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Description
Scope/Content: Abstract: The 2007 removal of Marmot Dam on Oregon's Sandy River presented a unique opportunity to study the geomorphic ramifications of introducing a large supply of sediment into an energetic gravel-bed river. Since 1913, Marmot Dam has influenced streamflow and impounded approximately 981,000 cubic yards (750,000 cubic meters) of sediment. In 1999, Portland General Electric (PGE) announced its decision to decommission Marmot Dam and in summer 2007, the concrete structure was removed and nearly all of the reservoir sediment was left in place to be subsequently transported by the river. In October 2007, a carefully planned coffer-dam failure was timed to coincide with the first storm of the year expected to produce roughly a 2,000 cfs flow. Following coffer-dam failure, subsequent storms removed a large percentage of the remaining reservoir deposit. Studies of the dam removal project included predictive (modeling) approaches and empirical (monitoring) efforts. The two approaches complement one another, as modeled predictions guided empirical studies (e.g. in choosing which locations were likely to show change) and data obtained from empirical geomorphic monitoring studies will help further refine the models (e.g. by providing data for calibration). Geomorphic monitoring parameters relating to sediment transport included: channel geometry; volume and particle size characteristics of bed sediments; and sediment transport rates and annual loads. Monitoring these parameters supports a sediment-budget approach to explaining how Marmot Dam sediments are routed through the river system. The objective of this 2007-2008 study by Graham Matthews and Associates (under contract to Sandy River Basin Watershed Council for the Oregon Watershed Enhancement Board) was to monitor sediment transport at a point approximately 5.6 miles downstream of Marmot Dam, at the first major expansion below a prominent bedrock gorge. A monitoring site was developed which included a temporary stream gauging station and a cableway from which a cataraft-based sampling platform was deployed during high flows to measure streamflow, suspended sediment concentration and bedload transport rate. These data were utilized to compute a continuous streamflow record, annual bedload and suspended sediment discharges and particle size characteristics of sediment samples. The resulting annual suspended sediment and bedload discharge values were computed at 425,000 tons and 160,000 tons respectively. This total of 585,000 tons represents a volume of approximately 390,000 cubic yards or 300,000 cubic meters. Other researchers will compare this value (1) to the volume of sediment excavated from the reservoir deposit by 2007-2008 winter flows (2) to sediment loads and stored volumes measured at various other points along the river, and (3) to values predicted by models. Water year 2008 presented a relatively modest maximum annual streamflow of roughly 9,000 cfs, well below the 1.5 year average annual peak of 11,400 cfs. Subsequent years, with higher annual peaks, will provide even more valuable sediment load comparisons. Additionally, much of the coarse gravel from the Marmot Dam impoundment has not reached the sampling site. Future measurements will better track the movement of the Marmot Dam sediment and will continue to guide the development of the next generation of mathematical models, thereby facilitating more accurate predictions of how large sediment pulses are routed through river systems following dam removal. This report describes sediment transport monitoring results obtained by Graham Matthews and Associates (GMA), under contract to the Sandy River Basin Watershed Council (SRBWC) and the Oregon Watershed Enhancement Board (OWEB), for the Sandy River above Revenue Bridge for Water Year (WY) 2008. Scope/Content: Dam type: concrete Scope/Content: Reservoir size: 928 acre-feet. Height: 30 feet Length: 4,690 feet Scope/Content: Date constructed: 1908. Date removed: 2008.
Type
text
Identifier
ark:/86086/n25b026g 1207
Language
eng
Subject
Pre- and post-project monitoring Sediment and channel dynamics Dams Dam retirement
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