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

河流是侵蚀和沉积的活跃媒介，沉积物的移动是一种后勤上的、有时是战略上的滋扰，影响到公路和铁路交通、河道稳定、河流生态系统，并威胁到水库容量的寿命。然而，泥沙输运过程仍不清楚，现有的预测方程往往大大低估或高估泥沙输运率。大部分的不确定性来自于这样一个事实，即河床是一个复杂的颗粒排列，很难被识别。河流砾石在单个颗粒的大小、间距、堆积和几何排列方面表现出很大的变化，并且经常形成小尺度的底形，如卵石簇。虽然这些特性联合收割机共同决定了河床的粗糙度、近床水流的特性和起动沉积物运动所需的水流强度，但冲积表面通常仅以颗粒大小为特征。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运过程知之甚少。该项目的目的是更好地了解砾石河床河流的床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，量化砾石表面结构特性的变化及其对水流和泥沙输运的影响。实验的目的是获得一套同时和详细的测量流速，推移质运动和表面粒度和结构的时间分辨率相称的时间尺度的床调整。将使用摄影和激光扫描技术来确定不断变化的砾石表面的表面粒度和微观形貌。近床流动的详细测量将通过粒子图像流速仪获得，并与通过常规捕获技术获得的沉积物运动和运输的测量相结合。泥沙夹带阈值的估计将通过测量颗粒的枢轴角和床面上方的突出来进行。这种组合的测量还没有作出之前的研究代表了一个重要的机会，以扩大现有的知识的性质，河床调整砾石河床河流，迄今为止，在很大程度上仅限于考虑粒度。通过更好地了解床面特性和控制其发展的流体/沉积物相互作用的性质，我们将提高我们预测河流系统中泥沙输移率的能力。

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.