Observations of the dynamics of mixed sediments on a nourished beach in a mixed-energy environment

Sand nourishments play a crucial role in safeguarding vulnerable urbanized coastlines in wave-dominated environments. Nowadays, nourishments are also implemented as sandy retrofits in front of hard infrastructure within mixed wave-current systems using poorly sorted sediments, where the coarse fraction is expected to minimize erosion. However, the dynamics of these mixed sediments in mixed-energy systems is poorly understood. Here, we combine hydrodynamic data and sediment samples collected from the Prins Hendrikzanddijk retrofit, Texel, the Netherlands, to demonstrate that frequent remixing of the bed (sub)surface disrupts sediment sorting processes, preventing the formation of a more erosion-resistant composition (armor layer). Despite the low-energy environment and the use of coarser-than-native sediments, tidal currents are shown to commonly mobilize the finer particles, while moderate storm waves also mobilize the coarser fractions. Additionally, the cross-shore exchange of sediments between the upper and lower beach in response to changing wave conditions further prevents long-term sorting, as this profile dynamic exposes new coarse, poorly sorted material at the surface. Our finding that a stable armor layer does not develop can lead to higher erosion rates over time than anticipated.