## 3 Results
### Reservoir Simulation Result  
#### 3.2.1 Zone A Simulation Results  
Figure 8 shows the simulation results of the T-P water quality changes with respect to Zone A in 2015 for the reservoir. As for the annual average water quality, T-P is 0.183 mg/L when no measure is applied, and the following results are obtained when the scenario is applied: S1, 0.175 mg/L; S2, 0.172 mg/L; S3, 0.175 mg/L; and S4, 0.171 mg/L. When conducting dredging only with regard to Zone A, in 2015, the target water quality standard of 1 mg/L was not satisfied. As for the overall water quality change tendencies, the slope is gradual when no measure is applied during the water quality deterioration process. This implies the water quality resilience of the estuarine reservoir is low when the load is high within the reservoir. Thus, it is difficult to accomplish the water quality objective of an estuarine reservoir merely by dredging. However, this measure can increase the resilience of estuarine reservoir systems. As a result of the simulation in Scenario 5, in which dredging was conducted in Zone B based on Scenario 3, and comparing the results with corresponding scenarios, it is found that a thorough circulation of the water quality does not happen upstream and downstream because of the influence of the embankment placed in the middle. Even if dredging takes place in Zone B, it cannot significantly influence the upstream water quality changes. The annual average water quality was slightly lower than that in the case where no measure was applied (T-P value 0.183 mg/L). However, the T-P water quality concentration in scenario 5 has an annual average value of 0.175 mg/L, which is similar to that of S3. As a result of the simulation in Scenario 7, which conducts dredging in Zone B based on Scenario 4 (unlike Scenario 5 in a state where Zone A dredging progresses to a significant degree), the water quality is partially influenced by Zone B dredging. Regarding the actual annual average water quality concentration, when compared with the T-P simulation result of 0.172 mg/L in S4, the T-P simulation result of Scenario 7 was 0.171 mg/L. When considering \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Location** & **Content** & **TEMP** & **DO** & **T-P (cal.)** & **T-P (cal.)** \\  & & & **(2016–2017)** & **(2015)** \\ \hline Zone & Observed Ave. & 16.64 & 10.34 & 0.242 & 0.201 \\ A-observation & Simulated Ave. & 17.17 & 10.41 & 0.232 & 0.186 \\ point & O/S & 0.969 & 0.993 & 0.957 & 0.927 \\ Zone & Observed Ave. & 16.60 & 11.99 & 0.143 & 0.105 \\ B-observation & Simulated Ave. & 17.32 & 10.62 & 0.148 & 0.101 \\ point & O/S & 0.959 & 1.128 & 0.964 & 0.961 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Calibration and validation results of EFDC model simulation. Figure 8: Simulated results of T-P concentration (Zone A, S0, S1–S4). not only the average concentration but also the water quality change tendency, unlike in the beginning of 2015, the water quality concentration is low at the point where water quality is aggravated over time. This implies a minor increase in resilience than the previous scenario. Figure 9 shows the simulation results of the water quality changes with regard to the cases of extending the phosphorus treatment facility at a PSTW located at the reservoir inlet (Scenario 5) and installing an advanced phosphorus treatment facility (Scenario 6). This scenario is the result of applying the discharge load reduction level of the same PSTWs over the entire period. When comparing the annual average T-P concentration, the results of Scenarios 5 and 6 have a concentration of 0.169 mg/L and 0.156 mg/L, respectively, which plays a major role in improving water quality when compared to the dredging scenario. In particular, unlike the dredging scenario, the case of aggravating the overall water quality compared to the scenario without any measure has a low maximum concentration and rapid water quality recovery. The resilience was significantly higher in Scenarios 5 and 6 than in the dredging scenario.