Fereshteh Bagherimiyab

Ulrich Lemmin

Effects of bed form structure on particle-turbulence interaction in unsteady suspended sediment-laden

laboratory open-channel flows

Introduction Experimental set-up Procedure Results Conclusion

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

Experimental set-up

side walls = transparent glass gravel layer= 0.1 m thick gravel size range = 3 to 8 mmD50 = 5.5 mm

L=17mh = 0.8m

B = 0.6m

ADVP with housing

Experimental procedure

Constant discharge ?

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

Results

Hydrograph: Low High

Pump discharge Q (l sec-1) 10 35Water depth h (cm) 10 15

Mean velocity u (cm s-1) 17 39

Reynolds Number 1.5 104 5.3 104

ResultsVelocity measurements without sediment

Longitudinal mean velocity and mean velocity from log fit during the unsteady flow stages against depth changes

Results

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

Flow with particle motion Sediment suspension

D50 = 0.16 mm

1 m

Thickness = 6 mm

Results

Mean particle velocities during the accelerating and the peak flow stages

Particle velocity with ADVP

Results

a) b)

PTV results during the (a) initial and (b) final phase of the accelerating stage. Arrows indicate particle velocity vectors

Mean particle velocities in ten image slices for the (a) initial and (b) final phases of the accelerating stage

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

Particle velocity

profiles extend higher into the water column in the final phase

Particle Tracing Velocimetry (PTV)

Particle concentrations in the same image slices

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

concentration (gr cm-3)

ResultsExample of PTV results during the final phase of the accelerating flow range. Arrows indicate particle velocity vectors

a) b)

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

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

Results

L= 0.8 water depth h=5 mm

fine particles rolled along ripples

ripples were observed

Ripples formed quickly, within about 3 sec after fine sediment particle saltation started

Results

Close-up view of sediment particle trajectories related to ripple formation

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

Results

Large scale image of ripple formation, seen from the top. Mean spacing of the

ripples is about 6 cm

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

Remains of the initial reference bed level

Sediment suspension strongly characterized by burst events 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

Conclusion