Subjective and Objective Methods in Multi-Criteria Decision-Making (MCDM) for Flood Mitigation: Implications on Policymaking

Flood risk management comprises risk assessment through robust modeling and mitigation through measure implementation. Decision-making on mitigation measures is complicated by the plethora of criteria, stakeholder influence, implementation scale and financial constraints. Multi-criteria decision-making (MCDM) methods have emerged as valuable tools in this context, allowing for the systematic integration of diverse factors and perspectives. Nonetheless, MCDM applications in mitigation measure ranking remain challenged by the lack of informed evaluation of criteria and the diversity of measures at local reach-scale. This work aims to develop a comprehensive methodology for prioritizing flood mitigation measures. An application is conducted on a Mediterranean catchment, the Ostouane River (144 km²), Northern Lebanon. The approach involves identifying 11 intervention reaches, proposing 38 mitigation measures, and evaluating a set of 7 primary criteria decomposed into 19 multidimensional secondary criteria. We introduce criteria of effectiveness, technical, exposure and vulnerability in addition to the commonly used criteria of environmental impact, socio-economic impact, and cost. The criteria are evaluated based on qualitative and quantitative inputs derived from the literature, surveys, questionnaires, hydrological and hydraulic modelling. The TOPSIS model is employed using 6 subjective stakeholder-driven weighting methods and 6 data-driven objective weighting methods. The methodology is evaluated through a sensitivity analysis that emphasizes on the importance of measure effectiveness, environmental impact, and cost criteria in the model. Results show that subjective weighting methods tend to prioritize structural measures at downstream areas with high-value assets, while objective methods show a more balanced distribution of measures, including green solutions and upstream reaches. The total cost of the 10 prioritized measures using subjective methods is 20% higher than that of objective methods. However, the specific choice of a weighting method can imply a substantial variation in total implementation and maintenance cost. Essentially, the choice of weighting method in MCDM can significantly alter the resulting strategies and management of risk. This contrast highlights the need for policymakers to develop flexible, adaptive strategies that balance immediate protection needs with long-term sustainability goals. Overall, this work provides a novel approach for integrated flood risk management based on adapted local-scale and informed decision-making.