| Abstract: | Suspended sediment is supplied from river bed sediment in Hiyamizusawa Brook, Hokkaido, Japan, during the early snowmelt season. The stirring up of fine grains from the river bed is an important control of the time variation of suspended-sediment flux. In this stream, about 10 per cent of the river bed is covered with sand sediment, 80 per cent with cobbles and/or pebbles and the remaining 10 per cent is exposed bedrock. A model previously used to explain the stirring up of fine grains within a cobble and pebble bed is applied to a sand bed, with the modification that fine grains in a sand bed are assumed to be stirred up from the tractive layer formed on the surface, whereas those in a cobble and pebble bed are assumed to be stirred up from the gaps formed by the selective movement of pebbles on the river bed. The lift force acting at the river bed is estimated from the bed shear stress, and the maximum grain size capable of being stirred up was calculated from the lift force. Consequently, the amount of fine material stirred up from the river bed is estimated from the grain size distribution of river bed sediment, and the suspended-sediment flux is thus calculated. All stirred-up fines are assumed to become suspended sediment. The simulated time variation of suspended-sediment concentration was similar to that obtained in the field study. The calculated grain size of suspended sediment was also equivalent to the field data. |
