Hydrologic models coupled with 2D hydrodynamic model for high-resolution urban flood simulation

Floods are the most frequent natural disaster and pose a very challenging threat to many cities worldwide. Understanding the flood dynamic is essential for developing strategies to reduce its risk and damages, thus ensuring the cities’ protection. This study evaluated the Capibaribe River basin's hydrological response to extreme events and its impact on the city of Recife, in the northeast of Brazil. The CAWM IV and HEC-HMS models were coupled with a high-resolution 2D HEC-RAS model to simulate the flood events of 1975 and 2011 in Recife. CAWM IV is a newly developed hydrological model that presented very promising results for the data-scarce watersheds of the Brazilian semiarid region. For the 2D hydrodynamic modeling, 1-m LiDAR DEM was used. A reservoir operation model was also applied to assess the effect of the basin's main reservoirs on the water system upstream from Recife. Lastly, the 2011 flood event was simulated under the scenario of an absence of this reservoir system. The strategy used to address flooding simulation in an urban area proved to be satisfactory. Of the events simulated with CAWM IV, 60% have at least a satisfactory adjustment with NSEsqrtQ coefficients greater than 0.36 in 95% of cases. With the reservoir operation model, it was possible to calculate the peak flow of the events of 1975 and 2011 as being 2574 and 731 m3/s, respectively. The 2D HEC-RAS model presented a measure of fit of approximately 0.7. The study showed that the reservoir system was responsible for reducing flood extent by 70.3% in the 2011 event, but even with this system, this event still caused a flood covering an area of 6.01 km2. The results indicate that although the reservoirs prevent severe flooding in the lower course of the Capibaribe River, Recife is still vulnerable to flooding.