Levee Breaches Statistics, “Geotechnical Uncertainty”, Residual Risk in Flood Hazard Mapping

The aim of this study is to implement a conceptual framework to consider, in a statistical sense, the residual risk related to possible levee failures in flood hazard mapping. Extending previous analyses on statistics of levee failure type and breach size on a 98 km-reach of the Po River, here we analyze flood events occurred in the Piave, Tagliamento and Adige rivers, in Northern Italy, providing statistics of some hundreds of historical breaches occurred in the last two centuries. Levee failure spatial density, temporal frequency and, partially, size are presented as a function of the geomorphology of the river (i.e. braided, sinuous and meandering) of levees geometry and of flood intensity. A levee fragility function is introduced to take into account the influence of hydraulic and geotechnical factors on the probability of failure. On average a spatial density and temporal intensity of 0.8 levee's failure km^-1 100 years^-1 was observed on the four rivers, in a range between 0.5 and 1.1 for levees designed with a return period ranging from 100 to 200 years and a freeboard between 0.5 and 1 m, on average. This means that the residual risk for the flooding areas is locally higher than the design one. A stochastic framework for probabilistic flood mapping is then discussed, taking into account in a Monte Carlo approach the effect of position, size, density of levees failures on the statistics of depth and velocity of inundated areas. An example is shown of the resulting residual hazard map for the 100-years return period flooding map for a 23km-reach in the middle-lower portion of the Po River catchment between the gauging station of Cremona (upstream) and Borgoforte (downstream), in a floodprone area of 100 km2 in size.