Monday, October 31, 2011

Breaching of Condit Dam

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2011. All rights reserved.

Last Friday, Condit Dam, on the White Salmon River was intentionally breached by blowing a hole in the spillway as part of a plan to remove the dam and restore the river to its natural state.  Although the actual breaching of the dam was not all that spectacular (i.e. the dam itself wasn't destroyed-only a relatively small hole was blown through the base of the spillway), the drawdown and subsequent erosion of the reservoir was very interesting.  It was all captured in "time-lapse" in a video posted at the following location:


Please enjoy this video.  I'm sure there will be more to come later.

5 comments:

  1. They are going to blow more?

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  2. Not that I'm aware of. From the Pacificorp website, "Once the reservoir is drained, activities will focus on addressing the sediment and slope stability within the former reservoir area. In the spring of 2012, the dam will be excavated and removed along with the flowline, surge tank, and penstocks. Concrete from the dam will be buried onsite; other materials will be salvaged or transported to the Klickitat County waste facility. The powerhouse will be left intact. The upstream cofferdam in the White Salmon River present from original dam construction will be removed from the river as soon as practicable after the breach. PacifiCorp Energy expects to complete the dam removal process within one year, start to finish. Subsequently, restoration of the former reservoir areas may take an additional year or more." http://www.pacificorp.com/condit#

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  3. Any idea why the reservoir wasn't slowly released? I'm not at all familiar with the configuration of the dam or the condition of the low-level spillway but seeing all of that erosion really has me questioning the method of decommissioning Condit Dam.

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  4. I had the same question and last week I emailed one of the engineers in charge of the the decommisioning. Here is his response:

    "One of the significant data sets on which the removal plan for Condit is based is the properties of the accumulated sediment, which did not offer the prospect of long term stability. At the downstream end of the reservoir, the predominant sediment was fine sand and smaller accumulated between steep rock slopes. Since the sediment was expected to erode, the consensus among the settlement parties and the regulatory agencies was to plan for it to mobilize quickly and get to a point of stability as soon as practical. As a free-flowing river, there will certainly be ongoing changes in the riverbed, so stability is defined by reducing public safety risks associated with slopes, sediment mobilization not causing obstructions to fish passage, and steady water quality conditions."

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