# Quantitative Assessment of the Environmental Impacts of Dredging and Dumping Activities at Sea
## 3 Results
### Result of Mud Transport (MT) in 2D Model  
The scenarios tested in the mud transport module are as below:  
Scenario 1 (S1): Using the reanalysis of wind data from September and October 2017 produced from the NOAA GFS model for the study area. Scenario 2 (S2): Calm wind  
Scenario 3 (S3): Using the wind direction from September and October 2017, and additionally, the wind velocity reaches Beaufort level 4, with a wind speed of 6,7 m/s. The simulation time was from 10 September 2017 to 30 September 2017, the time step for the simulation was 72 s, and the iteration was 60,000 steps. Scenario 1 (S1): This scenario applied the reanalysis of wind data in September and October 2017 produced from the NOAA GFS model for the study area. The results of the hydraulic simulation demonstrated that streamflow velocity fluctuated from 0.08-0.72 m/s in the dredged area, streamflow velocity fluctuated from 0.08-0.16 m/s in the dredged area, the tidal flow in high tide could reach 0.36 m/s, and the flow velocity in the dredged area ranged from 0.12-0.28 m/s in the southeast (Figure 8a,b). The hydraulic results affected the diffusion of total suspended solids (TSS) in the 2D MT model. Based on the results, the concentration of floating sludge under calm wind was 0.08 kg/m\({}^{3}\), which is below the acceptable limit (the tolerance limit of QCVN 10-MT:2015/BTNMT [54] is 0.5 kg/m\({}^{3}\)). At low tide, the concentration was higher, at 0.0072 kg/m\({}^{3}\) diffused in an area of 6 km\({}^{2}\), but at high tide, the concentration was greater than 0.0045 kg/m\({}^{3}\) in an area of 8 km\({}^{2}\) (Figure 8c,d). However, this concentration was much lower than the acceptable limit; therefore, the dumping of dredged materials in this area was unlikely to increase the concentration of suspended material to beyond the acceptable limit. Figure 7: The flow field results at a closer view of Quy Nhon port. Scenario 2 (S2): The simulation in calm wind showed that during peak and low tides, the flow velocity was smaller than 0.03 m/s. In the dredged area, flow velocity was in the southwest during the peak tide and in the west during low tide. In the ebb tide, flow velocity was northeast, at 0.04-0.36 m/s. At high tide, the flow was upstream, southwest, at 0.03-0.05 m/s (Figure 9a,b). Figure 8: The simulation results of S1. Streamflow field during (**a**) low tide; (**b**) high tide. Distribution of suspended sediment concentration (SSC) during (**c**) low tide; (**d**) high tide. The streamflow in the dredged area was quite stable, and the velocity was similar in both high and low tides. Therefore, the effect of streamflow on the concentration of total suspended solids was minimal. The suspended material moved to the northeast in the ebb tide and moved upstream to the southwest in the high tide. As a result, the distribution of the suspended material was an elliptical shape with the northeast-southwest axis. The simulation results showed that the maximum concentration of suspended sludge was under the acceptable limit [54] (the acceptance of QCVN 10-MT:2015/BTNMT is 0.5 kg/m\({}^{3}\)). The concentration of suspended sludge was greater than 0.004 kg/m\({}^{3}\), spread over a 3 km\({}^{2}\) area (Figure 9c,d). However, this concentration was much smaller than the acceptable limit, so dumping dredged materials in this area is unlikely to adversely increase the concentration of suspended material. Scenario 3 (S3): According to the simulated wind velocity during September and October 2017, the wind blew from the north to the south. At the dredged area, the direction of streamflow was southwest, and the current speed ranged from 0.08-0.24 m/s. At the outside of Hon Kho, the current speed could reach 0.6-0.75 m/s. During the ebb tide, the direction of run-off was similar, from the north to the south. At the dredged area, the streamflow was southwest, with speeds from 0.15-0.45 m/s. The streamflow outside Hong Kho extended to the south, with speed from 0.32-0.48 m/s (Figure 10a,b).