## 3 Results
### Flood Evolution Simulation Analysis  
The model is divided into six grid blocks of varying sizes, as shown in Figure 4, labeled Grid 1 through Grid 6, from right to left. Given that turbulent flow is likely to occur as the reservoir water flows downstream, Grid 1, located at the Shanmei Reservoir's dam  
Figure 3: Model of the road network downstream of Shanmei Reservoir. Figure 2: Three-dimensional topographic model of the Shanmei Reservoir dam and its downstream area. site, is locally refined. The refined grid cell size in this area is 2 m \(\times\) 2 m \(\times\) 2 m, while the other grids have a cell size of 6 m \(\times\) 6 m \(\times\) 6 m. The grid partitioning of the model is illustrated in Figure 4. The inflow boundary is set at the inlet point \(Y_{\text{max}}\) of Grid 1, and free outflow boundaries are placed at the surface \(Y_{\text{min}}\) of Grid 5 and the surface \(Y_{\text{max}}\) of Grid 6. The \(Z_{\text{max}}\) surface of all grids is set as a pressure boundary condition, while the connection surfaces between grids are defined with symmetry boundary conditions. The remaining grid boundaries are specified as wall boundary conditions. The boundary conditions for each mesh are set as shown in Table 1.  
The simulation models the scenario in which the Shanmei Reservoir is subjected to a 10,000-year design flood, characterized by a peak inflow discharge of 8040 m\({}^{3}\)/s and a corresponding water level of 102.28 m. The initial reservoir water level is set at 102 m, and the reservoir inflow follows the calibrated design flood hydrograph. Due to the unavailability of detailed bathymetric data for the reservoir area, the upstream boundary condition is represented by a breach outflow hydrograph derived from regulated flood routing calculations. The dam breach is assumed to initiate once the reservoir water level rises to the dam crest, with the breach initiation level set to the design flood level of 102.28 m. The total simulation time for the dam failure process is 2 h. The erosion rate of the dam body is assumed to be positively correlated with the velocity of overtopping flow. Simultaneously, the breach is allowed to expand both laterally and longitudinally. The soils on both sides of the breach undergo intermittent instability and collapse as the breach deepens. To assess the downstream inundation at various time points following a dam breach, eight monitoring points were strategically positioned based on the distribution of downstream villages for flood depth and flow velocity monitoring. These monitoring points, listed in order, include Shanmei Reservoir Management Office, Yanjiang Cement Company, Sanluo Village, Wancheng Paper Industry, Shinn Village, Jun Village, Xiawei Village, and \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \(\mathbf{X_{\text{min}}}\) & \(\mathbf{X_{\text{max}}}\) & \(\mathbf{Y_{\text{min}}}\) & \(\mathbf{Y_{\text{max}}}\) & \(\mathbf{Z_{\text{min}}}\) & \(\mathbf{Z_{\text{max}}}\) \\ \hline Grid 1 & W & W & S & Vfr & W & P \\ Grid 2 & S & W & W & S & W & P \\ Grid 3 & S & S & W & W & W & P \\ Grid 4 & S & S & W & W & W & P \\ Grid 5 & W & S & O & S & W & P \\ Grid 6 & W & W & S & O & W & P \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Boundary conditions for each mesh. Figure 4: Schematic diagram of grid meshing. Shikou Village, corresponding to Monitoring Points 1 through 8. The locations of these monitoring points are illustrated in Figure 5.  
Food depth and flow velocity were recorded at three specific time intervals following the breach: 15 min, 45 min, and 75 min. These data were used to analyze the evolving characteristics of the downstream flood. As shown in Figure 5, the breach progressively widens and deepens due to the erosive force of the water flow, while the floodwaters extend downstream. The distribution of flood depth generally follows a symmetrical pattern, with the deepest water in the center and progressively shallower water on either side. Meanwhile, the position of the maximum flow velocity shifts downstream as time progresses.