Abstract

A lot of field data is referred in the paper to describe the exchange between coarse and fine particles as a common phenomenon in most of the alluvial river. In the process of deposition and scouring along the river the coarse particles are deposited and the fine scoured. Especially at the river reach downstream the reservoir and the fluctuating backwater region of reservoir no matter in case of erosion or deposition the phenomenon of exchange is very evident. Further the theoretical expression of critical size for separating coarser and finer particles is derived and has been verified by the field data. The comparing of critical size based on exchange theory and traditional method for separating wash load and bed material load reveals that both are the same in most cases, but the former provides a theoretical calculation method. Finally in the paper it is revealed the influences of exchange between coarse and fine particles, including, armouring by exchange may occur during erosion, equilibrium and even deposition; the largest size of bed material after armouring may bigger then that of original bed material; and armouring caused by exchange will decrease the size distribution, increase carrying capacity and promote erosion and even change deposition to erosion.

Keywords: exchange between coarse and fine particles, division size between coarse and fine particles, armouring process, carrying capacity