The effects of sediment supply, width variations, and unsteady flow on riffle-pool dynamics
Date
2018
Authors
Morgan, Jacob A., author
Nelson, Peter A., advisor
Bledsoe, Brian P., committee member
Julien, Pierre Y., committee member
Wohl, Ellen, committee member
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Abstract
Channel geometry, water discharge, and sediment supply work together to influence gravel-bed morphodynamics. How these forcings change and interact affects instream meso-scale geomorphic units, such as riffles and pools, which are often important habitat areas for aquatic organisms. Riffles and pools, defined as vertical undulations in the longitudinal bed profile, are often co-located with variations in channel width and their maintenance in natural systems is often attributed to unsteady flow effects. However, little work has been done to investigate the interaction between unsteady flow and the periodic width variations that often accompany riffle-pool morphology. Surficial sediment sorting, which is largely dependent on sediment supply, is also invoked as an important factor for riffle-pool maintenance. However, there is a lack of studies exploring how riffles and pools respond, or are maintained, in the case of increased sediment supply, such as might be experienced due to dam removal. In general, little is known about how constriction-forced riffles and pools interact with unsteady flow and changes to sediment supply. This dissertation investigates the interplay between channel geometry, discharge, and sediment supply using numerical methods, laboratory experiments, and field exploration. Chapter 2 presents a one-dimensional morphodynamic model which was used to investigate the controls on sediment pulse evolution in coarse-bed rivers. The model uses the standard step backwater method to compute hydrodynamics, calculates bedload, and simulates elevation changes. A stratigraphy submodel retains data related to vertical grain size sorting in the channel subsurface. The results suggest that sediment pulses move downstream with a greater degree of translation with smaller pulse sizes, longer pulse feed times, finer pulse grain sizes, and prolonged higher discharges. In Chapter 3, a two-dimensional morphodynamic model was used to systematically investigate the influence of width variations, unsteady flow, and changing sediment supply rates on equilibrium morphodynamics. Multiple channels with various amplitudes and wavelengths of sinusoidal width variations were modeled under conditions of steady and unsteady discharge and different sediment supply rates. Results suggest that the amplitude of width variations exerts a primary control on riffle-pool relief and that under cycled hydrographs a reversal in the location of maximum shear stress occurs providing a riffle-pool maintenance mechanism. Complementary flume experiments are presented in Chapter 4, where two geometries (constant- and variable-width) were subjected to the same sequential phases of steady flow and constant sediment supply, unsteady flow and constant sediment supply, and unsteady flow and increased sediment supply. Results show that the variable-width channel adjusts to an increased sediment supply by reducing the elevation relief between adjacent riffles and pools and decreased cross-sectional elevation variability, effectively reducing the form drag, rather than increasing the overall bed slope. Finally, Chapter 5 presents a field investigation of the Elwha River downstream of the former Glines Canyon Dam site, using the dam removal as a natural experiment. Three annual topographic surveys were conducted along with hydrodynamic modeling to investigate the impact of increased sediment supply to a natural channel with riffle-pool morphology. Results show aggradation and channel widening have resulted in shallower, slower flows. Field surveys were complemented with historical aerial image analysis which suggests that channel widening and lateral migration rates have increased substantially since dam removal.
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Subject
gravel-bed rivers
riffle-pool sequences
flume experiments
topographic surveying
morphodynamic modeling