Assessment Frameworks and Performance Indicators for Hybrid Flood Risk Management Measures From a Scoping Review and Expert Perspectives

Hybrid measures, which combine engineered (grey) and nature-based (green/blue) infrastructure, are increasingly promoted to enhance flood resilience across pluvial, fluvial, and coastal systems. However, no standardised framework currently exists to assess their performance, limiting comparison across studies and practical implementation. This study investigates which assessment frameworks, methods, and key performance indicators (KPIs) are used to evaluate hybrid measures, and how these vary across contexts. A scoping review of peer-reviewed literature indexed in Web of Science and Scopus identified 187 eligible studies. This was complemented by eight semi-structured expert interviews with practitioners and researchers to validate literature findings and identify implementation gaps. Extracted data were analysed using descriptive statistics, thematic coding, and cross-comparison of methods, indicators, and governance dimensions. Results show a strong increase in publications after 2019, with most studies focused on urban settings and mitigation-oriented applications. Assessment is dominated by hydraulic and hydrological modelling, followed by cost-benefit analysis and multi-criteria decision approaches. Frequently used flood KPIs include runoff volume reduction (45.5% of studies), damage/risk metrics (38.5%), and peak flow reduction (31.0%). Ecological KPIs most commonly address biodiversity/habitat quality (45.5%) and water quality (42.2%), while socioeconomic evaluation is largely limited to cost indicators (72.2%). Few studies explicitly assess long-term adaptive performance, component interaction, or governance effectiveness. Interview findings further highlight gaps between theory and practice, particularly regarding fragmented governance responsibilities, limited monitoring of hybrid interactions, and uncertainty around long-term performance and maintenance. The results reveal a mismatch between the multifunctional ambitions of hybrid measures and the predominantly hazard-focused methods currently applied. Advancing hybrid flood resilience requires integrated, interaction-based, and temporally explicit assessment frameworks that jointly capture hydraulic performance, ecological development, and governance capacity.