On the reactivation of sand boils near the Po River major embankments

Backward erosion piping is one of the most critical issues for the stability of the Po River embankments, which are reported to have experienced more than one hundred episodes of sand boils over the last century. This phenomenon is particularly widespread in the Po River for structural, hydrogeological and climatic reasons. This paper describes an in-depth study recently carried out on a large diameter sand boil located at the toe of a Po River major embankment in Guarda Ferrarese (Ferrara Province), which reactivated during a high-water event in autumn 2018. First of all, a 3D FEM model, devised to identify the hydraulic boundary conditions causing the sand boil to reactivate, is introduced. The numerical analysis, while proving to be effective in simulating past reactivations, showed that the proper modelling of the phenomenon relies on a number of parameters which are usually unknown. Accordingly, a novel field investigation was carried out during the November 2018 reactivation in order to collect a few relevant data (i.e. pipe geometry, hydraulic heads, water flow and velocities as a function of the river level) and thus reduce the uncertainties in the numerical model. This study aims at providing a new insight into the backward erosion piping mechanism and a contribution to the development of a reliable tool for the management of potentially dangerous reactivations of sand boils during increasingly recurrent high water events.