Debris flows in southeast Brazil: susceptibility assessment for watersheds and vulnerability assessment of buildings

Abstract

Debris flows is one of the primary mass movement processes in the Serra do Mar, a system of escarpments and mountains that stretches more than 1,500 km in south and southeast of Brazil. Usually, these processes cause environmental and social damages. On March 1967 only one small city was affected by 947 mm, with 115 mm on the 17th and 420 mm on the 18th and triggered numerous landslides and debris flows with the great mobilization of material, reaching a 15 km radius, causing approximately 440 fatalities. Approximately 50 years later, another city, located in the same mountain range, was affected by cumulative rainfall of 150 mm/6 hours causing deaths and houses destroyed and structural damage to bridges. Thus, the objective of this work was to evaluate of vulnerability to debris flows in some watersheds located in two cities of the Serra do Mar affected in 1967 and 2014, respectively. For this purpose, some procedures were defined: (a) the evaluation of the vulnerability of buildings, considered, for instance, the number of floors, the presence of broad terraces, large doors, windows and high walls surrounding the buildings. (b) elaboration of indicators and maps of vulnerability that consider the hazard properties, the exposure, preparedness and prevention of elements at risk; (c) elaboration of indicators and map of risk perception. The results show38 areas with vulnerable construction to debris flows: 8%-high;70%-medium;22%-low vulnerability of buildings. As preliminary results, an inventory of damages was provided from the sectors of the affected districts and preliminary mapping of the debris flow.