Debris-Flow Watersheds and Fans: Morphology, Sedimentology and Dynamics

Debris flows typically originate in mountainous watersheds. At the base of these watersheds and where not truncated by a higher order stream or discharging into a body of still water, repeated debris-flow deposition often forms semi-conical debris-flow fans. To mitigate debris-flow hazards, it is important to identify debris flow-prone watersheds and to understand their dynamics. This chapter reviews the morphology, sedimentology and dynamics of debris-flow watersheds and associated fans. Debris flow-generating watersheds are generally smaller and steeper than those dominated by floods and debris floods, and host less well-developed drainage networks. They can be roughly categorized into transport-limited systems (in which each hydroclimatic event producing high discharge triggers debris flows due to an abundance of sediment and high recharge rates) or supply-limited systems (in which debris-flow activity is limited by sediment availability and corresponding slow recharge rates). Debris-flow fans have typical lengths of 0.5–10 km and slopes of 5–15° and develop through spatio-temporal shifts of the locus of deposition through avulsion. Their surface topography and stratigraphy consist of stacked and amalgamated lobe, levee, and channel deposits, with sediment of mud to boulder grade. They provide archives of past flow processes and sediment supply indicative of past climate and environmental change. However, our ability to interpret debris-flow successions is still limited by relatively poor knowledge of how catchment geology and flow properties and composition affect resulting depositional features, from bed scale to fan scale. Recent field-based, experimental, and numerical advances on these topics are starting to fill this knowledge gap.