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<br />flow and added to the sediment already in transport. During sa:1.iment <br />ranoval, the exchange beboieen the flow and the bed is assumed to take place <br />in the active layer at the surface. Thickness of the active layer is basa:1. <br />upon the relation defined by Borah, et al. This thickness is not only a <br />function of the material size and composition, but also reflects the flow <br />concH tion. During degradation, several of these layers may be scoured away, <br />resulting in the coarsening of the bed material and formation of an armor <br />coat. However, new active layers may be deposi ta:1. on the bed in the process <br />of aggradation. Materials eroded from the channel banks, excluding that <br />portion in the wash load size range, are includa:1. in the accounting. Bed <br />armoring develops if bed shear stress is too low to transport any available <br />size. <br /> <br />The non-equilibrium se:liment transport is also affecta:1. by diffusion, <br />particularly for finer sediments. Because of diffusion, the deposition or <br />entrainment of sediment is a gradual process and it takes certain travel <br />time or distance to reach the transport capacity for a flow condition. <br />Therefore, the actual sediment discharge at a section depends not only on <br />the transport capacity at the section but also on the supply from upstream <br />and its gradual adjustment toward the flow conditions of this section. In <br />the model, the sediment discharge is corrected for the diffusion effects on <br />deposi tion and entrainment using the method by Zhang, et al. (1983). The <br />procedures for computing sediment transport rate, sediment sorting and <br />diffusion are applied to the longitudinal and transverse directions. They <br />are also coupled with. bed-profile evolution described later in this section. <br /> <br />Sediment discharge may be limi ta:1. by availability, as exemplified by <br />the flow over a grade-control structure or bed rock. The very high <br />transport capacity at such a section, associated with the high velocity, is <br />limited by the supply rate from upstream; that is, the sediment discharge at <br />such a section is under upstream control. <br /> <br />Upstream Boundary Conditions for Sediment Inflow <br /> <br />The rate of sediment inflow into the study reach is provided by the <br />upstream boundary condition for sa:1.iment. If this rate is known, it may be <br />14 <br />