Avaliação dos parâmetros de terreno para proposta de vazão de lote unitária: estudo de caso no município de Itajubá/MG

Abstract

Flooding and inundations are frequent in many Brazilian municipalities, due mainly to uncontrolled urban growth and change in land use. Therefore, this study aims to evaluate the sensitivity of terrain slope and urbanization, represented by the Curve Number (CN) parameter, in runoff generation. Also, it aims to propose a discharge value to be retained by each lot in different scenarios in an allotment in the municipality of Itajubá/MG. To achieve this, hydrological modelling was performed in the allotment drainage area, which is part of the Ribeirão José Pereira basin, using the Storm Water Management Model (SWMM). At first, the physical and hydrological characteristics of the drainage area were delineated and analyzed, followed by the intense rainfall determination study. The design rainfall was obtained through precipitation disaggregation into sub-daily quantiles and temporal discretization using the Huff method, to obtain design rainfall for 5-, 10- and 25-year Return Periods. Regarding the hydrological modelling, a 10-minute duration rainfall was defined, while the runoff estimation was defined by the CN-SCS infiltration method and the hydraulic transport by the Kinetic Wave model. Initially, a scenario 0, representing pre-urbanization conditions, was simulated in SWMM, considering the original terrain slope and land use parameters. The other simulated scenarios were: scenario 1 with CN equal to 80, scenario 2 with CN equal to 85, scenario 3 with CN equal to 90 and scenario 4, with CN equal to 95, to each urbanized sub-basin. In each scenario, the average slope of urbanized sub-basins was reduced, gradually, 5% from the original slope. Therefore, each scenario was further sub-divided according to slope categories: predominantly strongly rolling (20-45%), rolling (8-20%), gently rolling (3-8%) and flat (0-3%). The discharge obtained in the pre-urbanization scenario served as the baseline to calculate the discharge by unit area to be retained by lots in the urbanized scenarios, ensuring that the discharge in the outlet does not exceed natural discharge. As a result, it was possible to develop abacuses that relate the CN parameter, slope and the discharge value to be retained in the lots for each analyzed Return Period, and the control of water discharged to natural water bodies. To the pre-urbanization scenario, the discharge in outlet calculated was 0,09m³/s, 0,15m³/s and 0,24m³/s to each Return Period analyzed. In urbanized scenarios, the outlet discharges were significantly greater, with values of 3,11m³/s, 3,20m³/s and 3,36m³/s for the maximum CN and slope values. Therefore, the discharge values to be retained by area are 0,30m³/(s.ha), 0,31m³/(s.ha) and 0,32m³/(s.ha). Finally, it was possible to conclude that the urbanization process, represented by the CN parameter, has a great sensitivity in the discharge generated in the watershed outlet, whereas terrain slope was more sensitive to CN equal to 85.