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Utilizing Lithologic Identification, Repeat Photos, and ArcGIS to Understand the Potential Bedload Contribution from Erosion of a Prominent Cut Bank Cliff
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Description
Scope/Content: Abstract: The Marmot Dam was a hydroelectric diversion structure on the Sandy River in Clackamas County, OR. The dam was recently decommissioned, in October of 2007, by the Portland General Electric (PGE) company, but stood for nearly one hundred years, first as a timber dam then as a modified concrete dam. PGE partnered with multiple local, state, and national agencies to assist in the assessment of the benefits and consequences of the Marmot Dam removal. One of the chief concerns in a project of this nature is the long term affect the removal of the dam will have on the river. Because of this, geomorphologists from Johns Hopkins University, in partnership with the National Center for Earth-Surface Dynamics (NCED), are involved in observing the changes of the Sandy River through the release of the 750,000 cubic meters of sediment (Tanner, 2009) stored in the Marmot Dam reservoir. While observing these changes, an estimated sediment budget was compiled to see what changes to bars, banks, thalwegs, and general river structures have to do with the sediment released from Marmot Dam. To calculate the certainty of this budget, it's important discover if there are any sediment contributors independent of the reservoir sediment below the Marmot Dam site. An obvious place to start checking for outside sediment contributors is erosion of local geologic strata that make up the banks, cliffs, and landforms adjacent to the Sandy River. Though these cliffs and ridges are experiencing failure from fracturing and erosion year-round, much of the sediment contributed from this sort of erosion is fine and suspended sediment. Because fine and suspended sediment will have such a miniscule effect on the changes to river forms and structures, they aren't considered in the sediment budget of the Marmot Dam ongoing study. Instead, this study is interested in bedload sediment, because boulders, cobbles, gravel, and sand each play a much larger role in the shape and form dynamics of the river. This project entails making a basic stratigraphic column of a prominent cliff in a key reach of the ongoing Marmot Dam study. This lithologic data will be used to better understand the compositional materials within each geologic layer and therefore the bedload contributions. Then, using repeat panoramic photos of the cliff area from the past two and half years, it will be possible to broadly calculate the quantity of sediment contributed from cliff landslides. Scope/Content: Dam type: concrete Scope/Content: Height: 30 feet. Length: 4690 feet. Scope/Content: Date constructed: 2013. Date removed: 2007.
Type
text
Identifier
ark:/86086/n2rx9bw9 1213
Subject
Pre- and post-project monitoring Sediment and channel dynamics Dams Dam retirement
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