Debris Flow Hazards
Debris flows and related sediment flows are fast-moving, flow-type landslides composed of a slurry of rock, mud, organic matter, and water that move down drainage-basin channels onto alluvial fans. Debris flows generally initiate on steep slopes or in channels by the addition of water from intense rainfall or rapid snowmelt and often occur after wildland fires.
Flows typically incorporate additional sediment and vegetation as they travel down-channel. When flows reach an alluvial fan and lose channel confinement, they spread laterally and deposit the entrained sediment. In addition to being debris-flow-deposition sites, alluvial fans are also favored sites for urban development; therefore, a debris-flow-hazard investigation is necessary when developing on alluvial fans. The hazard investigation may indicate that risk reduction is necessary for sustainable development on the alluvial fan.
Large-volume debris flows are low-frequency events, and the interval between large flows is typically deceptively tranquil. The debris-flow hazard on alluvial fans can be difficult to recognize, particularly on alluvial fans that are subject to high-magnitude, low-frequency events. Debris flows pose a hazard very different from other types of landslides and floods due to their rapid movement and destructive power. Debris flows can occur with little warning. Fifteen people have been killed by debris flows in Utah. Thirteen of the victims died in two different night events when fast-moving debris flows allowed little chance of escape. In addition to threatening lives, debris flows can damage buildings and infrastructure by sediment burial, erosion, direct impact, and associated water flooding. The 1983 Rudd Canyon debris flow in Farmington deposited approximately 90,000 cubic yards of sediment on the alluvial fan, damaged 35 houses, and caused an estimated $3 million in property damage.