as the flow declines, additional surveys will be needed, additional monitoring will be needed, but the major concern and the major problems, it looks like they have passed.

Rachael Hoagland: Well, Rick, I'd like to thank you for taking the time to talk with me today.

Richard Wilson: Thank you.

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