Science in the Aftermath of the Arkansas Flood

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Marisa Lubeck: Welcome and thanks for tuning in to this episode of USGS CoreCast. I'm Marisa Lubeck. Early Friday morning of June 11th the Little Missouri River in Southwest Arkansas experienced a deadly flash flood with waters that rose over 20 feet in just five hours. In response, the USGS deployed a team of scientists to study the flow and height of the water. This information is crucial for forecasters and emergency responders to warn the public, improve planning efforts and minimize the impacts of future floods.

With me on the phone is USGS flood expert, Robert Holmes who is currently on-site in Arkansas. Welcome Bob.

In the aftermath of this tragic flood, the USGS has been working to document the status of the river. Exactly what information are you collecting and why is the scientific response important so soon after the incident?

Robert Holmes: Well basically, we need to understand these -- killer floods like this, this has killed a great number of people in the area and we're essentially trying to go in and we have a pretty good idea on rainfall from National Water Service and an x-ray and doppler but we need to look at the flow rate.

01:06

What was the response of the channels of the landscape to the rainfall that they had. So basically, we're going in here and determine what the peak discharge is forensically or after the fact. We collect high water marks and then we use that information with some physics principles to essentially estimate the peak discharge at the number of locations.

We basically need to understand that this phenomenon from a scientific standpoint, we have a large land mass in the United States and there are a number of areas in the United States like the Washita Mountains where this particular flood disaster occurred that has deep slopes with the ready source of moisture. In this case, the Gulf of Mexico is that source of moisture. So if that moisture comes up from the Gulf and travels and it gets lifted up over the mountains it can drop off tremendous amounts of rain in a short amount of time and this triggered a flood happened in the early morning hours so it is dark and a nice warm time to be camping and so there's a large number of people in the camp grounds along the long Missouri River and its tributaries.

02:04

Marisa Lubeck: What have been some of the key findings about the flood that you've documented so far?

Robert Holmes: Well, we had extremes in some of the camp sites. There was probably five to 10 feet of water, there are 15 feet of water depending on where they were at. It was also a very powerful flood. I'm seeing areas where the pavement is totally ripped apart and moved up. We're seeing large boulders moved from a geographic standpoint or looking at the changes in the river through time we're seeing gravels and gravels that have been moved and re-deposited, re-worked. It was a tremendously powerful flood in the way that we think of this flood being able to move the entire channels around and so we're seeing a lot of cobbles and things like that making new bars and new formations that changed the course of the channel in a few areas.

Marisa Lubeck: And you're on-site now?

Robert Holmes: I am and we're basically trying to stay out of the way of the authorities but -- then collect the information we need to try to do a proper understanding of this particular flood. So, yes, I'm here. We have various crews here serving and flagging high water marks part of the USGS operations out in the Arkansas Water Science Center and the Missouri Water Science Center along with myself.

03:09

Marisa Lubeck: How accurate is your data after the fact? Is it difficult to collect this information, for example, considering you weren't on-site at the exact time this would happen?

Robert Holmes: It's very difficult science coupled that with two tours in the mid 90s out here and the humidity is up around 90%. We're slugging through really deep underbrush in some places. We try to look for tell-tale signs of what the high water was. We look at wash lines on the side of hill slopes where we have a very good demarcation from where the leaf litter was and what has been washed away. We look for areas where there was slack water that was deposited. Recede lines on the back side of trees. It's almost like being a detective to try to figure out how high the water got and then we do surveys of the channels and we tie it all together with the physics principles and estimate the discharge value at the peak of the flood.

Marisa Lubeck: What USGS technologies are you using and the scientific response?

04:02

Robert Holmes: Basically, we use surveying equipments ranging from global positioning satellite instrumentation to weather station surveying equipment what uses an electronic distance metering system with the ability to track its position in a data logger. So we've got some fairly sophisticated surveying equipments that we're using to try to tie everything together.

Marisa Lubeck: How did this happen so quickly and what kind of insight can science you're developing provide for future floods?

Robert Holmes: We had 20 feet of rise in our gauge 10 miles downstream of the camp ground areas. We have 20 feet of rise in about five hours. Upstream here is about 10 miles, the channel slopes are even steeper and their valley walls are steep. It's a very rugged terrain and as that water comes off the hill slopes from the rain, it congregates very rapidly and the water has no place to go, it spread out over a flood plain like it would do in a lower area. So it goes up and that's how you get this varied rapid rises and responses. Some of these people that were caught totally unaware. They were in their RVs or they were in their camp tents and the water was up in their camp ground areas before they even knew it, again it's the middle of the night.

05:07

Marisa Lubeck: Can dangerous floods like these be predicted?

Robert Holmes: Well, we try to. The weather service issues flood warnings on these smaller tributaries. It's a very difficult science to tell exactly where it is. As you look and trying to predict thunderstorms, areas and locations, and depth and that kind of thing, it's very difficult. We do a very good job as a federal government predicting floods on locations like the Mississippi River in some of the bigger streams. They go up a little slower and it's a lot easier once understood. And so what you got is a very difficult mechanisms to try to predict. And so the weather service does a fairly decent job of getting those warnings out but then trying to get that information out to local people in time to get them evacuated is very difficult especially in the middle of the night.

Marisa Lubeck: What facts about flood such as that should -- the public be aware of in the future? What are the scientific lessons learned?

Robert Holmes: I think the long-term action is the combination of things. There is a place for increased instrumentation for streams monitoring and rapid flood warning system and the USGS does a lot of those around the country.

06:08

But there's also a public education aspects of the natural disasters that occur, probably the largest loss of lives is from flooding and from flash flooding. And so this is a natural phenomenon and anytime you get people close to the water's edge when they're camping or living or whatever, there is always a significant risk of flooding and the loss of property and lives in the ensuing aftermath.

Marisa Lubeck: Bob, thanks for speaking with us today.

Robert Holmes: Thank you.

Marisa Lubeck: This podcast is a product of the US Geological Survey, Department of the Interior. I'm Marisa Lubeck. Thanks for tuning in.

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