Why Sediments Deposit in Lined Channels

Abstract

With a view to alleviate waterlogging, minimize maintenance costs, improve conveyance efficiency, etc., numerous unlined channels in India are replaced by parallel‐running, cup‐shaped (round‐bottom), brick‐lined (rigid boundary) channels. The operation of these newly lined channels manifests sediment deposition all along the wetted perimeter. This is quite surprising because: (1) The grade and concentration of sediment entering these lined channels (from their parent unlined branch canals) remain the same as in the case of unlined channels; (2) water surface or bed slopes are unchanged because the channel command is the same; and (3) the roughness coefficient rather appreciably decreases on account of the lining. This paper examines the causes responsible for sediment deposition in lined channels and suggests the most efficient shape of conveyance for sediment transport with no deposition. The rectangular section placed with its longer side horizontal is shown to be the most efficient for sediment transport, followed by the conventional trapezoidal section. The cup‐shaped section should not be used since it is the least efficient for sediment transport. Criteria for determining the limiting concentration of transported sediment are also suggested.