A microscale framework for harmonized flood and earthquake risk assessment and multi-hazard intervention appraisal

Multi-hazard risk reduction is increasingly recognized as essential for effective disaster risk management, yet its operationalization remains constrained by decision-making frameworks not designed for cross-hazard comparison. This paper presents a unified microscale framework for harmonized flood and earthquake risk assessment and multi-hazard intervention appraisal in residential buildings. Here, microscale refers to building-specific assessment in which hazard intensity, exposure attributes, vulnerability functions, and intervention appraisal are represented at the level of individual residential buildings rather than aggregated spatial units. The framework establishes a common structure for hazard representation, exposure characterization, vulnerability modelling, and decision variables across flood and seismic domains while retaining hazard-specific physical models, enabling consistent evaluation of single-hazard as well as multi-hazard intervention strategies. The methodology combines building-level flood and seismic risk assessment with multi-dimensional performance metrics, including expected losses, life-safety outcomes, habitability, and environmental impacts. Flood and earthquake risks are evaluated using hazard-specific models, while their annualized consequences and intervention outcomes are brought together at the multi-hazard appraisal stage. Intervention appraisal is supported through feasibility screening based on payback-period constraints and Pareto-efficient analysis to identify non-dominated retrofit options prior to a stakeholder-driven decision. The framework is demonstrated through an application to a portfolio of residential buildings in Pesaro (Italy) exposed to riverine flooding and seismic hazard under multiple hazard and vulnerability scenarios. Results show that changes in seismic demand, building vulnerability, and payback-period thresholds reshape the intervention decision space, shifting the portfolio from a feasibility-limited regime dominated by no action or flood-only measures toward broader adoption of integrated strategies. In the investigated case study, under lower seismic demand, adoption of interventions is limited, and integration emerges primarily as an extension of economically viable single-hazard measures. As seismic demand and vulnerability increase, integrated strategies become structurally competitive, expand the actionable portfolio, and significantly shorten payback periods through cost sharing and synergy effects, although benefits remain typology-dependent. More generally, the proposed framework provides an operational pathway for translating multi-hazard principles into building-scale decision support and offers practical insights for planners, asset managers, and policymakers seeking coherent and cost-effective risk reduction strategies.