Author(s): Fereshteh Bagherimiyab; U Lemmin
Keywords: Fine sediment; Resuspension; Unsteady flow; Accelerating flow; Acoustic Doppler methods; Bottom shear stress
Abstract: In this paper unsteady flow over a movable (but not moving) coarse gravel bed was investigated. In order to determine the effect of suspended sediment concentration on the flow field, experiments were first carried out without sediment and then repeated with fine sediments added. Using acoustic Doppler and imaging methods, quasi-instantaneous profiles of velocity and sediment concentration were taken simultaneously and collocated with spatial and temporal resolution of turbulence scales. They were complemented by synchronized direct bottom shear stress measurements by a hot-film sensor. Using a high-speed camera, flow visualization was made in parallel, in particular during the initial phase of sediment resuspension. In order to evaluate the processes in detail, the unsteady flow was segmented into several time slices. Emphasis in the analysis was put on the time-varying structure of turbulence characteristics, since it is assumed that organized and random variations of bottom shear stress and Reynolds shear stress in time and with depth are important in sediment suspension dynamics. Results with and without fine sediments were compared. This provided new insight into the process of sediment suspension in unsteady flow and allowed investigating the effect of sediment load on the flow characteristics.