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

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

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.