Beyond Levees: Controlled Overflows for Managing Residual Flood Risk in the Enza River

This contribution presents a systematic framework for managing residual flood risk in embanked fluvial systems, focusing on the Enza River (Italy), a right-bank tributary of the Po River. Even with planned structural and maintenance measures, the fluvial system cannot safely convey extreme flood events (e.g., 500-year floods). Under these conditions, controlled overflows implemented through engineered spillways offer a robust risk-mitigation strategy, enabling the controlled release of floodwaters and reducing the consequences associated with accidental levee failure.

The proposed approach integrates two-dimensional hydrodynamic simulations with the PARFLOOD model to delineate levee segments susceptible to overtopping, support the iterative optimization of spillway location and design parameters, and simulate flood inundation resulting from both uncontrolled levee breaches and controlled overflow conditions. Impact analyses are carried out using advanced tools developed under the MOVIDA project to quantify potential damage to population, infrastructure, and economic assets.

The analysis of multiple flood scenarios (ranging from uncontrolled breaches to controlled overflow configurations, with and without complementary mitigation measures) demonstrates the strong potential of controlled overflows through engineered spillways to reduce flood impacts. The results indicate that controlled overflows can reduce inundated areas by up to 80% and direct economic losses by up to 96%, while substantially decreasing population exposure from approximately 7,900 to 64 individuals.

These findings highlight the effectiveness of controlled overflows as a key element of residual flood risk mitigation, particularly when combined with conventional structural interventions. Such an approach enhances system adaptability and supports anticipatory, risk-informed floodplain management, representing a shift from passive flood defense toward proactive resilience-based planning.