Effect of water diversion flow on water quality improvement and saturation threshold in urban plain river networks

Despite the widespread application of joint sluice and pump operation in plain river networks, the quantitative relationship between diversion flow and water quality improvement under weak hydrodynamic conditions remains insufficiently understood. Thus, this study employed a coupled MIKE 11-21 hydrodynamic and water quality model to simulate 60 water diversion scenarios with gradually increasing inflow rates in the Yundong area of Shengze Town, Suzhou, Jiangsu Province. We then analyzed the response of water quality improvements-specifically for chemical oxygen demand (COD), total phosphorus (TP), and ammonia nitrogen (NH₃-N)-to variations in the diversion flow rate. Results show that improvement rates of COD and TP rise as flow increases and reach saturation thresholds at approximately 57 m³/s and 81 m³/s, respectively. In contrast, there is a limited NH₃-N improvement, with a saturation threshold of about 46 m³/s. In addition, water quality responds to flow differently across monitoring cross-sections, with some tributary water quality even deteriorating under special conditions, reflecting the complex interaction between local hydrodynamic conditions and pollution load distribution. This research reveals the quantitative relationship between water diversion flow and water quality improvement effect in plain river network areas, identifies saturation thresholds for water quality improvement, and provides scientific reference and decision support for dynamic water management and water environment treatment in similar regions.