Effects of constructed wetlands on hydrodynamic conditions and water quality in urban plain river networks

Plain river networks are typically characterized by low terrain, and slow and irregular flows, and are prone to water quality deterioration due to external inputs. Constructed wetlands are often applied for in- situ water environment management, but their overall impacts on river network hydrodynamics and water quality remain unclear. In this study, using the typical complex plain river network of Suqian, Jiangsu Province as a case study, eight rainfall-related field observations were conducted, and a one-dimensional hydrodynamic–water quality model coupling runoff generation, drainage network, and river network was developed and validated. The effects of constructed wetland layout on river network hydrodynamics and water quality patterns were investigated. The results show that the constructed wetlands significantly altered the hydrodynamic and water quality patterns of the river network. The increased resistance in wetland reaches adjusted the flow distribution, with the total discharge decreasing by 0.004 m³/s and the diversion ratio decreasing from 37.8% to 29.7%. The water quality in the wetland reaches and downstream areas improved notably, with the average NH₃-N concentration in the river network decreasing by 0.041 mg/L and the proportion of Class V water bodies decreasing by 6.5%. The impacts of constructed wetlands were reflected mainly in the increased Manning’s roughness, decreased dispersion coefficient, and enhanced decay coefficient, with the parameter sensitivity ranking as follows: decay coefficient ＞ dispersion coefficient >> roughness. Based on the cross-sectional characteristics of river reaches, recommendations for wetland layout were proposed, providing a theoretical basis and practical reference for the refined regulation of urban river networks and rational wetland deployment.