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Debris flows generated by rainfall runoff can occur in rocky alpine landscapes and burned steeplands. Runoff-generated debris-flow events are commonly composed of a series of dense granular surge fronts separated by water-rich flows. Owing to this intra-event variability in flow composition and mechanics, post-event interpretations of preserved sedimentary deposits, or lack thereof, can result in a dizzying mix of interpretations that range from clearwater flow to debris flow. Accurate identification of the presence or absence of a debris flow during a runoff event is critical for building empirical models used to predict likelihood of debris-flow occurrence, rainfall thresholds, and flow properties. Here, we propose a simple, quantitative method to identify the occurrence of a runoff-generated debris flow, based on a dimensionless discharge Q* calculated as the ratio of the peak event discharge Qp to the theoretical maximum clearwater runoff rate Qw. Using a preliminary compilation of Q* values from floods and runoff-generated debris flows, we find 98% of floods have Q* values < 1.6, whereas 91% of debris flows have Q* values greater than 1.6. Estimating Q* is typically straightforward as part of standard post-event reconnaissance if suitable rainfall estimates are available, and appears to be a robust indicator that runoff-generated debris flows traversed a particular portion of a valley network.

We evaluated the ability of three debris-flow runout models (RAMMS, FLO2D and D-Claw) to predict the number of damaged buildings in simulations of the 9 January 2019 Montecito, California, debrisflow event. Observations of building damage after the event were combined with OpenStreetMap building footprints to construct a database of all potentially impacted buildings. At the estimated event volume, all models overpredict the number of damaged buildings by a factor of 1.5–3.

In the semi-arid regions of the western United States, postfire debris flows are typically runoff generated. The U.S. Geological Survey has been studying the mechanisms of postfire debris-flow initiation for multiple decades to generate operational models for forecasting the timing, location, and magnitude of postfire debris flows. Here we discuss challenges and progress for extending operational capabilities to include modeling postfire debris-flow inundation extent. Analysis of volume and impacted area scaling relationships indicated that postfire debris flows do not conform to assumptions of geometric self-similarity. We documented sensitivity of impacted areas to rainfall intensity using a candidate methodology for generating inundation hazard assessments. Our results emphasize the importance of direct measurements of debris-flow volume, inundated area, and high temporal resolution rainfall intensity.

Debris flow erosion into bedrock helps to set the pace of mountain denudation, but there are few empirical observations of this process. We studied the effects of debris flows on bedrock erosion using Structure-From-Motion photogrammetry and multiple real-time monitoring measurements. We found that the distribution of bedrock erosion across the channel cross-section could be generalized as an exponentially decreasing function of height above the channel thalweg. Using this empirical function, we simulated the erosion at a cross-section after the theoretical passage of a migrating knickpoint effectively matching the upstream pre-knickpoint cross-sectional shape to the downstream post-knickpoint cross-sectional shape via debris-flow bedrock erosion.