Flow Dynamics over Estuarine Bedforms

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This project aims at characterising flow patterns over estuarine bedforms through flume experiments and numerical simulations. Bedforms (ripples and dunes) are ubiquitous features in rivers, estuaries, coastal environments and deep seas. They reflect the strength and pathways of sediment transport, strongly influence the overlying flow and have a major socio-economic relevance, for example through the control of navigational depth. In many estuaries, large bedform fields develop due to the strong hydrodynamics and high availability of sandy sediment. Flow above these bedform fields differs fundamentally from flow above well-known angle-of-repose (30° slope) triangular bedforms which have been until now the focus of laboratory and numerical modelling studies. Estuarine bedforms are mainly low-angle dunes, with mean lee slopes of 5 to 20°. Flow properties over such gentle slopes are currently not precisely known. For example, the relation between lee side slope and the absence or presence of intermittent or permanent flow separation is still poorly understood. Furthermore, estuarine bedforms have a relatively flat trough and a steep slope close to the crest, whereas river dunes have a flat crest and a steep slope close to their trough. The effect of this difference in dune morphology on the flow is currently unknown. Besides, the relation between reversing tidal flow and natural bedform morphology, including three-dimensional variations, have not yet been studied in detail.The proposed study will perform a series of experiments in a large flume facility in order to characterise flow properties (velocity and turbulence) above concrete bedforms resembling estuarine dunes. Three sets of bedform shapes will be modelled following bedform measurements from the Weser Estuary: steep asymmetric, low-angle asymmetric and symmetric dunes. Furthermore, high-resolution numerical simulations of flow above three-dimensional bedform fields will complement the flume experiments. The model simulations will allow to determine the patterns of tidal current velocities and turbulence above natural bedform fields from the Weser Estuary. The results of this project will allow a better understanding of the complex interaction between estuarine dunes and tidal flow, and enable a better parameterisation of small-scale processes into large scale hydro- and morphodynamic models.