Quantification and modelling of bedform dynamics in unsteady flows

大多数冲积河道的河床不是平坦的，而是由一系列起伏的沉积堆积组成，这些沉积堆积被称为“河床”，其中包括涟漪和沙丘。这些床型存在于一系列尺度上，并随着流量的变化而不断移动和改变其形状、大小和形状。这些床型是提供水流阻力的主要粗糙元素。因此，河床对流量变化的反应对于预报洪水淹没程度至关重要。流量的变化比床型的变化更快，因此床型通常与流量不平衡。这一点非常重要，因为我们的绝大多数河床相图(将水流速度和泥沙尺寸与可能存在的河床类型联系起来的稳定场预报器)、形态动力学模拟和数值模型预测都假设了基于平衡床态和恒定流量的简化河床形态。因此，我们的模型和预测中的许多反馈要么被忽略，要么被高度简化。这是一个重大缺陷，因为正是这些模型被使用，特别是在人口较多的地区和城市地区，以满足防洪、导航、水电、集体开采和供水方面的安全需求。对这些河流的精明管理至关重要，这对设计、实施和监测的准确性提出了很高的要求。如果要改进这样的模型，则需要对河床对非恒定流的动态调节的过程有新的基本理解，以及将这种知识整合到建模实践中的方法。作为实现这一目标的一步，需要将水力控制、泥沙输送过程和形态调整的反应以及形态阻力和床面阻力的变化与一系列非恒定流联系起来。一旦建立了这些关系，就可以帮助我们更好地了解这些动态过程，并更好地预测一组给定流量变化的河段。该项目将结合以下内容描绘这些过程：(I)在最先进的水槽中进行的新颖的实验室调查，将量化固定床和活动床上的水流结构和泥沙输送，随着水位的变化；(Ii)密西西比河洪水事件期间的密集实地工作，将绘制并量化河床形态、水流结构和泥沙输送的变化；以及(Iii)开发和应用创新的三维可变形边界上的非恒定流数值模型。这项建模工作将确保结果是通用的，并具有更广泛的吸引力，特别是在改进提供洪水预测和为环境管理决策提供信息的模型方面。所有数据和产出将通过科学渠道免费提供，也可以通过公共传播活动、互联网和基于GoogleEarth的XML接口免费提供。

The beds of most alluvial river channels are not flat, but comprise a series of undulating sedimentary accumulations termed 'bedforms' that include ripples and dunes. These bedforms exist over a range of scales, and are constantly moving and changing their shape, size and form in response to changes in flow discharge. These bedforms are the primary roughness elements that provide resistance to the water flow. The response of bedforms to a changing discharge is therefore critical for predicting flood inundation levels. Changes in flow discharge are more rapid than changes in the bedforms, such that bedforms are commonly out of equilibrium with the flow. This is very important as the vast majority of our bed-phase diagrams (stability field predictors that relate flow velocity and sediment size to the bedform types likely to be present), morphodynamic simulations, and numerical model predictions assume simplified bed morphologies that are based on equilibrium bed states and constant discharges. Consequently, many feedbacks within our models and predictions are either ignored or highly simplified. This is a significant shortcoming as it is these models that are used, especially in more populated and urban areas, to meet demands on safety against flooding, navigation, hydropower, aggregate mining and water supply. The astute management of these rivers is paramount, putting high demands on accuracy in design, implementation and monitoring. If such models are to be improved, then new fundamental understanding is required of the processes that underlie the dynamics of bedform adjustment to unsteady flow and ways of integrating such knowledge into modelling practice. As a step towards this goal, there is a need to link hydraulic controls, the response of sediment transport processes and morphological adjustment, and the changes in form drag and bed resistance to a range of unsteady flows. Once established, these relations can be used to help improve our understanding of these dynamic processes and predict better the river stage for a set of given discharge changes. This project will delineate these processes using a combination of (i) novel laboratory investigations in a state-of-the-art flume that will quantify the flow structure and sediment transport over fixed and mobile beds as stage varies, (ii) intense fieldwork during flood events in the Mississippi River that will map and quantify changes in bed morphology, flow structure and sediment transport, and (iii) development and application of an innovative numerical model of unsteady flow over a deformable 3D boundary. This modelling work will ensure that the results are generic and have a wider appeal, notably in the improvement of models that provide flood predictions and inform environmental management decisions. All data and output will be made freely available via scientific outlets but also through public dissemination events, the internet and via a GoogleEarth based XML interface.