# Analysis of Water Quality Improvement Effect of Controlled River by Multi-Functional Weir Operation
## 3 Results
### Analysis of Water Quality Improvement effect  
To analyze the water quality improvement effect, the water quality changes in Cases 2, 3, and 4 were compared with each other based on the scenario in Case 1, in which the actual pulsed flow was performed. Figure 8 is a graph showing the time variation of the water quality decrease rate in each scenario compared to Case 1, and Table 2 summarizes the change in the decrease rate over time. In Case 2, Chl-a, COD, and T-P were, respectively, 7.68%, 4.48%, and 3.99% higher compared to Case 1 for 24 h after the start of discharge, and the water quality concentrations were found to be higher due to the change in the fluid flow caused by the pulsed flow. For the following 48 h, the decrease rates of the water quality variables, except for DO, increased, and DO gradually increased, thereby exhibiting the water quality improvement effect. After 72 h, however, the decrease rate was close to 0% and the effect of pulsed flow showed a tendency to decrease. The initial increase in organic matter is considered to be the cause of the re-floating of organic matter in the sediment deposited on the river bed according to the pulsed flow. Therefore, it is considered that the effect of sediment organic matter re-floating should be carefully considered when performing pulsed flow. Therefore, when the pulsed flow time was extended, the initial water quality concentrations temporarily increased due to the upstream influence, but the water quality concentrations slowly improved over time due to the flushing effect. However, only a temporary water quality improvement effect was observed, and it seemed that effective countermeasures were necessary to obtain a continuous water quality improvement effect.  
For Cases 3 and 4, the flow velocity was found to be higher than that in Case 1 due to the opening of the gate. Therefore, the upstream water was temporarily introduced immediately after discharge, and thus the Chl-a, COD concentrations increased for 24 h after the start of pulsed flow similar to Case 2. The water quality concentrations of Case 4, in which there was a wider gap in the gate, were found to be higher than those of Case 3. T-N and T-P showed an immediate response to pulsed flow, and showed a tendency to improve compared to other water quality items due to the dilution effect as the flow rate increased. After 48 h, the flow was gradually stabilized, DO increased, and the decreased rates of Chl-a, COD, T-N, and T-P slowly increased, resulting in the improvement of water quality observed through the decrease in water quality concentrations. In addition, Case 4 exhibited a higher water quality improvement effect than Case 3. After 72 h, the decrease rate was close to 0% and the effect of pulsed flow disappeared as is seen in Case 2. These results indicate that the flushing effect is improved through the increase in the flow velocity caused by the opening of the downstream gate, and that the water quality improvement effect increased as the opening of the gate widened.  
Figure 8: Case 1 Contrast Scenario Reduction Ratio.