WHEN DEBRIS FLOWS LEAVE THEIR CHANNELS

A laboratory investigation into how channel geometry influences debris-flow avulsion and runout on alluvial fans.

Master's Thesis | East Carolina University | 2023

THE QUESTION

Debris flows can leave their established channels and spread across alluvial fans, threatening communities and infrastructure. These avulsions are often attributed to channel blockage by earlier deposits. My research tested a more difficult possibility: can a debris flow escape a clear channel simply because of the way the channel bends?

THE EXPERIMENT

I conducted controlled physical simulations in the ECU Geomorphic Modeling Laboratory using 3D-printed channels with different bend geometries. Sediment-water mixtures traveled through a debris-flow flume and entered a simulated alluvial plain.

A synchronized array of 20 Sony cameras documented each experimental surface. I processed the imagery in Agisoft Metashape to build dense point clouds and digital elevation models, then used ArcGIS Pro and statistical analysis to measure avulsion locations, inundated areas, runout distances, and deposited volumes.

WHAT I FOUND

The simulations showed that debris-flow avulsion can occur without a channel blockage. In the experiment, flow superelevation at outer bends was sufficient to overtop the channel and initiate avulsion.

Channel geometry also influenced where avulsions occurred and how the material spread after leaving the channel. Sharper bends produced distinct patterns in avulsion frequency, runout volume, distance, and surface coverage.

WHY IT MATTERS

Hazard assessments that only anticipate avulsion where a channel is visibly obstructed may overlook an important failure mechanism. Bend geometry and outer-bank superelevation can help identify places where a debris flow may leave its channel even before a blockage develops.

These findings can inform alluvial-fan hazard mapping, mitigation planning, and the placement of protective measures by expanding the range of scenarios considered.

METHODS & TOOLS

Physical geomorphic modeling | Experimental design | CAD and 3D printing | 20-camera photogrammetry | Agisoft Metashape | Point clouds and DEMs | ArcGIS Pro | Spatial and statistical analysis

FULL THESIS

Influence of Channel Bend Curvature on Debris-Flow-Driven Avulsion on Alluvial Fans, Explored through Discrete Simulations