Estudo hidrológico do efeito de barramento hidráulico no Rio Tarumã-Açu, Manaus-AM

Abstract

Amazonian rivers play a strategic role in the development of the region. Studying them allows guidance in decision-making, regarding the management of the conflicting uses of the resources of a watershed. However, there is still a great lack of hydrological data on them, especially in small basins. A river may have its attributes changed by another through the physical characteristics of the confluence and by the backwater effect. This is the case of a tributary of the Negro river, the Tarumã-Açu river, the largest river in the urban area of Manaus, Amazonas — Brazil. The objective of this study was to analyze how the backwater effect and the morphological characteristics of the confluence interact with the physical parameters of the Tarumã-Açu river. Four data collections were performed between 2018 and 2019 using the 600 kHz Acoustic Doppler Current Profile (ADCP), in five cross sections on the Tarumã-Açu river and the Negro river, before and after the river’s mouth, collecting discharge, flow velocity and direction, depth, width, surface temperature and backscatter data. Due to the absence of knowing water level measurements on the Tarumã-Açu river, the mean depth was added to the bed elevation to obtain the water level. The bed level was obtained through a bathymetric survey with ADCP, georeferenced at sea level by the Precision Point Positioning method. The hydraulic model HEC-RAS was used to estimate the behavior of the backwater effect of the Negro river on the water level of the Tarumã-Açu river. The flow has been considered as one-dimensional, permanent and subcritical. The results shows very dispersed discharge, highly influenced by the speed and depth of the section. It was observed through the velocity of the flow, surface temperature and backscatter that the entrance of the Negro River by the surface of the Tarumã-Açu river over a reach of 17 km upstream of its mouth, which causes alteration in its physicochemical characteristics. Tests with HEC-RAS indicate that Tarumã-Açu river assumes the same level of the Negro river when it reaches the quota of 19 meters (sea level), regardless of the upstream flow, which occurs during five months of the year, on average. The disagreement beds between the channels, the confluence angle and the momentum flow ratio between the rivers are the possible causes of the constant influence of the Negro river on the Tarumã-Açu river.