Environmental Engineering Reference
In-Depth Information
The median diameter of the bed sediment was about 60 mm and the median diameter of bed load was
only 3 mm. The flow energy was dissipated by stream bed roughness, as shown in Fig.5.53 (a).
Sediment was taken from the flood plain of the stream and sieved into four groups: D <2mm, 2-5mm,
5-15mm and 15-42mm. The four groups of sediment were fed into the stream. The sediment was
transported through the experimental reach and the bed structure was buried, as shown in Fig. 5.53(b).
The bed roughness was greatly reduced and the energy consumption by the bed roughness was less than that
of the stream power. A lot of bed load particles were initiated to move and collide with the bed to balance
the extra stream power. The rate of bed load transport was measured at the measurement section.
Fig. 5.53 (a) Almost no bed load motion under the natural conditions with a bed structure developed; (b) Incoming
bed load buried the bed structure and intensive bed load motion occurred
Figure 5.54 show the process of sediment feeding at constant rates (rectangular pulses) and measured
rate of bed load transportation for particle size groups of 2-5 mm and 5-15 mm (points and curves). The
sediment of 2-5 mm was transported as bed load, which buried the bed structures and flattened the bed.
This caused an intensive bed load motion to occur on the bed and approximately 35 min later the
sediment moved to the measurement section and the measured bed load abruptly increased from nearly
zero to a peak of 18 kg/min. The feeding of sediment of 5-15 mm also caused intensive bed load motion.
Approximately 90 minutes after this intensive motion the bed load moved to the measurement section
and the rate of bed load transportation increased from nearly zero to a peak of 7 kg/min. The integration
of the measured bed load transportation curves was almost equal to the total feeding amounts for the two
groups of sediment. The feeding of sediment of 15-42 mm did not cause intensive bed load motion. Most
of the particles piled on the bed and there was no obvious increase in the measured bed load motion.
Moreover, big particles piled on the bed, which created new resistance in the bed and, thus, further
increased the bed roughness. It is only during flood season these particles can be transported along the
channel.
Figure 5.55 shows the size distributions of sampled bed load sediment under natural conditions,
feeding sediments of 2-5 mm and 5-15 mm, and the bed material of the channel. There is a critical
diameter: sediment finer than the critical diameter may be transported through the channel. During the
transportation the sediment buries the bed structure and changes the bed roughness. The portion of flow
energy consumed on form drag reduced greatly and portion of flow energy for bed load transportation
increased greatly. All incoming such fine bed load particles may be transported through the channel. The
rate of bed load transport depends on the incoming amount rather than the flow intensity. Such a part of bed
load is named travelling bed load. The critical diameter is defined as D b 20 , where D b represents the size of
sediment on the surface of the channel bed. The critical diameter for the Diaoga River is about 15 mm.
Search WWH ::




Custom Search