Evaluations and proposals for the debris flow hazard mapping method of the GIDES Project

Abstract

On March 11 and 12, 2011, a large rainfall event caused floods and thousands of gravitational mass movements on the coast of the state of Parana, Brazil. This paper focuses on the large debris flows that reached the Jacarei river basin and uses them to test the new hazard mapping method proposed by the GIDES Project, a technology transfer partnership between Brazil and Japan. The method’s purpose is to be simple and fast, and, so, applicable in a developing country of continental dimension. Comparison of the maps generated by the GIDES methodology and the actual events that happened in the study area showed many discrepancies, that is, in its original form the methodology could not predict accurately the events. However, after adaptations to the studied case, major improvements were obtained. The adaptations aimed at testing morphometric parameters as the original technique to reach the best fit possible between the generated maps and the occurrence area of the studied event. For the debris flow transportation area, the parameters that produced better maps closer to the studied actual event was made with the relation between the maximum height reached by the debris flow and the maximum channel slope. No satisfactory morphometric relationship was found for the boulders’ deposition region. For the area of fine or less dense materials’ deposit, the most significant change was the incorporation of slopes from 0° to 2° in the mapping procedure. Finally, we found values and equations that could be used initially in the analysis of other sub-basins with some similarity with the studied area to map the debris flow hazard, although validation in other places is still needed. These different techniques adapted to the local reality can be used separately or together to create faster, simpler, cheaper, and reliable maps using relatively easy to find morphometric input data.