Debris flow experiments under various riverbed roughness conditions confirmed that debris flows are affected by riverbed roughness. To investigate the influence of riverbed roughness on debris flows, the coefficient f was defined by modifying the coefficient of resistance, f, which is the ratio of the energy gradient to the friction loss. The value of f calculated using constitutive debris flow equations is constant for the same sediment concentration, c. Experimental results indicate that f increases as riverbed roughness increases with larger riverbed particles: f increases when c is dense or the relative flow depth, h/ks, is small. The latter is determined by dividing the flow depth, h, by the roughness height, ks. That is, the flow resistance appears to be stronger than estimated by existing constitutive equations. This suggests that existing constitutive equations do not simulate debris flow in mountain rivers well, because the beds of such rivers are covered with large particles.

Key words:debris flow, riverbed roughness, constitutive equations