# Study on the Impact of the Coastline Changes on Hydrodynamics in Xiangshan Bay
## 2 Materials and Methods  
### Model Configuration  
The FVCOM ocean model uses a triangular mesh that can accurately fit complex coastlines and numerous islands of the Xiangshan Bay area, which is conducive to the accurate simulation of real topography. At the same time, FVCOM uses a method to determine dry and wet grids, which can better simulate the situation of tidal flats in the Xiangshan Bay. The hydrodynamic model constructed in this study sets two open boundaries in the Fodu channel and Niubishan channel, respectively. Due to the complex terrains in areas with drastic changes in topographies, the coastline is densified in areas such as islands and coastal boundaries. This study sets up a group of control experiments and five groups of comparative experiments. The above grid of Xiangshan Bay was nested into a larger domain, covering the entire East China Sea (Figure 2). In terms of  
Figure 1: Schematic diagram of the changes in the coastline of Xiangshan Bay. The red boxs indicate the main areas of coastline changes.  
experimental design for the control group, each of the comparative experimental groups only added or removed coastlines in different areas. Comparative experiment A targeted Area 1 near Meishan Island (Figure 3A); Experiment B targeted Area 2 on the east side of Dasong (Figure 3B); Experiment C targeted Area 3 on the east side of Tongzhao (Figure 3C); Experiment D targeted Area 4 on the north of Tie inlet (Figure 3D); and Experiment E targeted Area 5 near Qiangjiao (Figure 3E). To accurately simulate the horizontal and vertical profiles of the currents, the minimum grid accuracy of the above experiments was within 100 m, and the water column was divided into 11 uniform vertical layers using the \(\sigma\) coordinate system. The detailed descriptions and key parameters of these numerical experiments are shown in Tables 1 and 2.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Model Parameter** & **Value** \\ \hline Model time setup & 1.0 s \\ Bottom friction coefficient & 0.0003 \\ Vertical eddy viscosity & M-Y 2.5 turbulent closure \\ Vertical layers & \(11\) uniform sigma layers \\ Open boundary condition & Sea surface elevation time series from TPXO9 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Configurations of key model parameters.  
Figure 3: Mesh of comparative experiments (**A**–**E**) changed area. Subfigures (**A**–**E**) represent the zoomed-in meshes of coastline changed Areas 1–5, respectively.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Experiments** & **Descriptions** & **Nodes** & **Elements** \\ \hline Control experiment & 2002 coastlines & 15,412 & 29,089 \\ Experiment A & Changed Area 1 (near & 15,001 & 28,322 \\ Experiment B & Changed Area 2 (the & 15,159 & 28,613 \\  & east side of Dasons) & 14,905 & 28,160 \\ Experiment C & Changed Area 3 (the & 15,234 & 28,751 \\  & east side of Tongzhao) & 15,242 & 28,796 \\ Experiment D & Changed Area 4 (the & 15,234 & 28,796 \\  & north of Tie inlet) & 15,234 & 28,796 \\ Experiment E & Changed Area 5 (near & 15,242 & 28,796 \\  & Qiangjiao) & 15,242 & 28,796 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Various experimental model configuration.