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I I I I I I I I I I I I I I I I I I . 5.0 SEDIMENT TRANSPORT ANALYSES 5.1 Basic Sediment Transport Theory 5.1.1 Sediment Supplv versus Tran&port C~acity. A key factor in sediment transport analyses is a clear understanding of sediment supply and transport capacity. Sediment supply is primarily that sediment provided to the channel from watershed, tributary inflow, and the channel bed and banks. Transport capacity can be considered the actual physical capability of the channel to transport sediment, as defined by flow conditions. Transport capacity is a function of the size of bed material, flow rate, and geometric and hydraulic properties of the channel. It is significant that transport capacity is typically proportional to water discharge to the 1-2 power, and to velocity to the 3-5 power. In other words, small changes in discharge and velocity can cause large changes in transport capacity. Note that sediment supplied to the channel may be more or less than the transport capacity of the water in the channel, and it is this interaction which determines whether an aggrading, degrading or a stable condition exists. Either the supply or the transport capacity may limit the actual sediment transport rate in a given channel. The phenomenon of bed material yields being less than the transporting capacity of the river for the sizes of material in the river bed has long been recognized (Muller, 1955; Bagnold, 1966 and 1980; Nanson, 1974; Lauffer and Sommer, 1982; Beschta, et.al., 1980; Griffiths, 1980; Carson and Griffiths, 1987; Church, 1987; Pitlick and Thome, 1987; Whittaker, 1987). Citations in this section are in Chapter 8.0 and Appendix G together with additional summary material related to the supply limited condition. Bagnold, 1966, observed that the supply-limited condition could occur after a flood has removed much of the transportable material from the river bed, in which case the subsequent flows would transport less sediment than it could if more transportable sediment were available. In fact, one of the assumptions upon which his transport relation is based is an "unlimited supply of transportable solids". In a later paper (Bagnold, 1980), he observed that no general relation between stream power and sediment transport rate can be expected if the availability of the sediment is limited. Where a coarse surface layer overlies a finer layer of grave~ the total bed material yield for this fmer material will be less than the nominal capacity. This is common in mixed sand-gravel channels where, in many floods, the sand is moved along, and removed from the bed, without disturbance of the gravel layer (Carson and Griffiths, 1987). According to Carson and Griffiths (1987), this is common in upland channels that have coarse beds. Finer material is shed to the channel from hillslopes and moves downstream as bed material, often in waves associated with the point.. source inputs. In this case, the bed material yields are often substantially less than the transporting capacity of the river, being controlled by availability of the transportable material. Others have observed a progressive decline in the concentration of sediment in transport during extended high flow periods. For example, Beschta, et.al. (1981) noted a decrease in bedload transport over time in a coarse bedded stream during a prolonged reservoir release with nearly constant discharge. They concluded that the reduction was the result of limited sediment sources within the channel. Nanson (1974) proposed that the decline in bed material concentration was the result of a seasonal decrease in the intensity of geomorphic processes controlling sediment supply to the stream channe~ in contrast to the "generally accepted belief that bedload is controlled by the hydraulic and bed characteristics of a stream." Certain mountain streams are more than competent to transport the material supplied to them, but the limited period over which the processes supplying significant quantities of material to the stream act "restricts the supply to less than capacity." According to Muller (1955), in streams where the running bed load is much fmer than the bed material, there is no connection between the bed load transport and run-off, the stream is in a state of latent erosion, and the inflowing bed load runs off without scouring the coarse..textured bed. It should be recognized, of course, that even in a supply-limited stream, there is some correlation between the supply and the level of flow in the stream since the geomorphic processes that govern the input of material to the stream are related to the runoff conditions from the watershed. 5-1