Oblique sand ridges in confined tidal channels due to Coriolis and frictional torques
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Oblique sand ridges in confined tidal channels due to Coriolis and frictional torques Tjebbe M. Hepkema1
· Huib E. de Swart1 · Abdel Nnafie1 · George P. Schramkowski2 · Henk M. Schuttelaars3
Received: 25 March 2020 / Accepted: 21 September 2020 © The Author(s) 2020
Abstract The role of the Coriolis effect in the initial formation of bottom patterns in a tidal channel is studied by means of a linear stability analysis. The key finding is that the mechanism generating oblique tidal sand ridges on the continental shelf is also present in confined tidal channels. As a result, the Coriolis effect causes the fastest growing pattern to be a combination of tidal bars and oblique tidal sand ridges. Similar as on the continental shelf, the Coriolis-induced torques cause anticyclonic residual circulations around the ridges, which lead to the accumulation of sand above the ridges. Furthermore, an asymptotic analysis indicates that the maximum growth rate of the bottom perturbation is slightly increased by the Coriolis effect, while its preferred wavelength is hardly influenced. Keywords Coriolis effect · Sediment transport · Linear stability analysis · Sand ridges · Tidal bars · Residual circulation
1 Introduction Tidal bars are rhythmic bottom patterns that occur in many tidal channels (e.g., the Western Scheldt in the Netherlands, the Exe Estuary in England, the Ord River Estuary in Australia, and the Venice Lagoon in Italy). These bars are several meters high and have wavelengths of 1–15 km. Their characteristics are determined by channel properties (depth, width, tidal amplitude, etc.), which may change due to, for example, dredging, sea level rise, and land reclamation. Tidal bars are invaluable for many organisms that feed on their rich grounds, but they also may hamper marine traffic. For proper management of tidal channels, it is therefore important to understand their behavior. Seminara and Tubino (2001), Schramkowski et al. (2002), and Hepkema et al. (2019), among others, studied the physical mechanism that causes tidal bars to form, Responsible Editor: Emil Vassilev Stanev Tjebbe M. Hepkema
[email protected] 1
Institute for Marine and Atmospheric Research Utrecht, Utrecht, The Netherlands
2
Flanders Hydraulics Research, Antwerp, Belgium
3
Delft Institute of Applied Mathematics, Delft, The Netherlands
as well as the sensitivity of their wavelength to channel properties. They explained that the initial formation of tidal bars can be understood by analyzing the residual currents generated by the topography (using arguments similar to those by Zimmerman (1981)). Hibma et al. (2004) showed that the results of the linear stability analysis of Schramkowski et al. (2002) compare well with results of a numerical morphodynamic model, Delft3D. In these linear stability studies and in the study by Hibma et al. (2004), the Coriolis effect was neglected. However, tidal bars occur in natural systems (e.g., Western Scheldt) where the Coriolis force is a first-order term in the momentum balance. The importance of Coriolis
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