# Analysis of the Water Circulation Structure in the Paldang Reservoir, South Korea
## 3 Results  
### Analysis of Monitoring Data  
#### 3.1.1 Hydraulic Data  
Figure 3 shows the time series of velocity (speed and direction) at different layers of the PC1 station. Additionally, in Figure 3, velocity (m/s) is in vector form and comes from the average value of 10 min intervals. The hydraulic properties of PC1, which was located in the lower reaches of the North Han River, changed according to the diurnal discharge characteristics of the CPD located upstream the station. The flow velocity at PC1 remained low, i.e., below 0.1 m/s, in most cases during the zero-discharge period of the CPD, and it exceeded 0.2 m/s during the discharge period. Downstream flow was observed at the initial stage of observations, but two-layer flow accompanied by countercurrent flow extending down to 2-4 m from the water surface was observed after 25 May. Upstream flow (countercurrent flow) occurred in the surface layer immediately after the completion of discharge from the CPD and lasted right until the initiation of the next discharge. Downstream flow was observed in the next layer down. Upstream flow occurred in the lower layer, 3-4 m above the bottom surface, from May 19 onward when the discharge from the CPD ended, and this lasted for about three days until the discharge was resumed. The characteristics of flow velocity variations depending on two-layer flow were examined in this study, and the results showed that in the presence of two-layer flow, the flow velocity appeared higher, i.e., exceeding 0.2 m/s, than that in the presence of downstream flow throughout all layers. Moreover, during the period of upstream flow occurring in the surface layer, a high flow velocity was observed 4-7 m above the bottom surface. In contrast, during the period of upstream flow in the lower layer, a high flow velocity was observed in the surface layer. Additionally, in this study, data were classified into the following two categories: the vertical mixing period and the stratification period. At PC2, due to the continuous discharge from the Ipo Weir (IPW) and different from the discharge pattern of the CPD, no significant variations in the flow rate and velocity compared to PC1 were observed. Two-layer flow was present in the upstream direction at the lower layer and in the downstream direction at the upper layer, and the thickness of upper and lower layers varied depending on the discharge rate of the PDD. During the period of the lower discharge rate from the PDD, upstream  
Figure 3: Discharge of the Ipo Weir (IPW), the Cheongpyeong Dam (CPD), and the Paldang Dam (PDD); wind speed and wind direction at PA1; and velocity by depth at PC1 from 12 May to 17 June 2016.  
flow prevailed in the lower layer. On June 8, when the discharge rate of the PDD was recorded to have been below 200 m\({}^{3}\)/s, the thickness of the lower layer in which upstream flow occurred was measured at approximately 6 m. The upstream flow in the lower layer exhibited weaker diurnal variations, which can be attributed to the periodic discharge pattern of the CPD. As a result, at PC2, located in the lower reaches of the South Han River, upstream flow occurred in the lower layer of the South Han River during the discharge period of the CPD. At PC3, which was the location most affected by the discharge pattern of the PDD, downstream flow prevailed, and it exhibited periodicity when the instantaneous discharge rate from the PDD exceeded 400 m\({}^{3}\)/s. In particular, downstream flow with a velocity greater than 0.2 m/s was observed in all layers when the instantaneous discharge rate of the PDD exceeded 700 m\({}^{3}\)/s. The flow velocity in the lower layer decreased to below 0.2 m/s, or no flow occurred when the instantaneous discharge rate of the PDD remained below 400 m\({}^{3}\)/s for several days. To check the flow direction by layer, Figure 4 shows the velocity vectors for the different depth layers and periods. During the vertical mixing period, an average flow in the downstream direction was observed in all layers at all stations (Figures 4a and 5a).