Text / Marmot Dam Removal Geomorphic Monitoring & Modeling Project, Final Report

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Title: Marmot Dam Removal Geomorphic Monitoring & Modeling Project, Final Report
Creator: Charles Podolak
Date Created and/or Issued: 11-Nov
Publication Information: Johns Hopkins University
Contributing Institution: UC Riverside, Library, Water Resources Collections and Archives
Collection: Clearinghouse for Dam Removal Information (CDRI)
Rights Information: Copyrighted
Transmission or reproduction of materials protected by copyright beyond that allowed by fair use requires the written permission of the copyright owners. Works not in the public domain cannot be commercially exploited without permission of the copyright owner. Responsibility for any use rests exclusively with the user.
Description: Scope/Content: Abstract: This report describes the data collection and presents the results from an Oregon Watershed Enhancement Board-funded (OWEB) project, conducted by Johns Hopkins University, the National Center for Earth-surface Dynamics, and Graham Matthews and Associates (GMA) in cooperation with the Sandy River Basin Watershed Council, to monitor the geomorphic changes in the Sandy River, Oregon following the removal of the Marmot Dam in 2007. The 80 kilometer-long (50 mi) Sandy River drains 1,300 km2 (500 mi2) of the western Cascade Range and had been dammed by the Marmot Dam 48 km (30 mi) upstream from the Columbia River since 1913. As part of the decommissioning of the Bull Run Hydroelectric Project Portland General Electric opted to remove the 15 meter-tall (50 ft) concrete capped dam. Its 2007 removal, and subsequent exposure of 750,000 m3 (980,000 yd3) of impounded sand and gravel to fluvial erosion provided an excellent opportunity to measure the response of an energetic gravel-bed river to a temporary increase in sediment available for transport. We partnered with several federal agencies (US Geological Survey, US Forest Service, and the Bureau of Reclamation) and other universities to establish a comprehensive multi-year sediment and geomorphic monitoring program on the Sandy River. The program collected morphologic data (long profile surveys, monumented cross-section surveys, high density local topographic and bathymetric surveys, and LiDAR acquisition), grain size data (surface pebble counts and subsurface bulk samples), photographs, water discharge, and sediment transport measurements (suspended load and bedload). Measurements were taken throughout a 42 kilometer (26 mi) reach from well above the dam site downstream nearly to the Columbia. Our contribution to the group effort primarily focused on measuring 'sediment at rest' during the summer by surveying two downstream reaches and characterizing the grain size distribution of three downstream reaches. Additionally, during the winter high flows we measured 'sediment in motion' at several stations along the river to measure the sediment input into the study reach. By combining our measures of change from 2007 through 2011 and our measures of sediment transport with other contributors data we were able to describe the change in the river bed as a result of the dam removal. While our study focused on the 2007-2010 period, we made one last 2011 topographic survey of the reach immediately below the dam in order to describe the effect of the large flows of January 2011. In reaches 13 km (8 mi) and 20 km (12 mi) downstream of the dam we found no significant overall change in the river bed elevation following the dam removal. We measured a progressive downstream decrease in grain size which was unaffected by the dam removal, and a progressive downstream increase in sand present on the surface - which increased 10 percent after the dam removal. Upstream of the dam we computed an annual sediment load (in a slightly drier than normal year) equivalent to approximately one tenth of the volume of the impounded sediment. Changes in river bed elevation and grain size distributions over the three subsequent years were modest, despite the large discharges which occurred. We were able to show that the timing and gross patterns of erosion and deposition were well predicted by pre-removal numerical models. Our data collected after the Marmot Dam removal allows us to confidently describe the morphologic response of the Sandy River to the Marmot Dam sediment and these findings have been widely shared in local, regional, and national forums. In addition to helping people better understand river processes, this monitoring effort has taught us several lessons about monitoring a large restoration effort and we have used this opportunity to develop a tool to better integrate such monitoring efforts in the future.
Scope/Content: Reservoir size: 928 acre-feet. Height: 30 feet. Length: 4690 feet.
Scope/Content: Date constructed: 1913. Date removed: 2007.
Type: text
Identifier: ark:/86086/n2w66kf7
1191
Subject: Dams
Dam retirement
Place: Sandy River, OR
Marmot Dam

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