Abstract
Deepening of estuarine tidal channels often leads to tidal amplification and increasing fine sediment import. Increasing fine sediment import, in turn, may lower the hydraulic drag (due to a smoother muddy bed and/or sediment-induced damping of turbulence), and therefore, further strengthen tidal amplification, setting in motion a process in which the sediment concentration progressively increases until the river becomes hyper-turbid (Winterwerp and Wang, Ocean Dyn 63(11–12):1279–1292, 2013). To advance our understanding of the relative role of bed roughness and bed topography on sediment import mechanisms and sediment concentration, a Delft3D numerical model has been setup for an estuary which has been deepened and as a consequence experienced a strong increase in suspended sediment concentration: the lower Ems River. This model is calibrated against present-day hydrodynamic and sedimentary observations, and reproduces the basic sediment transport dynamics despite simplified sedimentological formulations. Historic model scenarios are semi-quantitatively calibrated against historic high and low water observations, revealing that changes in hydraulic roughness and deepening are probably equally important for the observed tidal amplification. This model is subsequently used to better understand historic changes in the hydrodynamic and sediment transport processes in the lower Ems River. Import of fine sediment has increased because of larger tidal transport, even though the degree of tidal asymmetry may not have significantly changed. The resulting rise in suspended sediment concentration reduced hydraulic drag, amplifying the tidal range. Export of fine sediment became less because the river-induced residual flow velocity decreased with deepening of the channel.
Highlights
Many estuaries worldwide, but in Western Europe, have been deepened in the past decades to centuries, allowing ship access to inland ports
This study aims to investigate this feedback in a heavily engineered estuary, the lower Ems River, using a numerical model
A numerical model was setup, in which water levels and flow velocities were extensively calibrated against data
Summary
But in Western Europe, have been deepened in the past decades to centuries, allowing ship access to inland ports. Both deepening and reclamation of intertidal areas have led to an increasing tidal range, with tides penetrating increasingly deeper up-estuary (Winterwerp and Wang 2013; Winterwerp et al 2013). Ocean Dynamics (2015) 65:589–605 result of increasing suspended sediment concentrations) which in turn strengthens tidal deformation—see Fig. 1 Because of this positive feedback mechanism, deepening or other engineering interventions may set into motion an evolution in which the tides become progressively more asymmetric, continuously more sediment is imported and the hydraulic drag becomes increasingly lower. This study aims to investigate this feedback in a heavily engineered estuary, the lower Ems River, using a numerical model
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