A Case Study of the Debris Flows Event in the Chalk Cliffs Basin, Colorado, USA: Numerical Simulations Based on a Multi-Phase Flow Model

Debris flows are among the severe gravity-driven mass phenomena that pose a significant threat to the environment and communities. Recent events and studies in the Chalk Cliffs basin in Colorado suggest that it is very susceptible to debris flow incidents that initiate from surface run-off, which involves significant entrainment of material along the hill slope and channel sediments. The entrainment of material along the flow makes these events destructive, with large travel distances s well as high velocity, flow pressure, kinetic energy, etc. This paper presents a case study of a debris flow event on 15 September 2009 based on a multi-phase flow model. The model provides the ability to investigate the effect of fluid and solid phases individually. Three sensitivity analyses are presented investigating the effect of bed roughness on solid and fluid phases separately, and also the effect of the entrainment of bed material. The findings demonstrate that the numerical model effectively replicates the observed field data, with the simulated peak discharge and runout distance closely aligning with the observed measurements. The analysis reveals that lower bed roughness promotes higher flow mobility and longer runout distances, while entrainment significantly influences flow height, velocity, and deposition pattern. Furthermore, the analysis highlights the dominant role of entrainment in debris flow evolution and emphasizes its importance in determining deposition and erosion patterns. These findings provide critical insights into the key processes of debris flows and could contribute to the development of accurate numerical models for debris flow events.