Fereshteh Bagherimiyab Ulrich Lemmin

1. Effects of bed form structure on particle-turbulence interaction in unsteady suspended sediment-laden laboratory open-channel flows Fereshteh Bagherimiyab Ulrich Lemmin • Introduction • Experimentalset-up • Procedure • Results • Conclusion

2. Introduction • Turbulence plays an essential role in • suspended sediment transport • Suspension of fine sediments • Generates bed forms • Affects Water quality in rivers • Impacts on the environment

3. Experimental set-up ADVP withhousing B = 0.6m h = 0.8m L=17m side walls = transparent glass gravel layer= 0.1 m thick gravel size range = 3 to 8 mm D50 = 5.5 mm

4. Experimental procedure Constant discharge ?

5. Results Range of variations of discharge, water depth, mean velocity and Reynolds number

6. Results • Velocity measurements without sediment Longitudinal mean velocity and mean velocity from log fit during the unsteady flow stages against depth changes

7. Results Friction velocity u* distribution for the unsteady ranges of 30s and 60 s

8. Flow withparticle motion Sediment suspension 1 m D50 = 0.16 mm Thickness = 6 mm

9. Results Particle velocity with ADVP Mean particle velocities during the accelerating and the peak flow stages

10. Results Particle Tracing Velocimetry (PTV) a) b) • suspension is nearly uniform in a shallow layer above the bed • Suspension into the water column above occurs in burst-like events in the final phase, strongest near the ripple crest • The highest concentration is found near the bottom • total suspension, increase in the final phase. This confirms the importance of burst structures with strongest bursts near the ripple crests PTV results during the (a) initial and (b) final phase of the accelerating stage. Arrows indicate particle velocity vectors • Particle velocity profiles extend higher into the water column in the final phase concentration (gr cm-3) Particle concentrations in the same image slices Mean particle velocities in ten image slices for the (a) initial and (b) final phases of the accelerating stage

11. Results Example of PTV results during the final phase of the accelerating flow range. Arrows indicate particle velocity vectors a) b) • The highest concentration is found in the position where ripples are formed (x = 10.2 cm with a strong gradient towards the trough • This confirms the burst structure pattern with strongest bursts near the ripple crests Particle velocities in six positions of images • Particle concentrations in six positions of images Bed form formation during the final phase of the accelerating flow range

13. Results ripples were observed L= 0.8 water depth h=5 mm fine particles rolled along ripples • Ripples formed quickly, within about 3 sec after fine sediment particle saltation started

14. Results • during the initial saltation of particles, the upper layer of the whole bed began to move (Fig. a) • Particles may rise about 1 cm above the bed • Saltation above the bed level occurs in bursts • The ripple started forming 2.5 s after (Fig. b) • vortex is formed in the lee side of the crest • the vertical velocity component is strong in the near bottom layer Close-up view of sediment particle trajectories related to ripple formation

15. Results Remains of the initial reference bed level • ripples are formed from some instability on the bed • towards the end of the acceleration range, ripples occurred nearly simultaneously along the whole bed and quickly formed a pattern • The whole pattern slowly moves in the direction of the flow, but the distribution does not change significantly • Over time during the steady peak flow of about 3 minutes, the colour of the dark area changes little Large scale image of ripple formation, seen from the top. Mean spacing of the ripples is about 6 cm

17. Conclusion • Sediment suspension stronglycharacterized by burstevents and the presence of ripples on the bed • High sediment suspension continued to occur during the decelerating flow even though the flow velocity decreased • Sediment suspension in unsteady flow is controlled by the same large scale turbulence processes as in steady flow • The wavelength of the ripples depended on the rate of acceleration • Faster acceleration produced shorter ripples due to a much stronger friction velocity • The shorter ripples produced stronger and higher suspension • Ripples did not change in appearance or dimension for the duration of the experiments