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Three dimensional flow field at confluent fixed-bed open channels

Alegria, F. ; Birhukova, O. B. ; Guillén Ludeña, S. L. ; Heleno Cardoso, A. H. C.

Three dimensional flow field at confluent fixed-bed open channels, Proc River Flow, Lausanne, Switzerland, Vol. 1, pp. 1007 - 1014, September, 2014.

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River confluences are known for their marked 3D patterns and, due to the complexity of local flow features and bed morphology, they require particular consideration. Although there are studies on the flow structure in junctions of two streams, a more detailed description of the flow patterns is still required. The objective of this work is to contribute to such description at river confluence for concordant bed, where both main and tributary channel beds have the same elevation. For that purpose, a set of experiments with fixed bed under quasi-uniform approach flow conditions was carried out. The experimental facility included a 12 m long and 1 m wide main channel, and a 4.5 m long and 0.15 m wide tributary channel. The confluence angle was 70◦ so as to represent common natural conditions and the discharge ratio between discharges in the tributary and in the main channel was kept constant. An Acoustic Doppler Velocimeter (ADV) with a side-looking configuration was used to measure the three components of flow velocity on a dense grid of measuring points, providing the accurate characterization of the mean flow and second-order velocity moments. The acquired data set constitutes the most refined characterization of river confluence flow structures known to date, due to the adopted temporal and spatial discretization, and may serve for the validation of 3D numerical models of complex confluent flows. Flow dynamics zones typical of confluent beds are present in the studied case. The separation zone limits the effective lateral flow cross-section, and, hence, results in the added acceleration of the mainstream flow near the downstream junction corner. The upward orientated vertical velocities of high magnitude were observed all along the separation zone.