Environmental Engineering Reference
In-Depth Information
in motion are quite different. Sediment in the channel bed is composed of much more coarse and much
less fine sediment than is the moving sediment. The fine sediment in the flow is not saturated to its capacity
and the rate of transport depends only on the amount contained in the oncoming flow. Thus, the amount
of coarse sediment carried by the flow depends on the sediment transport capacity and exhibits a well
defined relation with the discharge of water. In contrast, the concentration of fine sediment depends only
on the supply of the sediment from an upstream reach and shows no obvious correlation with the
discharge. Because coarse sediment always exchanges with bed material during transport it is called "bed
material load". In contrast, fine sediment, eroded and washed from upland watersheds that has been
transported through the channel over a long distance and is scarcely ever deposited in the channel, is called
"wash load". Wash load always moves as suspended load.
Wash load refers to the sediment washed away through the channel without any exchange with the bed
sediment and plays no role in the river morphological process. Wash load concentration is normally a
function of supply; i.e., the stream can carry as much wash load as the watershed and banks can deliver.
Bed material load is composed of the sediment of size classes found in the streambed. Bed material load
moves along the streambed by rolling, sliding, or jumping, and may be periodically entrained into the
flow by turbulence, where it becomes a portion of the suspended load. Bed material load is hydraulically
controlled and can be computed using sediment transport equations.
The previously defined terms can be combined in a number of ways to give the total sediment load in a
stream. However, it is important not to combine terms that are not compatible. For example, the suspended
load and the bed material load are not complimentary terms because the suspended load may include a
portion of the bed material load, depending on the energy available for transport. The total sediment load
is correctly defined by the combination of the following terms:
Total Sediment Load = Bed Material Load + Wash Load
or
Total Sediment Load = Bed Load + Suspended Load
or
Total Sediment Load = Bed Load + Suspended Bed Material Load + Wash Load
What is the criterion for distinguishing bed material load and wash load? Einstein suggested using D 5
of the bed sediment as the critical diameter. Sediment carried by the flow finer than D 5 is wash load and
those coarser are bed material load. Partheniades (1977) employed 0.06 mm as a critical diameter to
differentiate bed material load and wash load, because for sediment finer than 0.06 mm the cohesive force
among particles is important and coarser sediment is cohesionless. Some researchers have suggested that
the criterion for differentiating bed material load and wash load should include not only sediment size
but also flow intensity. Wang and Dittrich (1992) analyzed the rate of sediment transport in the Yellow
River at high concentrations under different flow intensities and found that sediment much coarser than
0.06 mm may wash downstream for several hundreds of kilometers without exchange with the bed
sediment. Thence they proposed that the bed load, suspended bed material load, and wash load could be
identified as follows:
Bed Load > Z = 3 > Suspended Bed Material load > Z = 0.06 > Wash Load (1.22)
In such a way the three kinds of load can be identified using the Rouse number. Table 1.2 lists the main
features and differences of the bed material load and wash load.
1.1.4.4 Bed Forms
Bed forms are wave-like regularities found on the bed of a stream that are related to flow characteristics.
They are given names such as "dunes," "ripples," and "antidunes." They are related to the transport of
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