Basalt ponders handling a roaring river
BASALT For the second time in two years, a detailed study has concluded Basalt faces potentially big problems when it comes to flooding.The first study by a company called McLaughlin Rincon suggested the town government would need to spend between $15 million and $20 million in mitigation. The consultants proposed aggressive alterations to the Roaring Fork River channel to control the threat.The cost of the mitigation made many town officials “gulp,” said town engineer Larry Thompson. The town sought a “peer review” of the first study by another consultant, much as an ailing person seeks a second opinion to a doctor’s prognosis.”We felt it was important to have another set of eyes to take a fresh look at this,” Thompson said.There was good news and bad news last week when the second opinion was delivered by Anderson Consulting Engineers, a firm from Fort Collins. The flood threat is very real and it will, indeed, be costly to mitigate, Greg Koch, vice president and principal engineer of the firm told the Basalt Town Council. However, it might not require the drastic action McLaughlin Rincon advised to try to tame the river, Koch said.A vast floodplainBasalt faces a drastic threat because the area has traditionally been a vast floodplain, explained Koch and Brian Hyde, a consultant on his team. The floodplain, or area affected by high water, spreads to between 1,300 and 1,700 feet in the Basalt area, Koch said. Immediately upstream and downstream from the floodplain is only about half that width.The slope of the Roaring Fork River decreases in Basalt, between the Upper Bypass Bridge on Highway 82 and just downvalley from the Midland Avenue Bridge. When snowmelt produces heavy runoff, the river wants to deposit cobble along the streambed as it snakes through town.Hyde likened Basalt to a compression in a downhill ski course. When a skier carrying a backpack full of rocks hits a compression, it creates problems with control because the skier’s velocity is so high. In the river’s case, raging flood waters hit Basalt and deposit cobble, flood the banks and create new routes throughout the area.Some of the more dramatic examples of flooding were in 1957, 1983, 1984 and 1995, Hyde said.Koch said the river’s power is evident by observing the size of rocks that have been deposited over the years alongside what is now the main channel.”While it looks placid normally, the river can roar when it gets a chance,” he said.Adding problemsThe river’s tendency to flood at Basalt has been exacerbated over the years by development, both studies said. Streamside development has tended to pinch the river rather than let it do what it wants at high water.The problem with that, Koch said, is the levees built to protect places like the Pan and Fork Mobile Home Park and the Roaring Fork Mobile Home Park won’t survive a major flood. Pinching the Roaring Fork has also made it harder to predict how it will react when a 100-year event strikes.It isn’t just buildings that have added to the problem. The Upper Bypass Bridge constructed in the 1980s by the Colorado Department of Transportation is at an odd angle with the river. Its piers tend to promote scouring and deposition of materials, its own studies indicate.The old Emma bridge by 7-Eleven is also a major pinch point that could add to flooding if trees and debris were caught there, Koch said. That bridge was taken out of commission for vehicular traffic in the 1990s.As development encroaches on the river corridor, even “minor adjustments” the river wants to make during times of high water are unacceptable, Koch said. In other words, what was once minor flooding now is a recipe for disaster. No cheap solutionsWhile Koch recommended different solutions than the McLaughlin study, the costs could be similar. The McLaughlin firm concentrated on getting into the riverbed with heavy equipment to build jetties and rock structures to stabilize the bank and control the flow. The idea was to move water through town fast and keep the cobble tumbling downstream so flooding wouldn’t occur.The work would be phased over several years and result in drastic alterations to a 1.5-mile stretch.Koch stressed he wasn’t trying to “beat up” the McLaughlin plan, but he had doubts about the approach. It’s hard to guarantee its success, he said. The materials carried by the river have to be deposited somewhere. His firm determined the McLaughlin plan might deposit materials at the Basalt Sanitation District’s wastewater treatment plant.He also questioned if the community would support an invasive solution or if Basalt could acquire a federal permit for such extensive work in the river.Koch recommended less alteration of the river channel, which would save millions of dollars. His alternative emphasized replacing the Emma bridge with a pedestrian span that doesn’t pinch the river so severely. If that step is taken, he advised also adding a span to the south side of the Midland Avenue bridge so it wouldn’t constrict flood waters.The major and costly piece of the mitigation plan that has to be determined is how to handle flood waters on the south side of Highway 82. One option would be improving a levee by the Upper Bypass Bridge to ensure water wouldn’t swamp the Southside subdivision, the study said. An alternative would be creating a causeway through the subdivision to direct floodwater and prevent it all from flowing to the north side of the highway.Ideally it would be best to allow a controlled flow through Southside, Koch said. Politically, that might not be acceptable, he noted.Koch’s firm strongly endorsed the first study’s conclusion that the two mobile home parks in the heart of Basalt must be relocated.”The two mobile home parks are in a tough spot,” Koch told the Town Council. “They really are in harm’s way.”The council members plan to hold meetings in coming months with the public and affected property owners to determine how to proceed.Scott Condon’s e-mail address is firstname.lastname@example.org.
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