Graduate Thesis Or Dissertation
 

On the formation of fluvial islands

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/76537335h

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  • This research analyzes the effects of islands on river process and the effects river processes have on island formation. A fluvial island is defined herein as a land mass within a river channel that is separated from the floodplain by water on all sides, exhibits some stability, and remains exposed during bankfull flow. Fluvial islands are present in nearly all major rivers. They must therefore have some impact on the fluid mechanics of the system, and yet there has never been a detailed study on fluvial islands. Islands represent a more natural state of a river system and have been shown to provide hydrologic variability and biotic diversity for the river. This research describes the formation of fluvial islands, investigates the formation of fluvial islands experimentally, determines the main relations between fluvial islands and river processes, compares and describes relationships between fluvial islands and residual islands found in megaflood outwash plains, and reaches conclusions regarding island shape evolution and flow energy loss optimization. Fluvial islands are known to form by at least nine separate processes: avulsion, gradual degradation of channel branches, lateral shifts in channel position, stabilization of a bar or riffle, isolation of structural features, rapid incision of flood deposits, sediment deposition in the lee of an obstacle, isolation of material deposited by mass movement, and isolation of riparian topography after the installation of a dam. A classification scheme is proposed in order to describe the islands and relate them to the river processes. Several physical experiments were performed to analyze both the processes involved with island formation and the effects islands have on river processes. The experiments performed herein ranged in scale from a 0.45-meter wide flume to the 100-meter wide Willamette River. The analyses describe the effects of islands on flow processes, such as drag force, energy loss, and flow patterns. Previous research has shown that the drag force on a streamlined object in a water flow can be minimized by setting the object's aspect ratio to about three. This research analyzed the flow patterns behind a blunt object in a streamfiow and showed that conditions can be conducive to creating a streamlined, depositional shape with an aspect ratio (length/width) of about three. By introducing islands of various aspect ratios into a streamflow and measuring the flow characteristics, it is shown that the energy loss is minimized with the island's aspect ratio around three. Aerial photographs of fluvial islands were analyzed for thirteen American rivers and a watershed-independent correlation was found for the shape parameters. The average length/width ratio of all analyzed fluvial islands was 4.14. By describing the island shapes with a lemniscate form, the islands were compared with dimensionless properties. The use of dimensionless properties allowed for the analyses of terrestrial fluvial islands to be compared to analyses of fluvially-formed residual islands in unique megaflood areas, such as the Channeled Scablands and Mars Channels. The shape characteristics of the islands were found to be similar, therefore similar relationships between the islands and flow processes are assumed.
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