# Analysis of the Water Circulation Structure in the Paldang Reservoir, South Korea
###### Abstract  
In this study, results are presented for depth-specific hydraulic and water quality surveys, as well as meteorological observations, at key monitoring sites in the Paldang Reservoir, South Korea. These results were used to determine the water circulation characteristics that represent the main contributors to water quality changes. In the section before the confluence point of the two rivers, a different type of two-layer flow appeared for each. In the North Han River, backwater flowing backward in the upstream direction occurred in the surface layer, and was accompanied by two-layer flow, during the zero-discharge period of the Cheongpyeong Dam. On the other hand, in the South Han River, two-layer flow was observed in the upper layer in the downstream flow, and the thickness of upper and lower layers varied depending on the discharge rate of the Paldang Dam. Strong flow fluctuations were observed near the Paldang Dam (3 km upstream), and these were dependent on its instantaneous discharge rate. In particular, if the instantaneous discharge from the Paldang Dam remained below 400 m\({}^{3}\)/s for several days, the flow velocity in the lower layer decreased below 2 cm/s or no flow occurred. The comparison between flows during different periods associated with depth-specific water temperature variations showed that the average flow prevailed across all layers in the downstream direction at all monitoring sites during the vertical mixing period. In contrast, spatiotemporal variations in flow, such as two-layer flow and backwater, were observed during the stratification period in the South Han River and the North Han River. This led to increased residence times and vertical stability, thus creating favorable conditions for algal blooms. P 2020 10 6822 10.3390/app10196822 10.3390/app10196822 10.3390/app10196822 10.390/app10196822 10. In order to successfully and effectively address the issues associated with reservoir water quality, analyses of various monitoring data collected on the flows and mixing patterns of all interconnected water bodies, as well as their hydraulic relationships, with reservoir water circulation should be performed [9; 10]; in particular, it would be desirable to develop strategies for improving water quality based on detailed quantification or prediction models of water quality issues. Furthermore, efficient water quality management strategies require continuing basic research that allows for all factors affecting water quality to be taken into account. Though many studies have been conducted by using statistical analyses, geographic information systems (GISs), artificial neural networks (ANNs), and numerical modeling for water resources management in South Korea and other countries [11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25], most of the ongoing studies currently suffer from an insufficient amount of high-quality monitoring data needed to ensure the reliability of outcomes. The Paldang Reservoir, which is one of the major drinking water sources in South Korea, is a key component of the stream-reservoir interface in the Han River system. The South Han River, the North Han River, and the Kyeongancheon Stream, which are characterized by distinctive watershed characteristics, meet within the Paldang Reservoir. Therefore, the Paldang Reservoir exhibits complex mixing patterns and distinguishing spatiotemporal variations in hydraulics and water quality. Such complexities and peculiarities of the Paldang Reservoir warrant continuous, precise, three-dimensional monitoring surveys in the region. Several comprehensive investigations have been conducted on the hydraulic properties and water quality of the Paldang Reservoir. The National Institute of Environmental Research (NIER) [4] examined the water quality at key sites in 2007, including the boundary points of three streams flowing into the Paldang Reservoir, while performing longitudinal measurements on various parameters, such as the flow velocity and direction, by using an acoustic Doppler current profiler (ADCP).