Optimization of layout plan for the sedimentation tank of the Middle Route of South-to-North Water Transfers Project
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Abstract:
South-to-North Water Transfers Project is a large-scale water diversion project to alleviate the severe shortage of water resources in northern China. Practical experience show that algae can grow rapidly under specific conditions along the main canal. The mixture of algae residues moves with water flow and is prone to siltation in the static water area near the discharge gate and diversion outlet, as well as in the front pool of the pumping station. Therefore, algae sedimentation tank was proposed to build near the Middle Route of South-to-North Water Transfers Project to settle the algae and other mixtures in the main canal. Due to being short of universal standard for the design of sedimentation tanks, the required planar distance for settling algae and other mixtures need to be studied, so as to provide key design parameters for its engineering layout.The method of on-site experiments was used to study the sedimentation characteristics of algae and other mixtures in the main canal of the Middle Route Project. In this way, the effectiveness of the length of the sedimentation tank could be estimated. Then, the method of numerical simulation was adopted to analyze the water flow characteristics in the sedimentation tank under design condition. The existing plan may not be effective in settlement efficiency. Thus, some improved schemes were proposed characterized by setting diversion piers, and their effects were explored by numerical simulation.The results indicated that with the increase of the average flow velocity, the effective settlement distance increases while the effective settlement rate decreases, when the water depth is basically the same. For the initial design scheme, the mainstream leans towards the left side of the tank, and there is a large area of low flow velocity reflux on the right side of the tank, which accounts for about 57% of the total area. When longitudinal diversion piers were arranged evenly along the cross-section in the inlet diffusion zone, the percentage of water area of the recirculation zone in the sedimentation tank was decreased from 57% to 7%, the average flow velocity in the tank was decreased from about 0.10 m/s to about 0.07 m/s, and the average water age was decreased from about 18 h to about 6 h under the design condition. When transverse diversion piers were arranged similarly, the percentage of water area of the recirculation zone in the sedimentation tank was decreased from 57% to 14%, while the decrease of average flow velocity in the tank and the average water age was almost the same as the plan of longitudinal diversion piers.The conclusion indicates that the effective settling distance of algae and other mixtures is 1,064 m under the design working conditions of the sedimentation tank, which means that algae and other mixtures could settle within the sedimentation tank. However, the utilization efficiency of the initial layout plan of the sedimentation tank is relatively low. With the deployment of longitudinal diversion piers, the flow smoothness and the water exchange capacity in the sedimentation tank could be improved. Findings of this study could provide technical support for the overall layout design of the algae sedimentation tank. In the subsequent in-depth research, three-dimensional simulation and other means should be used to fully study the sedimentation characteristics of the tank to further improve the design.
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