Scouring in movable bed expansions

The flow in suddenly abrubt expansions with movable bed with 0.28 mm material is studied experimentally using expansion ratios 1.5, 2.0, 3.0 and 4.5. The flow in movable-bed expansion is more symmetrical and stable than the flow in rigid-bed expansions (Fouladi, 1988 b). The symmetric flow in movable bed expansions seems to be partly due to a greater shear resistance of the bed causing the adjustment in flow and partly due to bed erosion. The longitudinal velocity distribution is found to follow Cosine law for small expansion ratios, which gradually changes to Ganssian distribution for expansion ratio of 4.5. The material scoured by the jet is deposited symmetrically on both sides thus yielding symmetric pattern of scour and deposition. The Cosine law seems to be valid for the scour depth profiles as well. The maximum scour depth occurred at the centre of the channel and the scour depth decreased along the length in the downstream direction as well as laterally.