Modelling flow over bedform fields in tidal environments

该项目旨在通过高分辨率数值模拟来描述潮汐环境中河床场的流动模式。河床（波纹和沙丘）是河流、沿海和深海环境中普遍存在的特征，它们反映了泥沙运输的强度和途径，强烈影响上覆水流，并具有重大的社会经济相关性，例如通过控制航行深度和近海建筑的安全。在浅海潮汐中，由于强大的水动力和沙质沉积物的高可利用性，形成了大的床状场。这些床形态场上方的流动与众所周知的休止角二维（2D）三角形床形态上方的流动有着根本的不同，后者一直是实验室和数值模拟研究的重点。床形态场本质上是三维的，具有弯曲的峰线、冲刷坑和峰线的分岔和终止，具有复杂的坡面不连续面和低角度背风面。在沿海地区，潮汐循环期间的流动逆转进一步复杂化了河床场和流体力学之间的相互作用。由于难以以适当的空间和时间分辨率精确测量水流速度和湍流，这些河床场上方的流动模式在很大程度上仍然未知，特别是与潮汐流有关的河床三维影响。该研究将建立、校准和验证一个三维数值模型，以表征天然河床场上方的流动和河床场的形态元素特征。这个新模型将第一次允许确定在逆转潮汐流的自然河床场之上的流速和湍流的模式，并认识到每种形态元素的影响以及它们如何相互作用。这些结果将使小规模过程更好地参数化成大规模的水力和形态动力学模型。

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.