Debris-flow hazard analysis and mitigation-an example from Glenwood Springs, Colorado. 45 pages. 8 figures. Digital PDF download. IS-08D
Debris flow is the most hazardous process affecting development in the three drainage basins discussed in this report. Large volumes of debris lie in relatively unstable positions in each of these basins. Therefore, the debris-flow process will continue to be a persistent hazard in the future .
The average return period of debris-flow events of approximately the magnitude of those that occurred in July 1977, is 50 years; thus a 2-percent chance exists that they will occur in any particular year. When they do occur, debris flows in these basins can be expected to be approximately 5 ft (1.5 m) deep, to transport large boulders on their upper surfaces, to flow at velocities of 10 to 15 ft/sec (3.0 to 4.5 m/sec) on the upper 600 ft (180 m) of the debris fan, and to produce impact pressures on exposed structures of 400 to 900 lbs/ft2 (19 kPa to 43 kPa).
Because of the physical characteristics and high probability of debris flows in this area, it is strongly recommended that specially designed structures be used to protect property. Two types of structures are recommended — reinforced lower building walls, and structural catching fences on the upper debris fans, above building locations. A preliminary economic analysis suggests that the annual amortized cost of these structures would be substantially less than the present annual cost of the debris-flows to the City of Glenwood Springs.