Ridge formation in deposits of rapid granular flows over slope–horizontal surfaces: An experimental study

Rapid granular flows commonly produce deposits exhibiting intricate transverse ridges, yet the underlying formation mechanisms remain poorly understood. This study presents a systematic experimental investigation of rapid granular flows over slope–horizontal composite surfaces, aimed at identifying the key dynamic controls on ridge development. A series of controlled flume experiments were performed using smooth glass beads as a baseline material and sand for comparative tests, in order to examine flow behavior and deposit morphology under varying conditions. High-speed imaging and particle image velocimetry (PIV) were used to quantify flow kinematics and the evolution of surface structures. Two recurring ridge morphologies—arc-shaped and V-shaped—were observed in the glass-bead experiments. Their occurrence is primarily associated with the inflow regime, characterized by the Froude number ( Fr ). Relatively high and stable Fr values favor secondary deposition and the development of V-shaped ridges, whereas lower and more variable Fr conditions promote shear failure and the formation of arc-shaped ridges. Variations in the initial granular volume influence the magnitude and temporal evolution of Fr , thereby affecting ridge geometry and transition thresholds without changing the fundamental ridge types. A depositional phase diagram in Fr − V space summarizes the conditions under which shear-failure-dominated and secondary-deposition-dominated ridge patterns occur. The sand experiments demonstrate how particle properties and basal friction modify the clarity and spacing of ridge structures without altering the underlying regime-dependent behavior. These results highlight the coupled roles of inflow dynamics, material properties, and basal boundary conditions in shaping depositional ridge patterns, and provide a physically grounded basis for interpreting ridge-bearing deposits observed in granular flows and landslides in natural settings. • Depositional ridge formation in granular flows over slope–horizontal surfaces is examined experimentally. • Inflow regime, characterized by the Froude number, governs ridge formation and transitions. • Initial granular volume modulates deposit geometry and shifts critical Froude thresholds. • A Froude-number–volume regime diagram distinguishes different depositional ridge types. • Natural sand experiments show how particle properties modify ridge expression without changing the regime-dependent mechanism.