Project Title: Self-Cleansing Urban Drainage System with the use of Sediment Flushing Gate

Funder: PRGS Grant


Duration: 15 September 2011 - 14 September 2014




Researchers :

                        Charles Bong Hin Joo <>

                        Prof. Dr. Aminuddin Ab. Ghani <>

                        Dr. Lau Tze Liang <>





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



1.0 m



1.5 m




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






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>




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.