Maria José Mesquita

The Hydrological Disasters Through Historical Survey in the Serra do Mar Range, Southern Brazil

The establishment of hydrological disasters frequency standard is a difficult task because of the lack of well fitted historical data series. The establishment of these standards through historical surveys in newspapers and historical archives is an option to set minimum frequency data. The data survey for floods and landslides in Southern Serra do Mar range, in southern Brazil, could establish parameters for frequency of hydrological disasters in that area. The research involved digital and analogical bases, including newspaper reports and historical monographs. Altogether 38 hydrological events were identified in the period of 1796–1913 for the region. The data focus on two main periods: the period of 1846–1913, which involved one event every 2.9 years, and 11.2 years for the large-scale events; and the period of 1960–1980, which presented one event every 1.7 years, and 7.4 years for the large-scale events. Based on the economic and human losses, the events of 1796, 1846, 1873, 1883, 1884, 1888, 1907, 1913, 1946, 1960, 1969, 1971, 1975 were very harmful. The most affected areas were the floodplains, with the occurrence of floods, and the mountain regions with landslides and flows. The major higher landslide incidences were associated with railways and highways. Debris flow deposits occurred mainly in the Piedmont areas. About 90% of the events occurred mainly from October to March, corresponding to the rainy season in the region. The survey of historical data proved to be an interesting source for data, which help to compose the picture, although incomplete, of the occurrence of these hydrological phenomena in the region.

Typology of Rainfall-Triggered Landslides in the Urban Area of Antonina, Southern Brazil

The Disaster of March 11, 2011, in Parana state, Southern Brazil, was triggered by concentrated rainfall in a short time, after an unusually wet and warm hot season. In Antonina, the little town most severely affected, the event triggered dozens of landslides in the urban area, with two deaths and about a thousand of affected people. Analysis of the landslide typology showed a correlation between geology and geomorphology of slopes in the urban area. Where granitic rocks occur, it develops a steep relief is and a relatively thick and heterogeneous silty weathering mantle, generated landslides in first order headwater drainages. Some of these landslides, coalescing along drainages, gave rise to highly destructive debris flows, such as the neighbourhood of Laranjeira, with a death and the destruction of several houses. On the Caixa D’Agua hill, where predominate mafic rocks (gabbro, mylonitic gabbro and diabase dikes), the landslides occurred on clayey steep slopes, controlled by a large diabase dike contact, causing hyperconcentrated flows in embedded drainages, provoked one death and the destruction of houses at the foot of the slope. In other areas dominated only by mafic rocks (gabbros and amphibolites), it develops concave slopes, with thicker and clayey weathering mantle. In these areas, landslides were mainly rotational. Where mylonitic rocks occur, we observe only shallow translational landslides. The strong correlation between typology of landslides, geomorphology (slope, drainage) and its connection with the geology (rock types, structures) is an important factor for future studies in the area of risk.