L. Coumou1*, J. Cleveringa1, S. Moons2
1 Arcadis Nederland B.V.; 2 Rijkswaterstaat Zee & Delta
The beach of the Dutch Wadden Island Vlieland will be nourished in 2023. Prior to a nourishment, the grain size (D50) of the sand at the extraction site is compared to the beach and dunes as part of the ecological impact assessment. In addition, we propose to look at the sand fractions that are subjected to aeolian transport. This allows for a more accurate assessment of the effects of nourishment grain size on dune development and associated ecological values. The potential aeolian transport sand fraction (PATSF) consists of the main grain size fractions that make up the wind-blown dunes.
The grain size distributions of 66 new samples from the dunes and beach at Vlieland give insight in the distribution of grain size fractions (Arcadis, 2022a). At Vlieland, the PATSF contains the sand fractions in between 125 μm and 355 μm, which make up 96% of the dune sediment. 83% of the current beach sediment falls within the PATSF. The intended sand extraction site at the North Sea is on average coarser than the beach and dune sand at Vlieland. The D50 of the extraction site (309 μm) is 30% coarser than the beach sand (238 μm). 66% of the sand in the extraction site falls within the PATSF and could be blown into the dunes, theoretically. Therefore, despite the coarse nature of the beach nourishment, aeolian transport from the planned nourishment towards the dunes is expected to occur, unless the finer particles get trapped beneath the bigger particles (desert pavement).
Among the design aspects of a beach nourishment, the height of the top of the nourishment is crucial for aeolian transport. The higher the top, the more prone it is to develop a desert pavement, which hinders aeolian transport and has little ecological value. This risk increases when using coarse sand. A reduced height promotes frequent reworking of the top layer by waves, preventing the development of a desert pavement. To accommodate natural dune development, the original height of the planned nourishment at Vlieland (NAP + 3.5 m) was therefore reduced by 0.5 - 1.0 m.
To assess the potential ecological impact of beach nourishments we recommend investigating the PATSF of the sand extraction site in addition to the median grain size (Arcadis, 2022b). Despite large differences in the D50, a major amount of the nourished sand can still become subject to aeolian transport. This can be stimulated by careful design of the beach nourishment.
Figure 1: Overview of grain size distribution in the dunes at Vlieland (left), at the beach at Vlieland (middle) and at the sand extraction site for the beach nourishment (right). Orange box indicates the main grain size range in the dunes, which is the potential aeolian transport sand fraction (PATSF).
Arcadis (2022a). Korrelgrootte strand en duinen Vlieland – Variatie in ruimte en tijd en de relatie met zandsuppleties (Dutch). Commissioned by Rijkswaterstaat Zee en Delta. Final report.
Arcadis (2022b). Ecologische gevolgen voor strand en duinen via morfologie en korrelgrootte van de geplande strandsuppletie Vlieland (Dutch). Commissioned by Rijkswaterstaat Zee en Delta. Final report.