HomeMy WebLinkAboutLake Oroville Community Update - Assessment of the Vegitation Area Menchaca, Clarissa
From: DWR Oroville Spillways <oroville@water.ca.gov>
Sent: Wednesday, August 30, 2017 3:39 PM
To: Menchaca, Clarissa
Subject: Oroville Dam Vegetation Area Assessment
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August 30, 2017
The California Department of Water Resources (DWR) today published the Assessment of the
Vegetation Area on the face of the Oroville Dam.
This past Friday, DWR shared these findings and conclusions with the independent Board of
Consultants convened for the Lake Oroville Spillways Emergency Recovery Project, who
provided feedback to the Department last week. The Board of Consultants agreed with DWR's
findings that the vegetation is caused by rain and there is no dam safety concern. The BOC
will provide official comments in their next memo which will be posted here.
Highlighted conclusions in the assessment:
• The vegetation area poses no threat to the integrity of the dam. The area is caused by
temporarily trapped rainwater.
• The area was observed in 1966 and 1967 during construction, before the reservoir was
filled. Construction operations during the 1966-67 rainy season resulted in ponding of
water on the fill surface and the creation of stratified lenses or layers within the fill in
the area where vegetation is now present. In addition, rainfall during the 1966-67 wet
season also resulted in small erosion channels, or rills and gullies in the area where
vegetation is now present. These rills and gullies were repaired with cobbles, not
regular dam fill, which allows rain water to percolate into the area rather than drain
quickly down the face of the dam. Numerous inspection reports and aerial
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photographs taken during construction and prior to reservoir filling document the
presence of wet areas seeping rain water that had percolated into the dam.
• Ongoing seepage measurements at the base of the dam have remained consistently
low and virtually unchanged since construction of the dam. Any increase in seepage
through the dam would be noticed almost immediately.
• The vegetation area dries out during the hot summer months, which it would not do if it
had a constant source of seepage through the dam. The vegetation growth cycle
begins again during the next wet season, indicating that it is rain-fed growth.
• The vegetation area is currently brown and dry despite the Lake Oroville lake level
sitting almost 100 feet higher than the vegetation area. If any water did make its way
through the dam's core, it would be intercepted by a vertical drain, preventing it from
flowing to the downstream area of the dam where the vegetation is located.
We hope this assessment clears up any misunderstandings about the integrity of Oroville
Dam. As always, please feel free to reach out if you have any additional questions.
-- Oroville Outreach Team
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