R.N. Szupiany & M.L. AmslerR.N. Szupiany & M.L. AmslerConsejo Nacional de Investigaciones Consejo Nacional de Investigaciones

Científicas y Técnicas (CONICET), Argentina.Científicas y Técnicas (CONICET), Argentina. Facultad de Ingeniería y Ciencias Hídricas, Facultad de Ingeniería y Ciencias Hídricas, Universidad Nacional del Litoral, Argentina.Universidad Nacional del Litoral, Argentina.

J.J. FedeleJ.J. FedeleSaint Anthony Falls Laboratory, University of Saint Anthony Falls Laboratory, University of

Minnesota, Minneapolis, USA.Minnesota, Minneapolis, USA.

SECONDARY FLOW AT A SCOUR SECONDARY FLOW AT A SCOUR HOLE DOWNSTREAM A BAR-HOLE DOWNSTREAM A BAR-

CONFLUENCE (PARANA RIVER, CONFLUENCE (PARANA RIVER, ARGENTINA)ARGENTINA)

INTRODUCTIONINTRODUCTION

Results are reported herein of a Results are reported herein of a project whose main purpose is tproject whose main purpose is to o study the flow pattern and its study the flow pattern and its relationships with the bed sediment relationships with the bed sediment transport along transport along the nodal units the nodal units (confluence-constriction-expansion) (confluence-constriction-expansion) of large braided streams as of large braided streams as the the Paraná RiverParaná River..

The investigation is based on field The investigation is based on field measurements.measurements.

INTRODUCTIONINTRODUCTION Most reported experimental and Most reported experimental and

numerical studies at river confluences, numerical studies at river confluences, has been performed in small streams has been performed in small streams with limited depths and widths.with limited depths and widths.

The study of confluences in large rivers The study of confluences in large rivers unlike the case in small streams are unlike the case in small streams are extremely scarce, essentially because of extremely scarce, essentially because of a lack of proper instrumentation.a lack of proper instrumentation.

TThe Doppler technology proved to be he Doppler technology proved to be useful for accurate determinations of useful for accurate determinations of flow discharges in large streams but the flow discharges in large streams but the reliability of its signals for other reliability of its signals for other calculations such as the secondary calculations such as the secondary circulation, still remain highly untested.circulation, still remain highly untested.

INTRODUCTIONINTRODUCTION

In this paper, a preliminary In this paper, a preliminary analysis of the secondary analysis of the secondary

currents measured at a scour currents measured at a scour hole downstream of an hole downstream of an

asymmetrical bar-confluence in asymmetrical bar-confluence in the Paraná River, Argentina, is the Paraná River, Argentina, is

presented.presented.

A methodology that allows for A methodology that allows for obtaining representative values of the obtaining representative values of the time-averaged velocities with moving-time-averaged velocities with moving-vessel ADP measurements, was used.vessel ADP measurements, was used.

MAIN FEATURES OF THE PARANÁ MAIN FEATURES OF THE PARANÁ RIVER (Middle Reach) RIVER (Middle Reach)

Mean Mean dischargedischarge of of 19500 m19500 m33 s s-1-1.. Water surface Water surface slopeslope in the order of in the order of

1010-5-5.. Channel bed composed largely by Channel bed composed largely by

fine and medium sandsfine and medium sands. . Channel planform pattern classified Channel planform pattern classified

as as braided with a meandering braided with a meandering thalwegthalweg. .

Mean Mean widths widths between between 600-2500 m600-2500 m DepthsDepths ranging ranging 5 to more than 12 m5 to more than 12 m..

STUDY SITE AND MEASURING CROSS STUDY SITE AND MEASURING CROSS SECTIONSECTION

S1

S2

S3

S4

S5 S6S7

PARANA RIVER

NO

RT

H

LEFT BRANCH

RIGHT BRANCH

ROSARIO CITY

0 1 km

Paraná River

Rosario City

NORTH Paraná R.

