Flow Dynamics over Estuarine Bedforms

该项目旨在通过水槽实验和数值模拟来描述河口河床上的水流模式。河床（波纹和沙丘）是河流、河口、海岸环境和深海中普遍存在的特征。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。在许多河口，由于强大的水动力和沙质沉积物的高可利用性，形成了大型的河床场。这些床形态场上方的流动与众所周知的休止角（30°坡度）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。河口坡面以低角度沙丘为主，平均背风坡度为5 ~ 20°。这种缓坡上的流动特性目前还不清楚。例如，背风边坡与间歇或永久流动分离的存在或不存在之间的关系仍然知之甚少。此外，河口河床有一个相对平坦的槽和一个靠近波峰的陡坡，而河流沙丘有一个平坦的波峰和一个靠近波谷的陡坡。沙丘形态的这种差异对水流的影响目前尚不清楚。此外，潮汐逆流与自然河床形态（包括三维变化）之间的关系尚未得到详细研究。拟议的研究将在大型水槽设施中进行一系列实验，以表征类似河口沙丘的混凝土床上的流动特性（速度和湍流）。根据威悉河口的床型测量，将模拟三组床型：陡峭不对称、低角度不对称和对称沙丘。此外，三维流场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潮汐流速度和湍流的模式，从威悉河口的自然河床场。该项目的结果将使人们更好地理解河口沙丘和潮汐流之间复杂的相互作用，并使小规模过程更好地参数化为大规模的水力和形态动力学模型。

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.