Fine sediment dynamics in unsteady open-channel flow studied with acoustic and optical systems

In order to simulate fine sediment dynamics over an armored bed in a tidal river, unsteady accelerating, then steady open-channel flow over a movable (but not moving) coarse gravel bed (D50=5.5 mm) was studied. A layer of fine sediment (D50=120 μm) was placed on the coarse gravel bed. The thickness of the fine sediment layer on the gravel bed was varied between 4 and 6 mm, but it was found that the thickness of the layer had no effect on the results. Quasi-instantaneous profiles of velocity and sediment concentration were taken simultaneously and co-located. An Acoustic Doppler Velocity Profiler (ADVP) was combined with Particle Tracking Velocimetry (PTV) for suspended sediment particle tracking. Measurements resolved turbulence scales. During the final phase of the accelerating flow range, fine sediment suspension from the bed started in packets and rapidly created a ripple pattern that remained nearly stationary. Thereafter, vortex shedding produced most of the sediment suspension into the water column in the form of events or packets, making suspension intermittent. Simultaneously, sediment particles rolled along the bed following the ripple structure, thus slowly advancing the ripple pattern in the direction of the flow without altering ripple geometry. Fine sediment particles and hydrogen bubbles were used individually or combined as flow tracers in the acoustic measurements. When used individually, hydrogen bubbles provided full depth flow and backscattering information, whereas sediment particles traced only the lower layers of the flow, indicating sediment suspension. When both tracers were combined, hydrogen bubbles could only be distinguished from sediment particles when results at two different acoustic carrier frequencies were compared. The intermittency was observed in the backscattering of the acoustic system. The event structure in fine sediment suspension is seen by the PTV method. PTV velocity vectors varied in speed and orientation, were organized in large coherent packets, mainly in the near bed layers, but also extended well above the bed. The two methods provide complementary information. ADVP measurements allow long time series analysis, whereas most of the spatial details seen in the PTV results cannot be resolved in the ADVP measurements.

► We studied fine sediment dynamics in an unsteady laboratory open-channel flow. ► Acoustic and optical measurement systems are combined and complement each other. ► Ripples are formed rapidly after bed-load movement started due to flow acceleration. ► Fine sediment is suspended into the lower part of the water column in packets. ► Most of these packets originate near the reattachment point on the ripple stoss side.