*3.3. Data Acquisition*

As part of the research, a precise record of Barcelona Metro Line 3's possible water accesses was obtained based on TMB provided data. Historical flood events confirm surface runoff reaches the Metro system by specific accesses and ventilation grates, set by visual reports authored by newspapers, also registered and validated by TMB staff. Metro Line 3 elements' representation as part of the coupled 1D/2D Barcelona drainage network model considers water flow through Metro station entrances, lifts, and ventilation grates, as Figure 8 shows. This inclusion allows improving model outcomes, as flow dynamics description between surface flow and urban drainage improves [61]. Figure 9 indicates the spatial location of the possible water entry points to Line 3 of the Metro system.

**Figure 9.** (**a**) Integrated Metro elements into the hydrodynamic model, such as possible water entry points for Barcelona Metro Line 3; (**b**) schematic representation of the Metro station accesses analyzed in the study.

### *3.4. Rainfall Data and Boundary Conditions*

The 1D/2D hydrodynamic model boundary conditions consisted of the result of the rainfall-runoff model (storm water management model based on the non-linear reservoir was chosen among the options provided by InfoWorks ICM®) applying the rain gauge data and the observed water depth in the Metro tunnels downstream. The upstream boundary conditions applied in the calibration and validation phase are two historical rainfall events with surface flooding over Barcelona.

#### 3.4.1. General Rainfall Conditions in Barcelona

The annual average rainfall is 460 mm. The Mediterranean rainfall pattern shows short-duration high-intensity events and spatial high-variability; 50% of the annual precipitation happens throughout variable rainfall events [62]. These events, in combination with city morphological characteristics and impervious areas, produce high flows in the sewer system. All these factors increase urban flood risk in city flatland areas. Barcelona has rainfall data since 1927 from Fabra Observatory; this long-time data series allowed the creation of the intensity duration frequency (IDF) curves for the city. Figure 10 shows new IDF curves based on 81 years data series (1927–1992 and 1995–2009) for some return periods [42]. Intensity values from these IDF curves are currently employed in local sewer network studies.

**Figure 10.** Intensity duration frequency (IDF) curves relating return periods (in years) with durations (in minutes) for Barcelona [63]. Points with information of two real precipitation events (2018-09-06, 2018-08-17), to compare them with the IDF curves.

#### 3.4.2. Calibration and Validation Data

Two recent rainfall events induced surface flooding in Barcelona. One of these events (2018-09-06) caused Metro service disruptions in five stations. Ergo, this study uses this flooding event to calibrate the hydrodynamic model considering a water level of 0.15 m, for which service availability is likely to be affected by flooding in Metro service tunnels. Figure 11 shows calibration, and validation rainfall events' hydrological features, including rain gauges' covered area, Thiessen polygon distribution, and cumulative rainfall depth.

**Figure 11.** Study area map indicating rain gauges used for rainfall data acquisition process, and the cumulative rainfall (mm) for every flooding event. Red dots reveal possible water entry points to Metro Line 3. Blue lines show the Thiessen polygon division for each gauge region (25). (**a**) Flooding event for calibration 2018-09-06; (**b**) flooding event for validation 2018-09-06.

The other flooding event (2018-08-17) produced surface flooding; nevertheless, this event did not affect Line 3 Metro stations. On this basis, this research considers this event valid for validation purposes, understanding the process to validate the surface 2D overland flow in the vicinity of the Metro station. For this event, the model registers surface flooding, but it does not indicate floods inside the station.
