Multiple reservoirs impact on flood frequency curves: a new global and physically-based index

The influence of the human activity (e.g. land use changes, alterations of the river system, hydraulic structures realization) on the flood frequency curves, represents a notable information to understand hydrologic changes identifying some of its drivers. In this work we provide a simple yet physically-based global index to assess the effect of multiple reservoirs, located in series along the main channel, on peak flood quantile at the catchment scale. The index formulation is based on an Instantaneous Unit Hydrograph (IUH) method, and it takes into account the main parameters of the system, such as the relative location and relative storage coefficient of the reservoirs, their number, and a climatic parameter. An analytical formulas of the index is provided, and it is independent of the return period. Numerical experiments and a comparison with the literature indices confirm the efficiency of the proposed index and allow to disentangle the role of the parameters on the flood peak reduction. Finally, we also present and discuss results of the index application for a real case study, that is the reservoirs system of the Tiber River (Central Italy).