An experimental study of the natural sedimentation-erosion patterns in the Nieuwe Waterweg

Abstract

The Nieuwe Waterweg is a heavily engineered estuary that forms the entrance to the Port of Rotterdam. To allow easy access for increasingly larger ships to the harbours, the Nieuwe Waterweg has been narrowed, straightened and embanked and the bed is continuously dredged to an unnaturally deep water depth. This has led to increased flood risks, salt intrusion and ecosystem degradation in the area and these issues are expected to worsen with sea level rise. To counteract these issues, it has been proposed to stop dredging and let the channel become shallower through natural sedimentation processes. However, it is unknown how the morphology would develop through natural sedimentation-erosion patterns due to the unnatural planform shape of the estuary. In this study, the morphological development of the Nieuwe Waterweg without dredging was simulated in the Metronome, a periodically tilting tidal flume capable of creating dynamic estuaries. Three experiments were performed to investigate how a harbour complex and mud supply influence the sedimentation-erosion patterns. The bed elevation was measured during the experiments to analyse the development of the sedimentation-erosion patterns. In addition, the hydrodynamics of the water flow in the experiments were measured and simulated with a numerical model, XBeach. Due to the fixed planform shape the morphology in the experiments developed in the vertical instead of the horizontal as in natural estuaries. This resulted in a pattern of increasing landward shallowing by net sedimentation and seaward deepening by net erosion in all experiments. The harbour complex acted as a sediment trap in both experiments, with most sedimentation in the case of mud supply. In general, the mud deposited mostly in inactive areas, on top of bars and in the landward end of the estuary. In reality the same general pattern of landward shallowing and seaward deepening is expected to develop. However, due to the large water depth in the Nieuwe Waterweg, this is expected to develop by increasing amounts of landward sedimentation only. This development would have large consequences for the accessibility for ships and thus harbour industry. However, it would also counteract salt intrusion, decrease flood risks and provide new areas for nature restoration which could turn the harbour areas into new attractive living and working spaces.