Estuarine intertidal areas are shaped by combined astronomical and meteorological forces. This paper reveals the relative importance of tide, surge, wind, and waves for the flow and sediment transport on large intertidal shoals. Results of an intensive field campaign have been used to validate a numerical model of the Roggenplaat intertidal shoal in the Eastern Scheldt Estuary, the Netherlands, in order to identify and quantify the importance of each of the processes over time and space. We show that its main tidal creeks are not the cause for the dominant direction of the net flow on the shoal. The tidal flow over the shoal is steered by the water level differences between the surrounding channels. Also during wind events, the tidal flow (enhanced by surge) is dominant in the creeks. In contrast, wind speeds of order 40 times the typical tidal flow velocity are sufficient to completely alter the flow direction and magnitude on an intertidal shoal. This has significant consequences for the sediment transport patterns. Apart from this wind-driven flow dominance during these events, the wind also increases the bed shear stress by waves. For the largest intertidal part of the Roggenplaat, only ∼1–10% of the yearly transport results from the 50% least windy tides, even if the shoal is artificially lowered half the tidal range. This dominance of energetic meteorological conditions in the transports matches with field observations, in which the migration of the creeks and high parts of the shoal are in line with the predominant wind direction.

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Despite the substantial progress in last decades in modelling short-term hydrodynamic and sediment transport processes as well as long-term (centuries) morphological development, the ability of morphodynamic models to simulate medium-term (decades) development of estuaries is still very limited. One of the causes for the limitation is the problem of spin up. Many application of morphodynamic models show that at the beginning of the model simulation model results are unrealistic, and only after a certain period the model gives meaningful results. A extreme example is presented by Dam et al. (2016), who simulated the morphological developments of the Western Scheldt estuary (The Netherlands) for decades to more than a century. They concluded that the performance of the model, expressed in the Brier Skill Score (BSS), first decreases and then increases. The model results are "good" only after many decades. In this study, we aim to obtain more insight into the problem. We try to answer the following questions: x What are the causes of the spin-up problem? x What is the time scale of the spin-up? x What can we do to avoid or minimise the problem? @en