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物组成的，这些堆积物被称为“河床”，包括波纹和沙丘。这些河床存在于一系列尺度上，并且随着流量的变化而不断移动和改变其形状、大小和形式。这些河床是提供水流阻力的主要粗糙元素。因此，河床对流量变化的响应对于预测洪水淹没水平至关重要。流量的变化比河床的变化要快，因此河床通常与水流失去平衡。这一点非常重要，因为我们绝大多数的床相图（将流速和沉积物大小与可能存在的床形态类型联系起来的稳定性场预测器）、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。这是一个明显的缺点，因为正是这些模型被使用，特别是在人口较多的城市地区，以满足对防洪、航行、水电、集料采矿和供水的安全要求。对这些河流的精细化管理至关重要，对设计、实施和监测的准确性提出了很高的要求。如果要改进这些模型，那么就需要对非定常流的床型调整动力学过程以及将这些知识整合到建模实践中的方法有新的基本理解。为了实现这一目标，需要将水力控制、泥沙输运过程的响应和形态调整、形态阻力和河床阻力的变化与一系列非定常流联系起来。一旦建立，这些关系可以用来帮助我们提高对这些动态过程的理解，并更好地预测一组给定流量变化的河流阶段。该项目将通过以下方法来描述这些过程：(i)在最先进的水槽中进行新颖的实验室调查，量化固定和移动河床的流动结构和沉积物运输，（ii）在密西西比河洪水事件期间进行密集的实地调查，绘制和量化河床形态、流动结构和沉积物运输的变化，(3)开发和应用了一种创新的非定常流场在可变形三维边界上的数值模型。这一建模工作将确保结果具有普遍性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、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.