Land reclamations influence the morphodynamic evolution of estuaries and tidal basins, because altered planform changes tidal dynamics and associated residual sediment transport. The morphodynamic response time to land reclamation is long, impacting the system for decades to centuries. Other human interventions (e.g., deepening of fairways or port construction) add a morphodynamic adaptation timescale to a system that may still adapt as the result of land reclamations. Our understanding of the cumulative effects of anthropogenic interference with estuaries is limited, because observations usually do not cover the complete morphological adaptation period. We aim to assess the impact of land reclamation works and other human interventions on an estuarine system by means of digital reconstructions of historical morphologies of the Ems Estuary over the past 500 years. Our analysis demonstrates that the intertidal-subtidal area ratio altered due to land reclamation works and that the ratio partly restored after land reclamation ended. The land reclamation works have led to the degeneration of an ebb- and flood channel system, transitioning the estuary from a multichannel to a single-channel system. We infer that the 20th-century intensification of channel dredging and re-alignment works accelerated rather than cause this development. The centennial-scale observations suggest that estuarine systems responding to land reclamations follow the evolutionary trajectory predicted by tidal asymmetry-based stability theory as they move towards a new equilibrium configuration with modified tidal flats and channels. Existing estuarine equilibrium theory, however, fails in linking multichannel stability to the loss of intertidal area, emphasizing the need for additional research.

In deltas and estuaries throughout the world, a fluvial-to-tidal transition zone (FTTZ) exists where both the river discharge and the tidal motion drive the flow. It is unclear how bedform characteristics are impacted by changes in tidal flow strength, and how this is reflected in the hydraulic roughness. To understand bedform geometry and variability in the FTTZ and possible impacts on hydraulic roughness, we assess dune variability from multibeam bathymetric surveys, and we use a calibrated 2D hydrodynamic model (Delft3D-FM) of a sand-bedded lowland river (Fraser River, Canada). We focus on a period of low river discharge during which tidal impact is strong. We find that the fluvial-tidal to tidal regime change is not directly reflected in dune height, but local patterns of increasing and decreasing dune height are present. The calibrated model is able to predict local patterns of dune heights using tidally-averaged values of bed shear stress. However, the spatially variable dune morphology hampers local dune height predictions. The fluvial-to-tidal regime change is reflected in dune shape, where dunes have lower leeside angles and are more symmetrical in the tidal regime. Those tidal effects do not significantly impact the reach-scale roughness, and predicted dune roughness using dune height and length is similar to the dune roughness inferred from model calibration. Hydraulic model performance with a calibrated, constant roughness is not improved by implementing dune-derived bed roughness. Instead, large-scale river morphology may explain differences in model roughness and corresponding estimates from dune predictors.

Existing tidal input reduction approaches applied in accelerated morphodynamic simulations aim to capture the dominant tidal forces in a single or double representative tidal cycle, often referred to as a “morphological tide”. These heavily simplified tidal signals fail to represent the tidal extremes, and hence poorly allow to represent hydrodynamics above the intertidal areas. Here, a generic method is developed to construct a synthetic spring-neap tidal cycle that (1) represents the original signal; (2) is exactly periodic; and (3) is constructed directly from full-complexity boundary information. The starting point is a fortnightly modulation of the semi-diurnal tide to represent spring-neap variation, while conserving periodicity. Diurnal tides and higher harmonics of the semi-diurnal tide are included to represent the asymmetry of the tide. The amplitudes and phases are then adjusted to give a best fit to histograms of water levels and water level gradients. A depth-averaged model of the Ems estuary (The Netherlands) demonstrates the effects of alternative tidal input reduction techniques. Adopting the new approach, the shape of the tidal wave is well-represented over the entire length of the estuary, leading to an improved representation of extreme tidal conditions. In particular, representing intertidal dynamics benefits from the new approach, which is reflected by hydrodynamics and residual sand transport patterns that approach non-schematized tidal dynamics. Future morphodynamic simulations forced with the synthetic signal are expected to show a more realistic exchange of sediment between the channels and tidal flats, likely improving their overall predictive capacity.