## 3 Results and Discussion
### Flood Mapping from Satellites  
Another step along the chain used in this event was the use of a recent tool developed by the CIMA foundation and the Italian Civil Protection Department, AUTOWADE, to detect flooded areas from satellites. For the Apollo Medicare, the first available S1 data were acquired on Oct 25 th 2021 at 05:00 UTC (orbit 124). As prefold image, an acquisition from the same orbit performed on 19 October 2021 at 05:00 UTC was used. The AUTOWADE-derived flood map was made available to the end user on 25 October 2021 at 13:00 UTC. By using both S1 and COSMO-SkyMed (CSK) data, a continuous monitoring of the flood evolution was performed. In particular, the following flood images were also used to produce flood maps in addition to those acquired on 25 October:  
* S1--25 October 2021 at 17.00 UTC
* CSK--27 October 2021 at 16.45. * CSK--28 October 2021 at 16.45. * CSG 2 nd generation--30 October 2021 at 04.30. * S1--31 October 2021 at 05.00 UTC. Figure 9, panel A, shows an RGB composite produced using the calibrated (backscattering coefficient s0 [dB]) and geocoded images acquired by S1 over eastern Sicily (Catania  
Figure 8: Results of TELEMAC-2D coupled with the Continuum; maximum water depth (on the left) and maximum water velocity (on the right) for the event that hit the Catanian floodplain 24–25 October 2021.  
and Siracusa districts) on 19 October 2021 (red) and 25 October 2021 (green and blue), both at 05:00 UTC. Flooded areas appear in red tone in the RGB composition, because the presence of floodwater produced a significant decrease in s0, as previously discussed. Note that permanent water appears in dark tone because s0 was low on 19 October, too. The same applies for very smooth surfaces such as airport runways. AUTOWADE is able to automatically detect floodwater and distinguish it from permanent water. The detection is based on automatic thresholding applied to both the flood image and the difference image (difference in dB between the flood and preflood images). Considering that automatic thresholding does not work well if water covers a small fraction of the image (as usually occurs in temperate regions), AUTOWADE uses an edge detection filter to reduce the number of input pixels to those placed near the water-land edges. For this purpose, an ISODATA clustering algorithm is preliminary applied for the purpose of a first-guess identification of water surfaces. After having determined the thresholds to identify permanent water and floodwater, AUTOWADE accounts for the spatial context by applying a region growing algorithm (RGA), whose seed region corresponds to the set of pixels identified by applying the thresholds. RGA grows these seeds into regions by successively adding neighboring pixels to them. The map of the flooded area generated by AUTOWADE is shown in Figure 9, panel B. As previously pointed out, AUTOWADE was used to monitor the flood evolution until 31 October 2021. The following S1 acquisition along orbit 124 was performed on 31 October, at 05:00 UTC. Figure 10, panel A, is analogous to Figure 9, panel A, but with the backscattering measured on 25 October replaced by that measured on 31 October. It can be noted that floodwater was still present six days after the day when the flood occurred and that, even in the Siracusa district, floodwater was present. Figure 10, panel B, shows the corresponding map. To cope with the latency of S1 images between 25 th and 31 st October, the Italian Space Agency (ASI) made available two COSMO-SkyMed (CSK) images and one COSMO-SkyMed second-generation (CSG) image. AUTOWADE was adapted to work with X-band SAR data, too, like those provided by CSK and CSG. On the one hand, the use of CSK and CSG allowed AUTOWADE to produce flood maps with higher spatial resolution (5 m instead of 20 m).