Author(s): M. Omid; A. Mahdavi; R. Narayanan
Linked Author(s): Mohammad Hossein Omid
Keywords: Resistance to flow; Bedload roughness; Shear velocity; Roughness scale; Bedload transport; Rigid boundary channel
Abstract: In the case of steady uniform flow in open channels with rigid boundaries, the mean velocity of flow is usually related to the hydraulic mean depth, the energy slope, and a measure of boundary roughness. Power law forms for velocity such as Manning's formula are used in the case of rigid boundary channels by choosing a suitable roughness coefficient. For a channel with a flat bed of sediment of certain thickness over the rigid bed, it may be assumed that the roughness is of the order of a characteristic grain size (for example d50) so long as there is no sediment motion. Movement of sediment particles essentially along the bed, however, produces roughness additional to that without sediment transport. The magnitude of this bed load roughness is expected to be proportional to the thickness of the sediment layer moving along the bed, the particle size and the sediment concentration. This paper presents the results of laboratory experiments to show that the friction factor of channels is substantially increased as the sediment discharge along the bed increases. The experiment consisted of creating flow conditions by injecting sand size particles (D50 = 0.5 mm) at the upstream end of the flume in order to produce bedload transport without deposition. Sediment concentration was increased progressively by increasing the injection rate of sand into the flow. The results indicate that for small sediment concentration the region of flow affected is restricted to the vicinity of the bed where it causes increase of shear stress and roughness scale. As sediment concentration increases, shear stress increases affecting the bulk of flow until a series of sediment bar forms on the bed.
Year: 2003