GUIDELINES FOR THE GEOLOGIC EVALUATION OF DEBRIS-FLOW HAZARDS ON ALLUVIAL FANS IN UTAH

Large debris flows and related sediment flows are generally low-frequency catastrophic sedimentation events that can strike with little warning and destroy structures in their path. Therefore, the evaluation of debris-flow hazards on alluvial fans is necessary for safe and appropriate land use to prevent loss of life and property damage. These guidelines outline techniques to address debris-flow hazards by evaluating 1) past flows on alluvial fans, and 2) drainage basin and channel sediment-supply conditions. Understanding the processes that govern debris-flow initiation, transport in the drainage basin, sediment bulking, and deposition on the alluvial fan are vital to hazard evaluation.

The geologic evaluation of past flows on alluvial fans follows a two-step procedure consisting of an initial delineation of the active (generally Holocene) depositional area, and a subsequent detailed, site-specific analysis of the hazard within the active depositional area. In the detailed fan evaluation, flow-type, frequency, volume, and runout data are collected to characterize the hazard based on past debris-flow deposits. Surficial geologic mapping, dating methods, and subsurface exploration are used to investigate and describe the geomorphology, sedimentology, and stratigraphy of alluvial-fan deposits. The character of past debris-flow deposits provides a basis for determining the nature of future debris-flow deposition and the associated hazards due to impact, inundation, and burial.

Drainage basin slopes and channels supply sediment to alluvial fans, and the sediment-supply conditions govern the volume and frequency of future debris flows. Historical records indicate that 80 to 90 percent of debris-flow volume is bulked from drainage-basin channels. Therefore, evaluation of the drainage basin focuses on determining the volume of channel sediment available for sediment bulking. The inventory of sediment supply provides information on the character, size, gradation, and volume of sediment available for incorporation into future flows. The flow volume determined from sediment-bulking estimates provides an independent check for flow volumes determined in the fan evaluation.