# 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
According to the information released in 2013 by the _Junta de Usaurios del Rio Rimac_ (i.e., Rimac River Users Board) [54], the basin is made up of marine and continental sediments, as expected for an alluvial valley. Sudden geomorphological changes also happen from one year to another, due to the magnitude of the highly dynamic deposition processes in the tributaries of the Rimac River. The tributaries pour the dragged material (blocks, stones, fine material) directly into the river, increasing the load of sediments which, in turn, contribute to increase flood susceptibility. From a climatologically point of view, the coastal part of the Rimac River basin stretches between 0 and 1000 m above sea level, and is characterized by a coastal desert climate, with a high frequency of fog. The latter consists of an almost permanent layer of stratocumulus [27], as well demonstrated by Sentinel-2 images (Figure 2a). This also explains why the application of SAR is ideal in Lima, not only for the coastal desert climate. Figure 1: (**a**) Geographic location of the Rimac River basin in the central-western region of Peru and (**b**) its boundaries, with the blue line marking the river course. (**c**) Hillshade and (**d**) slope of the basin downstream of the confluence between the Alto Rimac and Santa Eulalia rivers, with indication of locations and toponyms of interest. Google Earth StreetView pictures showing: (**e**) the geomorphology of the open canyon at Chosica, as opposed to (**f**) the narrow urban canyon downstream at El Ejercito bridge, with nearly vertical cliffs above which the _Margen Izquierda del Rio Rimac_ (MIRR) shantytown was built along the left riverbank. under ordinary conditions, but especially in case of flood events, when the cloud cover can be higher than normal. In this regard, Figure 2b shows the cloud cover of the Sentinel-2 scene of 22/03/2017 hampering the visibility (and thus the potential to carry out the damage assessment) of the urban areas in Carapongo and Santa Maria de Huachipa that were severely affected by the catastrophic flooding event analyzed in this research. Precipitation in the coastal part of the basin is one of the lowest in the whole South America, with an average of 5 mm of accumulated annual rainfall [55] (Figure 3a). However, the air in the city is relatively humid, due to frequent haze and strong dew. Differently, at elevations higher than 1000 m above sea level, i.e., the medium-upper part of the Rimac River basin, the climatic conditions change to subtropical, dry desert climate, with accumulated annual values of precipitation equal to 750 mm [27; 56]. This confirms that precipitation varies spatially across the basin depending on elevation, with the highest intensity upstream of the river basin, over the Cordillera of the Central Andes. Accounting that at these latitudes the hydrological cycle runs from September to August, the heavy rain season falls between November and April. Such seasonal rains in the central highlands of Peru cause an increase in the water flow of the Rimac River compared to the other months of the year. This happened, for example, during the flood event in mid-March 2017. According to the regional hydrological warning of \"red level\" issued by the _Servicio Nacional de Meteorologia e Hidrologia del Peru_ (SENHAMI) on the night of 17/03/2017, the maximum flow of the Rimac River was estimated to reach 95 to 100 m\({}^{3}\)/s, and 34 to 40 m\({}^{3}\)/s for the Chillon river, based on the measurements made at Chosica and Obrajillo hydrometer stations, respectively [57]. A precipitation anomaly with respect to mean historical records was indeed registered in the period December 2016-February 2017 (Figure 3b). Throughout its history, the Rimac River has caused floods along its path many times, as a consequence of such seasonal rains. There are logs of this type of events since 1578 [58]. While these flood events are recurrent phenomena, when they occur in conjunction with ENSO, the Rimac River water flow exceptionally increases up to catastrophic level.