Exploring debris flow deposit morphology in river valleys: Insights from physical modeling experiments

A comprehensive understanding of the deposit morphodynamics of debris flows in river valleys is crucial for disaster prevention and mitigation in mountainous areas. However, debris flows in river valleys are complex and variable, and the influence of the composition and terrain on deposit mechanisms and morphology remains unclear. In this study, a series of laboratory experiments were conducted to investigate the impact of the water content, slope, and grain size distribution on the deposit morphology of debris flows in river valleys. In the experiments, debris flows form narrow and elongated shuttle-shaped deposits upon entering river valleys. Increased water content and slope enhance the mobility of debris flows, resulting in wider deposit widths, gentler deposit slopes, and larger deposit areas. An increase in the gravel fraction partially facilitates the movement and deposition of debris flows. However, a large number of coarse particles may increase internal friction within the debris flow, hindering the spread of deposits and resulting in a relatively concentrated deposit with a steeper morphology. Furthermore, the debris flow regime shifts from friction to collision as the water content, slope, and gravel fraction increase. Debris flows dominated by collision forces exhibit higher mobility and a wider impact area, potentially leading to more severe disaster consequences. Compared with unconfined debris flows, the deposit area of debris flows in the river valley is smaller, and the deposit width to height ratio is larger. There is a power function relationship between deposition area and Savage number in river valleys. Additionally, particle sorting and flow regime transformation during the movement and deposition of debris flows can significantly impact the morphology and internal structure of the deposits in river valleys and then affect the development of debris flow dam breaching. In summary, this study utilized experiments to simulate natural debris flows, offering fresh insights into the depositional behavior of debris flows in river valleys.