Experiments on the Morphodynamics of Open Channel Confluences: Implications for the Accumulation of Contaminated Sediments

Abstract

Contaminated sediments are common in river networks. The flow convergence and particular flow structures in confluences, such as the flow separation zone, may result in greater accumulation of contaminated sediments than in other river locations, but this issue is rarely studied. In addition, the contaminated sediment transport is driven by the particular morphodynamics occurring at confluences. This article describes a novel confluence flume experiment on both morphodynamics and deposition patterns of contaminated sediments as a function of geometric and flow conditions. The initial equilibrium bed geometry was developed from a mobile bed and then fixed, allowing for subsequent flow velocimetry and sediment feeding. Colored sediments of fine gradation, which mimic contaminated sediments, were then fed to the tributary channel. The results suggest that the junction angle primarily determines the confluence bed morphology and sediment transport pattern while the discharge ratio is a secondary factor. It was also observed that most introduced sediments tended to deposit immediately after the cessation of feeding at the stoss of the bar in the flow separation zone. The time history of the transport of the contaminated sediments was also investigated. Sediment that initially deposited at the stoss of the bar eventually moved to the lee of the bar and deposited around the bar downstream of the confluence, demonstrating that the sedimentation pattern evolved to a state similar to the equilibrium bed morphology. Plain Language Summary This article describes a novel confluence flume experiment on the bed form and contaminated sediment transport patterns. The initial bed state was developed from a flat mobile bed until stable and was then fixed for flow field measurements and sediment feeding. Dyed sediments of fine sizes, representing contaminated sediments, were fed to the tributary channel. Variables including confluence junction angle, flow discharge ratio between tributary and mean channel, and sediment feeding location were all considered and investigated. It was observed that the confluence junction angle primarily determines the bed form and sediment transport pattern, while the discharge ratio is a secondary factor. The contaminated sediment deposition pattern tended to evolve to a state that is similar to the initial bed state. This study improves understanding of confluence bed form evolution and water quality maintenance in confluences. Confluences are characterized by complex hydrodynamics associated with particular morphological structures. Based on those defined by Best (1987) and Best and Rhoads (2008), the general hydrodynamic and morphological components in confluences can be summarized as follows, with numbers indicating labels in Figure 1: ① Flow stagnation zone, where the local velocity of the fluid is close to zero (Best, 1988).

Hydrodynamics, Sediment Transport and Morphological Features at the Confluence Between the Yangtze River and the Poyang Lake

“…A midstream deep scour hole and bank‐attached lateral bars, related to flow separation, are some of the principal morphological features identified at river confluences (Best, 1988). Field surveys (Biron et al., 1993; Boyer et al., 2006; Ianniruberto et al., 2018; Rhoads et al., 2009) and laboratory experiments (Best, 1988; Guillén‐Ludeña et al., 2015, 2016, Guillén Ludeña, Cheng, et al., 2017; Leite Ribeiro et al., 2012; Yu et al., 2020; Yuan et al., 2018) on sediment transport and morphology at channel confluences were conducted in the past to investigate the interaction between flow dynamics and the bed morphology. Many features, such as large flow velocity, strong turbulence, effects of the shear layer, or curvature‐induced helical circulations were believed to be the cause of the midstream scour hole formation.…”

Numerical Simulation of Confluence Flow in a Degraded Bed Under Different Turbulence Models with a Comparison of Rigid Lid and Volume of Fluid

“…Many factors including junction angle, flow discharge, downstream Froude number, channel geometry, and flow resistance, can affect the flow pattern in channel confluences [12]. There have been many studies of river confluence flows in both the laboratory (e.g., Weber et al (2001) [13], Pinto Coelho (2015) [14], Yu et al 2020) [15], and Canelas et al(2022) [16]) and the field (e.g., Ashmore et al (1992) [17], Biron et al (2002) [8], Rhoads and Sukhodolov (2008) [10], Constantinescu et al [7], and Sukhodolov et al (2017) [18]) to study the impact of the above-mentioned factors. However, due to the limitations of physical models (associated with scale effects) and field studies (labor-and time-consuming), as well as their dependency on the accuracy of measuring techniques, it is desirable to develop numerical simulation techniques to reduce the costs.…”

Effects of vegetation at a bar confluence on river hydrodynamics: The case study of the Arno River at Greve junction

“…The separation zone represents a preferential area of deposition of fine sediment, thus promoting the development of a bank-attached lateral bar (Boyer, Roy, & Best, 2006). Junction morphology, especially the confluence angle (Yu, Yuan, & Rennie, 2020) and momentum ratio between the tributary and the main channel (Boyer et al, 2006) influence the position and size of such sediment deposit, whose length and width can be estimated by using the empirical relationships of Best and Reid (1984).…”