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

河流是侵蚀和淤积的活跃因素，泥沙的移动是一种后勤、有时是战略上的滋扰，影响了公路和铁路通信、河道稳定、河流生态系统，并威胁到水库容量的持久性。然而，泥沙运移的过程仍然不清楚，以至于泥沙运移速率往往被现有的预测方程大大低估或过度预测。这种不确定性在很大程度上源于这样一个事实，即河床是复杂的颗粒排列，很难描述。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差异，并经常形成小规模的河床，如卵石簇。尽管这些特性结合在一起决定了河床的粗糙度、近床面水流的特性以及启动泥沙运动所需的水流强度，但冲积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泥沙输送过程知之甚少。这个项目的目的是为了更好地了解砾石河床的床面结构以及洪水期间这些结构的演变。我们将进行一系列水槽试验，以量化砾石表面结构特性的变化及其对水流和泥沙输送的影响。这些试验的目的是获得一套与床面调整时间尺度相称的时间分辨率的同时、详细的流速、推移质运动以及地表颗粒大小和结构的测量结果。摄影和激光扫描技术将被用来表征演变中的砾石表面的表面颗粒尺寸和微地形。将通过粒子图像测速仪获得近床面流动的详细测量，并结合通过传统捕集技术获得的泥沙运动和输运的测量。泥沙夹带阈值的估计将通过测量颗粒枢轴角和床面上方的突出量来进行。这种测量组合以前从未进行过，这项研究是扩大现有关于砾石河床床面调整性质的知识的重要机会，到目前为止，这一知识在很大程度上仅限于考虑颗粒大小。通过更好地了解河床-表面特征和控制其发展的流体/泥沙相互作用的性质，我们将提高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.