Author(s): Thomas J. Van Veelen; Pieter C. Roos; Suzanne J. M. H. Hulscher

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Keywords: Tidal sandbanks; Non-erodible layers; Morphodynamic modelling; Long-term evolution; Nonlinear tide-topography interaction

Abstract: Tidal sandbanks are large-scale marine bedforms observed at the bottom of shallow seas such as the North Sea and the East China Sea. They are tens of kilometres long, five to ten kilometres wide and tens of metres high, and provide ecosystem services to marine flora and fauna, and economic activities including wind farms, pipelines, and sand extraction. Importantly, tidal sandbanks are dynamic bedforms that evolve over centennial timescales. They grow in height, change cross-sectional shape, migrate, and induce new bedforms, as evidenced by numerical models that simulate their morphodynamic evolution (Roos et al., 2004; Yuan et al., 2017; van Veelen et al., 2018). These models assume that sediment supply is available without limitations. However, sediment scarcity occurs when non-erodible layers are present just below the seafloor, as is the case on the Belgian and French continental shelves (Le Bot, 2001). Here, we investigate how limitations in the sediment budget affect the cross-sectional evolution of tidal sandbanks, and related ecosystem services, using a novel process-based morphodynamic model. Our model solves the morphodynamic evolution that is driven by the nonlinear interaction between tidal flow and bed topography. We adopt a coupled-domain approach, which distinguishes between a domain with and without sediment available. Specifically, the domain without sediment cannot have a net sediment pickup during a tidal cycle. Both domains are coupled via the nonlinear hydrodynamic shallow water equations, and the advection-diffusion and mass balance in sediment transport. Following Roos et al. (2004), our focus lies initially with the evolution of the cross-section. Therefore, our model is run in 1D, i.e. depth-averaged flow and a topography that varies only in the cross-sectional dimension. Our results will show how the morphodynamic evolution of a sandbank in a sediment-scarce environment differs from a sandbank with unlimited sediment available. The crest height, trough depth, and the generation of adjacent bedforms will be limited by the presence of a non-erodible layer. This may limit its capacity to capacity to provide ecosystem benefits. Particularly, the capacity of a sandbank to recover from aggregate extraction may reduce due to the absence of new sediments. References: Le Bot, S (2001) Morphodynamique de dunes sous-marines sous influence des marées et des tempêtes: processus hydro-sédimentaires et enregistrement : exemple du Pas-de-Calais. PhD-thesis, University of Lille, pp. 311 Roos PC, Hulscher SJMH, Knaapen MAF (2004) The cross-sectional shape of tidal sandbanks: modelling and observation. J. Geophys. Res. 109, F02003. van Veelen TJ, Roos PC, Hulscher SJMH (2018) Process-based modelling of bank-breaking mechanisms of tidal sandbanks. Cont. Shelf. Res. 167, 139–152. Yuan B, de Swart HE, Panadès C (2017) Modelling the finite-height behavior of offshore tidal sand ridges, a sensitivity study. Cont. Shelf. Res. 137, 72–83.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221417

Year: 2022