The development of structure in coarse-grained river bed sediments: the key to predicting sediment flux

粗粒河床沉积物的结构发育：预测泥沙通量的关键

• 批准号: NE/H021973/1
• 负责人: Joseph Wood
• 金额: $ 2.19万
• 依托单位: City, University of London
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2011
• 资助国家: 英国
• 起止时间: 2011 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FH021973%2F1
• 关键词: development; structure; coarse; grained; river

Rivers are active agents of erosion and deposition, and the movement of sediment represents a logistic and sometimes strategic nuisance that affects road and rail communications, channel stability, river ecosystems and threatens the longevity of water-reservoir capacity. The processes of sediment transport are, however, still unclear to the extent that sediment transport rates are often greatly under- or over-predicted by existing predictive equations. Much of the uncertainty derives from the fact that the river bed is a complex arrangement of grains that is difficult to characterise. River gravels exhibit wide variations in the size, spacing, packing and geometrical arrangement of individual particles and often develop small-scale bedforms such as pebble clusters. Although these characteristics combine to determine the roughness of the bed, the character of the near-bed flow and the flow strength required to initiate sediment movement, alluvial surfaces are generally characterised solely in terms of particle size. Surprisingly little is known about the structural properties, or fabric, of river bed sediments and how they develop and influence flow and sediment transport processes. The aim of this project is to develop a better understanding of bed-surface structures in gravel-bed rivers and the evolution of those structures during flood events. We will conduct a series of flume experiments which will quantify the changing structural properties of gravel surfaces and their influence on flow and sediment transport. The goal of the experiments is to obtain a suite of simultaneous and detailed measurements of flow velocity, bedload movement and surface grain size and structure at a temporal resolution commensurate with time scales of bed adjustment. Photographic and laser scanning techniques will be used to characterise the surface grain size and microtopography of the evolving gravel surfaces. Detailed measurements of the near-bed flow will be obtained by Particle Image Velocimetry and coupled with measurements of sediment movement and transport obtained via conventional trapping techniques. Estimates of sediment entrainment thresholds will be made from measurements of grain pivot angles and protrusion above the bed. This combination of measurements has not been made before and the study represents an important opportunity to extend existing knowledge regarding the nature of bed-surface adjustments in gravel-bed rivers which has, hitherto, largely been restricted to considerations of grain size. Through a better understanding of bed-surface characteristics and the nature of the fluid/sediment interactions that control their development, we will improve our ability to predict sediment transport rates in river systems.

河流是侵蚀和沉积的活跃因素，沉积物的移动会造成后勤甚至战略上的公害，影响公路和铁路交通、河道稳定性、河流生态系统，并威胁水库容量的寿命。然而，沉积物输送过程仍然不清楚，现有的预测方程常常大大低估或高估了沉积物输送速率。大部分不确定性源于这样一个事实：河床是一种难以表征的复杂颗粒排列。河流砾石在尺寸、间距、堆积和单个颗粒的几何排列方面表现出很大的变化，并且经常形成小规模的河床形态，例如卵石簇。尽管这些特征结合起来确定了河床的粗糙度、近河床流动的特征以及引发沉积物运动所需的流动强度，但冲积面通常仅根据颗粒尺寸来表征。令人惊讶的是，人们对河床沉积物的结构特性或结构以及它们如何发展和影响水流和沉积物输送过程知之甚少。该项目的目的是更好地了解砾石河床河床表面结构以及洪水事件期间这些结构的演变。我们将进行一系列水槽实验，以量化砾石表面结构特性的变化及其对水流和沉积物输送的影响。实验的目标是在与床调整时间尺度相称的时间分辨率下获得一套对流速、床荷运动以及表面颗粒尺寸和结构的同步且详细的测量。摄影和激光扫描技术将用于表征不断变化的砾石表面的表面颗粒尺寸和微观形貌。近床流的详细测量将通过粒子图像测速获得，并结合通过传统捕获技术获得的沉积物运动和运输的测量。沉积物夹带阈值的估计将通过测量颗粒旋转角度和河床上方的突出量来进行。这种测量组合以前从未进行过，这项研究提供了一个重要的机会，可以扩展有关砾石河床河床表面调整性质的现有知识，而迄今为止，这些知识在很大程度上仅限于对颗粒尺寸的考虑。通过更好地了解床面特征以及控制其发展的流体/沉积物相互作用的性质，我们将提高预测河流系统中沉积物输送速率的能力。