*4.4. Main Results for Selected Adaptation Strategies and Climate Change Scenarios*

The three adaptation strategies selected for testing the proposed methodology, are some of those that can be simulated by hydraulic modeling and considered to be relevant by the stakeholders. These strategies are proposed in the Lisbon Master Plan 2016-2030 [36]. Even during risk identification, as presented in this paper, it is valuable to assess the impact of adaptation strategies in the flood related hazards.

The first strategy, CAS1—Adaptation of green infrastructure, corresponds to a significant increase of the total green area in the city (Figure 6). The second strategy, CAS2—Peak flow attenuation through the construction of two retention basins, includes the construction of two small retention basins, one of which has the main purpose of retaining solids (Figure 7).

**Figure 6.** Green areas relative change: % increase from BAU to CAS1.

The third strategy modelled, CAS3—Construction of new components in drainage system, proposes the construction of a large interception tunnel and improvement in the inlets to the sewer network (Figure 8).

The three strategies were simulated with the citywide simplified model 1 but for the detailed model 2 of catchments J and L, only CAS3 is relevant.

Results of model 1 (1D GIS based), for BAU, CAS1, CAS2 and CAS3, for each return period, for the metric C, use of sewer capacity, are given in Figure 9, in terms of the relative variations to CS. The results show an aggravation in the metric C for BAU situation as presented previously as response to increased flows generated in the scenarios of climate change, for the three return periods. CAS1 has only some effect in the areas downstream of the catchments but even in those areas the reduction is limited, since the area upstream to the basins is small and the basin volumes are also small. CAS2 has no substantial influence and results are similar for all return periods. This is attributed to the small influence of the green areas on the hydrological processes for intense rainfall events. CAS3 is the only strategy contributing to decrease the length of sewers in the most severe class. However, since the effect is mainly expected in the areas downstream of the tunnels, the effect is not evident when evaluated for the whole city. In Table 9, the results for the areas downstream of the tunnels are presented and the effect of the tunnels is clearly effective in the reduction of flooding.

**Figure 7.** Location of the retention basins planned in the CAS2.

**Figure 8.** Tunnels: associated drainage catchments and intersection locations [37].

Globally, the results indicate that from the three strategies analysed, only CAS3 has a significant effect on flood related hazards and is limited to the areas downstream of the tunnels.

(**a**) Use of sewer capacity: results for T010

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**Figure 9.** Citywide results for use of sewer capacity (model 1D GIS): results for BAU and CAS situations compared with CS.


**Table 9.** Results for use of sewer capacity (model 1D GIS) in catchments downstream of the tunnels: comparison between CS and CAS3.

The effect of CAS3 on the catchments J and L is also beneficial but is not improving significantly under the current situation. This can be explained by the existence of duckbill tidal valves that require a certain pressure upstream to allow flows downstream to receiving waters. The CAS-3 results for downtown catchments detailed modelling (model 2) show the increase of the carrying capacity at the downstream sewers as the main effects of construction of the diversion tunnels, as obtained for the 1D GIS Model simulations. Nevertheless, in some cases, considerable water depths still occur.

The results for flood hazard to pedestrians show a slight decrease for all return periods. The results for the hazard to vehicles follows a similar trend.
