Estimativa de produção de sedimentos em suspensão no trecho baixo do Rio São Francisco: subsídios para gestão ambiental

Abstract

Sediment flows are natural processes that occur in water courses, caused by ero sions in the environment. However, anthropogenic interventions can accelerate or increase erosive processes or even reduce sediment concentrations. The dams installed along the São Francisco River altered the natural flow regime in the regions downstream o f each dam and presented as a main consequence the flow regulation and the decrease of sediment concentrations. In this context, the objective of the research was to estimate quantitatively the suspended sediment flows in the Lower São Francisco after the Xingó dam. In order to evaluate the sedimentological behavior, flow data and sediment concentration in suspension were used, in addition to precipitation data in order to better understand the hydrological dynamics in the study area. The data collected wer e obtained from the Hidroweb system on the website of the National Water Agency (ANA) from 1999 to 2018. The Sediment Key Curve (CCS) method was used to estimate the suspended sediment load at the installed fluviometric stations in the Lower São Francisco. The results show that the estimated loads of suspended sediments were increasing along the river, starting at the Piranhas station with 1.98x10 6 t / year, at Traipu station with 7.4 x 10 6 t / year and at Propriá station, the closest to the mouth of the Sã o Francisco, with 8.0 x 10 6 t / year for the entire period of study. The section considered most silted was between the stations of Piranhas and Traipu, where the increase of the accumulated sediment in suspension reached 73.3% between the stations. For th e present day, covering the years 2017/2018, the estimated sediment load for Lower São Francisco was 1.8 x 10 4 t / yr in Piranhas, 8.5 x 10 4 t / y ea r in Traipu and 7 x 10 4 t / year in Propriá, which are considered very low results. The problem has been in the last five years where the Lower São Francisco has been suffering from the dry season. Since 2013 there has been a gradual reduction of the flows available in the Xingó dam. Consequently, there was also the gradual reduction of suspended sediment loads generated at the Piranhas, Traipu and Propriá stations. In this period, the lowest values already recorded in Lower São Francisco were found, corresponding to the lower historical series of both discharge and suspended sediments. In reference to water av ailability, these flow series were below Q 95 (permanence flow) for the study period, indicating serious situations regarding ecological flows in the region. This decrease in flow and sediment in recent years seriously compromises the hydrodynamic equilibri um of the course of the river to its mouth with the loss of energy due to the reduction of flow velocities. Consequently, sediments are deposited along the river's course, in addition to the invasion of the sea in the river with more intensity than before (drought), as well as saline intrusion. Because of this, the mouth of the Lower São Francisco is not managing to renew its waters (new waters) in order to obtain the necessary mixture for the environmental balance at the mouth of the river. In response to this water crisis, the study organized a summary table with the main causes and negative effects, as well as proposals for control actions for the region under study.