# ERS-1/2 and Sentinel-1 SAR Data Mining for Flood Hazard and Risk Assessment in Lima, Peru
## 2 Materials and Methods  
### Study Area and Susceptibility to Flooding  
The Rimac River basin is located in the central-western region of Peru (Figure 1a), west of the Cordillera of the Central Andes of Peru, between the Anticona, Pucacocha and Yuracocha mountains and the Pacific Ocean. Hydrographically, the river belongs to the Pacific Ocean (Figure 1b) and is formed by the confluence of the Santa Eulalia and San Mateo (or Alto Rimac) rivers at Chosica (Figure 1c), 55 km from the estuary into the Pacific Ocean, and crosses the city of Lima running east to west. Its main tributaries are the Huaycoloro, Bianco, and Aruri Rivers [45]. The catchment areas of the Alto Rimac and Santa Eulalia rivers at the confluence are 1228 km\({}^{2}\) and 1085 km\({}^{2}\), respectively. From a morphological point of view, the Rimac River basin has an elongated shape (Figure 1b) which, compared to a round basin, should be less exposed to flooding, owing to a better runoff of surface waters. The longitudinal slope (i.e., the ratio between the elevation difference and the stream length) of the Alto Rimac is 1:23 upstream of the confluence, and 1/65 at the lower reach after the confluence [46]. The valley is basically young, with a V-shape, steep banks and marked physical conditions between the surrounding summits and the valley depth [47]. The topographic configuration is narrow and steeper upstream, while it is wide and flat downstream, opening to the geomorphological unit of the alluvial plain (Figure 1d), where the historic core of Lima developed until 1924, and the first stages of modern urban expansion happened between 1940 and 1970 [48]. From a geomorphological point of view, the lateral dimension is not homogeneous along the river course [49]. In the middle section, i.e., approximately around Chosica, the river configuration partly widens, shaping a more open canyon with terraces along both riverbanks (Figure 1e). This is an ideal location for agricultural activities and minor urban settlements. The low section in the alluvial plain is instead characterized by the formation and consolidation of the floodplain. However, this assessment does not consider the anthropogenic impact on the river and its effects on increasing susceptibility to flooding and fluvial erosion. In this regard, it is worth recalling that the Rimac River is the main source of water for domestic, agricultural and energy use throughout the coast, including the capital Lima, the second largest city in the world located in a desert, after Cairo [50], and where a third of the total country's population resides (i.e., more than 9 million inhabitants censor within the metropolitan area [51]). As reconstructed by [PERSON] et al. (2011) [52], in order to cope with the urban development and the need to connect new neighborhoods, various bridges were constructed. The first was El Ejercito Bridge (Figure 1f), completed in 1936 to allow communication between Downtown Lima and the recently occupied northern sector. This engineering project led to an incision process downstream, started at least since 1944. In time, this resulted in the formation and narrowing of the Rimac River urban canyon, with the streambed deepening approximately 20 m downstream of El Ejercito Bridge (Figure 1f). Nowadays, this canyon runs towards the city airport and is clearly visible along Villa Maria del Perpetuo Socorro and Mirones, i.e., one of the shantytowns that formed without regulations on the left bank of the Rimac River and is locally known as MIRR (_Margen Izquierda del Rio Rimac_) [53]. There, the houses are perilously built at the top of steep, almost vertical cliffs, and thus are exposed to fluvial erosion and collapses (Figure 1f). Additionally, uncontrolled sewage discharge into the river further drives erosion of the cliffs, and thus of the foundation of houses [53]. Therefore, monitoring the increase of water flow in this section of the river course is required to assess the impact of hydrometeorological events and consequential sudden changes in water discharge.