A simple model for estimating the hydrograph at the outlet of a reach crossing a flood-prone area is investigated. The first part of the model involves routing of the inflow hydrograph through an ideal reach with infinite vertical extension of the walls. The second part routes the flow which determines a two-dimensional flooding (outbank flow) through a linear reservoir whose storage coefficient is linked with geometric features of the flood plain. The model was applied to various geometric configurations representative of real areas in Central Italy. Its calibration and validation were carried out by comparison of the results of a diffusion hydrodynamic model. The hydrodynamic model incorporates two-dimensional overland flow and one-dimensional open channel flow. The two models were in substantial agreement, as revealed by similar shape characteristics of the corresponding hydrographs and by the values of the peak runoff error which were lower than 15% of the maximum outbank flow. Model validity was also confirmed by a comparison of its results with observed outflow hydrographs.

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