Exchange Between Coarser and Finer Sediment and Comparison Between Bed Material Load and Wash Load

The exchange between the coarser and finer sediment particles are discussed in this chapter: (1) The massive field data indicate that the exchange between the coarser and the finer sediment particles constantly occur during the channel scour or deposition. In the sediment exchange process, the finer particles are eroded, and the coarser particles are deposited. Within a certain length of a reach, the threshold particle size for scour or deposition is relatively stable; (2) In the earlier studies it is considered that the laws for sediment transport were essentially different between the coarser and finer particles. As a result, the suspended load is divided into bed material load and wash load. It is considered that the bed material load is closely dependent of the flow. Coming from the watershed, the wash load is almost independent of the flow condition, and can be completely transported through the reach; (3) Utilizing the theory for carrying-sediment capacity and for the non-equilibrium sediment transport, the following results have been obtained; (a) The inevitability of the exchange between the coarser and finer particles is proved and the threshold particle size for scour or deposition is determined by the average gradation of bed material, which is approximate to the threshold particle size presented by experience for wash load and bed material load. (b) The relative water volume percentage to carry the finer and the coarser sediment are determined, and then the carrying-sediment capacities for the total load, the wash load, and the bed material load are proved to have the same form of expressions as well as to follow the united laws. The results are verified with the field data of the Yangtze River and the Yellow River of China; (c) Bed material load and wash load are a united whole. Compared to the wash load, the bed material load is coarser, and expends more flow energy. By adjusting the concentration of bed material load, the adjustment of sediment concentration is achieved more quickly with the least bed change. Hence, it lacks of theoretical basis to simply exclude wash load from the carrying-sediment capacity, considering it does not expend flow energy, and only consider the carrying-sediment capacity for bed material load; (d) The importance of the coarser and finer particle exchange in sediment transport and channel evolution is theoretically illustrated. This exchange increases the sediment transport capacity along the flow path, causes the exchange and coarsening of the bed material, and is responsible for the long distance scour downstream of a reservoir, which can reach the mouth of the stream in some cases; (d) the differences in the transport phenomena between the coarser particles and the finer particles are identified. The differences include: transport of wash load only expends a small amount of flow energy; and sediment carried by the density flow in a reservoir is typically wash load, In other words, while the bed material load and the wash load are proved to follow the same laws of transport, the differences in the transport phenomena can still be illustrated. Hence, it is unnecessary to differentiate the bed material load and the wash load.