Environmental Engineering Reference
In-Depth Information
(dashed line). The fluctuating discharge simulated the flood-low flow cycles. The duration of flood flow
generally lasted 1-2 hours in the experiment. The rate of bed load transport was measured, which is also
shown in Figure 11.35(b) (black pyramids). The total rate of bed load transport was very high during
flood flow but low during low flow. The width of the channel varied in a range of 1-3 m along the course
with an average of about 2 m. Therefore, the rate of bed load transport per width was about 3 kg/ms
during flood flow, which was extremely high. The fluvial process and the bed deformation were very fast
and generally reached a primary equilibrium after 6-10 hydro-cycles. The rate of bed load transport was
still high in this case, which was equal to the incoming bed load. Because the incoming bed load was not
controlled the measured rate of bed load transport fluctuated with fluctuation of discharge and incoming
bed load.
Fig. 11.35 (a) Original sediment size distribution of the materials in the experimental plot (debris flow deposit); (b)
Flow discharge (dashed line) diverted into the experimental channel and measured rate of bed load transport (black
pyramids) varying with time during the first experiment.
In the experiment, water was diverted from the Jiangjia Ravine and flowed back into the same ravine
through the experimental channel. The experiment was conducted in a 60 m long lower section of the
channel, on which 15 measurement cross sections were set. Between the experimental section and the upper
section a 2 m long concrete flume was constructed for measurement of flow discharge with a velocimeter,
which also acted as a base point for the measurement of the bed profiles. The bed structures, channel width
and depth, erosion, sedimentation, and migration of the channel were measured at the 15 measurement
cross sections.
Three experiments were done with different conditions. Figure 11.36 shows the bed profiles for the
experiments. In the first experiment, bed load, suspended load and water were diverted though the
concrete flume into the experimental section. The channel was scoured and widened but gradually reached
equilibrium. The S P value was measured at 0.08. The profile is shown in the figure marked with “With
loads.” In the second experiment, bed load was trapped in a pit at the upper end of the experimental
channel and only water and suspended load flowed into the experimental channel. The channel bed was
incised down until a bed structure similar to a step-pool system gradually developed. The bed incision
finally stopped and the bed profile reached equilibrium (profile marked with “No bed load”). The S P
value was measured at 0.13 in this case. Before the third experiment, bed load again was allowed to
transport into the experimental channel and the channel bed reaggraded because of bed load deposition.
The S P value reduced again to 0.08. The bed profile is shown in the figure marked with “Bed load”. Then
the third experiment started, no bed load was transported into the experimental section but the bed
resistance was enhanced by constructing an artificial step-pool system. Artificial steps were constructed
with cobbles 50-300 mm in diameter. The distance between two steps was 1-2 m. The S P value was
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