Controls on debris-flow avulsions: White Mountains of California and Nevada

The process by which debris flows shift from an active channel and branch out into new transport or depositional areas is termed “avulsion.” They pose serious risks for structures and populations on debris-flow fans, yet avulsion mechanisms are relatively unknown and unaccounted for in hazard assessments, as compared to avulsions of rivers and streams, which are better understood. This study analyzes six debris-flow fans in the White Mountains of California and Nevada to identify relationships between avulsion locations and channel characteristics, constrain the controlling factors on avulsion, assess the probability that avulsion will occur at specified locations, and develop a method to predict avulsion locations. A database of avulsion locations and their channel characteristics was compiled in the field. These were compared to the characteristics of other positions on the fan surface that show evidence of debris flows that did not avulse through stepwise, binary logistic regression. Results indicate that two-thirds of avulsion likelihood can be attributed to the percentage of boulders at the site, slope angle, channel width, and the ratio between flow thickness and average slope at the avulsion location. The accuracy of this model can be improved when it accounts for the presence of a coarse channel plug, which increases the likelihood of avulsion. Application of the model is demonstrated by runout simulations with forced avulsions from modeled channel plugs.