Assessing flood impacts in an Urban watershed in São Paulo City, Brazil, using a fully distributed and coupled Hydrological-Hydraulic model and demographic statistics.

The continuous expansion of impervious areas in megacities, combined with the increasing frequency and magnitude of extreme climatic events, has led to more frequent flood events in urban areas. Flooding, currently the most common disaster worldwide, is an adverse event that can result in significant human impacts (e.g., loss of life, injuries, and illnesses), material damage (e.g., destruction of private and public property), and environmental degradation. These damages also lead to economic and social consequences, such as psychological trauma and social disruption. The watershed of the Aricanduva River, located in the East Zone of São Paulo, Brazil, faces recurrent flooding issues, particularly along its main course, which is adjacent to a critical avenue. These flood events are primarily attributed to the watershed's physical characteristics, including its steep river gradient and extensive urbanization in the lower and middle sections of the basin. This study aims to assess socio-environmental impacts using hydrological modeling and demographic data provided by the Brazilian Institute of Geography and Statistics (IBGE). The methodology is divided into three main steps: (i) Generation of inundation maps for six events using HydroPol2D, a fully distributed and coupled hydrologic-hydraulic model that solves the shallow water equations (SWE); (ii) Spatial analysis of census data provided by IBGE to develop population and household density maps; (iii) Assessment of impact factors, termed “affected population” and “affected households”, through the overlay of flood maps with population and household density maps. It is important to note that the six analyzed events were selected based on alerts issued by the Flood Warning System of the State of São Paulo (SAISP). The study's results reveal how the impacts of recent rainfall events evolve over time and highlight areas with recurrent flooding. These analyses demonstrate that residents in the Aricanduva watershed face considerable flood risks. The methodology implemented for impact assessment can support the development of emergency plans and actions to mitigate the social and economic impacts of flooding. These measures are closely aligned with the Sustainable Development Goals (SDGs), specifically Goals 6.5, 9.1, 11.5, and 13.3, as well as the United Nations' Sendai Framework for Disaster Risk Reduction (SFDRR). The data generated in this study could serve as a reference for future analyses, such as evaluating the effectiveness of urban drainage plans, future flood warning systems, or other flood control strategies. Additionally, the watershed model could be utilized to develop an assessment framework for indirect impacts, including the potential effects of flood events on accessibility to critical areas, disruptions to economic activities, and transportation. This would enable proactive planning and the identification of alternative solutions in advance.

Keywords: Natural hazards, Urban flooding risk management, Socio-environmental impacts, Hydrological-hydrodynamic models, Climate change.