SWMM-based assessment of the effectiveness of water system connectivity in towns and cities

The Storm Water Management Model (SWMM) is widely used in urban drainage, water resources engineering, and environmental science, making it one of the key technical tools in urban water management and related research. In this study, ArcGIS is integrated with SWMM to develop a hydraulic model of the urban river network in Pingyu County, Zhumadian City, Henan Province. The model is used to analyze changes in the river system before and after the implementation of the water system connectivity project. Following the completion of the project, the number of rivers increased from 12 to 19, and the number of lakes grew from 4 to 8. The number of tributaries rose from 6 to 13, with the number of secondary tributaries increasing from 0 to 7. Analysis of a representative cross-section of the secondary drainage channel revealed that the peak flow and flow velocity at return periods of 10 and 20 years were delayed and reduced after the project. The concentrations of all monitored pollutants in the four primary rivers exhibited a decreasing trend. Specifically, the concentration of C6H5OH decreased by 95.77% to 93.33%, while the concentrations of TN (Total Nitrogen) and TP (Total Phosphorus) declined by 40–25% and 56–34.48%, respectively. The NH3-H concentration decreased by 45.83–40%, TSS (Total Suspended Solids) concentrations fell by 56–16.67%, and the reduction in COD (Chemical Oxygen Demand) ranged from 64 to 20.94%. Additionally, the river network density increased from 0.43 to 0.63 km/km2, the water surface area ratio grew from 0.95 to 2.26%, and the river frequency rose from 0.19 to 0.28 line/km2. The development coefficient of the first-order tributary river network increased from 4.27 to 4.68 (a 9.51% increase), while the coefficient for secondary rivers rose from 0 to 2.22. Both the area-to-length ratio and average branching ratio remained largely unchanged.