Flood Risk Mapping in Southern Brazil using a terrain descriptor and socio-economic indices

Floods are the most frequent natural disaster impacting society and all inhabited continents. Accurately mapping the extent of flooding is challenging as it requires a detailed, computationally demanding and data-limited representation of hydrological processes. This study focused on mapping the flood risk of the Itajaí River Basin, an important basin with an extensive history of flooding in Southern Brazil, using the HAND terrain descriptor and socio-economic indices.  A combination of three factors was used to define the flood risk: hazard, exposure and vulnerability. In order to characterize hazard, we use a parallel implementation in GPU of the HAND terrain descriptor combined with flood frequency analysis. Vulnerability was determined by combining the Human Development Index and population spatial distribution. Finally, the exposure was determined by using nightlights. Floods observed extent maps of the September 2011 50-year event in different municipalities of the Itajaí River Basin were used to determine the performance of the HAND terrain descriptor as a flood mapping tool. The best performance of the model was obtained for Rio do Sul municipality, with a correctness index of 86% and a fit index of 75%. Most of the Itajaí River Basin (93%) was classified as low risk. Of the remaining 7%, 90% was classified as medium risk, 8% as high risk and 2% as severe risk. By using the HAND terrain descriptor and socio-economic indices, a flood risk map of the Itajaí River Basin in Southern Brazil was developed which can be used as a valuable resource in urban planning, including the development of flood mitigation and response measures.