# Analysis of Water Quality Improvement Effect of Controlled River by Multi-Functional Weir Operation
## 1 Introduction  
The concept of pulsed flow was established by the Australian Government's National Water Commission (NWC). It was reported that the water quality as well as the natural ecology of the river environment can be improved by mimicking the conditions of natural flooding by adjusting the flow rate changes in existing controlled rivers through the discharge of dams and veins [1]. In the Nakdong River, located in the south-eastern region of South Korea, eight multifunctional wers have been constructed to prepare for droughs and floods. Each of these consists of a fixed weir and a gate for controlling the flow rate, their purpose is to adjust the flow of the Nakdong River according to precipitation. A river in which the flow rate is managed using gates, such as the multifunctional weirs of the Nakdong River, is referred to as a \"controlled river\". A controlled river allows for the benefit of manipulating the flow of the river depending on its purpose. However, the sediment accumulation in the river due to the implementation of hard management and the subsequent bed slope as well as particle size change become factors that can alter the river environment [2]. Because such a change in the river environment affects the water quality of the river system, it is necessary to monitor the change in water quality due to the operation of these gates. This is especially important for the Nakdong River system as itsupplies water to nearby cities and agricultural lands, in addition to some areas without any treatment, ergo it is imperative to determine gate operation plans for improvement in water quality [3]. Weir gate operation and pulsed flow are also interconnected with the algal bloom warning system, which has classified four cyanobacteria species (Microcysts, Anabaena, Oscillatoria, and Aphanizomenon) that produce toxic substances into three warning levels depending on the population of harmful cyanobacteria: attention, alert, and outbreak. The gates are used to perform a pulsed flow from the alert step through a dam-weir link operation for the purpose of improving the water quality in the main channel of the Nakdong River. In the case of the Murray River in Australia, the average annual flow rate was reduced by half through the use of weirs constructed upstream. Due to the decrease in the flow rate, river deformation was observed, such as the drying and exposure of some of the river beds [4; 5]. In France, studies on the effect of pulsed flow on canal water quality management and algae reduction in terms of water quality management have been reported [6]. In addition, in the United States, in-lake mesocosm experiments were conducted in small bays during an outbreak of harmful algae, and the effect of reducing algae by pulsed flow was demonstrated [7]. In South Korea, the water pollution phenomenon in which algal bloom occurs due to the construction of hydraulic structures is recognized as an important issue in managed river systems. To reduce water pollution by algae, a temporary water quality improvement effect induced by the discharge of a large amount of flow from an upstream reservoir using pulsed flow is often used. To analyze the flushing effect of pulsed flow, [PERSON] et al. explored the improvement in the Daecheong Dam reservoir water quality after pulsed flow [8]. Further, [PERSON] et al. studied water quality improvement and algal bloom due to pulsed flow in the Seungchon Weir of the Yeongsan River [9]. The pulsed flow, which does not take into account the algal growth characteristics and growth rate, increases the nutrient utilization rate of the algae, which can cause side effects such as increased algae generation or accumulation in downstream weirs. Therefore, for optimal weir operation, a detailed study on the pulsed flow effect is required. However, these studies, which applied actual gate operation cases of a multifunctional weir for their research on water quality improvement, are not sufficient.