River Levee Overtopping: A Bivariate Methodology for Hydrological Characterization of Overtopping Failure

This research presents a bivariate methodology for the hydrological characterization of the overtopping failure for an existing river levee. Conventional procedures usually consider only one variable for the hydrologic forcing (peak discharge, Q). Such an approach might fail if the volume of the hydrograph (V) is also significant. The proposed methodology is based on the generation of a set of plausible inflow hydrographs characterized by their peaks and volumes in the Q-V space. The shapes of the hydrographs are classified according to their tendency to produce overtopping, introducing a new index: the Overtopping Hydrograph Shape Index (OHSI). The levees in two river reaches located in Tuscany, Italy, were used as a case study. As a preliminary result, it was found that the hydrographs that produce overtopping failure in both cases lay on a zone in Q-V space delimited by a curve, the Critical Overtopping Flood Hydrograph (COFH) curve. The existence of COFH demonstrates that overtopping failure is not determined by a unique variable (Q) but rather by the combination of both Q and V. The limiting case corresponds to a family of hydrographs with varying Q and V values. The COFH allows dividing the Q-V space into two zones, the no-failure zone and the failure zone, that simplify the evaluation of the failure probability for a river levee.