Transport and Mixing at River Confluences

河流汇合处——河流彼此交汇的地方——是自然排水网络的基本组成部分。不同的地形、地质、土壤和流域在排水网络中的土地利用的差异产生了河流的保守性和这些河流所运输的物质的差异。这些不同的水流和水流成分在汇合处汇集在一起，导致河流混合。虽然在过去的几十年里，人们对汇合点的河流动力学进行了深入的研究，但汇合点在保守示踪剂、悬浮沉积物和床质沉积物混合中所起的作用还没有得到全面的研究，而且仍然没有完全理解。拟议的研究是由于这种缺乏理解而产生的，其目的是通过结合现场实验来评估半受控条件下的混合，在水文和形态变化的背景下进行实地调查来评估混合，从而在河流汇合处及其下游的横向混合知识方面提供革命性的改进。以及基于流动和质量输运基本物理原理的三维数值模拟。拟议的研究将旨在直接评估混合和运输过程以及控制因素，以便更好地理解流向导向的垂直细胞和湍流相干结构在这些过程中的作用。获得的数据集将促进定量理论模型的发展，这些模型将描述这些细胞的形成及其与大规模河床形态的相互作用，包括河床不一致对流向导向的垂直细胞的影响。通过对合流动力学的实验结果进行定量评估，研究河床粗糙度和空间非均匀性对压力分布的影响，并采用传统的水动力模型，得出混合速率将直接与传统的水动力特性（如湍流涡流粘度和体积流量参数）联系起来。这项研究的实地研究将在塔利亚门托国际实地研究站进行，并在莱德拉河、奥德-尼斯河和施普雷河汇合处进行。涡流解析数值模拟将扩大实验参数空间的范围和分辨率，并将提供关于混合和输送过程的精细知识，然后将用于扩展河流过程的理论。拟议的研究结果将有许多实际成果，因为它们将允许更好地预测河流系统中污染物、沉积物和生物物质的运输和混合。

River confluences - locations where rivers join one another - are fundamental components of natural drainage networks.Differences in the topography, geology, soils, and land use of watersheds within a drainage network produce differences in conservative properties of river flows and in the materials transported by these flows. These different flows and flow constituents are brought together at confluences, leading to river mixing.Although the fluvial dynamics of confluences have been studied intensively over the past several decades, the role that confluences play in mixing of conservative tracers, suspended sediment, and bed-material sediment has not been examined comprehensively and is still incompletely understood.The proposed research is motivated by this lack of understanding and aims to provide a transformational improvement in knowledge of lateral mixing at and immediately downstream of river confluences through a combination of field experiments designed to evaluate mixing under semi-controlled conditions,field investigations undertaken to assess mixing in the context of hydrological and morphological variability, and three-dimensional numerical modeling based on fundamental physical principles of flow and mass transport.The proposed studies will aim at the direct assessment of mixing and transport processes concomitantly with the controlling factorsin order to better understand the role of streamwise oriented vertical cells and turbulent coherent structures in those processes. The obtained data sets will facilitate development of quantitative theoretical models which will describe formation of those cells and their interactions with the large-scale riverbed morphology including the effect of bed discordance on the streamwise oriented vertical cells. The effects of riverbed roughness and spatial inhomogeneity in pressure distribution will be also explored by carrying out the quantitative assessment of experimental results on confluence dynamics with conventional hydrodynamic models with the outcome that mixing rates will be directly linked to the conventional hydrodynamics characteristics such as turbulent eddy viscosity and bulk flow parameters. The field studies of this research will be performed at the international field research station Tagliamento and on the confluences of rivers Ledra and Oder-Neisse, and Spree. Eddy resolving numerical simulations will expand a range and resolution of the experimental parametric space and will deliver a refined knowledge on the mixing and transport processes which will then be used to expand theory of fluvial processes. The results of the proposed research will have numerous practical outcomes because they will allow better prediction of transport and mixing of pollutants, sediments, and biological substances in fluvial systems.