Comportamento hidrodinâmico na confluência de rios: estudo de caso do Canal São Gonçalo - Arroio Pelotas

Abstract

In recent years, an increase in studies on hydrodynamics in river confluence zones has been observed, mainly to improve water management and use and to understand the water body as a whole. These confluence zones are responsible for the separation and formation of rivers, which are fundamental to understanding river networks and developing river basins. In this context, this study was conducted at the confluence of the São Gonçalo Channel and the Pelotas Stream, located near the urban area of the municipality of Pelotas-RS. Considering the lack of studies on this topic in the region, the objective of this research was to analyze the hydrodynamic behavior at the confluence between the São Gonçalo Channel and the Pelotas Stream. To this end, three sampling campaigns were carried out with different hydrological conditions, where flow velocity and discharge measurements were made using an acoustic Doppler current profiler (ADCP), the RiverSurveyor® M9 model; collection of subsurface water for salinity determination; and collection of bottom sediment using a Rock-Island sampler in the thirteen pre-defined transverse sections in the confluence zone (ZC), whose granulometry was characterized in the laboratory. After the surveys, the processing of the flow velocity data was carried out using the Velocity Mapping Toolbox - VMT software, which allowed for the reproduction of composite fields in the transverse sections to obtain the average three-dimensional velocity in the confluence zone. From the results, it was possible to verify the variations of the measured data with and without saline intrusion, the significant influence of hydrological conditions on the flow, where the main channel (São Gonçalo Channel - SG) showed higher discharge, flow velocity, and depth compared to the tributary (Pelotas Stream - AP), which showed negative discharge and velocity in the sample campaign with the lowest accumulated precipitation due to deflection and stagnation of flow, possible increase in pressure and depth (AP-SG), reflection of convergence and river backwater, taking into account that besides these factors, the São Gonçalo Channel, due to its characteristic of flow inversion, may have contributed to the flow to the Pelotas Stream. Distinct flow directions were observed for low, medium, and high rainfall conditions, as well as the influence of northeast, north-northeast winds in the summer and east in the spring on the magnitude of flow velocities when analyzed by microregion. Furthermore, it was also verified that the flows presented a subcritical regime and a variation of dominance between thrust and inertial forces in relation to the movement of flows at the entrance, center, and exit of the confluence zone, from the internal Froude number, which remained with varied values in both sample campaigns. The frictional factor was 0.24 for AP and 6.14 for SG, with dominance of the flow of the Pelotas Stream on the date of January 10th, 2022. The data obtained on July 21st, 2022 and November 4th, 2022 indicate the dominance of the São Gonçalo Channel in relation to the flow, with frictional factors of 0.45 and 26.76 for AP and 0.04 and 0.11 for SG, respectively. Based on the joint representation of the average velocities of the cross sections calculated in the VMT, a stagnation zone was identified; separation and recirculation zone; acceleration zone; and flow recovery zone. Finally, the presence of finer sediments upstream of the ZC in the Pelotas Stream and coarser sediments in the center of the ZC in the São Gonçalo Channel upstream and downstream of the ZC were noticed. With this granulometric identification, the roughness coefficient and the relative flow depth (H/D84) were obtained, these parameters allowed to infer that the resistance to the flow caused by the roughness of the bed was very small between the sampled points, indicating that there was no direct influence of the type of bottom sediment on the flow discharge.