اثر حركت بار بستر بر مشخصات جريان در كانالهاي رسوبي و مجاري با بستر ثابت

Effect of Bed load Transport on Flow Resistance in Alluvial Channels

The effect of bedload transport on flow resistance in alluvial and rigid boundary channels was experimentally investigated. The experiments were carried out in a tilting flume of 250mm width and 12.5m length with glass-sides of rectangular cross-section with on either a smooth or a rough bed. Steady flow of clear and sediment-laden water in different flow depths with various velocities went studied in the experiments while a fine sand (d50 = 0.5 mm) and a fine gravel (d5Q=2.84mm) being employed in the cours of the experiments. The bed was artificially roughened by the same sediment sizes as used for injection to produce a sedimentary plane bed without any bed undulations. The results indicate that the transport of fine particle (d53 =0.5mm) for low sediment concentration can increase the friction factor by up to 50% and 75% for the smooth and rough beds respectively. This additional roughness may increase the flow depth by 14% and 21% for smooth and rough beds. The increment will be reduced by increase in grain size, as for d=2.84mm it would be 50% for a rough bed. Increase in friction factor can be interpreted as the additional energy consumption associated with conflict in sediment particles as well as momentum extraction from the flow, which decrease the mean velocity while increasing shear velocity. As sediment concentration increases, shear stress and consequently friction factor increases affecting the bulk of flow to the extent that a series of sediment bars start forming on the bed.