Author(s): Johan Damveld; Harriette Holzhauer; Bas Borsje; Peter Herman; Kathelijne Wijnberg; Suzanne Hulscher

Linked Author(s): Peter Herman

Keywords: Ebb-tidal delta; Nourishment; Delft3D; Grain size sorting; Wadden Se

Abstract: Erosion of sandy coasts is often countered by applying beach and shoreface nourishments. As a pilot project, recently a new type of (mega)nourishment was carried out near the Wadden Sea, where the nourishment was placed on the outer ebb-tidal delta. This area is characterized by its complex hydro- and morphodynamics, and so it is unclear how this nourishment will perform in the long term. Moreover, as this area is of great ecological importance, the stresses to the ecological environment should be minimized. One of the issues is that the sediment composition of the nourishment is different to that of the target area. The goal of this work is to gain insight in the role of sediment sorting processes and to predict the potential changes of the sediment composition of the ebb-tidal delta. We build upon earlier work of Nederhofff et al (2019) and Brakenhoff et al (2020), who developed a morphodynamic model for the Amelander tidal inlet system. To study the role of fractional sediment transport, we allow for an arbitrary number of sediment fractions, and we adopt the active layer approach to account for bed stratigraphy (see e.g. Damveld et al, 2020). The first step is to calibrate the model in order to reproduce the field observations as obtained during a large measuring campaign in 2017. Subsequently we include the nourishment in the model and perform a forecast of several years. Preliminary simulation results show that the model is able to predict the current grain size distribution pattern fairly well in a qualitative manner. Already in the tide-only case the predicted distribution of sediments generally shows a good agreement to the field measurements. Extending this case with wind and wave effects further increases the predictive capacity of the model. However, sensitivity runs show that the quantitative predictions depend on several parameters, such as the initial definition of the sediment fractions, forcing, thickness of the transport layer and MORFAC (morphological acceleration factor). Once further simulations have revealed the most optimal settings to replicate the field conditions, the model can be applied to predict the effect of the nourishment on the ebb-tidal delta. Nederhoff, C.; Schrijvershof, R.; Tonnon, P.; Van Der Werf, J.; Elias, E. (2019) Modelling hydrodynamics in the Ameland inlet as a basis for studying sand transport. Coast. Sediments 2019. Brakenhoff, L.; Schrijvershof, R.; van der Werf, J.; Grasmeijer, B.; Ruessink, G.; van der Vegt, M. (2020) From Ripples to Large-Scale Sand Transport: The Effects of Bedform-Related Roughness on Hydrodynamics and Sediment Transport Patterns in Delft3D. J. Mar. Sci. Eng. Damveld, J.H.; Borsje, B.W.; Roos, P.C.; Hulscher, S.J.M.H. (2020). Horizontal and Vertical Sediment Sorting in Tidal Sand Waves: Modeling the Finite-Amplitude Stage. JGR: Earth Surface.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221270

Year: 2022