*4.3. Risk Assessment*

Applying the outlined methodologies, Figures 14 and 15 describe the comparison of the risk assessment outcomes among the real event (2018-09-06) and the 1-in-20 years return period flooding event with climate change impact, as the most critical scenario analyzed. Differences between the projections provided by each flooding scenario and applying a peak ridership flow pattern are highlighted by the risk assessment presented in Figure 16 as a summary, detailing the risk obtained for Barcelona Metro Line 3 stations facing flooding events. The findings presented here highlight the considerable influence of the water depth in the water entry points' surrounding areas when assessing the climate change impact upon flood risk. Although all the flood events used here do not result in quite different pictures of future flood risk on the Metro accesses or ventilation grates, there are some significant differences in terms of the risk magnitude.

**Figure 14.** Metro stations risk assessment map for real event pluvial flooding scenario (2018-09-06): (**a**) Hazard map considering real flooding event; (**b**) vulnerability map applying off-peak ridership flow; (**c**) risk map with flooding water depth.

**Figure 15.** Metro stations risk assessment map for synthetic event scenario with climate change influence (a 20 year return period event—"T20 CC"): (**a**) Hazard map considering future flooding conditions with climate change influence; (**b**) vulnerability map applying peak ridership flow; (**c**) risk map with flooding water depth; (**d**) Barcelona Metro Line 3 longitudinal profile scheme, with the location of the Metro stations located at the highest and lowest points of the network.

**Figure 16.** Risk assessment outcome for flood events on Barcelona Metro Line 3 stations, arranged by return period (T, in years) and climate change (CC) influence on the projected flood event. The green color means a low risk, the yellow color symbolizes medium risk, and the red color denotes a high risk.

The small difference between the maximum water depth affecting the water entry points, comparing T05 CC and T20 CC scenarios (0.08 m), produces significant differences for the risk assessment results. As Figure 15 illustrates, with a significant risk for 14 stations in T05 CC scenario and increasing to 20 stations having a significant risk in T20 CC scenario. It therefore seems necessary to incorporate a range of approaches in adaptation measures that are focused on coping with this increment in water depth. The majority of Metro Line 3 stations are highly susceptible to floods. Considering all risk scenarios, the maximum water depth in the surroundings of the water entry points, such as ventilation grates and Metro stations access, is not larger than 0.38 m.

### **5. Discussion**

This chapter draws together and discusses the key findings, analyzing the results obtained in the study, and indicates how adaptation measures can be applied, according to local conditions. To the best of our knowledge, this is the first study to have quantified the flooding risk to underground Metro stations for water intrusion hazard due to rainfall events, obtaining water depths estimation for projected extreme rainfall events. The approach used in this study differs substantially from other approaches studied previously [5]. In particular, although it uses geographic information systems (GIS) as the basis for collecting and organizing information from the Barcelona Metro system, the use of GIS is a secondary component which does not form part of the analysis in this study, contrasting other studies in the field [25].

This study considered only the water depth in the Metro station tunnels, obtained from hydrodynamic modelling, as a variable to characterize the hazard. Although the hazard due to flow velocity can be taken into account since there is sufficient information to carry out such an analysis from the hydrodynamic model, the objective of the analysis carried out in this research is the interruption of the Metro service. Therefore, the flow velocity is not a relevant parameter for this analysis, as it would be if this study were focused on passenger safety.

A hydrodynamic modelling software, combined with innovative analysis on how Metro systems can be represented into a drainage system, and assumptions about the fragility of transport infrastructure derived from pluvial flooding events have been used to assess flood risk in a Barcelona Metro line. The study demonstrates the potential for conducting transport infrastructure risk analysis through hydrodynamic modelling and a depiction of the real physical conditions for representing Metro systems. One of the main conditions of this study was to define a simplification of Metro system water accesses, such as ventilation grates, depicted as drainage grates adapted to each grate condition; the stations' access, neglecting the complexity inherent to each station, such as hallways, stairs, and additional architectural components before entering the station tunnel. All these elements were considered as pipes until arriving at the main tunnel; this enabled the Metro network to be integrated into the complex hydrodynamic model.

