Condit Dam Removal Condit Dam Removal - Access Washington

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Condit Dam Hydroelectric Project Final Supplemental EIS Table 3-2 (Continued) Proposed Project Schedule Time Period September – October 2008 October 2008 October 2008 – April 2009 November 2008 November 2008 – July 2009 February – April 2009 March – May 2009 March 2009 – March 2010 August 2009 Source: PacifiCorp 2004 Action Remove Obermeyer gates, Tainter gate and radial gates Install temporary water line Northwestern Lake Bridge modification Remove debris from drain tunnel location Remove debris from drain tunnel location Blast tunnel plug and drain reservoir Cofferdam removal Demolish headworks Demolish dam Fill in tail race Remove flowline, penstocks, and surge tank Remove transmission line and substation Demobilization for dam removal activities 3-24
Condit Dam Hydroelectric Project Final Supplemental EIS 4.0 AFFECTED ENVIRONMENT, IMPACTS, MITIGATION MEASURES, AND SIGNIFICANT UNAVOIDABLE ADVERSE IMPACTS 4.1 GEOLOGY, SOILS, AND SEDIMENTS This section describes the affected environment and the effects of dam breaching and removal activities and processes on geology and soils. It also describes sediment movement and potential sedimentation that could occur from the dam removal and other removal activities. For a discussion of the impacts of sedimentation on water quality, please see Section 4.2 (Water Resources). Mitigation for sedimentation effects on the environment is described below, along with significant unavoidable impacts. 4.1.1 Affected Environment The affected environment for geology, soils, and sediments includes Northwestern Lake (Condit Dam reservoir), the area immediately surrounding Condit Dam, access roads to work areas and staging areas, the tailrace from the power plant, the corridor occupied by the wood stave pipe and the power line, and the spillway from the surge tank to the White Salmon River. It includes the White Salmon River and its floodplain from the dam down to and including part of the pool for the Bonneville Dam on the Columbia River. The project site is located on the White Salmon River in an area of rugged topography approximately three miles upstream from the confluence of the White Salmon and Columbia Rivers (USGS 1994). Soils in the White Salmon River Basin are generally characterized as loams and silt-loams derived from the regional basaltic bedrock, andesite, and pumice. Near Condit Dam, these soils are shallow and typically underlain by an intermediate gravel unit above the bedrock. Bedrock outcrops are common in areas of steeper topography along the river (FERC 1996). Immediately west of the dam, the geology is dominated by colluvial deposits, capped by coarse glacial outwash material at the rim of the canyon. East of the dam, the topography is somewhat less severe, supporting a thin veneer of soil over the basaltic bedrock. North of the dam, the soils along the shores of Northwestern Lake are composed principally of the Hood loam (USDA 1990). Bedrock is present over much of the channel of the White Salmon River below the dam. At the time of dam removal, an estimated 2.7 million cubic yards of sediment (2.3 million according to Finley 2006), primarily silt and sand with some gravel, will have accumulated in Northwestern Lake since the construction of Condit Dam in 1913. More than 7,500 feet upstream from the dam and primarily upstream from Northwestern Lake Bridge, the size of reservoir sediments is dominantly in the sand (.075-4.75 mm) and gravel (4.75-75 mm) range. Closer to the dam, the sediments are primarily sand and silt (.0039-.075 mm), and finally silt and clay (
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Condit Dam Removal Final SEPA Suppl
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March 23, 2007 Dear interested part
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Condit Dam Hydroelectric Project Fi
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Appendix A Memorandum of Agreement
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Appendix C Supplemental Aquatic Res
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Agency and Organization Letters and
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Agency and Organization Letters Con
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Contents Public Meetings Response P
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Contents Individuals Individuals A-
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