M.J. Franca & U. Lemmin

RCEM, 6th October 2005

CROSS-SECTION PERIODICITY OF TURBULENT GRAVEL-BED RIVER FLOWS

OUTLINE

- Instrumentation (ADVP) - River measurements- Bed forms- Mean velocity- Turbulence production- Conclusions

ACOUSTIC DOPPLER VELOCITY PROFILER (ADVP)

- Acoustic sonar is based on the echo backscattered by moving targets.

- It allows the measurement of quasi-instantaneous 3D velocity profiles.

ADVP configurationdeployable structure

RIVER MEASUREMENTSInvestigation in the Swiss river Chamberonne under stationary and shallow water conditions.

Ven

og

e 1

Ch

am

bero

nn

eVen

og

e 2

Q

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

measuring section

measuring grid – across the section

BED FORMS

- Periodic bed shape across the section - b,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

MEAN VELOCITY DISTRIBUTION: U

- 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 CL+,y ≈ 2h

- The coexistence of CL and CH cells implies compensatory secondary motion

contour lines of the mean streamwise velocity across-section

TRANSVERSAL MEAN VELOCITY FIELD: V and W

- 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)

“detrended” mean transversal velocities

THE VELOCITY DIP

dU(y) umax(y)

u(y,z 0.90h)

Fr(y) U(y)

gh(y)

- 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

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

power spectrum density variation across the section

≈ 2 cm from the bottom

≈ 2 cm from the surface

CROSS-SECTION PERIODICITY OF THE FLOW STRUCTURE

- All flow characteristics are bed-form dependent

- The flow is influenced by the macro-scale roughness until the surface (undulation the dU curve)

- Bed forms: b,y ≈ 2h

- Roughness: k,yu*,y ≈ 2h, in

phase

- Momentum: q,y ≈ 2h, out of phase

- Velocity dip: CL+,y ≈ 2h out of phase

periodicity of different flow characteristics

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

M.J. Franca & U. Lemmin

RCEM, 6th October 2005

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: k,y = u*,y≈ 2h

- The flow resistance has a strong form dependence

CROSS-SECTION PERIODICITY OF THE FLOW STRUCTURE

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.

b,y ≈ 2h

k,yu*,y ≈ 2h, in phase

q,y ≈ 2h, out of phase

CL+,y ≈ 2h out of phase