In real two-phase (debris) mass flows there exists a strong coupling between the solid and the fluid momentum transfer, where the solid's normal stress is reduced by buoyancy, which in turn diminishes the frictional resistance, enhances the pressure gradient, and reduces the drag on the solid component. Buoyancy is an important aspect of two-phase debris flow, because it enhances flow mobility (longer travel distances) by reducing the frictional resistance in the mixture. Buoyancy is present as long as there is fluid in the mixture. It reduces the solid normal stress, solid lateral normal stresses, and the basal shear stress (thus, frictional resistance) by a factor (formula_1), where formula_2 is the density ratio between the fluid and the solid phases. The effect is substantial when the density ratio (formula_2) is large (e.g., in the natural debris flow).