How Important Is Tide for Extreme Flood Events? A Case-study for the Tubarão River Basin in Southern Brazil

Coastal cities are affected upstream by hydrological phenomena from watershed and downstream by oceanic level variation. Tide height variation influences river flow velocity and can contribute to the river channel overflow. Therefore, flood management in cities near the ocean must include tide height as a variable to estimate flood area bounds in numerical simulations. Tubarão city, in southern Brazil, has been developed on river Tubarão floodplains and has problems with flooding since XIX century, occurring in 1974 the flood with highest socioeconomic impacts. Mathematical model application provides flood area calculation, as well as flood velocity and water depth that can guide urban flood management. In the present research, the MGB-IPH hydrological large-scale model was combined with the HEC-RAS 2D Unsteady Flow (v. 5.0.3) hydrodynamic model to estimate hydrographs and to propagate them along the topobathymetry of Tubarão city. The IPH-ECO (v. 0.2) hydrodynamic model, based on shallow water equations, was used to estimate tide effect over flows and levels of Tubarão river, including the effects of its connection with the ocean by Santo Antônio lake. Storm designs with 4^th quartile of Huff method for 10, 25, 50 and 100 years return period, and a critical rainfall duration of 63 hours with 1 hour interval were adopted. MGB-IPH model was calibrated and validated with discharge stations data and used to calculate hydrographs with storm designs created in the upstream of Tubarão city. These hydrographs were used as upstream inputs for HEC-RAS model and tide effect in Tubarão river is added as downstream condition to calculate the flood event. The maximum water level created by the tide on Tubarão river downstream was 0.67m and the minimum was -0.40m. Six cases are simulated combining storm design return period and tide water level. The area relative difference (A_rel,dif) for 10, 25, 50 and 100-years return period (RP) were 60.5%, 41.1%, 18.3% and 2.77%, respectively. This relation provides an exponential trend line that fits to the equation RP = 0.939 exp(-0.035A_rel,dif) with R²=0.996. Numerical model application and its results interpretation can help city managers decision-making with technical information. The more accurate this model is the more useful it is for the city management. The Tubarão city flood simulation case shows that for small return periods, tide influence can be essential to provide a good representation of reality.