# Wind-Wave-Current Coupled Modeling of the Effect of Artificial Island on the Coastal Environment
## 4 Discussion  
Based on the verified coupled model driven by measured water and sediment data presented above, the evolution of the near-shore topography under the disturbance of the artificial island and its causes were analyzed, and the trend of the topographic evolution after the removal of the artificial island is discussed.  
The existence of the artificial island will interfere with the local flow field, causing the formation and shedding of vortices. Different hydrodynamic characteristics were shown during the different advances and retreats of the tides relative to the coastline, as shown in Figure 11a. In the tidal surge stage, an obvious beam flow area was formed between the artificial island and the coastline, and a vortex area was formed behind the island. In addition, there was a dry terrain area near Tannen Port, and the dry and wet terrains changed with the change in the tidal current, which also shows the superiority of SCHISM's dry and wet terrain processing algorithm. In the stage of an abrupt tidal current (Figure 11b), the channel runoff from Tannen Harbor was large, forming a significant coastal current from bottom to top (from west to east), constantly transporting the sediment from the Wanquan River. Meanwhile, the seabed around the artificial island was mainly composed of sand and silt, which were non-cohesive and easy to stir up, contributing to the sediment accumulation between the artificial island and the coastline. With the passage of time, the tombolo bar was gradually formed. In general, multi-directional currents and sediment transport from the artificial island and shoreline-confined areas, channel runoff and coastal currents, and ocean-directed tidal currents contributed to the local seabed topography of the artificial island.  
In order to further simulate the influence of the artificial island on regional changes in scour and siltation, we simulated an ideal scenario of the complete removal of the artificial island. The simulation compared the distribution of topographic scour and siltation thickness around the artificial island one year after its removal and combined the results with the actual situation of active beds of the seabed surveyed by drilling, that is, the thickness of the quaternary was generally less than 2 m. The results show that in the case of the artificial island, as shown in Figure 12a, about 0.1 m of siltation occurred between the island and shoreline and on both sides of the island, with a \(-0.1\) m scour thickness behind it. After the removal of the artificial island, as shown in Figure 12b, the erosion and siltation on both sides of and behind the island decreased (the thickness was less than 0.05 m) and tended to return to the natural bed morphology. However, the siltation morphology remained at the local location of the shore at 0.1 m.  
Figure 10: (**a**) Distribution of suspended sediment concentration [the \(x\)-axis is the east longitude axis and the \(y\)-axis is the north latitude axis]; (**b**) suspension sediment simulation validation.  
A comprehensive analysis of the sediment transport and topographic evolution mechanism around the artificial island concluded that when the artificial island exists, a beam will form between the artificial island and the coastline, which will meet the coastal current coming to Tannen Port (see Figure 11), resulting in the siltation between the artificial island and the coast. An eddy wake was formed on the back surface of the artificial island, which interacted with the tidal current and formed scour pits on the back surface (see Figure 3b). After the complete removal of the artificial island, the sea bed developed towards a balance of erosion and silt. In conclusion, due to the synthetic effect of the beam and the water-retaining effect on the man-made island, the coastal current and the tidal current led to the artificial island having a complex influence on the water and sediment transport and topographic evolution.