Sediment scarcity: modelling the morphodynamics of sandbanks when sand is limited

Sandbanks are large-scale dynamic bedforms of sandy sediment that are found in shallow seas. They may occur in areas where the sediment supply is limited such as on the Belgian Continental Shelf. These sandbanks serve important ecological and economic functions (e.g. sand extraction). However, our understanding of the long-term morphodynamics of tidal sandbanks under sediment-scarce conditions is limited due to a lack of process-based modelling support. Here, we study equilibrium cross-sections and the morphodynamic evolution towards an equilibrium by extending a process-based model to account for sediment scarcity. Our model includes a non-erodible layer, which limits sediment entrainment. The model results show that the presence of a non-erodible layer limits the amplitude of a sandbank and sharpens the crest in its equilibrium state. The bank width remains unaffected. Furthermore, our results show that sandbanks reach equilibria faster when a non-erodible layer is present. However, the rate of amplitude growth of a bank crest is unaffected for a long period after the non-erodible layer is exposed under the conditions in this study. Our results are valuable for identifying potential impacts of sand extractions in sandbank fields with limited sand available in the seabed.