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<br />SUMMARY OF REPORT <br /> <br />Debris flow is the most hazardous process affecting development in <br /> <br />the three drainage basins discussed in this report. Large volumes of debris <br /> <br />lie in relatively unstable positions in each of these basins. <br /> <br />Therefore, <br /> <br />the debris-flow process will continue to be a persistent hazard in the <br />future. <br /> <br />The average return period of debris...flow events of approximately the <br /> <br />magnitude of those that occurred in July 1977, is SO years; thus a 2-percent <br /> <br />chance exists that they will occur in any particular year. <br /> <br />When they do <br /> <br />occur, debris flows in these basins can be expected to be approximately <br /> <br />5 ft (1.5 m) deep, to tran!lport large boulders on their upper surfaces, <br /> <br />to flow at velocities of 10 to 1S ft/sec (3.0 to 'f.S m/sec) on the upper <br />600 ft (180 m) of the debris fan, and to produce impact pressures on exposed <br />structures of 400 to 900 lbs/ft2 (19 kPa to 43 kPa). <br /> <br />Because of the physical characteristics and high probability of debris <br /> <br />flows in this area, it is strongly recommended that specially designed <br /> <br />structures be used to protect property. Two types of structures are recommended-- <br /> <br />reinforced lower building walls, and structural catching fences on t1le <br /> <br />upper debris fans, above building locations. A preliminary economic analysis <br /> <br />suggests that the annual amortized cost of these structures would be substantially <br /> <br />less than the present annual cost of the debris-flows to the City of Glenwood <br /> <br />Springs. <br /> <br />- 1 - <br />