What Is a Debris Flood?

Debris floods have been defined descriptively as mineral and organic sediment-rich floods, occurring in a steep channel and potentially destabilizing the streambed and banks. While this definition allows one to visualize the process, it does not inform on the mechanics, nor does it recognize different types. We propose to define debris floods as floods during which the entire bed, possibly barring the very largest clasts, becomes mobile for at least a few minutes and over a length scale of at least 10 times the channel width. We define the onset of a debris flood by the exceedance of a critical shear stress threshold required to mobilize at least the D-84 of bed material. A threefold classification is proposed in which the first type is triggered by the shear stress exceedance. The second is initiated by transition from a debris flow either in the channel or by oblique impact of a debris-flow-prone tributary. In this context we highlight the importance of effective fluid density. The third type is associated with outbreak floods from artificial or natural dams. A further subdivision of debris floods is made by using the ratio of the actual shear stress to the critical shear stress, with higher values indicating damaging and finally catastrophic debris floods in which even preexisting channel bank and bed protection is mobilized. This contribution aims to provide a more succinct mechanistic definition of debris floods that can be implemented in hazard and risk assessments for steep streams and rivers. Plain Language Summary Debris floods are a potentially destructive form of streamflow in steep channels. They are often referred to by the media as flash floods and carry unusual amounts of sediment in the form of rocks, soil, and trees. Rather than defining them by their visual appearance, we provide a definition that responds to the question what forces are required for most of the channel bed to become mobile. We categorize three types of debris floods based on their triggering mechanisms and subcategorize further into damaging and catastrophic debris floods based on the forces imposed on the bed by the flowing water.