Initiation and Frequency of Debris Flows in Grand Canyon, Arizona.

摘要

Debris flows occur in 600 tributaries of the Colorado River in Grand Canyon, Arizona when intense precipitation causes slope failures in bedrock or colluvium. These slurries transport poorly sorted sediment, including very large boulders that form rapids at the mouths of tributaries and control the longitudinal profile of the Colorado River. Although the amount of rainfall on the days of historic debris flows typically is not unusual, the storm rainfall on consecutive days before the debris flows typically had recurrence intervals greater than 10 yrs. Four types of failure mechanisms initiate debris flows: bedrock failure (12 percent), failure of colluvial wedges by rainfall (21 percent), failure of colluvial wedges by runoff (the "firehose effect; " 36 percent), and combinations of these failure mechanisms (30 percent). Failure points are directly or indirectly associated with terrestrial shales, particularly the Permian Hermit Shale, shale units within the Permian Esplanade Sandstone of the Supai Group, and the Cambrian Bright Angel Shale. Shales either directly fail, produce colluvial wedges downslope that contain clay, or form benches that store poorly sorted colluvium in wedge-shaped deposits. Terrestrial shales provide the fine particles and clay minerals - particularly kaolinite and illite - essential to long-distance debris- flow transport, whereas marine shales mostly contain smectites, which inhibit debris-flow initiation.