Simulation Study On The Effect Of Pump And Gate Hydrodynamic Regulation On Water Environmental Quality Of Plain River Network

The Yangtze River Delta region has a dense network of rivers and is very rich in total water resources,but water environment problems are still prominent.As one of the regions with the most active economic development and high degree of urbanization in China,with the rapid development of social economy and urbanization in recent years,the problems of overloading water resources and deterioration of water environmental quality in the region have become increasingly prominent.In these areas of the river network,the existing gates and pumps and other hydraulic buildings are mostly built for flood control and drainage purposes during the flood season,with fewer daily openings and closings,making it difficult to connect the water system between the river network in the region,forming a large area of still water,which makes it difficult to dissipate and discharge pollutants in the water body in a timely manner,thus causing the water quality of the river network to deteriorate.With the promotion of the construction of water ecological civilization,it is urgent to accurately assess the health status of these rivers and improve the water environmental quality of the regional river network through scientific scheduling and joint regulation and control of pumps and gates by efficiently using the existing engineering facilities.In this study,Yaozhuang River network in Jiashan County,Jiaxing City was used as the research object,The river network is a typical plain orthogonal river network,with interlocking channels,slow flow velocity and poor self-purification capacity,which is very representative of a typical area for improving the water environment of a plain river network by applying engineering regulation technology.In this study,we comprehensively assessed the current situation of water environmental quality in the study area and explored the main pollutants affecting the water environmental quality of the river network through a comprehensive evaluation method of water environmental quality based on the entropy-weighted-cloud model and a single-factor evaluation method.At the same time,the numerical simulation model based on hydrodynamic-water quality coupling was constructed using Mike21 by combining the field survey and topographic information of the study area.According to the results of water environmental quality evaluation and the current situation of engineering arrangement in the study area,the river systems with poor water environmental quality in the river network in the study area were improved by designing different engineering regulation schemes,simulating and analyzing the improvement effect of different regulation schemes on the water environmental quality of the river network,and proposing a reference scheme for improving the water environmental quality in the plain river network in the study area.The main work and conclusions of the paper are as follows:（1）The average data of typical indexes of water environmental quality of each measurement point were obtained through the analysis of field monitoring and indoor experimental data of 18 positioning measurement points in the study area.The entropy-weighted-cloud model and single-factor evaluation method were applied to comprehensively evaluate the water environmental quality at each point.The results show that:DO and NH₃-N are two indicators that are more sensitive to the results of comprehensive assessment of water environmental quality.The comprehensive evaluation of the river water environmental quality in Yaozhuang polder is significantly worse than the other three polders in the study area.（2）Based on the field survey and relevant topographic information of the study area,a two-dimensional hydrodynamic model of the river network in the study area was established based on Mike21.Based on this,a two-dimensional water environment quality simulation model of the river network in the study area was constructed using the ECOLab water quality module.By comparing the measured and simulated values,it was found that the R² of both numerical simulation and measured values were within a reasonable range,which quantitatively verified the reliability of the constructed water quality simulation model of the river network.（3）A simulation study of different water environmental quality improvement schemes was conducted for Yaozhuang polder,which has the worst water environmental quality,and then the entropy-weight-cloud model comprehensive evaluation method was applied to evaluate the improvement effect of different schemes.A comprehensive analysis of the results of the water quality assessment and the distribution of flow fields within the polder area reveals that:Based on the pump and gate control and sluice control of water quality improvement engineering control scheme can make the polder area water environmental quality to achieve the goal of improvement,Among them,to ensure that the polder gates are fully open and maintain a large gate opening on the regional water environment quality improvement effect is optimal.The distribution of DO and NH₃-N concentrations are positively and negatively correlated with hydrodynamics,respectively.Ensuring the regional flow balance,reducing the area of relative hydrostatic zone,and making the flow field uniform and densely distributed have positive effects on the improvement of regional DO and NH₃-N concentrations.The effect of the large gate opening scenario on the improvement of water quality is slightly better than that of the small gate opening scenario.In this paper,through the combination of field data collection and the construction of evaluation models and simulation models.Based on the results of water environmental quality evaluation,different regulation schemes are designed for areas with poor water environmental quality,the improvement effect of regional water environmental quality under different schemes is simulated and evaluated,and the optimal improvement scheme is selected.The results can provide a decision basis for the improvement of water environmental quality of the plain river network in the study area,and can also provide reference for the engineering regulation of pump and gate projects in similar areas.