Flow Dynamics over Estuarine Bedforms

该项目旨在通过水槽实验和数值模拟来表征河口河床的流态。河床(涟漪和沙丘)是河流、河口、沿海环境和深海中普遍存在的特征。它们反映了泥沙运移的强度和路径，强烈影响上覆水流，并具有重大的社会经济意义，例如通过控制航行深度。在许多河口，由于强烈的水动力和高可利用性的沙质沉积物，形成了大的河床状场。这些床面场上方的流动从根本上不同于众所周知的休止角(30°斜率)三角形床面上方的流动，后者到目前为止一直是实验室和数值模拟研究的重点。河口河床以低角度沙丘为主，平均背风坡为5°~20°。这种缓坡上的流动特性目前还不是很清楚。例如，背风面坡度与间歇性或永久性水流分离的存在或不存在之间的关系仍然知之甚少。此外，河口河床具有相对平坦的谷底和接近谷顶的陡坡，而河流沙丘具有平顶和接近谷底的陡坡。沙丘形态的这种差异对水流的影响目前尚不清楚。此外，潮流逆转与自然河床形态之间的关系，包括三维变化，尚未得到详细的研究。建议的研究将在大型水槽设备中进行一系列实验，以表征类似河口沙丘的混凝土河床上方的流动特性(速度和湍流)。根据Weser河口的河床形状测量，将模拟三组河床形状：陡峭的不对称沙丘、低角度不对称沙丘和对称沙丘。此外，三维床面场上方流动的高分辨率数值模拟将补充水槽实验。模型模拟将允许确定来自威瑟河口的天然床面场上方的潮流速度和湍流的模式。该项目的结果将有助于更好地了解河口沙丘和潮流之间的复杂相互作用，并能够更好地将小尺度过程参数化为大尺度水动力和地貌动力学模型。

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.