*4.3. Main Results for Current Situation and Business as Usual with Climate Change*

Comparing the results of model 1 (1D GIS based), for the current situation (CS as baseline) and climate change scenarios (BAU, business as usual), i.e., BAU-CS, for each return period, allows estimating the effects of climate change scenarios simulated for the whole city, despite the limitations previously mentioned. The results show an aggravation in the metric C (use of sewer capacity) as response to increased flows generated, for the three return periods (Figure 4 and Table 5). The urban drainage overall performance decreases 8.6%, 7.8% and 10.0%, respectively for the return periods 10, 20 and 100 years. These percentages correspond to the sum of the values of classes "high" and "very high" from Table 5 and represent the relative increase in sewer length where capacity exceeds full pipe capacity and is a proxy of the relative effect of the simulated climate change scenarios.

**Figure 4.** Citywide results for use of sewer capacity (model 1D GIS): CS and business as usual (BAU) situations.

**Table 5.** Citywide results for use of sewer capacity (model 1D GIS): comparison between CS and BAU.


For Lisbon downtown catchments J and L, results of the assessment of flood water level hazard using model 2 (Table 6) show a predominance of very low water levels and a slight increase in flooded areas associated with climate change scenarios (BAU). The apparent small magnitude in the effect of precipitation and sea level increase is explained by the aggregation in hazard classes, a factor not affected by variations in level within the classes of hazard, which is observed in detailed simulation results such as the example in Figure 5, the map showing the results at a critical time step, for T100. The overall magnitude of this hazard is identical in BAU and CS simulation results.

**Table 6.** Catchments J and L results for flood water level hazard (model 1D/2D): CS and BAU situations summary.


(\*) Percentage in each class.

The results for hazard to pedestrians are shown in Table 7. The magnitude of this hazard increases for all return periods, reaching around 4.0, 3.6 and 2.9 for the T010, T020 and T100 return periods, showing a slight decrease with the return period. Results for the assessment of hazard to vehicles (Table 8), follow a similar trend.

**Figure 5.** Results from model 1D/2D, catchments J and L. for the BAU situation (T100): water level at a critical time.

**Table 7.** Catchments J and L results for hazard to pedestrians (model 1D/2D): CS and BAU situations summary.


(\*) Percentage in each class.

Considering the results from the methods applied, it can be concluded that current situation is already unfavourable in terms of flooding frequency in various locations in Lisbon, but the magnitude of the hazards is generally low. Generally, these results are in alignment with available historical observations. The overlapping of the maps for the use of sewer capacity ((model 1D GIS) with results of model 1D/2D, for catchments J and L, confirms the most overloaded parts of the sewer network.

The climate change scenarios simulated do not impose significant increase in the magnitude of flood related hazards to properties and infrastructures, pedestrians and vehicles. Consequently, implications for the mobility and to waste sectors, compared to current situation, are expected to be low.


**Table 8.** Catchments J and L results for hazard to vehicles (model 1D/2D): CS and BAU situations summary.

(\*) Percentage in each class.
