Sand boils are the surface manifestation of an erosion process, known as backward erosion piping, which may take place beneath river embankments during high-water events. The risk of embankment failure greatly increases in locations affected by sand boils. Numerous studies have been carried out, mainly at the laboratory scale, providing significant advancements in this field. Nonetheless, there is still a gap between research and practice that needs to be filled. This study presents a set of field measurements carried out on a large sand boil reactivated near the toe of an embankment along the river Po, Italy. Hydraulic heads, velocity and discharge, concentration and pipe geometry were measured as a function of the water level in the river during the November 2018 flood. The collected data are compared with predictions of a theoretical model that provides the head loss in the vertical pipe. Furthermore, the local exit gradients, as deduced from measurements, are discussed, together with the operational critical gradients adopted in current design practice. The collected data provide important input parameters for the calibration of analytical and numerical models, typically implemented to investigate the sand boil evolution and then to assess the backward erosion piping risk at real scale.