CHARACTERIZATION AND QUANTIFICATION OF ROCKFALL, AVALANCHES AND DEBRIS FLOWS FOR TELLURIDE'S PANDORA WATER PROJECT

The Town of Telluride's Water Project plans to take water rights from the upper San Miguel River basin and transport the water from a hydropower plant at Bridal Veil Falls, down a talus and colluvial slope, crossing three perennial and numerous ephemeral drainages, and terminating at a storage facility six miles down valley after treatment mid-way. This is the famous “box canyon” of Telluride with magnificent, vertical canyon walls that shed avalanches, debris flows and rockfalls of mega proportions. Adjacent peaks rise to 12,500 feet and the valley floor is at 8,800 feet. This extreme topography in a small area, thanks to brecciated volcanics overlying sedimentary beds oversteepened by glaciation, is the perfect recipe for geologic hazards on a grand scale. Rockfall can be the size of houses, avalanche blasts have concussions that decimate structures, and debris flows can bury the mouth of canyons by tens of feet with large rocky debris.

Constructing, operating and maintaining the town's water supply and water treatment plant in such conditions requires an accurate assessment of the magnitude and frequency of occurrence of these geologic hazards for proper design and protection of the project. This area has a rich mining history, with the Idarado Mining Company's Pandora Mill and other structures still located along the pipeline route. Due to miner's resourcefulness and tenacity, they have positioned and protected structures from recurring geologic hazards. They have also removed rockfall, debris flows, and avalanche debris whenever it is in their way. Therefore, a traditional field and aerial study of current conditions is not representative of the events that have occurred in the past 120 years. Since some of the evidence has been removed, our analysis required interviews with residents and miners and review of historic photographs and records to supplement our field observations, aerial photo analysis, and review of geologic maps. Findings and general recommendations for design and protection of the project are included.