Net sediment transport in tidal basins: quantifying the tidal barotropic mechanisms in a unified framework

Vincenzo Marco Gatto,Zheng Bing Wang,Bram Christiaan Van Prooijen

doi:10.1007/s10236-017-1099-3

Vincenzo Marco Gatto, Zheng Bing Wang + Show 1 more

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https://doi.org/10.1007/s10236-017-1099-3

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Journal: Ocean Dynamics	Publication Date: Sep 26, 2017
Citations: 25	License type: open-access

Affiliation: Delft University of Technology, Deltares

Abstract

Net sediment transport in tidal basins is a subtle imbalance between large fluxes produced by the flood/ebb alternation. The imbalance arises from several mechanisms of suspended transport. Lag effects and tidal asymmetries are regarded as dominant, but defined in different frames of reference (Lagrangian and Eulerian, respectively). A quantitative ranking of their effectiveness is therefore missing. Furthermore, although wind waves are recognized as crucial for tidal flats’ morphodynamics, a systematic analysis of the interaction with tidal mechanisms has not been carried out so far. We review the tide-induced barotropic mechanisms and discuss the shortcomings of their current classification for numerical process-based models. Hence, we conceive a unified Eulerian framework accounting for wave-induced resuspension. A new methodology is proposed to decompose the sediment fluxes accordingly, which is applicable without needing (semi-) analytical approximations. The approach is tested with a one-dimensional model of the Vlie basin, Wadden Sea (The Netherlands). Results show that lag-driven transport is dominant for the finer fractions (silt and mud). In absence of waves, net sediment fluxes are landward and spatial (advective) lag effects are dominant. In presence of waves, sediment can be exported from the tidal flats and temporal (local) lag effects are dominant. Conversely, sand transport is dominated by the asymmetry of peak ebb/flood velocities. We show that the direction of lag-driven transport can be estimated by the gradient of hydrodynamic energy. In agreement with previous studies, our results support the conceptualization of tidal flats’ equilibrium as a simplified balance between tidal mechanisms and wave resuspension.

Highlights

Tidal basins are threatened worldwide by accelerated sea-level rise and anthropogenic interferences
Concluding, we have identified three generally coexisting sub-mechanisms contributing to settling lag: velocity damping, bed-level variation and water-depth variation
If we introduce the condition Ue > Ud, settling lag is further enhanced by scour lag

Summary

Introduction

Tidal basins are threatened worldwide by accelerated sea-level rise and anthropogenic interferences (e.g., damming and dredging). A major concern is the progressive erosion and drowning of intertidal flats, which provide fundamental ecosystem services (Costanza et al 1997). It is still questionable whether sediment import is sufficient for the tidal flats to keep pace with accelerated sea level rise (Siefert 1990). We consider the Dutch part of the Wadden Sea (Fig. 1), UNESCO World Heritage since 2009. This area has been heavily impacted over the last century. Since 1990, the sandy foreshores of the barrier islands are maintained with regular nourishments (Wang et al 2012)

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