Rough meets smooth: Unifying granular landslide speed and runout

The flow of grains down an inclined plane is a paradigmatic system of granular physics. Even for this simple geometry, the flow behavior varies considerably depending on parameters such as inclination angle, injection flow rate, and substrate conditions. In the parameter space that yields steady-state conditions, there are generally distinct regimes for flow over smooth, rough, and erodible substrates. Here, using systematic experiments on rough and smooth surfaces at large inclination angles and with an injection that does not produce steady-state conditions, we find that the smooth and rough flow behaviors reunite. We also demonstrate that the travel distance following the steep incline depends on the roughness through the dynamic sliding friction with the substrate.