Influence of cross-sectional shape on flow patterns in an open channel confluence.

Laurent Schindfessel, Stephan Creelle, Tom De Mulder

Tuesday 30 june 2015

11:15 - 11:30h at Oceania (level 0)

Themes: (T) Sediment management and morphodynamics, (ST) River morphodynamics

Parallel session: 5B. Sediment - River

Open channel confluences are an essential element of a riverine network, and can be found in natural and regulated river systems, as well as in manmade canal networks. The flow patterns found in these confluences often induce important water level elevations, ecological and morphodynamic changes. This explains why confluences have been and still are intensively studied, both in realistic configurations as well as in schematized geometries. In most laboratory experiments on confluences so far, the channels have a rectangular cross-section with fixed vertical walls, irrespective of the use of a fixed or mobile bed in the experiments. In many natural or manmade configurations, however, the channel cross-sections have banks with a fixed or variable slope. For straight channels with trapezoidal cross-sections, the inclined walls are known to influence the distribution of longitudinal velocity and the secondary circulation. Therefore, it can be expected that the inclination of the side walls also has an influence on the flow patterns and mixing characteristics in a confluence. To investigate this hypothesized influence, large-eddy simulations (LES) are performed on a schematized, 90° confluence with fixed concordant bed, for different shapes of the cross-section. First, the numerical model is benchmarked against laboratory experiments for the case of a chamfered rectangular cross-section. Then, the cases of a truly rectangular and a trapezoidal cross-section are simulated. For all these cases, the (subcritical) Froude number, which is the dominant dimensionless parameter for channel flow, is kept constant. The results of the simulations are compared to deduce the influence of sloping banks on the flow patterns and mixing processes in the confluence.