Improved flood prediction in mountainous catchments requires knowledge of the hydraulic characteristics of mountain streams. The flow and flow resistance of steep mountain stream channels are not well understood because of a lack of measured data. Therefore, we simultaneously measured discharge, using a v]notch weir and the water depth 50m downstream of the weir in a small natural channel, in the Aono Research Forest of the Arboricultural Research Institute of the University of Tokyo Forests on the Izu Peninsula, Japan. The Manning's roughness coefficient decreased almost one order of magnitude from 1.88 to 0.14 as the water depth increased. Moreover, the coefficient was large and decreased dramatically with increasing water depth as the water surface was below the top of most of the gravel in the stream bed(maximum water depth<0.2m). Once the water surface was above most of the gravel (>0.2m), the coefficient then changed little with increasing water depth. The minimum Manning's roughness coefficient observed was similar to previously reported values of 0.09 to 0.23 in steep step]pool channels under high flow conditions. These findings indicate that the roughness characteristics of steep mountain streams can exceed a Manning's roughness coefficient of0.1.

Key wordsFdischarge, Manning's roughness coefficient, mean velocity, mountain stream, step]pool channel, water depth