flood hazard maps, corresponding to the Cildáñez, Maldonado and Vega basins, cover an area which is slightly larger than the _Ciudad Autonoma de Buenos Aires_ (CABA) or city of Buenos Aires proper and, therefore not the whole area of study which corresponds to the _Area Metropolitana de Buenos Aires_ (AMBA). It should be noted here that our study focuses on floods that occur mostly in CABA, a vastly smaller area than our area of study. There are two sets of flood maps: before and after works planned in the PDOH, aiming to reduce flood instances. The works planned to mitigate flooding consist mostly of stormwater drainage and retention capacity investments in the three water basins. In both cases a flood hazard map (in polygon form) is produced that provides the extent of a flood for a given return period and for a given flood depth. Return periods range from frequent events occurring every two years to rare events occurring once every 100 years. The list of return periods considered is the following: 2, 5, 10, 20, 50 and 100 years. Flood depths range from 15 centimeters to 160 centimeters. The full list of flood depths considered is: 15, 25, 40, 90 and 160 centimeters. In total, with each flood map being produced after flood protection works and in their absence, there are 59 flood maps.<sup>8</sup> Typically, the area covered by a given flood-depth for a specific flood return period is smaller after flood mitigation actions have been implemented. Each flood map is intersected (overlayed) with a grid that has homogenous 500m x 500m sized grid cells so that the flooded share of land, , of each grid cell ( ), is known for each of the 59 flood maps, depending on the return period of the flood ( ), the depth of the flood ( ) and whether flood protection jj,ττ,dd,ww ss jj works have taken place or not ( ). Given that for a given grid cell, a given return period and a given public ττ dd works scenario, the share of land flooded by 160 centimeters, is also by construction flooded by 15 ww centimeters of water, there is some obvious double counting of flooded areas that needs to be addressed. In a geometric sense, areas that are only flooded by 15 centimeters of water (not more) constitute the outer concentric ring of areas which are also flooded by 25 centimeters of water, by 40 centimeters and so on<sup>9</sup> (see Figure 3 for a visual representation of flood maps for a centennial event before flood mitigation investments). We address this double counting by defining as follows: jj,ττ,dd,ww ss_rriiaaAA (8) = �<sup>ssjj,ττ,dd,ww−ssjj,ττ,dd+1,ww</sup> f<sup>frr dd∈{15, 25, 40, 90}</sup> jj,ττ,dd,ww ss_rriiaaAA jj,ττ,dd,ww ss rr dd= 160 8 We would expect 60 distinct flood maps for each flood depth, return period and in the absence or after flood protection works: 5x6x2=60. However, after flood protection works, there are no locations that get flooded every two years with a water depth of 160 centimeters, therefore we have 59 flood maps rather than 60.

9 This pattern has some very limited exceptions: out of the sum of 10,699 instances where a grid cell is impacted by a specific flood event (before resilience investments), only in 97 of these do we observe unusual behaviors where a bigger flood-depth is not contained within the area of a smaller flood-depth for any given return periods. We also find that out of the 961 grid cells that get flooded in part or in whole by at least one flood event (before flood mitigation), in only 31 of these does the flood data show bigger flood depth areas that are larger than smaller flood depths. These localized discrepancies can possibly be explained by some small errors in the hydrological modeling or the transcription of these results into shapefiles. We deal with these outliers by assuming that a concentric ring for a smaller flood-depth cannot have a negative value: . jj,ττ,dd,ww jj,ττ,dd+1,ww ss 14 ≥ss