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Researchers :
Charles Bong Hin Joo <bhjcharles@feng.unimas.my>
Prof. Dr. Aminuddin Ab. Ghani <redac02@eng.usm.my>
Dr. Lau Tze Liang <celau@eng.usm.my>
Synopsis:
Sediment
deposits in drainage system have been found to be one of the causes of flash
flood due to loss of hydraulic capacity of drains besides causing blockages
and potential pollution to existing water. Drainage systems are usually
designed to be self-cleansing by adopting a minimum velocity to reduce
sedimentation. In order for drain to have self-cleansing properties,
increased slopes is required causing water to flow faster hence enhancing
erosion of sediment. However, this is not feasible for flat area due to the
cost of deep excavation beside the downstream condition which might limit
the depth of drain to be built. Periodic removal of sediment manually is
effective but requires man-power and expensive cost. More recent development
in European countries for combined sewers was to use hydraulic devices such
as chambers, tipping buckets and flushing gates which stores water to
certain amount before releasing into sewer system, thus generating
high-energy flushing waves that have scouring effects on sediment.
The use of
flushing devices in drainage systems is a new concept for storm drain like
the one used in Malaysia and requires further study to design a suitable
device for local condition. The characteristics of sediment commonly found
in Malaysian drains needs to be understood and take into account in
designing the flushing device. An understanding of the sediment
characteristics is important in the determination of critical shear stress
or critical velocity for the initiation of motion (incipient motion) of the
sediment. The scouring effect generated from the flushing device will be
dependent on the incipient motion criteria of the sediment.
The present study attempts
to understand the characteristics of sediment deposition commonly found in
Malaysian urban concrete open drain systems and to develop improved
incipient motion formulas for self-cleansing design. The concept and
effectiveness of using flushing gate as aid in removing sediment deposition
for open channel system will also be explore. Table 1 shows the
characteristics of sediment deposition from 58 sites in Kuching city,
surrounding urban towns outside Kuching city and Penang. Generally the main
component of the non cohesive sediment found in urban Malaysian storm drains
were sand, followed by gravel and a bit of slit and clay. Figure 1 shows the
incipient motion experiment setup. Results from the incipient motion
experiment were as shown in Figure 2 and Table 2.
The new equation
 which
incorporates the effect of sediment deposition thickness was able to predict
the critical velocity value slightly better than the existing equations as
shown in Table 2. Figure 3 shows the model of tipping flushing gate
installed in a flume in the laboratory and Table 3 shows the preliminary
results from the flushing experiment. Preliminary results showed that
generally the number of flushes required increased by nearly two times as
the sediment bed thickness doubled.
Table 1.
Summary of the average value of specific gravity, grain size distribution
and representative size according to land use and location.
 
Figure 1:
Sediment bed set up for the incipient motion experiment.
Figure 2:
Comparison between the predicted critical velocity values using the new
equation with the observed values.
Table 2
Comparison of discrepancy ratio value at different sediment deposit
thickness between the new equation with the existing equations.
 
Figure 3:
Model tipping flushing gate.
Table 3
Relationship between the initial distances of the sediment bed from the gate
with the number of flushes to totally remove the sediment for a distance of
1 m.
|
Distance of initial sediment position from flushing gate |
Number of flushes required to totally remove the sediment for a
distance of 1 m |
|
For
thickness of 24 mm |
For
thickness of 48 mm |
|
0.5 m |
7 |
12 |
|
1.0 m |
6 |
10 |
|
1.5 m |
4 |
7 |
Figure 4:
Installation of Sediment Flushing Gate at Taman Pekaka's Concrete Drain - 14th
November 2012
Figure 5: Operation of
Sediment Flushing Gate 19 November 2012
 Click
here to view CCTV
Publication:
Journal:
Bong, C.H.J, Lau, T.L. & Ab. Ghani, A. (2013).
Sediment Size and Deposition Characteristics in Malaysian Urban Concrete
Drains – A Case Study of Kuching City, Urban Water Journal, ISSN
1573-062X, In Press
<download>
Bong, C.H.J,
Lau, T.L. & Ab. Ghani, A. (2013). Verification of Equations for Incipient
Motion Studies for Rigid Rectangular Channel, Water Science and
Technology. Vol. 67, No. 2, pp. 397-405. ISSN 0273-1223
<download>
Conference:
Bong, C.H.J., Lau, T.L. &
Ab. Ghani, A. (2012). ‘Incipient
Motion of Sediment in Open Channel: A Comparison between Laboratory Data and
Site Observation’. 2nd International Conference on Water
Resources (ICWR2012), Langkawi, Malaysia, 5 - 9 November 2012.
Bong, C.H.J., Lau, T.L. &
Ab. Ghani, A. (2011). ‘Sediment Deposition Characteristics of Urban Concrete
Drains in Kuching City, Sarawak, Malaysia’. 3rd International
Conference on Managing Rivers in 21st Century (Rivers2011),
Penang, Malaysia, 6 – 9 December 2011, pp. 712-720. ISBN:9789833067350.
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