ABSTRACT
Debris flows and related sediment flows are fastmoving flow-type landslides composed of a slurry of rock, mud, organic matter, and water that move down drainage-basin channels onto alluvial fans (Fig. 1). Debris flows generally initiate on steep slopes or in channels by the addition of water from intense rainfall or rapid snowmelt. Flows typically incorporate additional sediment and vegetation as they travel downchannel. When flows reach an alluvial fan and lose channel confinement, they spread laterally and deposit the entrained sediment. In addition to being debrisflow-deposition sites, alluvial fans are also favored sites for urban development; therefore, a debris-flowhazard evaluation is necessary when developing on alluvial fans. A debris-flow-hazard evaluation requires an understanding of the debris-flow processes that govern sediment supply, sediment bulking, flow volume, flow frequency, and deposition. This paper is a shorter version of debris-flow hazard guidelines developed by the UGS to assist geologists in hazard evaluation, engineers in designing risk-reduction measures, and land-use planners and technical reviewers in reviewing debris-flow-hazard reports.
