Impact of incoming sediment grain sizes on channel sedimentation in Inner Mongolia Reach of Upper Yellow River

The sediment grain sizes,as well as the flow and boundary conditions,are important impact factors on sediment transport,contributing to the fluvial processes of the river.The sedimentation occurring in the Inner Mongolia Reach of the Upper Yellow River is affected not only by the incoming water and sediment volume,but also by the grain size gradation of the incoming sediment both from the main stream and the ten tributaries.Based on the measured data in 1960 to 2005 and the data of bed load samples of the ten tributaries,the characteristics of grain size gradation variation of suspended sediment and the contribution of the sediment of different grain sizes to the channel scouring or silting are analyzed.The impacts of the grain size gradation of the suspended sediment both from the main channel and the ten tributaries are further explored.The results show that the grain size of the suspended sediment decreases slightly along the river when less sediment comes from the ten tributaries,the incoming water and sediment condition from the upstream main stream is decisive to the adjustment of the river bed.When there is a large amount of sediment from the ten tributaries,the grain sizes of the suspended sediment increase along the river in scouring periods,while it decrease a little in silting period;but in both occasions there is no obvious change of the grain size at Toudaoguai,which means most of the coarse sediment from the ten tributaries almost deposits,contributing much to the channel sedimentation.The results about the source of the sedimentation put forward in this paper provide reference for the research on flood control and sedimentation reduction in this region.