Modelling water and sediment flow in branching channel system
Operational problems associated with branching channels and water intakes increase over time due to localised sediment accumulations. The success of branching channel projects depends on the right design to increase the unit discharge and decrease the sediment concentration as much as possible. I...
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| Format: | Thesis |
| Language: | English |
| Published: |
2017
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| Online Access: | http://psasir.upm.edu.my/id/eprint/71186/1/FK%202017%2056%20-%20IR.pdf http://psasir.upm.edu.my/id/eprint/71186/ |
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| Summary: | Operational problems associated with branching channels and water intakes increase
over time due to localised sediment accumulations. The success of branching channel
projects depends on the right design to increase the unit discharge and decrease the
sediment concentration as much as possible. In addition, a branching channel from
rivers and channels affects the bed morphology and causes erosion and sedimentation
in the branching junction. There is still a lack of studies on the flow pattern in movable
bed branching channel systems for different branching channel angles and bed widths
to quantify the amount of water and sediment concentration in the branching channel
and investigate its effect on the bed morphology. In this study, the hydraulic
performance of differently angled branching channels was compared in an effort to
maximise discharge, minimise sediment concentration and decrease its effect on the
bed morphology.
The objectives of the study are to investigate the effect of the branching angle and the
bed width ratio on the water and sediment flow in the branching channel and scour
hole characteristics (scour depth and scour length). The scour hole is formed in the
main channel just downstream from the location of the branching channel entrance.
This study also investigated the variation in velocity vertically and horizontally at the
junction region and determined the total energy loss coefficient across the junction
region. The objectives of the study were implemented experimentally using a physical
model of 30, 45, 60, 75 and 90° branching channel angles with the main flow direction.
In addition, three bed width ratios (30, 40 and 50%) and five total discharges (7.25,
8.5, 9.75, 11, and 12.25 L/s) were investigated for each branching angle scenario. A
sand bed with d50 of 0.4 mm was used for all the experiments. In order to ensure
sediment movement in the main channel at the upstream and to quantify the branching
channel sediment concentration, a live-bed condition with flow intensity (Vu/Vc) of
1.1–1.5 was maintained in all experiments. The results indicated that branching angles of 30° and 45° increased the relative
discharge ratios (QR) by approximately 5–10% compared with the discharge ratio for
the 90° branching angle. The results also indicated that the branching channel sediment
concentration and scour depth decreased as the branching channel angle decreased.
The branching angles of 30° and 45° reduced the sediment concentrations by an
average of 64% and 37%, respectively, compared with the concentration for the 90°
branching angle. With respect to scour depth, the branching angle of 30° reduced the
scour depth by approximately 14.4-46.7% compared with the scour depth for the 90°
branching angle. The main reasons for forming the scour hole are the vortexes
generated due to diverting some of the flow towards the branching channel and the
downstream branching channel entrance sharp edge.
The 30° branching angle recorded the smaller low velocity region at the beginning of
the upstream side wall of the branching channel than other branching angles.
Moreover, the velocity distribution in this branching angle is more uniform along the
branching channel width than others. The outcomes from this study indicate that a
branching angle of 30°– 45° is the best arrangement to increase the branching channel
discharge, decrease the branching channel sediment concentration and decrease the
scour depth at the junction region. Reducing the amount of branching channel
sediment maintains the project efficiency of those depending on the branching channel
flow. In addition, a high water unit discharge means a lower initial construction cost
for the channel. Moreover, decreasing the scour depth helps to reduce the risks of a
scouring effect on the side bank of the main channel or any nearby structures. |
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