Evaluation of Roughness Coefficient in Convergence Transition under Various Hydraulic Conditions

In alluvial streams, roughness coefficient (RC) is one of the essential parameters which should be considered in study of sediment flows in rivers. The height of bedforms directly affects RC based on flow hydraulic conditions.In this research some experiments were done to evaluate the effects of convergence transition on the roughness coefficient. Three kind of transitions with different angles of inflow direction to the flume wall were used (5, 10 and 15 degrees). In each transition after the completion of the test and slow draw down of the water level, the sediment profile of the bed was recorded and the bedform height was determined. The results showed that the channel width reduction in transitions had a significant effect firstly on the ripple bedform height and secondly on the RC of the flow. The Froud number in the all of experiments was in the range of 0.2-1.05. With increase of the transition angle from 5 to 15 degrees, at the discharge of 10 L S-1, the RC was reduced about 17.5 percent and in the discharge of 14 L S-1 it was increased about 11 percent.