Influence of junction angle on three‐dimensional flow structure and bed morphology at confluent meander bends during different hydrological conditions

Abstract

Recent field and modeling investigations have examined the fluvial dynamics of confluent meander bends where a straight tributary channel enters a meandering river at the apex of a bend with a 90° junction angle. Past work on confluences with asymmetrical and symmetrical planforms has shown that the angle of tributary entry has a strong influence on mutual deflection of confluent flows and the spatial extent of confluence hydrodynamic and morphodynamic features. This paper examines three‐dimensional flow structure and bed morphology for incoming flows with high and low momentum‐flux ratios at two large, natural confluent meander bends that have different tributary entry angles. At the high‐angle (90°) confluent meander bend, mutual deflection of converging flows abruptly turns fluid from the lateral tributary into the downstream channel and flow in the main river is deflected away from the outer bank of the bend by a bar that extends downstream of the junction corner along the inner bank of the tributary. Two counter‐rotating helical cells inherited from upstream flow curvature flank the mixing interface, which overlies a central pool. A large influx of sediment to the confluence from a meander cutoff immediately upstream has produced substantial morphologic change during large, tributary‐dominant discharge events, resulting in displacement of the pool inward and substantial erosion of the point bar in the main channel. In contrast, flow deflection is less pronounced at the low‐angle (36°) confluent meander bend, where the converging flows are nearly parallel to one another upon entering the confluence. A large helical cell imparted from upstream flow curvature in the main river occupies most of the downstream channel for prevailing low momentum‐flux ratio conditions and a weak counter‐rotating cell forms during infrequent tributary‐dominant flow events. Bed morphology remains relatively stable and does not exhibit extensive scour that often occurs at confluences with concordant beds. Copyright © 2014 John Wiley & Sons, Ltd.

Variability in the vertical hyporheic water exchange affected by hydraulic conductivity and river morphology at a natural confluent meander bend

“…They consist of (a) a flow stagnation zone near the M–T junction corner, (b) a deflection zone within the confluence, (c) two flow separation zones near the two sides of the MT channel entrance, (d) a flow acceleration zone near the centre of the MT entrance, (e) a flow recovery zone located in the centre of the downstream area, and (f) a shear layer or mixing interface zone between the two confluence flows. The results of the investigations of the locations of hydrodynamic zones were similar to the study results reported by Riley, Rhoads, Parsons, and Johnson () for high‐angle CMBs and were significant in the identification of locations of sediment erosional and depositional zones at the CMB (including M, T, and MT segments).…”

Hydrodynamics of mountain‐river confluences and its relationship to sediment transport

“…[], and Riley et al . [], for concordant bed confluences, were present in the predicted flow field at initial state. The formation of these cells forming in the vicinity of the mixing interface was inhibited by the strong three‐dimensional effects induced by the bed discordance and high momentum flow from the tributary.…”

Effect of the Junction Angle on Turbulent Flow at a Hydraulic Confluence

“…The effects of these factors have been examined with both experimentally derived junctions in laboratory (e.g., [1,3,4]), and field investigations (e.g., [5][6][7][8][9]), recently integrated with numerical modelling (e.g., [10][11][12]). In particular, with laboratory experiments, it was possible to isolate and accentuate the effect of certain parameters and processes under controlled conditions, which then were varied by using numerical simulations in order to overcome limitations of the physical models.…”