Dynamics of Flow at Concordant Gravel Bed River Confluences: Effects of Junction Angle and Momentum Flux Ratio

Abstract

Although the dynamics of secondary flow at river confluences have received considerable attention, a lack of theoretical insight in exploratory field and experimental research has led to different conclusions about the mechanisms driving these dynamics. This study revisits the problem of secondary flow at confluences by examining responses of the flow to controllable variations in curvature of flow trajectories in a series of field‐based experiments. The experiments were performed in a physical model designed to eliminate the influence of bed morphology, while using a large width‐to‐depth ratio to enable proper identification of flow structures at different scales. The results show that no secondary flow is detectable in runs with parallel merging flows irrespective of the momentum ratio. In runs with converging channels the secondary flow is represented by large‐scale motions consisting either of a pair of counterrotating helixes or a single helix depending on the momentum ratio. Comparison of scaled measured and theoretically predicted vertical profiles of lateral velocity shows that the helical secondary flow is primarily driven by the curvature of flow trajectories. This study also indicates that small‐scale secondary motions that have the same sense of rotation as the large‐scale helixes may develop in the immediate margins of the mixing interface. Differing only slightly from the flow depth in size, they are manifested by local upwelling within the large‐scale secondary motions. Several reasons to believe that interactions of large‐scale motions with opposing sense of rotation drive these small‐scale secondary flow cells are discussed in the paper.

Density Effects at a Concordant Bed Natural River Confluence

“…In other words, flow in the CS curves to a greater extent than flow in the KR, which explains the formation of strongly coherent SOV cells on the CS side of the MI only (Rhoads & Kenworthy, ). Recent field experiments show that curvature‐induced effects facilitate the formation of SOV cells and/or increase their coherence (Sukhodolov & Sukhodolova, ). The development of SOV cells on the KR side of the MI is not documented for the flows simulated here but can develop when the MR is close to 1 and/or when curvature of flow entering the confluence from this tributary is pronounced (Rhoads, ).…”

“…The dynamics of mixing within the CHZ are important because if diffusive and dispersive mechanisms are ineffective at producing mixing within this zone, the two flows may remain unmixed for large distances downstream of confluences (Bouchez et al, ; Lane et al, ; Mackay, ; Rathbun & Rostad, ; Umar et al, ). The influence of controlling factors, such as the MR of the two incoming streams, the planform symmetry of the confluence, the junction angle, and bed discordance, on the position and structure of the MI, on flow hydrodynamics, and on mixing (Kenworthy & Rhoads, ), has been the focus of numerous field, experimental and, numerical studies (Best & Rhoads, ; Constantinescu et al, , ; Dordevic, ; Guillén‐Ludeña et al, ; Leite Ribeiro et al, ; Rhoads & Kenworthy, ; Rhoads & Sukhodolov, ; Roy et al, ; Roy & Roy, ; Sukhodolov & Sukhodolova, ). The effects of density differences between incoming flows on confluence hydrodynamics have, at least until recently, received less attention (e.g., see Cook et al, ; Laraque et al, ; Prats et al, ; Ramón et al, ; Lewis & Rhoads, ; Martí‐Cardona et al, ; Cheng & Constantinescu, , ).…”

Advective Lateral Transport of Streamwise Momentum Governs Mixing at Small River Confluences

“…As the name suggests, the mixing interface is an important hydrodynamic feature related to lateral mixing at confluences. Turbulence within this interface is generated by lateral shear associated with differences in velocity between the two flows (Babarutsi & Chu, 1998; Guillén Ludeña et al, 2017; Sukhodolov & Rhoads, 2001; Sukhodolov & Sukhodolova, 2019; van Prooijen & Uijttewaal, 2002) and by shedding of vortices from interacting shear layers that bound the stagnation zone (Lewis & Rhoads, 2018b). Large velocity differences between the incoming flows result in Kelvin‐Helmholtz (KH) instability and the generation of co‐ (co‐ rotating)rotating vertically oriented vortices within the mixing interface (KH mode) (Constantinescu, 2014; Rogers & Moser, 1992; Sukhodolov & Rhoads, 2001).…”

“…The orientation of the mixing interface and the structure of turbulence within it can be modified by the bathymetry of the confluence, especially pronounced discordance between the bed elevations of the two confluent channels (Biron et al, 1996; Boyer et al, 2006; Bradbrook et al, 2001; de Serres et al, 1999; Gaudet & Roy, 1995; Parsons et al, 2007). In any case, lateral transport by vertically orientated turbulent eddies is the main lateral mixing mechanism within the mixing interface (Sukhodolov & Sukhodolova, 2019).…”

Dynamics of shallow wakes on gravel-bed floodplains: Data set from field experiments

“…A side branch located at 46°12ʹ9.45ʺN, 12°58ʹ14.55ʺE is an ideal site for conducting field-based experiments (Sukhodolov and Sukhodolova, 2014;Sukhodolov et al, 2017;Sukhodolov and Sukhodolova, 2019). During base flow conditions, a gravel bar blocks surface flow from the main river, which means that the branch is fed only by perennial springs.…”