Granular Flow Behavior over Complex Topography: Insights from Flume Experiments

Gravitational mass movements, such as debris flows, rock and snow avalanches, move downslope through complex terrain, where bends, undulations, and channel constrictions strongly influence flow behavior and deposition patterns. Accurate understanding and modeling of these processes are critical for effective hazard assessment and risk management. However, most experimental flume setups idealize the terrain as planar. Therefore, the verification and validation of numerical models is often performed in simplified terrain conditions, where curvature effects, flow detachment from the terrain, and slope-normal accelerations do not emerge and, thus, do not challenge the assumptions and simplifications of various numerical models.

In this work, we performed flume experiments on complex topographies to generate a novel dataset of flow kinematics and depositional patterns under varied initial and boundary conditions. We used dry granular flows (quartz sand, 0.7 − 1.2 mm), released into a half circular channel with diameter 0.2 m. Along the 4 m inclined slope, complex topographical features are introduced through a modular 3D printed configuration. Slope (25°, 35°), release mass (2 − 20 kg), bed roughness (smooth and rough, 0.7 − 1.2 mm), and terrain features such as bends and bumps are tested. Flow fronts are tracked with video cameras, flow depths are measured with laser sensors, and deposition is quantified via pre- and post- experiment laser scans. We compare the runout length and mobility angle throughout the different experiments. As expected, basal roughness reduces flow velocity and runout
distance, and alters deposition patterns. Increasing the flow mass promotes the formation of roll waves and longer runout. Bends lead to energy dissipation, thereby promoting upstream deposition. Longitudinal terrain bumps induce the detachment of material from the channel bed at the higher slope angle or the deposition upstream of the bump at the lower slope angle.

These experiments provide a unique dataset of granular flows with systematically varied terrain features and well defined boundary conditions. They are designed for direct comparison with numerical models, providing a realistic benchmark to assess model performance and to identify limitations of different approaches for simulating granular flows over real topographies.