The comparative sediment processes in channel and overbank

The question posed in this study is why alluvial soil on a floodplain is finer than the bed material of the river that supplied the sediments deposited to form the floodplain. A schematic, simplified river/floodplain system is used in an approximate analysis to find the size distribution of the floodplain soil. It is assumed that the stable limiting condition is a suspended load in the floodplain flow of the same concentration and composition as the sediment load in that portion of the river channel flow above the level of the floodplain. It was found that floodplain soil should be finer than the channel bed material; how much finer depending on the bank height, and to a lesser degree the width of the floodplain