Guidelines for the geologic evaluation of debris-flow hazards on alluvial fans in Utah (MP 05-6)

By: R. E. Giraud

The Utah Geological Survey (UGS) developed these guidelines to help geologists evaluate debris-flow hazards on alluvial fans to ensure safe development. Debris-flow hazard evaluations are particularly important because alluvial fans are the primary sites of debris-flow deposition and are also favored sites for development. The purpose of a debris-flow-hazard evaluation is to characterize the hazard and provide design parameters for risk reduction. The UGS recommends critical facilities and structures for human occupancy not be placed in active debris flow travel and deposition areas unless the risk is reduced to an acceptable level.

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. The hazard evaluation relies on the geomorphology, sedimentology, and stratigraphy of existing alluvial fan deposits. Analysis of alluvial-fan deposits provides the geologic basis for estimating frequency and potential volume of debris flows and describing debris-flow behavior. 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.

The debris-flow hazard depends on site location on an alluvial fan. Generally, sediment burial and impact hazards are much greater in proximal fan areas than in medial and distal areas down fan. Hazard zones may also be outlined on the alluvial fan to understand potential effects of debris flows and determine appropriate risk reduction measures. Geologic estimates of debris-flow design parameters are necessary for the engineering design of risk-reduction structures.

Other Information:
Published: 2005
Pages: 16 p.
Location: Utah
Media Type: Paper Publication