Large Confluences Background and Aims: River channel confluences are ubiquitous features of all fluvial networks & represent sites of complex, three-dimensional flow & sediment dynamics. Current understanding of these sites is based largely on laboratory experiments & study of small-scale natural junctions.
These have identified the nature of coherent flow structures at junctions (both time-averaged & periodic) & their controls (e.g. streamline curvature, bed topography, flow separation). To date, there have been few studies of the dynamics of large, natural channel confluences, & we do not know the extent to which conclusions reached from these small-scale studies can be extended to larger spatial scales.
Although this scaling may be expected to be complex, we currently have neither the data nor the level of understanding required to evaluate the results from small-scale laboratory & field studies. Our ability both to model & to manage these key elements in the fluvial network is thus severely limited.
This UK-NERC funded study seeks to use recent technical developments in acoustic Doppler current profiling, together with multi-beam echo-sounding & side-looking sounders, to quantify the morphology and fluid dynamics of one of the world's largest channel junctions between the Paraná & Paraguay Rivers in Argentina at several flow stages.
Results: This research project sought to quantify & model the nature of flow & bed morphology at a large river junction, & examine the nature of sediment transport at these important sites. Eight principal findings can be drawn from this research:
1) The general form of confluence bed morphology at the junction of large rivers is similar in many aspects to that of smaller scale confluences (Fig. 1). The presence of a central scour & avalanche faces at the mouth of one, or both, tributaries appears characteristic & scale-invariant. The depth of scour at the junction of the Paraná-Paraguay junction (>30 meters) is greater than may be expected, due to bedrock constriction of flow at the lower junction corner.
2) Mixing at large confluences does not appear to mimic models derived from smaller junctions that describe the dominance of counter-rotating helical cells, with downwelling at the centre of the junction where the two flows meet. At the junctions studied here, flow at the centre of the junction is dominated by upwelling along, or adjacent to, the mixing interface. This upwelling is a function of: a) discordance between the tributary bed heights; b) topographic forcing of flow through the upstream thalwegs, which is significant at the Paraná-Paraguay junction due to scour generated by bedrock constriction & an upstream meander bend, & 3) the density differences between the flows (see (3) below).
3) Density differences between the two incoming flows are a significant, but as yet unrecognised, influence upon fluid mixing at channel confluences. We have quantified, for the first time, the presence of density underflows where one river has a higher suspended load than its confluent partner. At such sites, mixing is dominated by: i) the underflow of denser fluid; ii) entrainment from the top interface of the density underflow, generated by Kelvin-Helmholtz instabilities; iii) movement of the density underflow over bed roughness, which may create enhanced vertical mixing. We conjecture that the density difference between confluent rivers is more likely to be greater at the junction of bigger rivers, due to the increased probability of these rivers being sourced in regions of differing geology & climate regime. Consequently it is likely that density-controlled mixing will be more influential at larger river junctions (see Best et al., in review).
4) The mixing length required for complete mixing between the confluent flows is a function of the velocity gradient across the shear layer, & how quickly this gradient becomes negligible, as well as the density contrast between rivers (see Lane et al., in review). The velocity gradient has been shown to be a function of the momentum ratio between the two flows & the form, & activity, of Kelvin-Helmholtz instabilities generated along the shear layer. Depending on these factors, complete mixing may vary from as little as 6km to upwards of 400km.
5) We have proven the utility & huge advantages of using multibeam echo sounding (MBES) within large rivers. This project, allied to NER/B/S/2003/00243/2, has shown the capacity of MBES to quantify bed morphology & give indications as to the suspension of sediment within the water column, for instance at the shear layers associated with rivers junctions & in the leeside of sand dunes.
6) MBES surveys of the Paraná-Paraguay junction show that there is only slight divergence of bedload through the scour zone, & that sand bedload (as shown by dunes) is moving through the deepest scour. We have been unable, however, to examine the spatial distribution of bedload transport rates at this large junction.
7) CFD modelling has been completed of: i) flow within a large confluence-diffluence unit; ii) flow over 3D dune bedforms, and iii) models to investigate the effects of density differences on mixing. However, the bathymetric surveys of the Paraná-Paraguay junction revealed a complex geometry that has not been possible to model.
8) Two other principal research problems were addressed during this grant:
i. The nature of flow over 3D dunes (see details in Parsons et al., 2005; Hardy et al., 2006; Parsons et al., 2006).
ii. Flow within confluence-diffluence units: this work has demonstrated the nature of flow at a low-angle confluence that develops into a downstream divergence. The thalweg is seen to cross the post-confluence channel into the opposite downstream distributary, & flow is not dominated by back-to-back helical flow cells at the junction. Instead, flow is characterised by a simple convergence-divergence, with the width:depth ratio & role of high relative roughness (due to dune bedforms) dominating the flow field (see Orfeo et al., 2006; Parsons et al., submitted).
Publications:
Journal Papers (published):
1) Best J.L. (2005) The fluid dynamics of river dunes: a review and some future research directions, Journal of Geophysical Research, 110.
2) Parsons D., Best J.L., Hardy R.J., Kostaschuk R.A., Lane S.N., and Orfeo O. (2005) The morphology and flow fields of three-dimensional dunes, Rio Paraná, Argentina: results from simultaneous multibeam echo sounding and acoustic Doppler current profiling, Journal of Geophysical Research, 110.
Papers in Books/Conference Proceedings (excluding Abstracts, see ROD for listings):
1) Best J.L., and Lane S.N. (2004) Confluence, channel, and river junctions, In: A.S. Goudie (ed), Routledge Encyclopedia of Geomorphology, pp.180-183.
