Occurrence of sand boils landside of a river dike during flooding: A geophysical perspective

The study of internal erosion of earth dams, dikes, and levees is an essential ingredient to address their safety. Indeed, internal erosion is known to be a major cause of failure for dams and levees. Several sections of the Agly river dike (south-west France) are affected by the appearance of sand boils on the side of the protected zone. These phenomena generally occur without any significant erosion of the dike itself. In order to better understand the reasons for these phenomena, we took advantage of a drought (dry riverbed) to image, using induced polarization tomography, the subsurface from the riverbed to the plains extending inland behind the areas prone to these pathologies. Induced polarization imaging is a geophysical technique that extends the classical electrical resistivity tomography to include low-frequency polarization mechanisms. This approach shows that the river dike is partly built on a sand-filled paleochannel, allowing water to flow under the dike and its clay core during river floods. During floods, underground flow under the dike is responsible for sand boil phenomena on the side of the protected zone. Numerical hydraulic simulations show that the discharge zone has specific concentration points, particularly at the toe of the dike even when the permeability of the sand filling the paleochannel is homogeneous. These concentrated discharges are controlled by the water level in the river, the geometry of the dike and the shape of the clay-sand interface of the paleochannel. This study highlights the role of geophysical techniques, especially induced polarization, in providing key information to better understand erosion and fluidization phenomena affecting river dikes.