Mechanistic Insights into Flood Pulse-Induced Ecological Benefits and a Balanced Eco-Risk Assessment Framework.

Climate change has significantly intensified flood regimes in recent decades. While traditional flood management emphasizes disaster prevention through structural interventions, it often overlooks the ecological functions of flood pulses. This study first explores how flood frequency and phases affect riverine habitat quality, integrating two-dimensional hydrodynamic modeling with ecological habitat assessment. Using the Qingyi River as a case study, we quantified the habitat responses of multiple aquatic organisms under different flood frequencies and different hydrological stages. Results show that moderate floods (with a 10-year return period) enhanced fish habitat functionality (Weighted Usable Area, WUA) by up to 120% compared to multi-year average discharge, primarily by expanding shallow zones and improving lateral connectivity. Fish species reached peak habitat suitability during flood crests, while Benthic organisms favored recession phases, revealing phase-specific ecological responses. To further account for flood risks-often neglected in ecological evaluations-we introduced a Flood Assessment Method Coupling Ecology and Risk (FAMCER) and in this method we proposed the Flood Efficiency Index (FEI) to integrate ecological gains and hazard costs. This index identifies trade-offs between ecological benefits and flood risks, showing that 10-year floods achieve an optimal balance in our case study. The flexible FEI-based framework enables targeted evaluation across different conservation priorities and supports adaptive decision-making for ecological flow releases and floodplain restoration. These findings highlight the need to shift from purely defensive flood control to integrated flood management that balances ecological enhancement with risk mitigation.