Graduate Thesis Or Dissertation

 

Evaluating downstream channel changes in response to a small dam removal on the Calapooia River, Oregon with respect to measurement errors and prior aerial photo observed changes Public Deposited

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/736667168

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  • The objective of this study was to evaluate the use of aerial photos as a substitute for multiple-year pre-removal field data to assess the downstream channel changes associated with a small dam removal. The Brownsville Dam, a 2.1 m tall concrete dam on the Calapooia River, Oregon, was removed in 2007. We surveyed cross sections below the dam during the summers prior to (2007) and follow (2008) removal, and we analyzed aerial photos from 1994 to 2006. The field surveys and aerial photos were used to map the low flow channel and bars over 1.3 km downstream from the dam and in an upstream control reach. Three types of measurement error (position error, identification error, and wetted boundary datum error) were assessed for the field surveys and aerial photos in order make the two methods of measurement more comparable. The first year after the dam removal, we observed changes in bar width, wetted width, and wetted width midpoint as large as 17 m ± 3 m immediately below the dam. We inferred that channel changes below the dam after removal were a result of the removal through comparison with pre-dam removal aerial photos, where changes up to 67 m ± 24 m were predominantly far from the dam, and an upstream control, where changes were only as large as 9 m ± 5 m. We conclude with suggestions of best practices for reducing errors, including establishment of permanent monuments for field surveys, taking measurements at similar, well-known discharges, and generating the highest-quality digital aerial photos through source and scanning choices.
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