We have found that improving some ventilation grates and Metro access designs, by increasing their minimum height to 0.35 m, could yield a potential decrease in flooding events for Metro Line 3 tunnels and stations. Of course, care should be taken with the interpretation of these results, as local conditions for each of the ventilation grates and accesses must be considered in detail. This simplification could affect water transport times from water entry points to tunnels.

Service disruptions due to flooding events are not only by water depth increase in Metro tunnels, but also by impacts on ridership stability inside stations, corridors, and hallways. Flooding events can also affect the electrical equipment and increase maintenance costs, as Figure 2 illustrates. Service disruption does not occur mainly because of the water level itself but because of the misleading interaction between water and the Metro blocks-based electrical system. The higher the water depth, the more likely a block can be misleadingly occupied, and thus the service disrupted.

The outcomes of this study display that by accounting for increased possible water entry sources, water depth range forecasts in Metro tunnels widen. It is reasonable to assert that the principal source of flooding is a local level phenomenon of water entry. However, differences in topographic altitude also influenced flooding cases in the Metro tunnels for our case study. The locations of water entry points to the Metro identified in the real flood event (2018-09-06) are near the two most affected stations, Paral·lel and Drassanes. Nevertheless, flood events with projected high-rainfall intensities also affect high sub-basin areas in our case. These outcomes suggest an association between water entry from the upper sub-basin and the increased water entry risk for upper and lower Metro stations. It is therefore likely that such connections exist between a water transport phenomenon into the system tunnels and the increased water depth in Paral·lel and Trinitat Nova Metro stations.

Flooding events also impact the system's vulnerability, as the presence of water in the Metro tunnels affects the continuity of the service, also impacting the passengers' flow. By preventing passengers' access to the stations due to an extreme precipitation event and the consequent flooding that this causes, the vulnerability decreases because the flow of passengers entering the system also decreases. By preventing water from entering the Metro system, both the flooding hazard and the vulnerability expressed in the flow of ridership decrease. Nevertheless, such vulnerability is never zero, and in fact, it is high, since the disruption of the service also impacts passengers who are already on the trains, inhibiting their adequate transportation.

Our findings draw attention to the importance of considering that the low level of some ventilation grates and Metro stations' accesses may increase probabilities for water entering in flood events, as Figure 17 illustrates.

Moreover, the significant water depths in upper-located stations such as Vall d'Hebron associated with high-return periods under observed conditions may further question rainfall events' role in impacting water intrusion to the Metro system. Addressing the spatial and temporal full range uncertainty related to future rainfall events poses a significant challenge, therefore, we believe that having a homogeneous rainfall distribution over the city is adequate to cope with most possible climate change situations to come.

Concerning the research methods, some limitations need to be acknowledged. The interaction at the hydraulic level between the 1D and 2D model is not fully understood, for example, during the transformation of the surface flood water level to the entry points of the Metro system. Due to the lack of modelling elements that competently render physical structures that are not common in hydraulic systems. A possible source of bias for the study is the influence that unexamined physical details of stairs, access corridors, and vents, as water access points to the Metro, could have physical details beyond the roughness, sizes, and lengths that were considered.

**Figure 17.** The visual contrast among a Paral·lel station entrance facing a flood event, and its regular service; (**a**) flooding event 2009-10-22; (**b**) current situation.

This study emphasizes that this risk assessment combines water level projections with present ridership flows and traveler density. Outcomes should hence not be used as projected impacts. Instead, they ponder the current risk, which we define as a threat indicator for Metro system sustainability in the future. Future research should consider and integrate all the major possible flood hazard modes for Metro systems, such as the influence of groundwater infiltrations. A full climate risk assessment is required for the Metro system. It must have a high priority within a comprehensive, Barcelona-wide climate risk assessment and adaptation effort, involving all governmental agencies.
