Sediment Deposition in A Rigid Monsoon Drain: Case Study of Raja River, Alor Setar 

Urban drainage system in Malaysia is usually based on conventional system made up of concrete drains which convey surface run-off from rapid development urban areas. This conventional design could increase sedimentation in the drains due to surface run-off which carried along sand and soil particles into the drains. This sedimentation will decrease the cross-sectional area of the drains and slowly reduce the designed hydraulic capacity of the drains. The incipient motion and sediment transport are influenced by water velocity, particles sizes (d₅₀), concentration of suspended bed load (C_v) an other factors.

The objectives of the research are to establish the deposited sediment particle size, to determine sediment deposition and corresponding slope and also to evaluate the existing incipient motion equations for self-cleansing design purposes.

The study conducted includes sampling works at Sg.Raja (10 stations) and twelve cities of Peninsular Malaysia. Particles sizes distribution analyses were carried out to determine particles sizes for use in the existing incipient motion equations. The most suitable equations can be determined to predict sediment deposition occurred in the drains based on measured deposited sediment slope. 

Based on particle size distribution analyses, the sediment average size for twelve cities in Malaysia is 0.75mm while for Alor Star the average sediment size is 0.93mm. The existing incipient motion equation assessments show that Equation 2.19 (Novak & Nalluri, 1975) is able to predict satisfactorily the sediment deposited slope while Equation 2.27 (Ab.Ghani et al., 1975) is able to predict well the design slope.