Debris flow hazard and susceptibility zonation in small watersheds in Itaoca municipality, São Paulo state, Brazil

摘要

This paper presents the regional analysis of debris flow susceptibility and hazard among 106 watersheds of the municipality of Itaoca and surroundings, State of São Paulo, Brazil. For each watershed were considered eight attributes: slope between 30° and 40°, relief or height range, drainage density, circularity ratio, watershed length, relief ratio, Melton ratio and landslide hazard index. The method combines two approaches: (1) numerical calculation and preliminary classification based on a linear proportional variation of the eight selected attributes, which gives a relative gradual variation of the potential intensity and frequency of the process for each watershed; (2) application of reclassifying rules based on Melton ratio, which qualifies the type of process prone to occur, namely debris flow, debris flood or flood. The susceptibility values and classes were maintained for the hazard index and classes, but rather than the whole area of the watersheds, the debris flow hazard were restricted in four ranges of width along the river channels. This new attribute obtained from the characterization of previous events occurred in 2014 gives an estimative of the area potentially affected by debris flow along the river channels and is an indicator of the absolute intensity in each hazard class. The Very High and High hazard classes takes place in 41 watersheds comprising 3.5% of the area of the municipality arising concerns and restrictions to be considered by the procedures, instruments and legal framework of development and land use planning.