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The fringes of estuaries are often characterized by the presence of side embayments (secondary basins), with dimensions in the order of hundreds of meters to tens of kilometers. The presence of secondary basins significantly alters the hydrodynamic and sediment characteristics in the main estuary, implying that loss of secondary basin area due to human interventions might affect the estuarine morphodynamics. Analysis of historical bathymetric data of the Western Scheldt Estuary (Netherlands) suggests that closure of its secondary basins has triggered the observed lateral displacement of the nearby channels. This analysis motivated investigation of the impact of secondary basins on decadal evolution of estuarine channels, using the numerical model Delft3D. Model results show that channels that form near a secondary basin are located farther away from the bank of the estuary with respect to their positions in the case without a basin. Overall, results in cases with two or three basins are similar to those in case with one single basin. The wider the basin, the farther away the nearby channel forms. Removing a secondary basin causes a lateral displacement of the nearby channel toward the bank, indicating that the observed lateral displacement of channels in the Western Scheldt is triggered by closure of its secondary basins. The physical explanation is that tidal currents in the main estuary are weaker and more rotary near secondary basins, favoring sediment deposition and shoal development at these locations. Model results are particularly relevant for estuaries with moderate to high friction and converging width.

Sandy beaches are often characterized by the presence of sand bars, whose characteristics (growth, migration speed, etc.) strongly depend on offshore wave conditions, such as wave height and angle of wave incidence. This study addresses the impact of a sinusoidally time‐varying wave angle of incidence with different time‐means on the saturation height, migration speed and longshore spacing of sand bars. Model results show that shore‐transverse sand bars (so‐called TBR bars) eventually develop under a time‐varying wave angle. Depending on the time‐mean, amplitude and period of the varying angle of wave incidence, the mean heights and mean migration speeds of the bars can be larger or smaller than their corresponding values in the case of time‐invariant angles. Bars might not even form when the wave angle varies around a too large oblique mean value, whereas bars exist in the case of a time‐invariant wave angle. The oscillations in both bar height and migration speed are large if the period of the time‐varying wave angle is close to the adjustment timescale of the system and if large differences in the local growth and migration rates of the bars occur during one oscillation period. The oscillations in bar height are a combination of harmonics with the principal period and half the period of the time‐varying wave angle, whereas those of migration speed contain only the principal period. Bars that are subject to time‐varying wave angles have larger longshore crest‐to‐crest spacings than those which form under fixed wave angles. Physical explanations for these findings are given. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

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