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

The Utah Geological Survey has developed guidelines to help geologists evaluate debris-flow hazards on alluvial fans to ensure safe development. The purpose of a debris-flow-hazard evaluation is to characterize the hazard and provide design parameters for risk reduction. These guidelines use the characteristics of alluvial-fan deposits as well as drainage-basin and feeder-channel sediment-supply conditions to evaluate debris-flow hazards. Geologic estimates of debris-flow parameters are necessary for the engineering design of risk-reduction structures.

Analysis of alluvial-fan deposits provides the geologic basis for estimating frequency and potential volume of debris flows and describing debris-flow behavior. Surficial geologic mapping, dating, and subsurface exploration can provide data to determine deposit thicknesses, flow volumes, flow types (debris, hyperconcentrated, and stream flow), maximum grain size, and flow frequency. 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 and feeder-channel characteristics determine potential debris-flow susceptibility and the volume of stored channel sediment available for sediment bulking in future flows. Historical records indicate that 80 to 90 percent of debris-flow volume is bulked from drainage-basin channels. Therefore, evaluation of the feeder channel focuses on measuring the volume of erodible channel sediment available for sediment bulking. The debris volume determined from sediment-bulking estimates provides an independent check for debris volumes determined in the fan evaluation. Additionally, hazard zones may be outlined on the alluvial fan to understand potential effects of debris flows and determine appropriate risk-reduction measures.