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River Confluences
Farid KarimpourJosh Smalley
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Definition/ObjectiveThe meeting of two or more rivers is called a
river confluence.Usually a river is the main stream and others
are the tributaries.Objective is to study the river confluences
effects and characteristics
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Physical ProcessesDownstream of a river confluence, if the sediment concentration and grain size remain constant, can be expected to be:
Significantly widerDeeperExperiences more shear stressSlightly fasterSlight decrease in slope
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Physical Relationships
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Confluence Formation
Confluences primarily form as the drainage network originally forms, but can also form through:River captureHuman induced
changes
Teay River capture by the Ohio
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Confluence Migration
River Confluences tend to be static through time
Slackwater zones form downstream of wide barsThis induces rapid
deposition and maintains a streamline confluence From Julien, 2002
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Mississippi confluence with the RedAtchafalaya provides
shorter distanceNaturally would
have captured Mississippi
Flow diversion from Mississippi is managed
Human managed confluence
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Amazon River
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Amazon ConfluencesConfluence of the
Marañón and the Ucayali
15,000 sub-branches join the Amazon
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Flow diagram at a 90 river confluence10
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Mixing ZonesMixing zones occur downstream of river
confluencesDistance till mixing can be approximated by:
Where:
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Mixing length depends primarily on width, depth, velocity, and shear velocity
An increase in shear velocity or depth will decrease mixing length
An increase in width or velocity will increase mixing length
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Numerical Simulations
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Iso-concentration curves(left figure), Dispersion (right figure)
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Distribution of the vertical vorticity, in the instantaneous flow in a horizontal surface situated at (left) 0.1 D and (right) 0.5 D
below the free surface.
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Left) Distribution of the mean streamwise velocity, us (m/s), in section A1.Right) Distribution of the streamwise‐oriented vorticity, in the mean flow at sections A1, A, and C (As shown in slide 15)
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Glacier and spring fed streams in Ecuador
Natural Confluences
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Shallow mixing layers developing at the river confluences
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Photographs of Lake Powell at the confluence with the Dirty Devil River (entering from left).
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References Constantinescu, G., Miyawaki, S., Rhoads, B., Sukhodolov, A., Kirkil,
G., “Structure of turbulent flow at a river confluence with momentum and velocity ratios close to 1: Insight provided by an eddy‐resolving numerical simulation”, Water Resources Research, vol. 47, w05507, doi:10.1029/2010wr010018, 2011
Julien, P. Y., “Erosion and Sedimentation”, Cambridge Press, 2010
Julien, P. Y., “River Mechanics”, Cambridge Press, 2002
Petrescu, V., Sumbasacu, O., “Comparison between numerical simulation and measurements of the pollutant dispersion in a river. case study”, U.P.B. Sci. Bull., Series D, Vol. 72, Iss. 3, 2010
Roca, M., Martín-Vide, J.P., Moreta, P.J.M., ” Modelling a torrential event in a river confluence”, Journal of Hydrology 364 (2009) 207–215
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