PREDICTION OF THE THALWEG ASPECT RATIO AT A LARGE ALLUVIAL RIVER: THE PARANA RIVER (ARGENTINA)

 

Horacio TONIOLO (*), Mario L. AMSLER (**), Carlos RAMONELL(*)

 

(*)C.C. 217 (3000), Santa Fe, Argentina; Fax: (54) (042 571143);

e- mail:htoniolo@fich1.unl.edu.ar . Facultad de Ingeniería y Ciencias Hídricas (FICH) Universidad Nacional del Litoral

(**)C.C. 217 (3000), Santa Fe, Argentina; Fax: (54) (042 571143)

Consejo Nacional de Investigaciones Científicas y Técnicas

 

 

ABSTRACT

The Paraná river is one of the largest alluvial rivers of the world (mean discharge: 21,000 m3/s; total length: 3,943 km) and, as such, it is a fundamental communication way of great economical importance for several countries in South America. These facts have prompted recent studies about its behavior, specially those concerning the improvement of navigation that strongly depends on a proper knowledge of the stream thalweg dynamics.

Through measurements performed along this century, it was observed that the Paraná river thalweg has a sinuous and highly unstable pattern with nearly permanent shiftings, and with such a flow concentrated in it as to govern, essentially, the whole hydrosedimentologic dynamics of the channel. On this base, an empirical method is presented here in order to compute the geometric (mean widths and depths) and hydraulic parameters that define the thalweg area of the Paraná river channel. The method predicts that parameters for different morphological features of the main channel (enlargements and nodes). Moreover, it is valid for river stages near to those of the dominant discharges, a key requirement in a current investigation concerning the Paraná river thalweg shiftings.

 

Keywords: Paraná river thalweg; characterization of the thalweg area; enlargements and nodes; dominant discharges; morphologic investigations.

 

INTRODUCTION

The Paraná river is the principal stream in one of the major fluvial systems of the Earth. Its drainage area have 2,3.106 km2, encompassing surfaces of four South American countries -Brazil, Paraguay, Bolivia and Argentina-. It is the sixth largest river in the world as measured by discharge (average discharge: 21,000 m3/s), and the thirteenth one as measured by length (total length: 3,943 km) (Schumm and Winkley, 1994). These quantities explain why the main channel, as well as some tributaries, were used profitably as communication and commercial ways since the conquest of America. Such use was growing continuously during the current century, with an important increase during the nineties, due to the starting of the Paraná - Paraguay Rivers Waterway Project, directed to insure the inland navigation deeply inside the continent from its mouth in Argentina (Fig. 1). This project, a consequence of the economic growth of the region, was conditioned by the morphologic and sediment transport characteristics of the Paraná river, which requires the sailing route maintenance by means of nearly continuous dredgings. Historically, the dredging works were made downstream of the Paraguay river confluence (km 1240; Fig. 1), principally between the Paraná Port (km 601) and the river outlet (into the de la Plata river, km 130), a reach with oceanic ships traffic.

After the Paraguay river confluence, the Paraná main channel has similar morphologic, hydraulic and hydrologic properties, all in a reach of nearly 1000 km long. The river forms successive enlargements in which the stream divides into two or more branches around islands, separated by narrower, shorter and deeper reaches (or nodes). Upstream from Paraná Port, the relationship enlargement / node lengths are of 4 to 2.3, and the medium values of maximum width / maximum depth are 300 and 15 at both, enlargements and nodes, respectively. The water surface slope is in the order of 10-5 at both type-reaches (smaller at the narrower zones), and the bed sediment is composed by medium sized sands along the whole reach (Drago and Amsler, 1998).

Regarding the average main channel discharge, it is nearly constant downstream from Confluencia. Its minor changes are related with the maintenance of anabranches 1 to 170 km long, that flow along the Paraná alluvial plain between Confluencia and its mouth. The anabranches carry 5 to 15 % of the average discharge measured at Corrientes section (Fig. 1).

