In the Interreg-funded Polder2C’s project, large-scale overflow experiments were conducted from 2020 to 2022 on levee slopes along the Scheldt River in Belgium and the Netherlands. These tests assessed surface erosion resistance under varied conditions, including levee sections containing animal-induced damages. Electrical Resistivity Tomography (ERT) was employed as a non-invasive monitoring tool to observe subsurface changes, particularly those linked to erosion-prone animal burrows. A unique system configuration enabled detailed imaging of the levee’s internal dynamics during overflow testing. Post-processed ERT data effectively captured subsurface changes during these events, including water infiltration into existing burrows, cavity formation and collapse, and the interconnection of subsurface voids. The study demonstrates ERT’s ability to identify critical subsurface features, with low resistivity zones indicating water-saturated areas and high resistivity zones marking air-filled voids. Time-lapse ERT imaging successfully captured dynamic resistivity shifts, correlating with key processes like soil displacement around burrows. Despite potential limitations, such as environmental noise and the influence of synthetic road plates used as protective coverings, ERT proved effective in detecting internal erosion patterns and pre-existing structural weaknesses. The results indicate that ERT offers a feasible, scalable approach, also for real-time levee monitoring in overflow scenarios, enhancing its applicability for validation of erosion models. Future studies should investigate the effect of cumulative damage during overflow testing and optimize forms of data presentation to improve interpretability, ultimately refining ERT’s potential as a reliable tool for predicting levee vulnerabilities.

Scholte wave surveys were conducted at both the laboratory and field scales to evaluate the reliability of distributed acoustic sensing (DAS) with a fiber-optic cable resting on top of unconsolidated sedimentary deposits to determine the distribution of S-wave velocity underneath. Laboratory measurements performed in a controlled environment at the Deltares Laboratory facility demonstrated that DAS retrieves low- and high-frequency energy associated with Scholte and guided waves. The recorded DAS signals provided consistent Scholte wave signals, which depicted coherent phase velocity energy that was used to accurately depict S-wave velocity layering. We observed the presence of guided waves at higher frequencies, which appeared to be enhanced as the source position was moved away from the fiber-optic cable. A field survey was carried out using a linear set-up in a shallow lake, where a fiber-optic cable was placed on top of a sediment layer with a thickness of 5–10 m. The results from DAS were validated using standard hydrophone measurements performed simultaneously. The 2D S-wave velocity cross-section retrieved by DAS appeared to be in good agreement with the results obtained from hydrophone measurements, especially when detecting the main velocity transition occurring at a 7–10 m depth from the free surface.