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- 000892 o I. INTRODUCTION 1.1 General The city of Ouray is located in southwestern Colorado in Ouray County. The location is shown on the map provided in Figure 1.1. It has a full-time population of approximately 570 citizens, with an estimated 2,500 residents during the summer. The town is situated in a narrow valley formed by the Uncompahgre River and surrounded by the San Juan Mountains. As a result of the City.s location in an extremely scenic area with an interesting mining history, much of the City's economy is based on tourism. The elevation of the town is approximately 7,800 feet above sea level; however, the mountains imme- diately surrounding it rise to elevations in excess of 13,000 feet. Besides the Uncompahgre River, six named tributaries of the Uncompahgre flow through the city and into the Uncompahgre, including Canyon Creek, Oak Creek, Bridalveil Creek, Skyrocket Creek, Cascade Creek and Portland Creek. Of these creeks only Portland and Cascade flow through densely populated areas of Ouray for an appreciable distance. To convey the runoff of Portland and Cascade Creeks throug? Ouray, a concrete flume was constructed on each creek in 1909. Both flumes are approximately 2,300 feet in length and terminate at the Uncompahgre River. Figure 1.2 provides photographs of typical reaches of each flume. Problems associated with the runoff from Portland and Cascade Creeks are the major focus of this report. These watersheds have periodically loaded large volumes of water mixed with a high percentage of sediments, rocks and timber into the flumes, resulting in blockage and overtopping of the flumes. These flows are referred to as mud or debris flows.. A debris flow is a rapid downslope movement of fine to coarse grained material which has become fluidized by water entrainment. Flows are thought to originate as small landslides in an unstable catchment basin during intensive summer thun- derstorms. Debris flows are relatively short lived events and are either con- fined to the pre-existing channel or take erratic paths across debris fans. Damages have occurred from the floods leaving the flume and spreading out through streets, businesses and homes. The majority of property damage is nonstructural and consists of removing mud and debris from houses, yards, streets, businesses, and the flume itself. However, structural damages have occurred, with houses being torn off their foundations. Most of the struc- tural damage has been confined to smaller structures such as fences and sheds. '.