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

许多砾石层河流的特征是深浅区域的准规则交替，称为浅滩-水池序列，在河流的直线段，经常受到河道宽度变化的影响。这些区域提供了重要的多样性的栖息地，和浅滩池序列的重建可能是河流恢复项目的预期成果。如何池浅滩序列持续是一个世纪的问题，在地貌学仍然没有完全解决，目前缺乏基于实地的，理论的，或实验研究，探索非稳定流，沉积物供应，分选和浅滩池形态之间的关系。本研究旨在通过数值模拟、水槽实验和实地观测来填补这一空白。 该项目将培训一名研究生，让几名本科生参与有监督的独立研究项目，并将活动纳入河流力学和沉积物运输的本科生和研究生课程。本研究将利用二维形态动力学模式探讨河道宽度变化、水文线形状及泥沙供应量变化对地形平衡模式及床面分选的影响。模型的结果将指导移动床，混合粒径的实验中进行的可变宽度水槽。恒定流量和重复过程线下的实验运行将提供非稳定流对浅滩-水池动力学影响的见解，而减少和增加泥沙供应的实验运行将研究浅滩和水池如何受到大坝安装或拆除的影响。实验室规模的模拟和实验将与从美国华盛顿的埃尔瓦河上的浅滩-水池序列收集的数据相补充，在那里，正在进行的大坝拆除项目创造了一个自然实验，其中河流受到大量增加的沉积物供应。

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.