Paraguay R

DESCRIPTION OF FIELD DESCRIPTION OF FIELD MEASUREMENTSMEASUREMENTSEquipments:Equipments: ADP of 1000 kHz (moving-vessel ADP of 1000 kHz (moving-vessel

measurements were performed).measurements were performed). Cell size of 0.75 mCell size of 0.75 m Averaging interval of 10 secondsAveraging interval of 10 seconds

DGPSDGPS Echo-sounderEcho-sounder

HydrographicHydrographic

vesselvessel

DESCRIPTION OF FIELD DESCRIPTION OF FIELD MEASUREMENTSMEASUREMENTSMethodology:Methodology: Bathymetry of the confluence zone Bathymetry of the confluence zone

based on cross section based on cross section measurements each separated 100 measurements each separated 100 m.m.

ADP velocity measurements at ADP velocity measurements at seven cross sections located along seven cross sections located along the braid-bar confluence. the braid-bar confluence. A procedure was used that requires A procedure was used that requires at least five transects recorded with at least five transects recorded with the ADP at each cross section to the ADP at each cross section to obtain representative time-averaged obtain representative time-averaged horizontal velocity data (Szupiany & horizontal velocity data (Szupiany & Amsler, 2005).Amsler, 2005).

ANALYSIS OF VELOCITY DATAANALYSIS OF VELOCITY DATASECONDARY CURRENTSSECONDARY CURRENTS

Methods classification Methods classification (Lane et al. 2000)(Lane et al. 2000):: The centerline definition.The centerline definition. The discharge continuity definition.The discharge continuity definition. The zero net cross-stream discharge The zero net cross-stream discharge

definition.definition. The Rozovskii definition.The Rozovskii definition.

Much discussion still remains regarding Much discussion still remains regarding which of the above methods yield the most which of the above methods yield the most realistic secondary velocity field. In this realistic secondary velocity field. In this study in a large alluvial river with its study in a large alluvial river with its constraints for data collection, constraints for data collection, the discharge the discharge continuity procedurecontinuity procedure and and the centerline the centerline definitiondefinition were discarded. were discarded.

The Rozovskii definition The Rozovskii definition (basic (basic

expressions)expressions):: )(cos vv p

)(sin vv s

: 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

where:

22EN vvv

PRIMARY COMPONENT

SECONDARY COMPONENT

North

Depth average velocity vector Point velocity vectors

pv

sv

RESULTSRESULTSMAIN HYDRAULIC CONDITIONS AT THE CONFLUENCEMAIN HYDRAULIC CONDITIONS AT THE CONFLUENCE

RRLL VQVQM /

SECTIONSSECTIONS S4(LB)S4(LB) S3(RB)S3(RB) S5S5Mean flow velocity (m sMean flow velocity (m s-1-1) 0.90 ) 0.90 1.12 1.12 1.251.25Width (m)Width (m) 680 680 750 750 900 900Discharge (mDischarge (m33 s s-1-1)) 7036 7036 10610 10610 17646 17646Maximum depth (m)Maximum depth (m) 18 18 16 16 31 31----------------------------------------------------------------------------------------------------------------------------------------------Junction angle (degrees)Junction angle (degrees) 65 65 Flow momentum ratio, Flow momentum ratio, MM** 0.53 0.53

Discharge ratioDischarge ratio 0.670.67

((, water density,, water density, Q, Q, total discharge, total discharge, V,V, cross-section average velocity and the cross-section average velocity and the subscriptssubscripts L L and and RR denote left and right branch) denote left and right branch)

*

STUDY SITE AND MEASURING CROSS STUDY SITE AND MEASURING CROSS SECTIONSECTION

S1

S2

S3

S4

S5 S6S7

PARANA RIVER

NO

RT

H

LEFT BRANCH

RIGHT BRANCH

ROSARIO CITY

0 1 km

Paraná River

Rosario City

NORTH Paraná R.

Paraguay R

RESULTSRESULTSMORPHOLOGIC CONDITIONS AT THE MORPHOLOGIC CONDITIONS AT THE

CONFLUENCECONFLUENCE

S5

N

River stage at Rosario Port gage: 3.86mDate: 21/12/04

RESULTSRESULTSHORIZONTAL SECONDARY CURRENTSHORIZONTAL SECONDARY CURRENTS

Horizontal secondary currents in section S5 Horizontal secondary currents in section S5 (average values of 5 transects).(average values of 5 transects).

