A robust quantitative method to identify the occurrence of a runoff-generated debris flow

Debris flows and floods generated by rainfall runoff can occur in rocky mountainous landscapes and burned steeplands. Flow type is commonly identified through interpretation of depositional structures, but this evidence may be poorly preserved or misinterpreted, leading to inaccurate characterization. Prior research indicates that peak discharge is amplified in debris flows owing to volumetric bulking and increased frictional resistance. Here, we take advantage of this amplification to develop a quantitative flow-type diagnostic Q* based on the ratio of observed peak discharge to theoretical maximum clear-water discharge from rainfall runoff. We compile 648 observations of floods and debris flows and demonstrate that this metric predicts flow type to ~93% accuracy. This metric allows for quantitative identification of debris flows through channel cross-section surveys rather than through qualitative interpretation of deposits, and therefore should increase the accuracy of models and suitability of engineered structures that depend on flow-type observations.