Case study: the levee failure along the Foenna stream on 1st January 2006 causing the flooding of Sinalunga urban area (Tuscany region, central Italy).

The levee failure event occurred along the Foenna stream (Tuscany region, central Italy) on 1st January 2006 is here investigated. The levee body vulnerability to seepage is first assessed through a procedure here proposed and based on a simple vulnerability index. In particular, the index is defined for dimensionless geometry of levee body and related to the probability of occurrence of the levee seepage through the identification of ?fragility curves?. The procedure can be applied for embankments whose soil hydraulic parameters are unknown or partly known. The obtained results indicate that the investigated levee, if intact, would not have to be vulnerable to seepage for the flood event occurred on January 2006 when, instead, the failure happened. The analysis shows that the failure was likely due to the presence of burrows at middle height of levee body riverside drawing the flow into the embankment triggering the piping phenomenon. This study indicates that to prevent the failure due to burrows presence, the levees must be actively maintained through both the burrows closing and the capture of wild animals. Finally, considering that scarce data are usually available for this type of events worldwide, this paper aims also to illustrate the collected data in terms of: maximum stage recorded at a gauged section close to the failure location, status of the levee, evolution of the failure event.