A planície costeira do Rio São Francisco: retrabalhamento costeiro, formas de leito, vazão e clima

Abstract

The São Francisco River is located in northwestern Brazil and is the longest river that runs entirely within the country of Brazil. This river has been greatly affected by coastline retreat, with a history of severe coastline erosion. These changes in the coastline have been attributed to anthropogenic changes such as the construction of dams. The purpose of this study is to understand what factors control short-term changes in the coastline of this river. Three separate satellite images were used, and the coastlines were drawn for the time periods of each cruise: 2009, 2012 and 2017. The lines were drawn using Landsat 7 ETM+ scenes for 2009 and 2012 and with Landsat 8 OLI scenes for 2017. Accretion and erosion were calculated, as well as the accretion/erosion ratio for areas northeast and southwest of the São Francisco River Mouth. Sonographic and seismic data were acquired from 2009 and 2012 to identify the changes in bedforms during those periods. The sonographic data were acquired using a Side Scan Sonar model 272-TD from Edgetech, and seismic data were acquired using a parametric sub-bottom profiler model SES 2000 from Innomar Technologie. Hydrodynamic data were acquired in 2009 using an Acoustic doppler current profiler (ADCP) to understand the behavior of the river mouth. Sediment samples from 2009 were collected with a Van Veen Grab to analyze sediment distribution in the area. Additionally, historical data regarding rainfall, river runoff, Oceanic Nino Index (ONI) events and South Atlantic Convergence Zone (SACZ) events were obtained to find any correlations with coastal changes. The analyzed and processed data showed that the bedforms had not undergone any significant changes, indicating that the river was still transporting enough sediments to the coast. The SACZ events had a greater influence on the rainfall regime than ONI events. However, the anthropogenic interference of rivers dams also had a greater influence on coastline changes than rainfall regimes. We therefore conclude that the São Francisco River is the main provider of sediment to the coast; in this way, this river will always find a new equilibrium when its discharge is controlled.