Basalt river work overtopped, mostly by design
June 15, 2015
BASALT — The Roaring Fork River, flowing at over 3,000 cubic feet second through Basalt since Wednesday, has overtopped the rock work installed last fall on the right bank of the river just below the Midland Avenue Bridge, and it has washed away some topsoil and portions of a recently installed gravel path.
From the bridge, the project looks like it might have failed, but the rocks put in place along the river-right bank to stabilize the main river channel were designed that way, not to hold back the river like a levee would.
"It's designed to overtop and dissipate the energy over a broad floodplain," said Robert Krehbiel, an engineer with Matrix Design Group in Denver, who designed the river-management aspect of the project. "We're trying to return the river to its natural function. We're not trying to channelize the Roaring Fork River."
However, Krehbiel acknowledged that the rock work was designed to be overtopped by higher flows on a uniform basis, and it was not supposed to be breached in several areas, as it has been.
On Monday, one such outflow from the river was knee deep and was moving fast enough to make crossing it tricky.
"We're dealing with a few areas that have localized sags," Krehbiel said. "We want the river to uniformly spill over the bank, and we don't want any concentrated flow.
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"It seems to coincide with the jetties, that they are backing up a foot of water or so, and that seems to be leading to more concentrated overtopping. Our design is to spread it uniformly," he said. "So now we have an opportunity to refine it now."
The rock work includes several small jetties that extend into the river. They are designed to create pillows of water that serve as a buffer between the bank and the corrosive, fast-moving water in the river's main channel.
Additionally, the rock work was designed so that when it was overtopped, the water would flow over a mix of vegetation and not onto open topsoil. But there hasn't been time to establish plants since the rocks were installed in September.
"Right now it is at its most vulnerable situation," Krehbiel said. "None of the vegetation has taken hold yet. We have rock and we've got soil. We don't have any root mass. That is going to change with all the plantings. That vegetation will withstand shallow bank overtopping."
The river has swept away some topsoil, dismantled portions of a gravel path installed along the rock work, and left piles of driftwood and construction debris on the rocks and the river bank. In all, it's a bit of a high-water mess.
The Roaring Fork River, as measured at a gage in Emma, jumped over 3,000 cfs on June 10, reached about 3,500 cfs, and then dropped below 3,000 cfs on Monday afternoon. Peak water was early Thursday morning.
The overtopping flows have also sent a steady flow of water into the bypass channel that runs past the new Rocky Mountain Institute building — now a blue color while under construction — and into Old Pond. From there, the water returns to the river.
But the overtopping has not caused any harm to the new RMI building or, apparently, to the rock structure itself, according to Larry Thompson, the engineer for the town of Basalt.
"While we're seeing some minor damage, if you will, from overtopping of the channel, in the bigger picture, I think we've got a lot less potential for damage than if we had a mobile home park there," Thompson said.
The site where the river is overtopping the new rock work used to feature a taller and more solid levee, which was built to protect the trailers in the former Pan and Fork mobile home park, half of which was built in the floodplain.
The site of the RMI building, and the balance of the open construction site between the building and Midland Avenue, was raised up to be a foot above the 100-year flood plain. No development is slated for the lower-lying flood plain, which is designed to absorb water that overtops the river's bank.
Thompson said while the loss of topsoil and the gravel path this week is regrettable, at least the river is giving the town valuable feedback on the recent work.
"There may be some minor adjustments that we decide we need to make after observing what's going on," Thompson said.
Editor's note: Aspen Journalism is collaborating with the Glenwood Springs Post Independent and The Aspen Times on coverage of rivers and water. More at http://www.aspenjournalism.org.
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