This study aims to clarify the erosive power of forest road-surface flow depending on flow velocity. We measured the volume-flow rate of the road-surface flow and the dry weight of sediment discharge at two test sections, the longitudinal grades of which differed; one was relatively gentle (3.7%) and the other steeper (8.2%). We also conducted cross-sectional surveying to identify the shape of ruts functioning as channels. Flow velocity was estimated from the observed volume-flow rate and hydraulic radius gained from cross-sectional surveying using the deformed Manning's equation. Although runoff on the gentle test section exceeded that of the steeper section, the weight of sediment discharge was much larger on the latter. Our calculation suggested that a considerable volume of runoff on the steeper test section flowed at 0.10 m/s, which did not appear on the gentle test section. Multiple regression analysis suggested that the higher velocity runoff primarily contributed to coarse sediment discharge while the lower velocity runoff made little contribution. Although the absolute value of the estimated flow velocity remains controversial, given that the flow velocity estimated by Manning's equation relates to the volume of discharged sediment, it is possible to quantify the influence of longitudinal grade, rut shapes and volume-flow rate, as the parameters of Manning's equation, on sediment discharge.

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