City worried about the big one
The city of Aspen has invited Pitkin County to join in its preparations for mudslides and flooding from Aspen Mountain.
City Engineer Nick Adeh, in a detailed presentation Tuesday, told the county commissioners the risk of mudslides warrants big expenditures for preventive actions. Two alternative solutions have been evaluated, he said, and neither is cheap.
The first option – a drainage system of ditches and pipe to “dewater” the mountain – would cost an estimated $10 million to $12 million, he said.
The second solution, a series of low retaining walls across the mountain slope, would help by blocking the movement of soil. But Adeh said these could not be expected to completely prevent sloughing, and such a project could cost up to $27 million.
The Aspen Skiing Co. could be asked to share some of the expense if the Skico wants to pursue its plans for more snowmaking on Aspen Mountain, noted Lance Clarke, deputy director of planning.
The threat of mudflows and flooding prompted the city to study the potential for damage from these events. The study located the likely routes of slides and the boundaries of the drainage basin that would catch the brunt of flooding and debris from the mountain.
Almost three years after starting the study, the city is asking Pitkin County to take a role in the project. The city is ready to upgrade its underground storm drainage, Adeh said. But controlling flooding or mudflows which would originate on Aspen Mountain, outside the city limits, requires county cooperation.
Aspen has some underground storm drains, Adeh said, but some of these are not adequate to handle runoff from even a minor storm. The city hopes to upgrade the drainage system to be able to clear storm water from a “10-year event” – a flood of a size probable once in 10 years – at a cost of $4 million.
But in a major flood, say a 50-year or 100-year event, a large portion of the core of Aspen could end up under a foot of water, Adeh said. And, he warned, certain conditions could lead to soil saturation, causing huge quantities of soil and rocks to slough off the mountain and flow through Aspen. That level of saturation could happen if the summer rains come before the winter’s snow accumulations are gone, Adeh said.
According to the study, because large areas of unstable soil exist within the drainage basin – the part of the mountain which drains through the town – up to 10 feet of mud could descend on parts of the city. The Aspen Alps Condominiums is in one of those areas.
Even more catastrophic events are possible, Adeh warned. He told of an Australian ski village which was buried under 30 feet of mud, killing everyone in town except for one man.
Commissioner Patti Clapper asked Adeh if the network of mine tunnels that riddles Aspen Mountain could be employed to drain excess moisture from the face of the mountain. Adeh replied that too little is known about the tunnels to attempt to use them. He said directing water into the tunnels could conceivably help to saturate the soil more deeply and cause a collapse of the magnitude of the Australian slide.
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