*2.3. Description of how the Metro Service is A*ff*ected by Flooding*

Depending on the type of tunnel, existing floor, sleeper type, and other variables unique to each Metro system in the world, the characteristics of the infrastructure that perform primary flood control in Barcelona Metro service tunnels are summarized as follows.

The dimensions presented in Figure 4 are estimated, since they may vary depending on the sort of tunnel, among other factors, and should be taken as a theoretical reference. Therefore, depending on the existing drainage in each case, it will be more or less rapid for the water to reach the base of the rail.

**Figure 4.** Approximate scheme of the Barcelona Metro's railway infrastructure.

One of the essential parts of the train movement is railway signaling, which always allows safe movement by regulating the speed and location of trains. The basis of the railway signaling is based on the track circuits, whose theoretical schematic configuration is shown in Figure 5 (it does not precisely represent the reality for the whole Line 3 of the Barcelona Metro), where the track is electrically isolated in sections of a certain length. In the case of Metro Line 3, the track circuits are about 25 and 35 meters long.

The railway signaling equipment is qualified to work under wet conditions; therefore, the circuit could work in case of flooding. Depending on the amount of water in the surrounding area and its conductivity, it may not complete the circuit, then diverting the electric current between the power supply and the receptor, producing the receptor is not over-excited by the electric current and the block is assumed occupied. It is not conceivable to set only a water level which produces the false occupation phenomenon. However, it could be established as an inaccurate reference level when the water level reaches half of the railway rail (0.08 m from the rail bed), we could have an occasional false occupation.

**Figure 5.** Occupied block—track circuit scheme. Modified from [43].

In the case of having a higher water level, in many cases, there would be false occupation, and therefore no Metro service would be provided. Due to these circumstances, this study takes a level of 0.15 m as an approximate reference level in which, in any road configuration, it is inevitable that because of a flooding event, false occupation is produced. It must be considered that even if we could find a situation where the water reaches the height of the head of the rail, there would be no false occupation, due to there being no short circuit of the electrical circuit. At this point, it would be necessary to analyze whether it is mechanically viable for trains to transit, given the possibility of slippage in the wheel-rail contact and the impossibility of visualizing the rail with the danger that this entails. Furthermore, concerning the ATP (automatic train protection) system, the closer the water gets to the head of the rail, the easier it is for the train's ATP antennas not to be able to obtain correct track circuit information, causing the train to brake for lack of obtaining the ATP code. When the water covers the whole head of the rail, accurate ATP interpretation is not possible.
