Nonlinear flood peak mitigation driven by initial reservoir conditions

Reservoirs are critical infrastructures for regulating natural flow regimes and reducing flood discharges, yet their effectiveness during extreme events strongly depends on operational strategies, particularly the initial storage level at the onset of a flood. Here we investigate the non-linear relationship between initial reservoir conditions and flood-attenuation efficiency for about 250 large dams across Italy, adopting a comprehensive, data-driven modelling framework. Flood hydrographs are generated using a simplified hydrological model and subsequently routed through each reservoir, using a “no gates management” approach and full hydraulic routing. We investigate different scenarios of input hydrograph and initial reservoir storage, derived from historical time series of stored volumes from approximately 70 reservoirs across the country and informed by regional flood seasonality.The results indicate that the reduction in peak discharge is neither spatially homogeneous nor uniform with increasing flood return periods when initial storage levels are reduced. This relationship is strongly non-linear; for instance, as reservoirs reach their capacity limits, doubling the incoming flood peak leads to abrupt reductions in attenuation efficiency. Based on the initially available storage capacity for flood control and the actual reservoir geometries, dams were classified according to the flood severity level needed to cause a significant reduction in their attenuation capacity. This classification allows us to distinguish between dams that experience a gradual decline in performance with increasing flood return periods and those that undergo a threshold effect, which is often not accounted for in conventional regional dam-safety assessments. Notably, the commonly used assumption of a fully filled reservoir at the onset of a flood proves to be overly conservative: under this scenario, about 20% of dams reach their maximum allowable water level for events with return periods of 100 years or less.By providing a national-scale assessment, the findings of this study can offer helpful insights for dam managers on the effectiveness of maintaining unfilled storage capacity for flood mitigation: by quantifying the impact of initial reservoir storage on flood attenuation, this research provides a data-driven basis for optimizing reservoir operations, particularly in balancing water storage needs with flood risk management. For instance, notable differences can be recognized between reservoirs in the Alpine region, primarily used for hydropower generation, and those in southern Italy, which serve mainly for irrigation and drinking water supply.