Flood hazard assessment on wastewater treatment plants: a case study of the Metropolitan Area of Barcelona (Spain)

Abstract Wastewater Treatment Plants (WWTPs) are often located in flood-prone areas, particularly those situated adjacent to rivers and streams. This specific location has been identified as a significant risk factor for recurrent floods exposing the facilities. In this context, the present study aims to assess the flood hazard affecting these essential infrastructures. The research proceeds to evaluate a specific case study focusing on six WWTPs of the Metropolitan Area of Barcelona (Spain) managed by Aigües de Barcelona. Two-dimensional hydrodynamic models have been developed to simulate design flood events with different return periods, generated by rivers and other ephemeral watercourses. Flood-hazard indicators, including flooded area, maximum and average floodwater depth within the facilities, are obtained from the simulations. Furthermore, recognised hazard assessment criteria are applied to identify high-hazard areas for pedestrian, thus evaluating potential safety implications for plant workers and staff. The results show that streams and ephemeral watercourses play a relevant role in the flood hazard of several of the analysed WWTPs, generating significant inundation within the plant boundaries and localised high-hazard conditions. In some facilities, these watercourses contribute to flooding of operational areas and access routes for medium and high return period events. The findings highlight that flood hazard assessments, which often focus exclusively on major rivers, may overlook relevant sources of flooding at the facility scale. Explicitly incorporating streams and small catchments provides a more complete characterisation of flood drivers and supports more robust decision-making for adaptation planning and resilience enhancement of urban wastewater infrastructure.