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.
The fluid dynamics of open channel confluences are highly complex due to the flow separation, mixing and secondary currents. Although numerous studies in the past few decades have focused on the numerical simulation of confluence flow, deformed beds were rarely used. This study attempts to address this issue through numerical simulation of the flow behavior in an open channel confluence flume with an equilibrium degraded bed in OpenFOAM. In the present study, six turbulence models, including Reynolds-Averaged Navier-Stokes (RANS) (standard k–ε, realizable k–ε, k–ω SST, V2-f), large-eddy simulation (LES) and detached eddy simulation (DES) models were performed using rigid lid and volume of fluid (VoF) methods. The accuracy of the models was statistically analyzed by comparing with the observation data, and their performance was prioritized accordingly. The results of this study demonstrated that the LES model had the best performance, with a minimum average normalized mean square error (NRMSE) of 9% under the VoF assumption and 27% under the rigid lid method.
Due to the lack of numerical modeling and continuous in-situ monitoring
of flow and sediment transport data, the mechanism of local-scale
morphodynamic processes during very large floods are not fully
understood. Thus, this study employs Delft3D to develop a
two-dimensional (2-D) morphodynamic model to simulate and analyze the
morphodynamic process of a gravel-bed river during an 80-year flood
event in Calgary, Canada. The model was calibrated using velocimetry
data and validated against the measured post-flood bed elevation data.
The coefficient of determination (R2) and ratio of the root-mean-square
error to the standard deviation (RSR) between the modeled and measured
bed elevation was 90% and 0.33, respectively, which demonstrates the
reliability of the model. The modeled flow velocity, bedload transport
rate, surface sediment sizes, and corresponding morphological changes at
different flood stages are presented and analyzed. Results show that bed
incision and the mid-channel bar continuously developed throughout the
flood while bank erosion and the growth of bank-attached bar mainly
happened during the rising and peak periods. We found that the timing
and duration of major morphological changes during a flood event varies
from site to site within a reach, but is similar for similar
morphological units. We also found that the spatial variation of channel
planform is the dominant determinant of major morphological changes
during floods, while flood events trigger the sediment motion and result
in actual deposition or erosion. Improvements are needed in terms of the
modeling of bedload transport and bed stratigraphy.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.