1. CROSS-SECTION PERIODICITY OF TURBULENT GRAVEL-BED RIVER FLOWS
M.J. Franca & U. Lemmin
RCEM, 6th October 2005

2. OUTLINE Instrumentation (ADVP) River measurements Bed forms
Mean velocity
Turbulence production
Conclusions

3. ACOUSTIC DOPPLER VELOCITY PROFILER (ADVP)
ADVP configuration
deployable structure
Acoustic sonar is based on the echo backscattered by moving targets.
It allows the measurement of quasi-instantaneous 3D velocity profiles.

4. RIVER MEASUREMENTS Investigation in the Swiss river Chamberonne
under stationary and shallow water conditions.
measuring grid – across the section
measuring section
Mean slope – S
(%)
Discharge – Q
(m3/s)
Mean water depth
(m)
Width – B (m) Re
(x104) Fr
D50 from the bottom (mm)
D84 from the bottom (mm)
0.67
0.55
0.29
5.75
4.5 – 12.3
0.24 – 0.44 49 81

5. BED FORMS Periodic bed shape across the section - lb,y ≈ 2h ≈ 10%B
Signatures of streamwise sediment stripes produce during high water events when bed load transport occurs
Prandtl’s secondary motion of the second type may take place during high water events due to the decrease in the aspect ratio
cross-section of the riverbed

6. MEAN VELOCITY DISTRIBUTION: U
contour lines of the mean streamwise velocity across-section
The structure of the flow is 3D
Mean flow distribution and flow resistance are strongly form-dependent
Periodically distributed high and low velocity regions were detected in the surface layer: CH and CL regions or cells
More intense CL cells coincide with the deeper profiles - periodicity lCL+,y ≈ 2h
The coexistence of CL and CH cells implies compensatory secondary motion

7. TRANSVERSAL MEAN VELOCITY FIELD: V and W
“detrended” mean transversal velocities
Permanent organized structure: SLOM – Surface Layer Organized Motion
Lateral mass transfer between CH and CL regions
A rotating movement is induced by the lateral transfer - streamwise vorticity
SLOM vortical cells scale with the water depth (density of 4 cells per meter)

8. THE VELOCITY DIP D-shaped profiles correspond to the CL regions
The occurrence of the d-shaped profiles is related to the local flow regime
For Fr<≈0.35 the dip phenomenon is important
relation between the velocity dip and local Froude number

9. PERIODICITY OF THE TURBULENCE PRODUCTION
The spectral dynamics is also conditioned by the bed form periodicity
The extent of the productive plateau varies as function of the bed forms
≈ 2 cm from the surface
≈ 2 cm from the bottom
power spectrum density variation across the section

10. CROSS-SECTION PERIODICITY OF THE FLOW STRUCTURE
Bed forms: lb,y ≈ 2h
Roughness: lk,y = lu*,y ≈ 2h, in phase
Momentum: lq,y ≈ 2h, out of phase
Velocity dip: lCL+,y ≈ 2h out of phase
periodicity of different flow characteristics
All flow characteristics are bed-form dependent
The flow is influenced by the macro-scale roughness until the surface (undulation the dU curve)

11. CONCLUSIONS
Existence of periodically distributed streamwise coarse sediment ridges formed during flood events
The flow structure is essentially 3D, hence 2D concepts are to be used with care
The hydraulic characteristics of the river flow are conditioned by the periodic bed forms: mean and turbulent flow
Existence of an organized 3D flow in the surface layer, SLOM, conditioned by the bed forms/local flow regime
A general wake effect induced by the large-scale roughness may confine the flow response to the layer z/h>0.80
These results are important in respect to transport and mixing processes in rivers

12. M.J. Franca & U. Lemmin
RCEM, 6th October 2005

15. FLOW RESISTANCE
The influence of the bed forms is also visible in the roughness parameterization of the flow (k and u*)
The Nikuradse equivalent roughness and the friction velocity are in phase with the bed forms: lk,y = lu*,y≈ 2h
The flow resistance has a strong form dependence

16. CROSS-SECTION PERIODICITY OF THE FLOW STRUCTURE
lb,y ≈ 2h
lk,y = lu*,y ≈ 2h, in phase
lq,y ≈ 2h, out of phase
lCL+,y ≈ 2h out of phase
All flow characteristics are influenced by the bed forms; in the presence of macro-scale roughness elements like these ones, the bed forms control the flow character up to the surface; the inviscid response of the flow is important all the way to the surface, as can be demonstrated by the undulation form of the dU curves.