Levee breaches and “geotechnical uncertainty” in flood risk mapping

The capability of river embankments to protect surrounding areas does not depend only on the return period of the water level and discharge of the design flood. It depends also on other uncertainty factors related to the safety level of the embankment itself, viz to the risk of piping, erosion and global stability. As it is difficult to predict the position where levee breaches can occur and their size, we intend to investigate in the following the “geotechnical uncertainty”, i.e. the uncertainty related to these factors, aiming at assessing its influence on flooding hazard and risk mapping. The aim of this study is to implement a stochastic approach to model the position, length and depth of the levee breaches in order to assess the flooding volume and area. Extending a previous analysis 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 Piave and Tagliamento rivers and provide statistics of 248 historical breaches occurred in the 1800—1966 period in three different stretches (i.e. braided, sinuous and meandering) of the floodplain course of the Piave river (65.1 km long) and of the Tagliamento river (91.4 km) in Northern Italy.