2) Hardy RJ., Parsons DR., Best JL., Lane SN., Kostaschuk R. and Orfeo, O. (2006). Three-dimensional numerical modelling of flows over a natural dune field. In: IAHR Third RiverFlow 2006, International Conference on Fluvial Hydraulics, Lisbon, Portugal.
3) Parsons D.R., Best, J.L., Lane S.N., Orfeo, O., Kostaschuk R.A., Hardy R.J., Franklin M.C. (2004) The morphology and sediment dynamics of a large river confluence: the Rio Paraná and Rio Paraguay, Argentina. In: Proceedings of the 2nd International Conference on Fluvial Hydraulics, Riverflow2004, Naples, Italy.
4) Parsons D.R., Best J.L., Lane S.N., Hardy R.J., Kostaschuk R., Shugar D., & Orfeo O. (in press) Morphology, flow and sediment transport over a natural 3D dune field: Rio Paraná, Argentina. In: IAHR Third River Flow 2006, International Conference on Fluvial Hydraulics, Lisbon, Portugal.
5) Orfeo O., Parsons D.R., Best J.L., Lane S.N., Hardy R.J., Kostaschuk R., Szupiany R., Amsler M.L. (in press) Morphology and flow structures in a large confluence-diffluence: Rio Paraná, Argentina. In: IAHR Third River Flow 2006, International Conference on Fluvial Hydraulics, Lisbon, Portugal.
Papers (in press/in review/in prep):
1) Best J.L., Parsons D.R, Kostaschuk R., Lane S.N., Orfeo O., Amsler M., Fraser J., Hardy R.J., Shugar D., and West G. (invited). Characterizing the dynamics of shear layers and sediment transport in large rivers: approaches using acoustic Doppler and multibeam sonar methods. To be submitted to Journal of Hydraulic Engineering - Acoustic Measurements in Rivers.
2) Best, J.L., Parsons, D.R., Lane, S.N., Amsler, M., Kostaschuk, R., Orfeo, O. and Hardy, R. (in prep) Fluid mixing at river junctions with unequal density confluent channels. To be submitted to Science.
3) Hardy R.J., Parsons D.R., Best J.L., Lane S.N. and Kostachuk R. (in prep) Porosity based modelling of flow over three-dimensional dune fields. To be submitted to Journal of Hydraulic Research
4) Lane S.N., Parsons D.R., Best J.L., Hardy R.J., Kostaschuk R. and Orfeo O. (in prep) Why do some big rivers take so long to mix? To be submitted to Nature.
5) Lane S.N., Parsons D.R., Best J.L., Hardy R.J., Kostaschuk R. and Orfeo O. (in prep) Mixing processes at large river channel junctions revealed using combined multibeam echo sounding and acoustic Doppler profiling. To be submitted to Journal of Geophysical Research.
6) Parsons D.R., Best J.L., Lane S.N., Orfeo O., Hardy R.J., Kostaschuk R. (in review) Form roughness and the absence of secondary flow in a large confluence-diffluence unit, Paraná River, Argentina. Earth Surface Processes and Landforms.
7) Parsons D.R., Best, J.L., Lane S.N., Orfeo, O., Kostaschuk R.A., Hardy R.J., Franklin M.C. (in prep) The morphology and flow structure at a large river confluence: the Rio Paraná and Rio Paraguay, NE Argentina. To be submitted to Journal of Geophysical Research.
Science in Society:
Articles published in Magazines:
Parsons, D. and Best, J. (2006) Sounding out coasts and rivers. NERC Planet Earth, Spring Issue.
Articles published in National Newspapers:
- "El Paraná bajo la lupa de expertos." EL CLARÍN, Argentinean National newspaper, Buenos Aires, May 29, 2005, page 45. Circulation is circa 1,740 000. (www.clarin.com/diario/2005/05/29/sociedad/s-04501.htm). This is Argentina's biggest selling newspaper and the 2nd largest selling Spanish language newspaper in the world.
Articles published in Provincial Newspapers:
- "Una investigación científica para develar los secretos del Paraná." ÉPOCA, Corrientes provincial newspaper, Argentina, May 18, 2005, page 16. Circulation is circa 134,000. (www.diarioepoca.com/2005/05/20/infogeneral/1_una_investigacion.php)
- "Un grupo de expertos analiza en comportamiento de las aguas del Paraná en Paso de la Patria." Diario NORTE, Chaco Provincial newspaper, May 30, 2005, page 40. Circulation is circa 83,000
- "El río Paraná está siendo estudiado en tres dimensiones." Diario ECOS, Corrientes Provincial weekly, May 31, 2005, page 4. Circulation is circa 245,000.
- "Científicos de diferentes latitudes estudian el río Paraná como nunca antes." Diario LA REPUBLICA, Corrientes Provincial, May 31, 2005, page 3. Circulation is circa 19,000.
- "El Paraná en profundidad." Diario EL LITORAL, Santa Fe Provincial, June 4, 2005, page 21. Circulation is circa 334,000. (www.litoral.com.ar/index.php/diarios/2005/08/13)
Radio Station interviews:
- Radio SUDAMERICANA (FM), Corrientes, Argentina, May 27, 2004.
- Radio CHACO (FM), Resistencia, Chaco, Argentina. May 31, 2004.
- Radio LITORAL (FM), Paraná, Entre Ríos, Argentina. June 2, 2004.
- Radio BARRANCAS (FM), Santa Fe, Argentina. June 2, 2004.
- Radio MAR DEL PLATA (FM), Buenos Aires, Argentina, June 3, 2004.
Research Projects
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