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.
