# Quantitative Assessment of the Environmental Impacts of Dredging and Dumping Activities at Sea
## 4 Conclusions and Discussion  
The study simulates and evaluates the effects of dumping dredged spoils at Quy Nhon Port, which is one of the largest ports in Central Viet Nam. The dredging and maintenance of Quy Nhon Port play an important role in the transportation, evaluation and selection of dredging position and dredging volume. This study applies a 2D numerical model to simulate the propagation of contaminants in dredge materials to the marine environment, with the assumption that the total volume of dredged materials from Quy Nhon Port is transported by barges to the dumping location. The reanalysis of wind data from the NOAA GFS model, with a high resolution of 0.5 degrees and a time span of 3 h, was used to simulate the wave-wind propagation in the 2D model. The simulation results were from 1 September 2017 to 31 October 2017. The calculated results of wave characteristics showed that the simulated and observed wave height were in good agreement in terms of the vibration amplitude, absolute value, and tidal phases in both the calibration and validation processes. The parameters were reliable enough to conduct wave field simulations under different scenarios in the study area. Calibration and validation of hydrodynamics in the 2D model used an observed water level tide at Quy Nhon station during 1-30 September 2016 and 1-31 October 2016, respectively. The results of the calculated and observed water levels were in good agreement in terms of the vibration amplitude, absolute value, and tidal phases, during both the calibration and validation processes. The results of hydrodynamics and spectral wind-wave in the 2D model were used to simulate three scenarios using the mud transport module in the 2D model. The simulation results showed that the maximum concentration of suspended sludge was below the acceptable limit (the acceptance of [54] QCVN 10-MT:2015/BTNMT is 0.5 kg/m\({}^{3}\)) for all three scenarios. The study used the particle tracking (PT) module in the 2D model to evaluate the environmental impact of dredge materials. The dredged materials were transported by hopper barge at 2304 m\({}^{3}\)/day; the concentration was 0.04 m\({}^{3}\)/s at the dredging position and 0.5% particles split from the dumped materials. The simulated results of the 2D PT model showed that the element block in scenario 3 moved the furthest from the dredging position, i.e., approximately 2.9 km. The results of the simulation were qualified, as the dredging position was situated far from the sea; therefore, it was significantly affected by the direction and velocity of wave and wind at the dredging position. For a future study, it is recommended that the uncertainty of the modeling needs is reduced. The following aspects may be improved:  
* The computation time steps of the 2D model are currently large, although it takes approximately 4 to 5 h for one simulation of the 2D SW, HD, MT, PT model. A smaller time step can refine the results and provide more clarity, especially for sedimentation and particle tracking. * The number of calibration and validation locations in the 2D models are few. It may not be enough to evaluate the adequacy of the model in the study area. \begin{table}
\begin{tabular}{c c c c} \hline
**No.** & **Parameter** & **Value** & **Unit** \\ \hline
1 & Flux & 8.3 & kg/s \\
2 & Number of particles per & 20 & Integer \\  & time step & & \\
3 & Decay Rate & 0.05 & /sec \\
4 & Settling velocity data & 0.1 & m/s \\
5 & Settling parameters & & \\
5.1 & Minimum concentration & & \\  & for & 0.01 & kg/m\({}^{3}\) \\
5.2 & Maximum concentration & & \\  & for & 10 & kg/m\({}^{3}\) \\
5.3 & Alpha & 1.01 & \\
6 & Dispersion coefficient & 0.05 & m\({}^{2}\)/s \\  & formulation & & \\ \hline \end{tabular}
\end{table} 
Table 3: The parameters of particle tracking (PT) in 2D model. The results of the 2D PT model only evaluate the distribution concentration and distance transmission of particles after the dredged materials.