Flood changing river channel

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The flooding, fast-moving Missouri River is changing in ways that won't be fully known for months.

“It's an untamed river,” said John LaRandeau of the U.S. Army Corps of Engineers in Omaha. “I'm worried about what's going to be there when it goes back to normal.”

Historic flooding along the Missouri's 384.6-mile course past eastern Nebraska — including parts of South Dakota, Iowa and Missouri — is washing across an estimated 450,000 acres of farmland, swamping rural dwellings, overflowing highways and threatening cities, railroads, power plants and airports.

The river in some places also is digging itself a deeper channel — a phenomenon called “scouring” that could reduce flooding at those locations because the stream is deeper, said LaRandeau, a civil engineer.

“Scour may be good at one location and problematic elsewhere,'' he said. “Omaha actually could get higher flood protection for a while if degradation of the riverbed occurs here.''

Rivers naturally erode their beds as they flow. They pick up sediment from the river bottom in one place and then deposit it someplace else. This scouring process is accelerated during flooding, when rivers flow faster, LaRandeau said.

The Grand Canyon is an extreme example of river scouring, LaRandeau said.

“The goals of a river are to take mountains and send them to the sea — and to get longer,” he said.

South Sioux City, Neb., officials said this week that the river channel there was 1 to 2 feet deeper than before the Corps increased releases of water from nearby Gavins Point Dam to accommodate historic snowmelt runoff and rainfall in the Rocky Mountains and northern plains.

Corps of Engineers crews eventually will conduct on-river surveys to determine where the Missouri scoured and deepened the bed, and where it deposited new sediment that made the bed shallower.

Stream gauges have indicated a possible change in the riverbed at some sites already, but the information hasn't been fully analyzed, LaRandeau said.

Scour holes can occur at bridge piers, the pillars supporting spans such as Omaha's Interstate 80, I-480, I-680 and South Omaha bridges and the Bob Kerrey Pedestrian Bridge.

But motorists have no reason for concern, said Mark Traynowicz, state bridge engineer for the Nebraska Department of Roads.

Missouri River bridges are elevated to accommodate barges passing below and designed and constructed to higher standards, he said. The structures, for example, must withstand a collision by a runaway barge or other vessel.

The bridges are inspected top to bottom every 24 months, and divers check below the water line every five years, Traynowicz said.

“Streambeds are always moving, but the Missouri has a stable channel,” he said.

The corps has spent millions of dollars in recent years building new chutes and wetlands along the Missouri in Nebraska and Iowa to improve fish and wildlife habitat that was lost when the federal government channelized the river south of Gavins Point Dam in the mid-20th century for flood control, barge navigation and other purposes.

LaRandeau said layers of rock were placed at the inlets of chutes to prevent scouring and keep the river from creating a new channel.

“We hope the controls will survive the flood,” he said.

The Missouri generally flows about 4 miles per hour. “About walking speed, if you walk briskly,” LaRandeau said.

Today, flood flows in the main channel generally are about 5 mph to 6 mph, perhaps faster in some locations, he said.

“You could keep up with a stick floating by in the middle of the river,” LaRandeau said.

Water on the edge of the river flows more slowly.

The pace of the flood flow can be observed from the Kerrey Pedestrian Bridge spanning the river at Omaha's riverfront, LaRandeau said.

“Look down at the water around the bridge pier. When water hits the pier the velocity accelerates,” he said. “The backwater flows on the Iowa side are a couple miles an hour slower.”

It normally takes 10 days for water released from Gavins Point to reach the mouth of the Missouri at St. Louis, LaRandeau said. It's about a seven-day journey right now.

The Missouri still hasn't inundated all the corners of Nebraska and Iowa that it will eventually flood after several weeks of the high releases from Gavins Point, LaRandeau said.

Once it does, he said, the river's flow will settle into a consistent speed during the next two months of planned high releases, and the natural process of scouring will continue.

Contact the writer:

402-444-1127, david.hendee@owh.com

* * * See how high the Missouri River reaches on the bronze statues at Lewis and Clark Landing in this World-Herald video.

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