PROCESS-BASED MODEL OF GRAIN LIFTING FROM RIVER BED TO ESTIMATE SUSPENDED-SEDIMENT CONCENTRATION IN A SMALL HEADWATER BASIN

Suspended sediment is supplied from river bed sediment in Hiyamizusawa Brook, Hokkaido, Japan, during the early snowmelt season. The stirrin g 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 pe r cent of the river bed is covered with sand sediment, 80 per cent wit h cobbles and/or pebbles and the remaining 10 per cent is exposed bedr ock. 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 mod ification 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 cobb le 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 ac ting 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 simulat ed time variation of suspended-sediment concentration was similar to t hat obtained in the held study. The calculated grain size of suspended sediment was also equivalent to the held data.