A importância do coeficiente de rugosidade de Manning na avaliação numérica do assoreamento de reservatórios a fio d'água

Abstract

This work proposes a methodology for the characterization of the Manning roughness coefficient in water reservoirs and the variation analysis of the silting process along the operational life. This becomes especially important for small reservoirs, which has a small relation between reservoir volume versus mean annual flow. To support this methodology, the physical model of the Salto Paraopeba Small Hydroelectric Power Plant (SHP) was used, located in the municipality of Belo Vale, MG, which stopped operating due to complete siltation of its water intake. In the physical model, built at the Hydraulic Research Center (CPH), fields of flow velocities were measured on low and high flow conditions, in scenarios of clean and silted reservoir. The sediments were represented by tires granular rubber material capable of reproducing sediment transport characteristics. After data treatment, simulations were performed in HEC-RAS, in a two-dimensional environment for calibration and validation of the roughness coefficient in the previously described scenarios. The results indicate that although the water level in the reservoir is conditioned by the free-spillway discharge curve of this SHP, the circulation patterns and the velocity distribution varied significantly, obtaining a roughness coefficient of Manning average of 0.018 in the model under clean reservoir conditions, corresponding to a value equal to 0.033 in the prototype, compatible with the expected values for the river reach. These results demonstrate the importance of surveying water levels and discharge measurements in cross sections along the reservoirs when numerically evaluating the behavior of reservoir silting and defining control measures.