# ERS-1/2 and Sentinel-1 SAR Data Mining for Flood Hazard and Risk Assessment in Lima, Peru
## 3 Results
### Zoning of Areas at Risk in Case of Events of Equal Severity  
For the generation of the Rimac River basin flood risk map, three risk levels were defined: high, medium and low risk. The factors determining the level category were, first of all, the terrain slope, the surface occupied with alluvial deposits, and/or the presence of urban or non-urban area. For this reason, the characteristics of the changes taken into account for risk analysis are summarized in Table 7. At a glance, it can be noted that all the changes are located onto alluvial deposits, their most common slope class is A to B, and they split among urban and non-urban areas.  
The areas identified at a high risk of flooding are those that in the years 1997-1998 and 2017-2018 were directly affected by the flooding event, and where satellite data and/or ground evidence suggest  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type of Change** & **Slope (Class)** & **Alluvial Deposits** & **Urban** & **Non-Urban** \\ \hline F1 & B-D & ✔ & ✔ & \\ F2 & A-B & ✔ & & ✔ \\ F3 & A-B & ✔ & ✔ & \\ NF1 & A & ✔ & & ✔ \\ NF2 & A & ✔ & & ✔ \\ NF3 & A-B & ✔ & ✔ & ✔ \\ NF7 & B & ✔ & ✔ & \\ NF9 & A & ✔ & ✔ & \\ NF12 & A & ✔ & ✔ & \\ NF13 & A & ✔ & & ✔ \\ \hline \hline \end{tabular}
\end{table}
Table 7: Characteristics of the observed change patterns in the radar backscatter.  
Figure 17: Example of NF13 change “Archaeological excavations”: (**a**) Sentinel-1 RGB composite and (**b**) ratio map highlight a decrease in radar backscatter (dark lilac polygon) in an archaeological site where fires may have altered the vegetation coverage, as identified in the optical imagery acquired on (**c**) 08/02/2017 and (**d**) 03/03/2017 (Google Earth images © 2020 Maxar Technologies). Contains modified Copernicus Sentinel-1 data.  
that material loss occurred (e.g., collapse of riverbanks), slope was classified as A or B, and urban fabric is onto alluvial deposits. Categories F1 and F3 correspond to this description, with a total of 22 changes and a total area of 19.44 km\({}^{2}\).  
The medium risk was attributed to non-urban areas that could be affected by flooding events as a result of the combination between slope class A to B and presence of alluvial deposits. The categories that correspond to this description are F2, NF1, NF2, NF3, and NF13. The medium risk classification was also applied to those urban areas, characterized by slope class A or B, and the presence of alluvial deposits, that changed during the studied period and that due to their proximity to the Rimac River could be affected by flooding events (i.e., change categories NF7, NF9, and NF12).  
In the end, urban areas where neither previous evidence of flooding nor changes in the satellite data were found, and where it is very unlikely that they would be inundated due to their slope characteristics (up to D) and/or local geology, were classified at low risk. A total of 158 polygons of the newly identified urban areas fall within this category (15.54 km\({}^{2}\)).  
Figure 18 shows the obtained risk map for the hydrographic basin of the Rimac River and city of Lima. Because this map is the outcome of combining hazard and risk factors (i.e., via analysis of the BTM), and the changes (both due to flooding or not) found in the SAR data, it provides the zoning of the areas at risk with respect to flooding events of equal or greater magnitude than those occurred in 1997-1998 and 2017-2018, if no hazard and risk mitigation measures are undertaken.