Dinâmica hidrossedimentar na confluência dos rios Beni e Mamoré

Abstract

The Beni and Mamoré rivers, located on the border between Brazil and Bolivia, are the main tributaries of the Madeira River. Together, these rivers contribute a significant volume of water, with an average annual discharge of 9,578 m³/s and 7,847 m³/s, respectively, and they supply approximately 282 million tons of suspended sediments to the Madeira River annually. This research aimed to analyze the hydro-sedimentological variability in the confluence region of the Beni and Mamoré rivers. To achieve this objective, in situ measurements were combined with remote sensing data analyses. The fluvial dynamics parameters included measurements of water discharge (Q), solid discharge (Qs), flow velocity, and suspended sediment concentration (SSC) at the surface and depth, using data from the Cachuela Esperanza, Guayaramerin, and Porto Velho stations, provided by the HYBAM Observatory. Additionally, remote sensing data were employed to analyze land use and cover changes (Mapbiomas), precipitation (CHIRPS), and suspended sediment concentration obtained from PlanetScope satellites. The annual solid discharge was calculated based on data from the HYBAM Observatory and ins situ measurements collected during the 2021/2022 hydrological year. The results indicate that significant changes in land use and cover occurred in the Beni and Mamoré river basins in Bolivia between the 1990s and 2020s, with a notable expansion of areas dedicated to agriculture, urban infrastructure, and mining. Despite these intense changes in land use and land cover, no clear trend of increase or decrease in precipitation was observed over the analyzed period in these basins. Regarding solid discharge, was found a reduction of 15% in suspended sediment from the Beni and Mamoré rivers between their confluence and the Jirau Hydroelectric Power Plant (HPP) reservoir on the Madeira River, corresponding to approximately 40 million tons per year over a 400 km stretch, with a small reduction observed in the 200 km downstream of the confluence. The hydro-sedimentological dynamics in this region exhibit a significant deposition zone in the Madeira River, upstream of the Jirau HPP, where approximately 15% of the suspended material from the Beni and Mamoré rivers is trapped. It was observed that the region closest to the confluence has the largest deposition area, although no direct relationship with the reduction in average flow velocity in that section of the river was found. The SSC estimates using PlanetScope images showed significant discrepancies compared to the in situ data in the Beni, Mamoré, and Madeira rivers. This study on the hydro-sedimentological dynamics at the confluence of the Beni and Mamoré rivers provides relevant data for a better assessment of the impacts of the Jirau HPP on sediment transport in the Madeira River and discusses the relationship between hydro-sedimentological dynamics and extreme climatic events in the region. This dissertation is part of the Research and Development Program (R&D) of Energia Sustentável do Brasil (ESBR), aligned with ANEEL's guidelines. The research project "Spectroradiometry” for Sediment Monitoring in Reservoirs" (SPECTROSED), started in 2020, conducted campaigns to measure parameters such as suspended sediment concentration, granulometry, mineralogy, flow, and radiometry in the area upstream of the Jirau Hydroelectric Power Plant on the Madeira River. The project developed an automated experimental station for continuous sediment monitoring, funded by the University of Brasília Foundation (FUB), the Research Support Foundation (FUNAPE), and the João Neórico Institute (FARO).