Sediment Suspension Dynamics in Turbulent Unsteady, Depth-Varying Open-Channel Flow over a Gravel Bed

Abstract

Even though flow in natural rivers and channels is generally unsteady, only a few studies on turbulent structures in unsteady open-channel flows have been carried out. In hydraulic engineering problems, unsteady flow is often approximated with concepts of steady flow, because the treatment of unsteady flow can be difficult. Many variables enter into the mathematical relationship and the differential equations cannot be integrated in closed form, except under very simplified conditions. The limited number of studies of unsteady flow may also be due to the lack of experimental equipment capable of capturing small-scale, unsteady open-channel flow dynamics. Therefore, it is important to know when unsteady flow can be approximated by steady flow concepts and which simplifications are acceptable. In most cases, especially when simulating unsteady flood flow by a steady flow approach, the calculations may not produce reliable results. The mechanism of sediment transport in rivers and open channels is governed by complicated interactions between unsteady accelerating and decelerating turbulent flow, particle motion and bed configuration. Understanding the dynamics of unsteady sediment-laden water flows and characterizing the velocity of suspended particles is essential for enhancing the predictive accuracy of sediment transport and its impact on environmental processes in the water column. In order to simulate fine sediment dynamics over an armored bed in a river during the passage of a flood wave, unsteady accelerating, and decelerating open-channel flow over a movable (but not moving) coarse gravel bed (D50 = 5.5 mm) first without and then with fine sediment were studied. A layer of fine sediment of mean particle size about 120 µm was placed on the coarse gravel bed. The thickness of the fine sediment layer on the gravel bed was varied between 4 mm 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 and imaging method, using an Acoustic Doppler Velocity Profiler (ADVP) was combined with an optical method, using Particle Tracking Velocimetry (PTV) for suspended sediment particle tracking. Measurements resolved turbulence scales. Unsteadiness strongly affects the profile shape of velocity and friction velocity, particularly in the final phase of the accelerating range. Flow in the decelerating range approaches steady flow. Systematically higher friction velocities were observed in the accelerating flow than in the decelerating flow for comparable flow depth. This indicates that for the same change of relative submergence, different flow dynamics are generated during accelerating and decelerating flows. For the lowest unsteadiness (90 s hydrograph), the differences between velocity profiles in the accelerating and the decelerating ranges become small, indicating that for this unsteadiness, steady state conditions are approached. During the accelerating flow range, fine sediment suspension from the bed started in bursts and in the final phase of the accelerating flow range, a ripple pattern is rapidly created that remained nearly stationary. Thereafter, vortex shedding produced most of the sediment suspension into the water column in the form of events, 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. However, ripple geometry and ripple shape were not altered by this process, despite the fact that flow velocities changed. Due to the ripple structure, high sediment suspension events continued to occur in bursts during the decelerating flow even though mean flow velocity and friction velocity decreased. The dynamics of sediment suspension observed in this study indicate that mean value concepts cannot be applied in unsteady flow. Fine sediment particles and hydrogen bubbles were used individually and 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 not be distinguished from sediment particles. 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, but were organized in large coherent packets, mainly in the near-bed layers. They also extended well above the bed, supporting the concept that coherent structure events contribute to sediment suspension over ripples. The two methods provide complementary information. ADVP measurements allow long timeseries analysis, whereas the spatial details seen in the PTV results cannot be resolved in the ADVP measurements.