Basalt engineer’s report says flooding occurred in way that was intended
A report to the Basalt Town Council by town staff concludes that Roaring Fork River restoration work at the former Pan and Fork Mobile Home Park site “functioned as expected, and the flooding that did occur did so in the areas that it was expected.”
The report was prepared at the council’s request by Town Engineer Larry Thompson with an introduction by Town Manager Mike Scanlon. Councilman Mark Kittle said at the council’s June 23 meeting that he is “embarrassed” that the Roaring Fork River topped rock and dirt work the town deposited on the north bank. He asked for a report on the performance, and the other council members concurred.
Responding to the request for information, Thompson said the project was intended to allow flooding.
“One of the objectives of the project was to create a more natural environment for the river than what existed in this reach prior to construction,” Thompson wrote. “The banks of the main channel were stabilized and reinforced, but were purposefully left at a relatively low elevation to allow flows during peak runoff to overtop and spill out of the main channel into the adjoining flood fringe area.”
Allowing the river to overtop the restructured bank allows the water to spread wider and lowers the volume and velocity of flows in the main channel, Thompson wrote.
“High velocities contribute to more bank erosion and transport of cobble and boulders during flooding,” he wrote.
Thompson noted that a portion of the property owned by the town — the section closest to the Two Rivers Road and Midland Avenue intersection — as well as the site where the Rocky Mountain Institute is building its Innovation Center were elevated above the 100-year floodplain last year by moving in rock and dirt. Neither the town property nor the institute building were affected by runoff, he said.
“Grading on the south portion of the RMI site hasn’t been completed, and a small portion of the site where construction materials were being stored was affected by the river flows that overtopped the channel. However, no damage is believed to have been done,” Thompson wrote. “Once site grading has been completed, it is expected that all of the RMI site will be protected from flooding, although the stream channel immediately to the south will remain in the floodplain and can be expected to see flows again in the future comparable to this year’s flows.”
Thompson acknowledged that some topsoil and part of a soft-surface trail were lost and need to be replaced. Part of the problem was that no vegetation has been established yet to help protect the ground from erosion. That will improve with time, Thompson wrote.
The report was accompanied by a project description prior to the work and a map showing how the river would be allowed to flood a portion of the property.
The report will be official presented to the council at tonight’s meeting.
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