Incipient Motion Of Sediment Particles Over Loose Deposited Beds In A Rectangular Channel



The presence of sediment deposits in a drainage system or rigid channels as monsoon drains could produce flash flood.  The ability of the drainage system to erode sediment deposit will allow the system to carry the designed discharge.

The aims of the present study are to gain an improved understanding of the incipient motion processes in urban drains and to provide improved incipient motion relationships.  This is achieved by investigating the important parameters governing the incipient motion of particles for several bed  thicknesses.

In the first part of the study, the characteristics of flow in a rectangular cross section channel with different water flow depths and various slopes were studied. A series of incipient motion experiments were then carried out in a rectangular channel 300 mm wide and 10 m long.  Four different bed thicknesses (ts) were used namely d50, 5 mm, 10 mm, and 24 mm utilizing sediment with size between 0.55 mm and 4.78 mm.  The sediments were collected from existing drains in Ipoh City and Batu Gajah Town. 

The present data were utilized to appraise the existing incipient motion criteria for rigid channels. The results show that the existing equations are valid only for the case of bed having thickness equals sediment size (d50) only. 

New incipient motion criteria applicable for various bed thicknesses were then developed for 1% 5% (h is the channel depth  = 450 mm). These new equations could be used to design new drainage systems with the ability to erode deposited sediment.