Basalt prepares for Roaring Fork flood
It will cost between $15 million and $20 million to protect the town of Basalt from the potentially ravaging effects of a major flood on the Roaring Fork River, a town consultant disclosed this week.
“This is not the first time a little town has had a really big problem,” said Rick McLaughlin of the engineering firm McLaughlin Rincon.
Basalt’s problem is this: Development of structures, bridges and the Highway 82 bypass have reduced the area’s ability to absorb flood waters. So a big event like a 100-year flood poses a “substantial” threat to lives and property.
McLaughlin’s preliminary work indicated that the Pan and Fork, and Roaring Fork mobile home parks, at the center of town, were at high risk from a major flood. More extensive computer modeling of floods shows that problem is even greater than previously thought, he said.
He said relocating the residents of the trailer parks ” home to about 100 families ” should be the town’s top priority.
McLaughlin said that steeper stretches of the Roaring Fork River upstream from Basalt dump cobblestones into the flatter stretch running through the town. “The cobble then ‘falls out’ within this reach, making the river unstable and unpredictable,” said his report to the town.
His firm conducted extensive research into how a flood would affect various neighborhoods in Basalt. Then it devised a plan to soften the blow.
Typically the plan would be to construct rock weirs, or subsurface dams, that slow the velocity of the river and, therefore, stabilize the riverbanks. But reducing the velocity through Basalt would further limit the river’s ability to transport cobble ” making the river’s actions even more unpredictable during high water.
Instead, McLaughlin wants to keep the river flowing fast in its confined channel. That requires extensive work creating riverbanks and a riverbed capable of withstanding high-velocity flows when lots of rocks are rolling.
“These high velocities are required because the encroachment of development reduces other options that enable the river to transport cobble,” the firm’s report said.
One of the most damaging developments to the river’s flow has been the Upper Bypass Bridge constructed by the Colorado Department of Transportation in the late 1980s. “We all think of the Upper Bypass Bridge as evil ” and it is,” said McLaughlin.
It was constructed in a way that creates an unnatural angle for the river flow, he explained. The bed-loading from cobble is extreme there; tree trunks and other debris carried by high water tend to get caught on the bridge’s piers, and ditch diversions upstream from the bridge were poorly located in terms of handling flood waters.
Along with relocating the mobile home parks, McLaughlin identified interim improvements to the bridge as another priority. Ultimately, replacing the bridge with a free-spanning structure would be best, but that would double the cost of the $15 million to $20 million flood-proofing project, he said.
The interim steps include altering the piers so they deflect debris, relocating the diversion structures and stabilizing the river bed and banks around the bridge.
Basalt’s Town Council didn’t challenge the overall strategy, but members cringed at the price.
“It’s a long-term, grandiose plan,” McLaughlin agreed. “It’s the right thing to do.”
The expense is worth it when the cost of the project is weighed against the threat to people and property from a large flood, he added.
Assistant Town Manager Betsy Suerth said the town should tackle the plan in phases as grants and other funds become available rather than think of it as a gigantic $20 million project. She maintained that the funds are available through grants via agencies like the U.S. Army Corps of Engineers. However, it will become available in bits and pieces rather than a lump sum, she said.
Before it can even embark on flood-proofing the town must come up with funds for final design of the project.
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