Distinction between watersheds prone to debris flow, debris flood, and flood using morphometry in Serra do Mar, Brazil (São Paulo State North shore).

Among the hydrogeomorphologic processes, debris flows, debris floods, and floods stand out as the most destructive due to their potential to transport a variety of materials over long distances and the damage caused to urban infrastructures. The watershed dynamic and morphometry stand out as one of the paramount factors related to triggering the processes. The Serra do Mar, on the southeast coast of Brazil, is a mountain range with about 1,500 km where landslides are frequent, causing a high number of casualties and economic losses, especially when related to debris flows/floods.  Thus, studies focusing on the evaluation of the susceptibility of the areas are needed, aiming for mitigation and planning actions. In this way, the aim of this research was the characterization of watersheds for the occurrence of hydrogeomorphologic processes using morphometry on the North shore of Serra do Mar in São Paulo State. To reach this goal, the following methodological steps were carried out: (a) delimitation of the watersheds considering the proximity of the scarp of the Serra do Mar, altimetric range, angle, confinement of the channel, and evidence of past events; b) mapping of morphometrics parameters (Area, Length, Basin relief, Relief ratio, and Melton ratio) using a TanDEM-X Digital Elevation Model with 12 meters spatial resolution; and c) identification of watersheds prone to debris flows, debris floods, and floods according to the morphometry results. A total of 355 watersheds were mapped in three cities (Ubatuba, Caraguatatuba, and São Sebastião). The results show that according to the Melton ratio, 67% of the watersheds are prone to debris flows, with values > 0,60, followed by 30% prone to debris floods and floods, and only 3% prone to fluvial processes. Values vary from 0,16 to 1,70, with a mean of 0,68. Values for Area were between 0,07 to 8,02 km² (mean 0,70 km²), in the range for areas prone to debris flows, according to the literature (up to 10 km²). For the Length, the values vary from 0,47 to 6,73 (mean 1,30 km), with most of the watersheds (94%) with values up to 2,7 km (the threshold indicated for debris flow prone areas). Basin relief and Relief ratio presented values varying between 0,26 to 1,27 km (mean 0,57 km), and 0,09 to 0,79 (mean 0,44), respectively. The next step of the research is the investigation of past events using aerial photos and satellite images. The results of this research can contribute to government response and the reduction of the damage caused by natural hazards in Brazil.