Though the fluvial system might be classified as an anastomosing one due to these anabranches, both, the planform pattern of the main channel, and the tendency of the stream to form mid-channel bars at the widenings, make the morphology to resemble the dynamic properties of a braided river. Notwithstanding, there is an important difference with a typical braided stream: the Paraná river has a well-defined thalweg both, in a morphological and a hydraulic-hydrological sense. Based on measurements performed along the current century, it was suggested that this channel area governs the whole hydrosedimentologic dynamics of the channel (Reposini, 1913; Soldano, 1947). Latter studies (Parody and Estruco, 1975; FICH, 1997), demonstrate that the flow concentrated in the thalweg zone forms a sinuous stream wandering inside the major one, in which typical processes of meandering rivers -such as translation and exaggeration of meander bends, meander cutoffs, etc.- are verified. The direct consequences of such dynamics are, for example, bank erosion rates from 50 to 100 m/year (at the apex's waves), or local stretching/shortening of the sailing route lengths up to 4.5 %/year.

In this context, this paper presents a methodology designed for the rationale characterization of the Paraná river thalweg, as a previous step to investigate about the factors responsible of its horizontal and vertical mobility. The latter are the goals of an investigation recently started at the FICH.

 

DATA BASE AND METHODOLOGY

To define the hydraulic and geometric characteristics of the Paraná river thalweg, 65 selected flow measurements performed at 16 cross sections of the Middle Paraná main channel, were examined. All the sections are distributed in a reach of 771 km long. The upper end section is set at Corrientes (Km 1204) and the lower end one, is at Diamante (Km 533) (Fig. 1). All of them are located at points representing different morphologic characteristics of the main channel, i.e., enlargements and nodes sectors. In brief, 7 sections pertain to single channel reaches, and 9 represent channel sectors with at least two branches (main and secondary).

The selected flow measurements cover a period of 83 years of the present Century. The measured flows correspond to river levels in the vicinity of the mean one. It has implicit the concept that the morphologic processes like those to be studied are related with a "dominant" discharge which is similar to the mean discharge in the Paraná river (FICH, 1997). In the selection of these discharges growing and falling stages of the river were also included.

By making use of the above flow measurements, the geometric and hydraulic parameters of the Paraná river thalweg were computed through the following steps:

 

·        The discharge distribution curve at each cross section was built in non dimensional coordinates. The shape of these curves allowed a first identification of sectors with concentration of flows at each section.

·        For each flow measurement the mobile averages of unit discharges across the section were computed, beginning with the two verticals of highest unit discharges. By considering the partial widths of each average, a diagram like that of Fig. 2 was built at each section. The particular flow measurements considered at the section are represented by different groups of data points. The curve in Fig. 2 was obtained by averaging the different series of points selected at that section.

 

It is clearly seen from this curve, that a Bt width zone exists at the section, where the gross of the discharge is transported. Outside this sector, the discharges flowing through the section decrease markedly.

 

·        Defined, Bt, the mean depth of that sector (thalweg zone), ht, was computed by averaging the measured depths at Bt.

 

·        From the geometric parameters defined in the previous steps at each section, the coefficient I1/2/ n in the thalweg area was calculated by means of the Manning equation.

 

GEOMETRIC AND HYDRAULIC CHARACTERISTICS OF THE PARANA RIVER THALWEG

For each selected cross section the whole aspect ratio Bo/ho (referred to the 0 m level at the local gage), was computed. With these values and the thalweg geometric parameters, Bt, and, ht, two functions were empirically defined: Bt/ho vs. Bo/ho and Bt/ht vs. Bo/ho (Figs. 3 and 4). It is observed at both graphs that the data points corresponding to channel enlargements clearly cluster separated from those corresponding to other morphologic channel features (confluences, diffluences and constrictions). The curves fit to each group produced fairly good determination coefficients (r2 » 0.8 - 0.85).

By means of the functions of Figs. 3 and 4, it would be possible to know the thalweg main geometric parameters at river reaches with different morphologic characteristics only by measuring the width and mean depth of the whole cross section.

 

In connection with the I1/2/n coefficient, the results were the following:

 

·        At sections in single channel sectors, the computed mean value in the thalweg area was 0.185 with a standard deviation of 0.026, while those of the total section were 0.195 and 0.028, respectively.

