Granular debris flows in nature are generally composed of a wide range of solid particles and viscous pore fluids, moving at high traveling velocities down sloping channels. Multiple parameters govern a granular debris flow's rheological properties and affect the interactions between the solid and fluid phases of these flows. Study of the solids contact behavior (i.e., contact shearing and collisions) is essential for understanding the high flow mobility of catastrophic geophysical granular flows/avalanches. This study critically reviews two dimensionless numbers with clear physical meanings (e.g., the Savage number and the friction number), then demonstrates a new application of field monitoring data of natural debris flows to obtain specific values of these numbers for classifying flowing regimes of granular debris flows on large scales. These numbers are then used to analyze and illustrate the contact behavior of solid particles in different flow regimes. This study shows that the flow regimes of granular debris flows are governed by the relative dominance of contact shearing and collisions between solid particles, as well as being significantly affected by the flow's pore fluid viscosity.