On the impact of flood-control reservoirs on downstream flood characteristics

A distributed model of catchment dynamics has been used in order to evaluate the impact of flood-control reservoirs on downstream flood characteristics under variable storming conditions. The catchment model calculates local contributions to infiltration excess runoff at each elemental cell of the discretized catchment by means of a time compression approximation water balance model and routes thesecontributions throughout a drainage network extracted from digital elevation model data via a diffusion wave scheme based on the Muskingum-Cunge method with variable parameters. The reservoir is described in terms of elevation-water surface area and elevation-discharge curves. Level pool routing is modeled via a fourth-order Runge-Kutta routine, allowing the dynamic computation of the reservoir outflow during flood events, in response to the inflow hydrograph from the controlled upstream drainage area. The obtained model is applied to the approximately 840-km$^2$ Sievecatchment (Central Italian Apennines), where the $69 \times 10^6$-m$^3$ multipurpose Bilancino reservoir controls an upstream drainage area of about 150 km$^2$. The impact of the reservoir on downstream flood characteristics is evaluated by comparing hydrograph peaks along the mainstream, as simulated by themodel in the natural (without reservoir) and controlled (with reservoir) scenarios. The obtained results indicate that a profound impact may be produced on the scaling properties of peak discharges with upstream drainage area and thus a critical use of such scaling relationships is suggested when dealing with controlled river systems.