ABSTRACT
A sorting model based on a sediment transport model is formulated. The model describes the development of grain sizes of the bed material and the longitudinal profile of a river. Two time scales exist in the model: one for the grain sorting and one for the change in the profile, the latter being by far the largest. This implies that for a given longitudinal profile of a river the variation of the mean grain size along the river is uniquely determined. The analysis is in agreement with the common observation that the grain size decreases in the down stream direction of a river. Results from the model are compared with observations from three rivers and with laboratory experiments. The model also describes the grain size distribution curves. The grain size distributions given by the model have properties which are frequently observed in natural sediments e.g. break points in log-normal plots.
