Tracing sand transport pathways using a Lagrangian sediment tracking model

Quantifying sediment transport is crucial for thoroughly understanding coastal systems and accurately designing coastal management interventions (e.g., sand nourishments). Lagrangian particle tracking models are valuable tools for investigating sediment transport, as modelling in a Lagrangian framework provides complete records of particle transport sources, sinks, and the pathways between them. Here we present two examples of application of Lagrangian sediment tracking modelling in coastal settings. Both studies are conducted using SedTRAILS [1], a Lagrangian particle tracking model that derives particles position from flow velocity fields generated from hydrodynamic models.

In the first application we simulate the dispersal of a nourishment on the ebb-tidal delta of Ameland Inlet (Wadden Sea, Netherlands). The flow velocity fields employed by the SedTRAILS simulation are generated from a Delft3D simulation of Ameland Inlet. The nourishment is modelled as a sample of representative sand parcels randomly sourced within the nourishment area and its gradual erosion is modelled by continuously releasing the parcels at regular intervals for the duration of the simulation. The accuracy of the Lagrangian simulation results are validated by comparing maps of particles position generated at different time steps of the SedTRAILS simulation with maps of sand spatial distribution derived from the Delft3D simulation at the same time steps. Ultimately, we are able to model the pathways of individual nourishment particles up to six months after its displacement.

In the second application we couple SedTRAILS with measurements of sand grains luminescence (i.e., the ability of a mineral grain to store energy when buried and release it upon exposure to sunlight) to reconstruct sand transport history in coastal settings. In order to do so, we combine SedTRAILS with a model that quantifies sunlight exposure of a given sand particle as a function of turbidity and its position in the water column [2], allowing to compute the cumulative sunlight exposure of such particle during its transport history. Since luminescence signals produce evidence of how long a sand particle was buried for, we are able to infer and simulate the forcings that the particle was exposed to before burial (i.e., during its transport history). As luminescence signals also yield information on how much sunlight the sand particle was exposed to before being buried, we can eventually combine the modelled cumulative sunlight exposure with evidence on resetting of luminescence signals as a function of light exposure [3] to infer, for the first time, coastal sand transport history from luminescence measurements.

Overall, the two studies provide an overview of how Lagrangian particle tracking modelling can (on its own and in combination with other techniques) provide unique insights on where, when and how sand is transported across coastal systems, which can advance our understanding of coastal systems and be exploited for accurately designing coastal management interventions.

References

[1] Pearson S.G. et al. (2023). Proceedings of the Coastal Sediments 2023, 1212-1221.

[2] Storlazzi C.D. et al. (2015). Coral Reefs, 34 (3), 967-975.

[3] de Boer A.-M. et al. (2024). Netherlands Journal of Geosciences, 103, 22.