Numerical modelling of the geologically reconstructed Oer-IJ estuary.
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The Oer-IJ estuary near Castricum was active between 2500 BC and 200 BC. Geologic reconstruction resulted in paleographical maps and in hypotheses explaining its development and closure. The shape of the estuary was convergent, but with an uncharacteristic bend over the entire estuary that was perhaps caused by the washover-type initiation of the estuary. Reconstruction suggests that in the lower estuary bars were present. The estuary was connected with the Hollandse Vecht river and the Almere lake, which must have provided freshwater but barely bed sediment. The reconstruction showed that a period of activity, represented by intertidal bars and a widely extended estuary between 650 and 200 BC coincided with the activity of the Vecht river and the closure of the estuary after 200 BC coincides approximately with the opening of the Vlie inlet, which connects lake Almere with the Waddensea and the North Sea. This leads to the hypothesis that the freshwater inflow kept the estuary open and that tidal bars form in the outer part of the estuary (Vos et al. 2010). This study will systematically test the hypotheses raised by geologic reconstructions by scenario modelling in Delft3D, in depth averaged approach. The model consist of a 45 km long confined curvilinear grid based on the geologic reconstruction. Tidal components M2, M4 and O1 were applied at the offshore boundary, realistic river discharge up to 500 m3/s was prescribed at the landward boundary. The influence of the relative curved Oer-IJ estuary shape was tested with similar straightened and similar idealized grids. Model results show similar results in bar and channel pattern along the estuary for all model settings. Increased river discharge has primary an effect on bed level elevation in the middle of the estuary, which increases with higher discharges. The bended estuary reduces tidal range rapidly in particular when combined with high discharges. The study concludes that the effect of river discharge on tidal inflow and on estuary filling is significant even when fluvial discharge is only ten percent of the tidal discharge. The bended estuary possibly reflect a part of the tidal energy but definitely reduces tidal influences in the inner part of the estuary. Nevertheless this study shows that river flow is not a prerequisite to keep the estuary open.