State investigates pipes at Nebraska dams

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June 6, 2011 - 7:00pm

On a sunny, windy spring day, two dam safety workers waded into a drainage pool on the backside of Mud Creek dam just east of Filley in southeast Nebraska. Tim Gokie is an engineer who inspects dams.

"This is our remote operated camera, we've got an 18-inch corrugated metal conduit here," he said. "The dam was built in 1962, so this pipe is approaching 50 years old. We can drive the camera through the conduit, that will allow us to look for any holes, any signs of serious corrosion."


WHEN NEBRASKA'S 2,379 DAMS WERE BUILT

Decade

Percentage of total dams built

1880s less than 1%
1890s less than 1%
1910s less than 1%
1920s less than 1%
1930s 2%
1940s 3%
1950s 11%
1960s 27%
1970s 22%
1980s 12%
1990s 10%
2000s 12%

Nebraska's water supply is often recognized as one of the state's most vital natural resources, but sometimes with heavy rainfall, there can be too much of a good thing. That's when the system of dams in the state plays a role in keeping excessive water in control. With new video technology, the Nebraska Department of Natural Resources is trying to keep Nebraska's dams in good working order.

The special camera equipment was purchased with a grant from the Federal Emergency Management Agency, or FEMA. Equipped with LED lights and able to rotate 360 degrees, the camera is remotely operated to crawl through conduits. Gokie watched a live feed on a video monitor near the dam.

The age of this and many other dams in the state has the attention of state Dam Safety officials: about one-fourth of the state's dams were built in the 1960s. Many of them have corrugated metal drainage pipes, which typically have a serviceable life of 50 years.

Gokie said factors like soil condition and water levels play a part in the condition of the conduits.

"All those kind of factor in to how fast the corrosion process happens, so just because the dam is 50 years old doesn't mean the conduit is going to be corroded and failing," he said, adding, "but the potential is definitely there."

Flood control, agricultural use and recreation are primary functions of dams across Nebraska. The water held by the structures can be utilized for many purposes when dry spells come around. Nebraska has just under 2400 in the state; 96 percent are owned privately or by local governments, like Natural Resource Districts, counties and cities. Pat Diederich is chief of Dam Safety. He says typically staff spends three days a week inspecting the dams, and they commonly find problems centered around the drainage pipes.

"One of the main ways that a dam will fail is through internal erosion," Diederich said. "There'll be water pressure on the front side, so there's a big differential head, (and) if there's any discontinuities in the embankment, the water can flow through there and wash the dam away."

The 32 Mile Creek Dam near Kenesaw is an NRD dam. The site holds runoff from about 6,000 acres of land. Diederich said it's a "pretty typical" watershed dam.

"It's catching a lot of sediment that would otherwise go down stream end up in the Gulf of Mexico and contribute to the hypoxia down there, and also storing a lot of water so the flooding downstream isn't so bad. That's really it's main purpose: flood control."

A riser is part of the dam where the water flows through to a drainage pipe. One of those routine inspections found a problem with the concrete riser at the dam, located in south central Nebraska. Diederich speculated that the problem occurred in the winter.

"We think that the ice pressure the ice expands, it was up against the riser and it pushed it over, can't be really sure though. There's a draw-down pipe underneath that might have rusted out, water entered it, created a big vortex and undermined the structure ... we'll probably never know, but it won't matter 'cause we are going to fix it either way."

So heavy machinery was brought in to rip out the riser and replace it. If the broken riser remained in place, water could continue to erode dirt around the drainage pipe. That could lead to a much bigger hole in the dam and, Diederich said, a bigger problem.

"We're not really sure that we have good contact between the dirt and the pipe and so were thinking that we are going to do some excavating see what the situation is. But there might be a big pipe that's going through the dam that's really not a pipe it's just an open part through the dirt which could lead to a breach of the dam wipe out the highway downstream and the railroad.

"So we have the dam as a significant hazard dam. Probably nobody should get killed by it but it would cause pretty good damage downstream to the infrastructure."

Three days later, the 32 Mile Creek dam near Kenesaw is repaired. Back at the dam near Filley, Tim Gokie continued the pipe inspection with the remote camera.

"Were getting up there a little further now and it's looking a little better, it looks like the pipe was coated with an asphalt coating. A lot of that asphalt coating was missing in the last section, but we're up in the pipe a little further and a lot of that asphalt coating is still in the pipe."

Gokie said a good assessment of the pipe's condition helps determine what repair options to consider.

"You know, it depends really on the dam site. If the dam isn't very tall and it's a shallow pipe it may be just as economical to replace the pipe. A lot of our dams are 35 to 40 feet tall and in that case, the amount of earth that you have to move to get to the pipe really makes it a lot more expensive."

Two repair options often less expensive are proving to be effective. One is called slip lining. Instead of removing existing pipe a slightly smaller pipe is slid inside and the space between the old and new pipes is filled with grout. Another approach is called cast-in-place. That also repairs the situation without removing the existing pipe. Gokie said that decades ago, this area near Filley often flooded, and that's why the Mud Creek dam was built.

"But even dams that we consider to be low hazard potential like this one here today. It's 30- to 35-feet tall; if this dam were to fail, it would cause a lot of destruction downstream. There's no houses downstream, there's no highways downstream there's a lot of ag land it would cause a lot of destruction to farm ground and also any county roads in the flow path, it would do a lot of damage to any bridges on those county roads."

Sometimes when it rains, it pours. The result can be devastating, and that was seen about this time last year in the state. It's part of Dam Safety Engineers job to try to prevent that, Gokie said.

"Last year we had 7 dams that failed in Nebraska due to heavy rains and I believe it was last June went out and met with some of those people that lived downstream and they had pastures that were damaged and corn fields that were damaged and a lot of destruction that was caused by those dam failures. Our primary function is to protect downstream life and property and basically represent those people that live downstream."

Dam Safety approves plans for new dam construction in the state. It also reviews Emergency Action Plans for dams. The Plans directs officials on action to take and informs what areas could experience flooding should a dam fail.

"It provides a contact list, a calling tree of who gets contacted first," Gokie said. "Basically, National Weather Service would be contacted first so they could send out a flash flood warning and basically local emergency managers so they could get first responders on the site. Most importantly the plan includes an evacuation map, which has been mapped out prior to the event, which basically says these are the areas that could experience flooding, so that is provided to the first responders so they know what areas to be evacuated downstream in the case of a dam emergency."

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