# Analysis of Water Quality Improvement Effect of Controlled River by Multi-Functional Weir Operation
## 4 Discussion  
In this study, the water quality improvement effect of pulsed flow was analyzed for the section between the Gangjeong-Goryeong Weir and the Dalseong Weir of the Nakdong River, which became a controlled river due to the construction of multifunctional weirs. To determine an efficient pulsed flow method, the water quality improvement effects according to the upstream and downstream gate operation systems were compared via numerical simulation using the EFDC. To perform numerical simulation using EFDC, the parameters of the hydrodynamic model and water quality model were corrected through a comparison of the simulation results with the water quality observation results. The hydrodynamic model exhibited an R\({}^{2}\) value of 0.84, indicating that the model could effectively reproduce the flow of the target section. In addition, after correcting the parameters of the water quality model, the average error range of each water quality factor was found to be within 20% and the model accurately represented the water quality change of the target section. Various scenarios were analyzed using a physics-based water quality model. Finally, an effective multi-functional weir operation plan was attempted to improve water quality. To analyze the water quality improvement effect of pulsed flow, a numerical simulation was performed for a total of four pulsed flow scenarios, including an actual pulsed flow case and three gate operation scenarios. Water quality improvement was observed mostly in the upstream section, and the water quality concentrations hardly changed at the 15-km downstream point. Furthermore, the water quality improvement gradually decreased with time. When the discharge time was doubled, the water quality concentrations became lower for 24-48 h. In addition, when the gates of the upstream and downstream weirs were opened instead of maintaining the discharge time, the water quality improvement effect was higher than when the discharge time was extended. In general, the pulsed flow is used as a method for controlling the algal bloom. Flow management strategies such as pulsed flows, weir drawdown, and artificial de-stratification have been proposed as means of controlling cyanobacterial blooms in regulated rivers [24]. The algal management strategy for the Murray-Darling Basin aims to reduce the frequency and intensity of algal blooms and other water quality problems associated with nutrient pollution in the basin through a framework of coordinated planning and management actions [25]. The success of pulsed flows to disperse established algal blooms has been demonstrated in the Murray Darling Basin [26; 27; 28; 29]. The provision of pulsed flows above production targets has become part of the operating protocols for many systems including the Murray River. While the successes of pulsed flows to disperse algal blooms are numerous, there are documented cases of negative impacts. For example, the dispersal of an established bloom with pulsed flows can also create impacts in downstream reaches. In highly regulated systems such as the Menindee Lakes, blooms dispersed from upstream can simply reestablish in regulated reaches or impoundments further downstream where conducive flow conditions return [30]. The mitigation of algal bloom by pulsed flow in the Yeongsan River in Korea can be seen as an attempt to improve the efficiency with the minimum flow rate in the seasonal situation of the dry season when the flow rate is insufficient. However, it was a tool for solving problems in some sections, not macroscopically considering the condition of the downstream river. It contained limitations in controlling high concentrations of algal biomass [11]. When algal blooms occur frequently or for a long period of time, the economic loss caused by the suspension of small hydro power generation and the downstream impact of the transfer of algal blooms must be considered at the same time [1].