AbstractThe rheological behavior of soil–water mixtures can affect the flow dynamics of natural processes such as flow-type landslides and sediment transport. In this study, an experimental investigation was carried out on soil–water mixtures containing particles up to 0.425 mm with volumetric solid concentrations of 0.05≤CV≤0.35 using a wide-gap rotational viscometer, and the contributions of particle–fluid, collisional, and colloidal interactions to their rheological behavior were evaluated thoroughly. The torque-scaling method was applied to identify the flow regime and extract the effective rheological parameters from turbulent flows. A transition between dilute and concentrated mixtures can be identified based on their sediment type, CV, and the predominance of particle–fluid and interparticle interactions. For dilute mixtures (CV<0.25–0.28) the particle-fluid interaction is dominant. The colloidal interactions induce another force scale that is independent of the shear rate. Thus, the Bingham model can be applied to such mixtures (debris or mud flood). For concentrated mixtures (CV>0.25–0.28), the shear-thickening rheology can be applied for granular mixtures (debris flows), because the collisional interactions are dominant, whereas shear-thinning rheology can be applied for fine-grained mixtures (mudflows), because colloidal interactions are dominant.