Could a huge mudslide ooze into Aspen?
On some spring day, not too far in the future, is it possible that a sudden cloudburst atop Aspen Mountain could send a wall of mud and water down the hill and into town?
Is the potential of chest-deep mud oozing all the way through Aspen to the Roaring Fork River great enough that we should contemplate spending $15 million or more to deal with it?
The answer to both questions is a cautious but provocative “yes,” according to a team of local officials and consultants who have been studying the matter for more than a year.
Two principals from the WRC Engineering, Inc. company in Denver were in town last week to give city officials some of the details of their drainage study of Aspen Mountain. The company has been working with City Engineer Nick Adeh and other officials.
The consulting firm, which has worked in various flood-hazard areas around Colorado and the West, has put together a set of drawings and illustrations that indicate three potential routes for a massive mudslide down the face of Aspen Mountain. These three routes follow Pioneer, Vallejo and Spar gulches, which converge at the base of the mountain.
According to the “models” worked up by WRC, a sudden downpour of rain on top of leftover snow and saturated ground in the springtime could trigger a massive movement of mud streaming down the gulches and into town.
City Council member Jim Markalunas, who was at the meeting last Monday, noted that the Little Nell slope itself is actually a deposit from just such a massive slide sometime in the distant past.
Although such things are hard to predict, Alan Leak and Bruce Curtis of WRC indicated that a sustained, heavy rainfall of a half an hour or more on top of Aspen Mountain could produce such an event if the ground was already saturated by previous storms and the annual melting of the winter’s snow cover.
This, the two experts said, would amount to a “100-year flood,” which is predicted to periodically occur in areas throughout the region.
City officials recalled a sudden storm late last summer that sent water cascading down the mountainside and into buildings and basements on the east side of town, but Leak said the type of slide being contemplated in the drainage study would be very different.
“That was mostly water,” he said. “What we’re talking about would be made up of much greater amounts of debris – boulders, mud, rocks.”
That kind of massed, sodden earth can move at different speeds, the consultants said, ranging from a few yards per hour to 15 mph, depending on the makeup of the slide.
On one transparency projected at a screen for the assembled officials, Leak described a flow of mud that would reach all the way to the Roaring Fork River and could be as deep as or deeper than the mudslide that buried the Aspen Music Festival School campus in 1996.
At that time, a mass of sodden earth about 50 feet above the school was set in motion by a blocked drainage system. It ultimately sent down a wall of muck about 4 feet deep that covered the school’s parking area, buried several vehicles and muddied Castle Creek.
The team from WRC is still working on its study, which is costing between $80,000 and $90,000 and is being paid for out of a $250,000 drainage study fund set up by Savanah Limited Partnership. Savanah is in the midst of a development review process for the Top of Mill project at the base of Aspen Mountain, which could very well lie in the path of a mudslide on the mountain.
Leak said his firm would continue to refine their data about the drainage situation on Aspen Mountain, and will present a draft of their findings to city officials in December.
The plan will include a series of alternative action plans incorporating such features as rock- or concrete-lined channels that could be built on the mountain to direct water and mud into a new storm sewer system that would end at catch basins near the river. Another alternative calls for construction of “cutoff walls” sunk into the ground at various elevations, to actually stop the movement of the earth.
Officials were wary of saying how much it might cost to build diversion or other structures that might be able to deal with the predicted flood, since the cost depends on whether the goal is thwarting a 10-year event or a 100-year event.
But Adeh said he believes it could cost in the neighborhood of $15 million or more to effectively ward off a disaster.
Leak said that under the contract with the city, the study is intended only to “identify the hazard” and suggest several potential methods for channeling, blocking or slowing the mudflow. Any actual plan to deal with the potential debris flow, Leak said, would entail an entirely new contract with the city.
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