RESULTSRESULTSVERTICAL SECONDARY CURRENTSVERTICAL SECONDARY CURRENTS

Vertical secondary currents in section S5 Vertical secondary currents in section S5 (average values of 5 transects).(average values of 5 transects).

RESULTSRESULTSHORIZONTAL SECONDARY CURRENTSHORIZONTAL SECONDARY CURRENTS

Horizontal secondary currents in section S5 (average Horizontal secondary currents in section S5 (average values of 5 transects).values of 5 transects).

Horizontal secondary currents in section S5 (values from Horizontal secondary currents in section S5 (values from 1 transect).1 transect).

CONCLUDING REMARKSCONCLUDING REMARKS

The secondary circulation in the The secondary circulation in the scour hole downstream of a bar-scour hole downstream of a bar-confluence in a large braided river confluence in a large braided river was obtained applying the Rozovskii was obtained applying the Rozovskii method. method.

The results presented herein must be The results presented herein must be viewed as the first step of an viewed as the first step of an investigation that includes investigation that includes intercomparison of methods to check intercomparison of methods to check which one better represents the which one better represents the secondary structures at confluences and secondary structures at confluences and nodes of large rivers like the Paraná.nodes of large rivers like the Paraná.

CONCLUDING REMARKSCONCLUDING REMARKS

Making allowance for the statement Making allowance for the statement above, two counter-rotating surface above, two counter-rotating surface convergent secondary currents clearly convergent secondary currents clearly characterize the mean flow structure characterize the mean flow structure at the scour hole downstream the at the scour hole downstream the studied bar confluence. studied bar confluence.

Similar patterns have been observed in Similar patterns have been observed in laboratory and in small natural streams laboratory and in small natural streams (McLelland et al. 1996, Rhoads 1996, (McLelland et al. 1996, Rhoads 1996, between others). The two secondary cells between others). The two secondary cells locate on either side of the mixing locate on either side of the mixing interface at interface at 260-300 m from left bank. 260-300 m from left bank.

CONCLUDING REMARKSCONCLUDING REMARKS

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

CONCLUDING REMARKSCONCLUDING REMARKS

An averaging method specifically An averaging method specifically designed (Szupiany & Amsler designed (Szupiany & Amsler 2005) to record representative 2005) to record representative mean flow velocity values with mean flow velocity values with moving-vessel ADP moving-vessel ADP measurements was used to measurements was used to obtain the previous results.obtain the previous results.

In this manner a large number of In this manner a large number of average point primary velocities at a average point primary velocities at a given cross-section of a large river is given cross-section of a large river is rapidly possible to obtain.rapidly possible to obtain.

CONCLUDING REMARKSCONCLUDING REMARKS

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

The condition of M<1, would have The condition of M<1, would have important morphologic consequences important morphologic consequences since the scour hole appears displaced since the scour hole appears displaced towards the left bank thus preventing towards the left bank thus preventing the existence of the separation zone the existence of the separation zone and bar region observed downstream and bar region observed downstream the junction corner in small streams the junction corner in small streams (Best 1987, McLelland et al. 1996)(Best 1987, McLelland et al. 1996). .

(Bathymetric measurements (Bathymetric measurements complemented with visual inspections complemented with visual inspections by the authors, revealed an intense by the authors, revealed an intense erosive process on the left bank at S5)erosive process on the left bank at S5)

SOME APPLIED IMPLICATIONSSOME APPLIED IMPLICATIONS

S5

N

River stage at Rosario Port gage: 3.86mDate: 21/12/04

0

5

10

15

20

25

30

900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900

Progresive (m)

Dep

th (

m)

Year 1970

Year 2004

RIGTH BANK LEFT BANK

S5

N

River stage at Rosario Port gage: 3.86mDate: 21/12/04

THE ENDTHE END

THANK YOU !THANK YOU !

The zero net cross-stream discharge The zero net cross-stream discharge definitiondefinition

Considering the entire cross sectionConsidering the entire cross section

Dividing in 2 sectors the cross sectionDividing in 2 sectors the cross section

The zero net cross-stream discharge The zero net cross-stream discharge definitiondefinition

Applying the Rozaovskii methodApplying the Rozaovskii method

Dividing in 3 sectors the cross sectionDividing in 3 sectors the cross section

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M

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