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物流，有时是战略上的麻烦，影响公路和铁路交通、渠道稳定性、河流生态系统，并威胁到水库容量的寿命。然而，泥沙输移的过程仍然不清楚，以至于泥沙输移率常常被现有的预测方程大大低估或高估。这种不确定性很大程度上源于这样一个事实，即河床是一种复杂的颗粒排列，难以描述。河流砾石在单个颗粒的大小、间距、堆积和几何排列上表现出很大的变化，经常形成小尺度的河床，如卵石簇。虽然这些特征结合在一起决定了河床的粗糙度、河床附近水流的特征和引发沉积物运动所需的水流强度，但冲积层表面通常仅以粒度来表征。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运输过程所知甚少。该项目的目的是更好地了解砾石河床河流的河床-地表结构以及这些结构在洪水事件期间的演变。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送的影响。实验的目的是在与河床调整时间尺度相适应的时间分辨率下，获得一套同时和详细的流速、床质移动和表面粒度和结构的测量数据。摄影和激光扫描技术将被用来表征不断演变的砾石表面的表面粒度和微地形。将通过粒子图像测速法获得近床流的详细测量，并通过常规捕获技术获得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枢轴角和床上的突出量来实现。这种测量的组合以前没有做过，这项研究代表了一个重要的机会，可以扩展关于砾石河床河流河床面调整性质的现有知识，迄今为止，这些知识主要局限于考虑粒度。通过更好地了解床面特征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沉积物输运率的能力。

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.