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).
To investigate reaction order and kinetic parameters of the reaction between crystal violet (CV) and sodium hydroxide (NaOH), various concentrations of the reactants were applied. The present work also verifies the unknown solid product produced under highly concentrated conditions. The reaction orders of CV and NaOH were determined to be 1 and 1.08 by pseudo rate method, respectively, with a rate constant, k , of 0.054 [(M −1.08 ) s −1 ]. In addition to pseudo rate method, the half-life approach was used to calculate the overall reaction order to verify the accuracy of pseudo rate method. The overall reaction order was determined to be 1.9 by the half-life method. The overall reaction order based on the two methods studied was approximately 2. The precipitate formation was observed when high concentrations of CV (0.01–0.1 M) and NaOH (1.0 M) were applied. Fourier transform infrared (FTIR) spectroscopy was used to compare the spectra of the precipitate generated and a commercial solvent violet 9 (SV9). Based on the FTIR spectra, it was confirmed that the molecular structure of the precipitate matched that of solvent violet 9.
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