Characterization of subsurface heterogeneities in river embankments using geophysical and fiber optic techniques coupled with saline injection testing

The behavior of river embankments in face of flood events depends not only on the levee structure itself, but also on the nature and state of the underlying sediments. In particular, the presence of natural heterogeneities in the hydraulic conductivity distribution may easily lead to preferential water flow pathways during extreme river events. In this paper, we present an integrated approach for the characterization and monitoring of embankment/subsoil structures based primarily on the use of spatially distributed techniques, and specifically 3D electrical resistivity tomography (ERT) and distributed fiber optic sensing (DFOS), both used as permanent installations in 25 m deep boreholes. Two similar field sites have been setup along the banks of the Adige river, Northern Italy. In this paper, we discuss the application details and the monitoring results at both sites, that differ in the presence of granular material and subsoil heterogeneities. In particular, in both sites, saline water injection tests have been performed and monitored using both ERT and DFOS. These tests provided fundamental information about the motion of water under changing hydraulic head in the sediments below the embankment structure, showing the presence of heterogeneities and assessing the overall permeability of these sediments.