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~ ~ III'lllll'lllllllll ~~ ~~ ~~ ~~ ~~ ~~ TUTTLE APPLEGATE RINDAHL, INC. Consultants for Land and Resource Development February 16, 1989 Hr. Terry HcKee Army Corps of Engineers 9300 Colorado State Highway 75 Littleton, CO 80123-6901 RE: Hining Haul Road Crossing at Willow Creek Southwest of Castle Rock, Douglas County, Colorado Dear Hr. HcKee~ We are currently working with a mining operator interested in mining rhyolite resource south of Franktown between Castlewood Canyon Road and Ridge Road (please see enclosed Vicinity Hap). As you may be aware rhyolite is used as a light aggregate for production of lightweight concrete. There is currently no significant rhyolite production along the Colorado Front Range. The resource is currently brought in by rail from pits in Kansas. Our client has identified a very good quality source of the rhyolite for the Front Range market. The transport of this material can be accomplished with the least impact to county roads, traffic, local noise, visual effect, and air pollution by means of a mining haul road across Willow Creek. The construction of this mining road would commence sometime in 1989. The operation plans are currently being submitted to the Colorado Hined Land Reclamation Division and Douglas County for review. The operation is expected to have a life of between 40 to 50 years maximum. Willow Creek in the area of the project is a sandy ephemeral drainage, conveying relatively minor snow melt and runoff flows. At the point of the haul road crossing, Willow Creek flows from approximately 1,840 acres with an estimated 100-year storm event flow of 959 cfs (per CUHP analysis). The creek has no designated flood hazard or floodplain areas per the Douglas County FIRH Community Panel X080049-0310-B, September 3, 1980. The creek crossing would be made with culverts installed to convey at least a 5-year storm event prior to road overtopping as a low water crossing. From CUHP runoff analysis this flow would be 112 cfs, requiring the installation of double 36 inch CMP culverts, or triple 30 inch CHP culverts, or similar. £rom a Nannings flow analysis for an assumed typical creek cross section it is estimated the 100-year storm event would flood approximately a 30 to 50 foot width of the creek channel.