Laboratory study of the effect of Bendway weirs on pollution transfer in meandering rivers with sedimentary bed

Abstract
Background and objectives: In recent years, the rise in pollutant entry into surface waters and rivers has made studying the mixing process of pollutants increasingly important. Mixing in streams influences turbulence, nonuniform velocity, and molecular motion. Secondary current plays a significant role in determining mixing coefficients; enhancing these flows can improve the overall mixing process. One critical mixing coefficient is the longitudinal dispersion coefficient, which can be estimated through both theoretical and experimental methods. Increasing turbulence in rivers is one way to enhance their self-purification capacity. Installing structures like river wall protection systems, including various types of weirs, Bendway weirs, and submerged plates, can enhance turbulence and secondary flow patterns. This, in turn, helps improve the mixing of pollutants in the water. This present study's aim is to investigate the effect of the bendway weirs on the mixing process and the longitudinal dispersion coefficient in the laboratory meandering channel. To achieve this, tracking experiments were conducted in a laboratory meander channel with a sedimentary bed, comparing conditions both with and without bendway weirs.
Materials and methods: The experiments in this study were conducted in the Hydraulics and Physical Models Laboratory within the Water Engineering Department at the University of Guilan. The experiments took place in a rectangular flume with a length of 16 meters and a width of 1.5 meters. A salt and water solution was used as a tracer In each experiment, the flow concentration was measured with two EC meters at two points: at the beginning and at the end of the tested intervals. Additionally, the changes in the salinity concentration curve were assessed at two points: at the start of the meandering path and at the end of each curve. By using the Temporal concentration profiles, How the tracer is distributed, transferred, and changed in different intervals and flow rates is examined and then the longitudinal dispersion coefficient was estimated with the routing method, and the effect between hydraulic parameters and bendway weirs was compared.
Results: The results indicate that any increase in discharge per unit width from the minimum value (0.014 m3/m.s) to the maximum value (0.022 m3 /m.s), Cr, Tpr, and Tdr decreased up to 97% and U and DL increased by 98% in most experiments. The experiments indicated that most of the considered parameters increase with distance and number of bends and an increase in the lengths of sections leads to the improvement of the indicators related to the reduction of salinity concentration as a symbol of pollution. A comparison of the results showed that by installing the bendway weirs on both sides of the channel with a ratio of 2.5 times the distance to the length of the spillways, the Cr and DL parameters increased by 94% and decreased by 95%, respectively. Cr and D‌L were achieved by installing the weirs in the inner arch with a ratio of 2.5 times the distance to the length of the spillways with an increase of 94% and a decrease of 97%, respectively. Additionally, the structure was installed on both sides of the channel at a ratio of 5 times the distance to the length of the weirs, mentioned parameters were decreased. Furthermore, the distance of the bendway weirs was reduced to 1.7 times the distance to the length of the weirs, resulting in another increase of 94%. Comparison of the time measurement parameters of the distribution and transfer of the tracer, including Tpr and Tdr, obtained from experiments without structures and with the installation of bendway weirs with different arrangements showed that the aforementioned parameters increased with the installation of structures. So the Tpr parameter increased by 66% after the installation of bendway weirs on both sides of the meandering path with a ratio of 2.5 times the distance to the weir length, increased by 61% with the installation of bendway weirs only on the inner curve with a ratio of 2.5 times the distance to the weir length, increased by 49% with the installation of bendway weirs on both sides of the path with a ratio of 5 times the distance to the weir length, and increased by 48% with the installation of weirs on both sides of the meandering path with a ratio of 1.7 times the distance to the weir length compared to the case without weirs. The Tdr parameter has increased by as much as 84% in most cases after the installation of bendway weirs in various configurations. The analysis of the results shows that installing these bendway weirs, along with the increase in obstacles, has resulted in a decrease in tracer transfer speed (denoted as parameter U).
Conclusion: The analysis of the results showed that with the increase in the average velocity and turbulence of the flow in the experiments conducted on the sedimentary bed, the Cr was decreased and the DL (Longitudinal dispersion coefficient) was increased. Also, the increase in the distance of sections and the number of arches of the meandering flume has led to an increase in Cr and DL parameters. The results indicate that the installation of bendway weirs in various arrangements generally led to an increase in the Cr parameter and a decrease in the DL parameter. Notably, placing weirs on both sides of the channel, at a distance that is 1.7 times the length of the weirs, demonstrated the best performance. This configuration resulted in simultaneous improvements in both the Cr and DL parameters.