Analysing dispersal of sand nourishments using SedTRAILS

Abstract

In 2019, a 5 million m³ sand nourishment was placed at the Ameland ebb-tidal delta. This pilot nourishment aimed to investigate the feasibility and effectiveness of ebb-tidal delta nourishments and to understand the dynamics in this system. To assess how this nourishment has dispersed through the ebb-tidal delta system, SedTRAILS was used. SedTRAILS is a particle-based sediment transport model currently in development. A new process for interaction with the bed and burial of sediment was recently added to the model. The impact of this process on dispersal and sediment pathways needed to be analysed, as not much is known about the effects of its inclusion on transport and dispersal.

Several schematisations of wave- and tidal forcing were tested. The wave forcing selection had little effect on the overall model results. Storm conditions included in the more detailed schematisation were underrepresented in the model results. The implementation of wave-driven bed velocity had little influence on result as well due to small magnitudes that were calculated for this. A significant difference was found between using a morphological tide and an artificial spring-neap tidal cycle however, requiring further investigation.

Investigation of the burial formulation gave insight into the response of the model to changes in the different parameters. Direct dependence of erosion probability on deposition probability presents a major limitation in the current implementation of burial. Additionally, deposition probability in its current form is positively dependent on the maximum bed shear stress. Inclusion of burial does provide significant room for calibration of results.

Through modelling these various configurations of SedTRAILS, results consistently showed a recirculation of sediment from the nourishment on the ebb-tidal delta together with bypassing around the edge of the ebb-tidal delta, as seen in Figure 1. However, the degree to which bypassing occurs depends heavily on the chosen burial configuration. The bypassing is largely a result of flow around the periphery of the ebb-tidal delta and differs from known bar migration and shoal attachment pathways. Transport into the Wadden Sea basin was not found, likely due to the underestimation of influence of storm conditions.

To improve dispersal modelling with SedTRAILS, the impact of storm conditions needs to be more accurately included in the model. An under-representation of energetic wave conditions leads to a consistent underestimation of wave-related transport. Additionally, more investigation into the effects of the spring-neap tidal cycle is required. The significant difference presented here is an early indication that this is an important factor for SedTRAILS. Validation of dispersal modelled with SedTRAILS can be achieved through quantification of transport, assigning a volume of sediment per particle.