Erosão nas margens do Rio Amazonas: o fenômeno das terras caídas e as implicações para a cidade de Parintins-AM

Abstract

The lateral migration of the channels results from the processes that involve several factors that act together within the fluvial channel. The current discussion about the topic presents different conclusions, not having, therefore, a consensus regarding the mechanisms of the cause. In the Brazilian Amazonia, the complex process of erosion of the margins of the Amazon River, a phenomenon known regionally as fallen lands, has affected not only the populations scattered along its channel, but also agglomerated such as communities villages and cities. The capacity of this phenomenon in transforming the landscape would already impress naturalists, travelers and chroniclers that navigated across the rivers of the Amazonia. In the City of Parintins the problem is of long standing and is being notified, in accordance with the Minutes of the Municipal Chamber, since the 7th Legislature of city councilors (1973-1976). The concern of the executive e legislative powers increased until during the 8th Legislature (1977-1982), Together with the State Government, resources for the building of a support wall in a section in front of the city were obtained. However, during the following decades, the pressure Amazon River associated with strong rains and infiltrations of the rainwaters, made the construction, besides being affected, suffer constant fracturing and shaking. Parintins is located in the easternmost part of the State of Amazonas in a concave section of the left riverbank of the Amazon River, being built on an island of lowlands that belong to a complex river-lake system of the Amazonia flatland. Because of its position, it receives the almost frontal attack of the Amazon River, thus involving the section that is a part of the front of the city. This work had as a main objective principal to understand the process of lateral erosion of the Amazon River in front of the City of Parintins and the socioeconomic implications of this phenomenon for the city. The technical-methodological procedures of the research consisted of transversal bathymetric measurements of the Amazon River; analysis of hydrological data; granulometric analysis; analysis of the rainfall data and of the speed of the wind; multi-temporal analysis by remote sensing data; monitoring of the margin with erosion pins; and interviews with the residents and public institutions. The interpretation is that the process results from the combination of factors such as the shape and geometry of the channel; the pressure and the hydraulic forces that act in the channel; granulometric composition of the riverbank; climatic conditions and anthropic action. It is important to mention the fact that the bathymetric measurements revealed that the thalweg of the river gets to be at 328m near the riverbank, whose depth reaches 98m, which facilitates the erosion in the depth and impedes any work of engineering to restrain the process. Regarding the implications, we point out the high financial cost for the municipality as the building of support walls and loss of streets; devaluation of properties facing and/or bottom for the river; risk the circulation of vehicles and to the navigation; and the difficulty of loading and unloading during the low tide. With this research, we believe that we have contributed for the advance of knowledge and mainly to call the attention of the constituted authorities towards the problems generated by the phenomenon, particularly for the City of Parintins. Because it is a dynamic process and there are still few studies, it is understood that a discussion about the topic is not stopped in this work and therefore, it is necessary to generate data and new knowledge to raise deeper questions that instigate the scientific debate about this fluvial dynamic.