·        At sections in channel enlargements, the computed mean value in the thalweg area was 0.236 with a standard deviation of 0.023, while those of the total section were 0.221 and 0.039, respectively.

 

It is seen that the hydraulic conditions at the thalweg area are very similar to those for the whole section. The I1/2/n mean value at nodes is less than the one at enlargements, a fact compatible with the smaller gradients observed at constrictions in the Paraná river (FICH, 1997). Other differences between the absolute values obtained in the I1/2/n coefficients are being examined yet, by studying the bed resistance conditions at both, nodes and enlargements.

 

CONCLUSIONS

Based on observations and studies about hydraulics and morphology of the Paraná river during the current century, a methodology was developed in order to compute the thalweg aspect ratio, Bt/ht, of the channel. The thalweg area in the Paraná river has a sinuous planform shape resembling a meandering stream wandering inside a larger one. This stream concentrates the largest flows and depths at a given section, and seems to have a decisive incidence in the hydrosedimentologic behavior of the entire river.

The method presented in this paper allows the thalweg width and depth calculation, knowing the width and mean depth of the whole section. Moreover, the method shows that the thalweg dimensions would be different depending on certain morphologic characteristics of the main channel (enlargements, confluences, constrictions, etc.). The empirically fitted equations are valid for river stages near to its dominant discharge, a key requirement for the further morphologic studies concerning this investigation.

The values of the thalweg I1/2/n coefficient, were also computed. On comparing these values with those of the whole section, it is stressed the stated concept about the importance of the thalweg flow on the whole river dynamics.

 

ACKNOWLEDGEMENTS

The authors are grateful to state agencies and private companies of Argentina for supplying the basic information without which it had been impossible to carry out this work. It deserves to be mentioned: the National Institute of the Water and Environment; the Middle Paraná District of the National Direction of Port Constructions and Navigable Ways and the Resources Evaluation Co S.A. The Littoral National University supports this investigation through the grant N° 368910/18.

 

REFERENCES

1.      DRAGO, E. C. E. and AMSLER, M. L. (1998). "Bed sediment characteristics in the Paraná and Paraguay rivers". To be published in Water International. International Water Resources Association (IWRA).

2.      FICH (Facultad de Ingenieria y Ciencias Hídricas). (1997). "Análisis de la Evolución de los Principales Parámetros Hidráulico - Morfológicos que podrían influir sobre el Comportamiento Futuro de la Obra de Protección del Túnel". Informe Final. Comitente: Comisión Administradora Interprovincial Ente Túnel Subfluvial "Hernandarias". Santa Fe. Argentina. Abril, 1997.

3.      PARODY, H. O. and ESTRUCO, J. (1975). "Racionalización del Dragado del Río Paraná - Teoría de las Mutaciones Periódicas". IV Reunión General de la Asociación Latinoamericana de Dragado (ALAD). Buenos Aires. Argentina.

4.      REPOSINI, J. (1913). "Memoria sobre el río Paraná". Ministerio de Obras Públicas. República Argentina

5.      SCHUMM, S. A., and WINKLEY, B. R. (1994). "The Character of Large Alluvial Rivers" (pp. 1-9). In: "The Variability of Large Alluvial Rivers" (S. A. Schumm and B. R. Winkley Eds.). ASCE. 467 p.

6.      SOLDANO, F. A. (1947). "Régimen y Aprovechamiento de la Red Fluvial Argentina". Editorial Cimera. Buenos Aires. Argentina.

 

 

Figure 1. Studied cross section along the Parana´River.

 

1: Diamante; 2: Aldea Brasilera main channel; 3: Aldea Brasilera secondary channel; 4: Embarcadero; 5: Nafta; 6: "Hernandarias" Subfluvial Tunnel; 7: Aguas Corrientes; 8: Chapeton main channel; 9: Chapeton secondary channel (Zapata Channel); 10: Cerrito; 11: Curtiembre A; 12: Curtiembre B; 13: La Paz; 14: Pati main channel; 15: Pati secondary channel; 16: Corrientes.

 

 

Figure 2

 

 

Figure 3

 

 

Figure 4