Environmental Engineering Reference
In-Depth Information
Fig. 5.51 Measured rate of bed load transport per width in the Diaoga River in 2007: (a) as functions of stream
power P ; and (b) as a function of Shields dimensionless shear stress
Figure 5.52 shows measured rate of bed load transport as a function of flow discharge per width in 2006
and 2007. The rate of bed load transport differed by up to 1000 times for the same flow rate. Considering
the sediment size and bed gradient were almost the same the great difference in the rate of bed load
transport implies that the bed load transport in the stream can not be calculated with any formula based on
the relation of bed load motion with water flow.
Fig. 5.52
Rate of bed load transport versus water discharge per width
Bed load motion in mountain streams is a complex process, which is influenced by many factors.
Among these factors the incoming sediment load and bed structures are the most important. In Chapter 4
the bed structures, including cluster, ribbing structure and step-pools are discussed. These structures consume
flow energy and prohibit sediment particles to be initiated from the bed. Moreover, the structures may be
buried and the bed may be flattened if the incoming bed load is high. It is because the changing bed
structure and varying incoming bed load the rate of bed load transport varies in several orders under the
same flow conditions. Therefore, none of the bed load formulas can be used to calculate bed load transport,
because these formulas were developed based on data from lab experiments with uniform sediment and
simple boundary conditions.
To study the influence of bed structure and incoming bed load an experiment was carried out in the
Diaoga River during non-flood season. Sediment was fed at a cross section 22 m upstream from the
measurement section. The bed slope of the experimental reach was 0.05. During the experiment, the flow
discharge ranged from 0.09 to 0.126 m 3 /s. There was very low sediment transportation under the natural
conditions due to a bed structure of cobbles clusters. Less than 1 kg of bed load were collected in 2 hours.
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