Effect of accelerating and decelerating flows on incipient motion in sand bed streams

The effect of flow acceleration and deceleration on velocity, von Kármán constant, Reynolds and normal stress distributions under incipient motion were experimentally investigated in this study using eight positive and negative bed slopes (± 0.7%, ± 0.9%, ± 1.25% and ± 1.5%) and three uniform sediments with median grain sizes d50 = 1.8, 1.3 and 0.8 mm. By using an Acoustic Doppler Velocimeter (ADV), the instantaneous velocities were measured at three cross sections 5, 7 and 9-m from the flume entrance giving a total of 72 measured profiles. The results showed that: (1) the shear and normal Reynolds stresses in decelerating flow are greater than those in accelerating flow; (2) the normal Reynolds stress is considerably higher than shear stress in all measured profiles; (3) the von Kármán constant associated with non-uniform flows has an average of κ¯=0.26, which is lower than the classical value of 0.4; (4) the critical shear stress and Shields parameter for incipient motion in accelerating flow are considerably higher than that in decelerating flow; and (5) acceleration and deceleration along with slope variation are key factors governing incipient motion in non-uniform flows.