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Internal erosion, a major cause of failure of earthen dams and levees, is often difficult to detect at early stages using traditional visual inspection. The passive seismic-interferometry technique could enable the early detection of internal changes taking place within these structures. We test this technique on a portion of the sea levee of Colijnsplaat, Netherlands, which presents signs of concentrated seepage in the form of sandboils. Applying seismic interferometry to ambient noise collected over a 12-hour period, we retrieve surface waves propagating along the levee. We identify the contribution of two dominant ambient seismic noise sources: the traffic on the Zeeland bridge and a nearby wind turbine. Here, the seawave action does not constitute a suitable noise source for seismic interferometry. Using the retrieved surface waves, we compute time-lapse variations of the surface-wave group velocities during the 12-hour tidal cycle for different frequency bands, i.e., for different depth ranges. The estimated group-velocity variations correlate with variations in on-site pore-water pressure measurements that respond to tidal loading. We present lateral profiles of these group-velocity variations along a 180-meter section of the levee, at four different depth ranges (0 m-40 m). On these profiles, we observe some spatially localized relative group-velocity variations of up to 5% that might be related to concentrated seepage.

Given the large investments in dike construction and reinforcements, optimisation of these interventions could contribute to significant cost savings. In this study, a probabilistic method is developed for obtaining the geometry of a river dike cross‐section that fulfils a specified target reliability at a minimal cost. The method takes into account multiple relevant failure mechanisms and various geometric parameters of the dike. Large numbers of cross‐sections are generated and the reliability for each section is computed using FORM, Monte Carlo sampling, and numerical integration. The method is applied to several cases representing typical river dike profiles that are found in the Netherlands. Results show that for the optimal geometry the largest fraction of the target failure probability is assigned to those mechanisms with a relatively high cost of reinforcement. Also, comparison with the current semi‐probabilistic design guidelines shows that a decrease in costs of up to 15% could be reached with the new optimisation method.

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