Investigating the effects of sediment supply, width variation, and unsteady flow on riffle-pool dynamics

许多砾石河床河流的特点是深水和浅水的准规则交替，这种交替被称为冲沟-水池序列，在河流的笔直河段，这种序列经常受到河道宽度变化的影响。这些区域提供了重要的栖息地多样性，重建冲水池序列可能是河流恢复项目所希望的结果。冲刷序列如何持续存在是地貌学中一个尚未完全解决的百年问题，目前缺乏基于现场的、理论或实验研究来探索非恒定流、泥沙供应、分选和冲刷池形态之间的关系。本研究旨在通过数值模拟、水槽试验和现场观测来填补这一空白。该项目将培养一名研究生，让几名本科生参与指导下的独立研究项目，并将活动纳入本科生和研究生的河流力学和泥沙输运课程。对计算机模型的改进将被纳入免费提供的软件包中，以便这项研究的结果可以应用到其他环境中，以解决广泛的科学问题。二维地貌动力学模型将被用来探索河道宽度变化、水位线形状和泥沙供应的变化如何影响地形和床面分选的平衡模式。模型结果将指导在变宽度水槽中进行的移动床、混合颗粒尺寸试验。恒定流量和重复过程线下的试验运行将深入了解非恒定流对冲刷池动力学的影响，而泥沙供应减少和增加的试验运行将调查冲沟和水池可能如何受到大坝安装或大坝拆除的影响。实验室规模的模拟和实验将与从美国华盛顿州埃尔瓦河的冲洗池序列收集的数据相辅相成，在那里，正在进行的大坝拆除工程创造了一个自然实验，在这个实验中，河流的泥沙供应大大增加。

Many gravel-bedded rivers feature quasi-regular alternations of deep and shallow zones known as riffle-pool sequences, which, in straight sections of the river, are often influenced by variations in channel width. These zones provide an important diversity of habitats, and the reestablishment of riffle-pool sequences can be a desired outcome of river restoration projects. How pool-riffle sequences persist is a century-old question in geomorphology that is still not fully resolved, and there is currently a lack of field-based, theoretical, or experimental studies that explore the relationship between unsteady flow, sediment supply, sorting, and riffle-pool morphology. This study aims to fill this gap through numerical modeling, flume experiments, and field observations. The project will train a graduate student, engage several undergraduate students in supervised independent research projects, and incorporate activities into undergraduate and graduate classes in river mechanics and sediment transport. Improvements made to the computer model will be incorporated into freely-available software package so that findings from this study can be applied to other settings for a wide range of scientific questions.A two-dimensional morphodynamic model will be used to explore how river channel width variations, hydrograph shapes, and changes in sediment supply influence equilibrium patterns of topography and bed-surface sorting. Model results will guide mobile-bed, mixed-grain-size experiments conducted in a variable-width flume. Experimental runs under constant discharge and repeat hydrographs will provide insight on the effects of unsteady flow on riffle-pool dynamics, and experimental runs with decreased and increased sediment supply will investigate how riffles and pools might be affected by dam installation or dam removal. The laboratory-scale simulations and experiments will be complemented with data collected from a riffle-pool sequence on the Elwha River, Washington, USA, where an ongoing dam removal project has created a natural experiment in which the river is subject to a greatly increased sediment supply.