## 3 Simulation of Water Intrusion into Subway Station Operating Conditions
### Analysis of Flood Invasion Situation  
In accordance with the aforementioned operational conditions, the flood-spreading area continues to expand over time until it encompasses the entirety of the subway station and subsequently flows into the tunnel. The progression is illustrated in Figures 5a-g and 6a-g, which depict the distribution of the flood at various time intervals when the flood height outside the station is 1500 mm, referred to as Condition 1 in Table 3. At the 10 s mark, the floodwaters had all passed through the stairwells at the entrances (Figure 5a). The phenomenon of rapid air movement above the water current was observed at all entrances (Figure 6a). This was due to the fact that when the velocity of the water flow was higher, the pressure above it was lower, which resulted in an acceleration of the flow of air. As the floodwater reached the bottom of the stairs, its velocity increased. The lowest point of the staircase in Entrance B was situated in close proximity to the elevator wall. As the floodwater approached the wall, it was observed to move at a relatively high velocity. Concurrently, the floodwaters descended the elevator shaft at high velocity (Figure 6b). The floodwater proceeded to flow towards the wall at each corner of the double-run staircase at a high velocity (Figure 6c). At time point \(t=20\) s, the floodwater entered the platform level at A, while the floodwater at the other entrances had not yet reached platform level (Figure 5b). At \(t=30\) s, the floodwater from all entrances reached the platform level (Figure 5c). Some floodwater gained access to the station floor via S1 and S2. As the staircase proceeded in a downward direction, the velocity of the water flow increased (Figure 6d,e). At \(t=60\) s, the entire station level was inundated (Figure 5d). The floodwater gained access to the station floor via three distinct routes. A vortex was observed to form at passage S2 and subsequently flowed in a downward direction (Figure 6g). Analysing the 90 s, 150 s, and 480 s cloud diagrams, it was found that the height of the flood in the subway station gradually increased with time after the flood had completely inundated the \begin{table}
\begin{tabular}{c c} \hline \hline
**Highest Water Level Information** & **Water Level/Elevation (m)** \\ \hline The most unfavourable water level within the station section with a 100-year return period & 76.40 \\ Station entrance ground elevation & 73.50 \\ Highest water level & 2.90 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Maximum water level at subway station. \begin{table}
\begin{tabular}{c c} \hline \hline
**Condition Number** & **Depth of Water** \\ \hline Condition 1 & 1500 mm \\ Condition 2 & 2000 mm \\ Condition 3 & 2450 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Working condition simulation settings. subway station (Figure 5e-g). The mass flow rate of the floodwater at the tunnel outlet at 480 s of floodwater intrusion was determined to be 41,728 m\({}^{3}\). The mass flow rate at the entrance was consistently recorded at 43,303 m\({}^{3}\). At this juncture, the discrepancy between the two values is 1575 m\({}^{3}\), which represents 3.64% of the mass flow rate at the entrance. The percentage is relatively insignificant, and it can be posited that the total volume of floodwater within the station remains largely unaltered following 480 s of floodwater intrusion. The intrusion process thus reached a conclusion. The station is comprised of a straightforward configuration, with a discernible trajectory of floodwater intrusion. The floodwater flows from a higher elevation to a lower elevation, gradually covering the entire subway station. Upon entering the subway station, the flood flows into the station hall floor, subsequently traversing the staircase openings situated in the centre of the station hall floor, and finally, into the platform floor. Ultimately, the floodwater enters the tunnel and flows out of the subway station.