# Study on the Impact of the Coastline Changes on Hydrodynamics in Xiangshan Bay
## 4 Conclusions  
Based on the FVCOM ocean numerical model, this study designed a set of control experiments and five sets of comparative experiments targeting the areas with significant coastline changes in Xiangshan Bay over the past 21 years. We analyzed the impact of coastline changes on the hydrodynamics of the Xiangshan Bay in different sections. The results obtained were as follows.  
The control experiment showed that Xiangshan Bay was a typical semidiurnal bay, with the amplitude of M\({}_{2}\) tide in the bay above 1.0 m. The amplitude increased from the Fodu channel and Niubishan channel to the inner bay, and then gradually decreased towards the head of the bay. The contour lines of M\({}_{2}\) tidal phases were constrained by the orientation of Xiangshan Port, transitioning from a southwest-northeast direction near the open sea to a northwest-southeast direction in the inner bay. There was a significant correlation between the distribution of residual current and the distribution of coastline in Xiangshan Bay. The residual current direction and velocity in the inner bay were chaotic, whereas the currents in the mouth of the bay were uniform and faster. They were divided into two flows toward the Fodu channel and Niubishan channel, respectively.  
Experiments A and B had a greater impact on the amplitude and phase because the coastline change area was close to the mouth of the bay. However, experiments C, D, and E had less significant changes, as the coastline changes were near the head of the bay, with only a slight decrease in amplitude around the coastline change area.  
Through the comparison of tidal currents and residual currents in each experiment, the coastline changes near Meishan Island and the east side of Dasong had a significant impact on both non-periodic residual currents and periodic tidal currents. The coastline changes at these two locations influenced the direction of residual currents over a considerable range. There was a significant variation in the velocity of periodic tidal currents, with regions of increased and decreased velocities alternating. The coastline changes north of the Tie inlet had a certain impact on both the residual current and the periodic tidal current, mainly reducing their velocities. The coastline changes on the east side of Tongzhao and Qiangjiao had little effect on the residual current, whereas the impact scale of the tidal currents was relatively small.  
In summary, coastline changes at the mouth of the bay had a greater impact on hydrodynamics, changing the amplitudes and phase distributions of tides. However, coastline changes in the inner bay have lesser impacts on hydrodynamics, mainly affecting surrounding areas, which may accelerate the transport of pollutants and nutrients.  
It is worth noting that in this study, the friction coefficient values are assumed to be uniformly consistent. However, in reality, the bottom friction may vary across different regions, which will be taken into consideration in future studies.  
**Author Contributions:** Conceptualization, S.H. and J.Z. ([PERSON]); methodology, Y.X.; validation, Y.X., Y.W. and Y.Z.; formal analysis, Y.X.; data curation, Y.W.; writing--original draft preparation, Y.X.; writing--review and editing, J.Z. ([PERSON]); visualization, Y.X. and Y.W. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by National Key Research and Development Program of China, grant number 2021 YFC3101702.  
**Data Availability Statement:** No new data were created or analyzed in this study. Data sharing is not applicable to this article.  
**Conflicts of Interest:** The authors declare no conflict of interest.