Modelling flow over bedform fields in tidal environments

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This project aims at characterising the patterns of flow over bedform fields in tidal environments through high-resolution numerical modelling. Bedforms (ripples and dunes) are ubiquitous features in rivers, coastal and deep sea environments, where 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 and the safety of offshore constructions. In shallow tidal seas, 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 two-dimensional (2D) triangular bedforms which have been until now the focus of laboratory and numerical modelling studies. Bedform fields are intrinsically three-dimensional (3D), with sinuous crestlines, scour pits and crestlines bifurcations and terminations, they have complex profiles with slope discontinuities and low-angle lee sides. In the coastal zone, flow reversal during a tidal cycle further complicates the interaction between bedforms fields and hydrodynamics. The patterns of flow above such bedforms fields are still largely unknown, especially the influence of bedform three-dimensionality in relation to tidal flows, due to the difficulty in measuring accurately current velocities and turbulence at an appropriate spatial and temporal resolution. The proposed study will set up, calibrate and validate a 3D numerical model in order to characterise flow above natural bedform fields and morphological elements characteristic of bedform fields. This new model will allow for the first time to determine the patterns of current velocities and turbulence above natural bedforms fields in reversing tidal currents and to recognise the influence of each morphological elements and how they interact with each other. These results will enable a better parameterisation of small-scale processes into large scale hydro- and morphodynamic models.