Resolving Debris-Flow Avulsion Using Sub-Second Laboratory Topographic Time Series

Debris-flow avulsion controls the shifting of active flow pathways across an alluvial fan and strongly influences fan construction. In turn, fan morphology and its evolution are major factors governing where avulsion occurs. Yet the avulsion process exhibits strong uncertainty and stochastic behavior that remains poorly resolved in field records. Most existing datasets are restricted to event- or annual-scale observations and therefore lack the temporal resolution needed to quantify how channels relocate during an event.

This study introduces a laboratory framework that directly resolves debris-flow avulsion using high-frequency topographic data. Debris flows built an alluvial fan within a 60 × 90 cm basin while eight synchronized 2K industrial cameras captured continuous multi-view imagery for Structure-from-Motion reconstruction. Spatial targets and an automated workflow yielded sequential point clouds, orthophotos, and DEMs in a fixed coordinate system, resolving fan-surface evolution at 0.1-s intervals and capturing rapid adjustments associated with avulsion.

We also obtain a consistent space–time record of channel traces during fan building, allowing relocation to be tracked at sub-second intervals. This record is derived by applying short-window long-exposure stacking to successive image frames, where locally disturbed areas reveal instantaneous channel footprints.

The resulting database captures sub-event-scale coupling between avulsion and fan morphology, clarifying how avulsion both responds to and reorganizes the fan surface. It also enables direct quantification of avulsion geometry, recurrence, and lateral displacement. Building on the empirical constraints provided by these measurements, the study aims to formulate a stochastic framework based on Gamma-subordinated OU processes to represent the episodic and bounded properties of debris-flow avulsion and to assess their implications for long-term fan morphology.

Figure. High-frequency SfM measurements of alluvial-fan evolution.
Top row: DEMs; middle row: orthophotos; bottom row: channel footprints extracted by short-window long-exposure stacking, revealing debris-flow avulsion.