SECONDARY FLOW AT A SCOUR HOLE DOWNSTREAM A BAR-CONFLUENCE (PARANA RIVER, ARGENTINA)

1. R.N. Szupiany & M.L. AmslerConsejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina.Facultad de Ingeniería y Ciencias Hídricas, Universidad Nacional del Litoral, Argentina.J.J. FedeleSaint Anthony Falls Laboratory, University of Minnesota, Minneapolis, USA. SECONDARY FLOW AT A SCOUR HOLE DOWNSTREAM A BAR-CONFLUENCE (PARANA RIVER, ARGENTINA)

2. INTRODUCTION • Results are reported herein of a project whose main purpose is to study the flow pattern and its relationships with the bed sediment transport along the nodal units (confluence-constriction-expansion) of large braided streams as the Paraná River. • The investigation is based on field measurements.

3. INTRODUCTION • Most reported experimental and numerical studies at river confluences, has been performed in small streams with limited depths and widths. • The study of confluences in large rivers unlike the case in small streams are extremely scarce, essentially because of a lack of proper instrumentation. • The Doppler technology proved to be useful for accurate determinations of flow discharges in large streams but the reliability of its signals for other calculations such as the secondary circulation, still remain highly untested.

4. INTRODUCTION In this paper, a preliminary analysis of the secondary currents measured at a scour hole downstream of an asymmetrical bar-confluence in the Paraná River, Argentina, is presented. A methodology that allows for obtaining representative values of the time-averaged velocities with moving-vessel ADP measurements, was used.

5. MAIN FEATURES OF THE PARANÁ RIVER (Middle Reach) • Mean discharge of 19500 m3 s-1. • Water surface slopein the order of 10-5. • Channel bed composed largely by fine and medium sands. • Channel planform pattern classified as braided with a meandering thalweg. • Mean widths between 600-2500 m • Depths ranging 5 to more than 12 m.

6. STUDY SITE AND MEASURING CROSS SECTION

7. DESCRIPTION OF FIELD MEASUREMENTS Equipments: • ADP of 1000 kHz (moving-vessel measurements were performed). • Cell size of 0.75 m • Averaging interval of 10 seconds • DGPS • Echo-sounder • Hydrographic vessel

8. DESCRIPTION OF FIELD MEASUREMENTS Methodology: • Bathymetry of the confluence zone based on cross section measurements each separated 100 m. • ADP velocity measurements at seven cross sections located along the braid-bar confluence. A procedure was used that requires at least five transects recorded with the ADP at each cross section to obtain representative time-averaged horizontal velocity data (Szupiany & Amsler, 2005).

9. ANALYSIS OF VELOCITY DATASECONDARY CURRENTS Methods classification (Lane et al. 2000): • The centerline definition. • The discharge continuity definition. • The zero net cross-stream discharge definition. • The Rozovskii definition. Much discussion still remains regarding which of the above methods yield the most realistic secondary velocity field. In this study in a large alluvial river with its constraints for data collection, the discharge continuity procedure and the centerline definition were discarded.

10. D epth average velocity vector North v P oint velocity vectors p q a v s The Rozovskii definition (basic expressions): PRIMARY COMPONENT SECONDARY COMPONENT where: : orientation of v respecting the north; : orientation respecting the north of the depth average velocity vector vN, vE: north and eastward components of the point velocity vectors

11. RESULTS MAIN HYDRAULIC CONDITIONS AT THE CONFLUENCE SECTIONSS4(LB)S3(RB)S5 Mean flow velocity (m s-1) 0.90 1.12 1.25 Width (m) 680 750 900 Discharge (m3 s-1) 7036 10610 17646 Maximum depth (m) 18 16 31 ----------------------------------------------------------------------- Junction angle (degrees) 65 Flow momentum ratio, M* 0.53 Discharge ratio0.67 * (, water density, Q, total discharge, V, cross-section average velocity and the subscripts L and R denote left and right branch)

12. STUDY SITE AND MEASURING CROSS SECTION

13. RESULTS MORPHOLOGIC CONDITIONS AT THE CONFLUENCE

14. RESULTS HORIZONTAL SECONDARY CURRENTS Horizontal secondary currents in section S5 (average values of 5 transects).

15. RESULTS VERTICAL SECONDARY CURRENTS Vertical secondary currents in section S5 (average values of 5 transects).

16. RESULTS HORIZONTAL SECONDARY CURRENTS Horizontal secondary currents in section S5 (average values of 5 transects). Horizontal secondary currents in section S5 (values from 1 transect).

17. CONCLUDING REMARKS • The secondary circulation in the scour hole downstream of a bar-confluence in a large braided river was obtained applying the Rozovskii method. The results presented herein must be viewed as the first step of an investigation that includes intercomparison of methods to check which one better represents the secondary structures at confluences and nodes of large rivers like the Paraná.

18. CONCLUDING REMARKS • Making allowance for the statement above, two counter-rotating surface convergent secondary currents clearly characterize the mean flow structure at the scour hole downstream the studied bar confluence. Similar patterns have been observed in laboratory and in small natural streams (McLelland et al. 1996, Rhoads 1996, between others). The two secondary cells locate on either side of the mixing interface at  260-300 m from left bank.

19. CONCLUDING REMARKS • Apparently the strength of the left secondary cell is larger than the right one, something that could be related with several factors such as: the flow momentum value M (<1), flow scale and confluence asymmetry.

20. CONCLUDING REMARKS • An averaging method specifically designed (Szupiany & Amsler 2005) to record representative mean flow velocity values with moving-vessel ADP measurements was used to obtain the previous results. In this manner a large number of average point primary velocities at a given cross-section of a large river is rapidly possible to obtain.

21. CONCLUDING REMARKS • The previous conclusion would be also an indirect evidence in the Paraná River of the statement by Rhoads (1996), referring to the two secondary cells as a mechanism to transport high-velocity, near-surface fluid downward toward the bed over the zone of maximum channel depth and inward along the bed toward each bank. Note that the secondary velocities near the bed of the left cell appear to be of the same order than those near the water surface.

23. SOME APPLIED IMPLICATIONS The condition of M<1, would have important morphologic consequences since the scour hole appears displaced towards the left bank thus preventing the existence of the separation zone and bar region observed downstream the junction corner in small streams (Best 1987, McLelland et al. 1996). (Bathymetric measurements complemented with visual inspections by the authors, revealed an intense erosive process on the left bank at S5)

26. THE END THANK YOU !

27. The zero net cross-stream discharge definition Considering the entire cross section Dividing in 2 sectors the cross section

28. The zero net cross-stream discharge definition Dividing in 3 sectors the cross section Applying the Rozaovskii method