Discussion Topic |
|
This thread has been locked |
neebee
Social climber
calif/texas
|
|
|
Feb 28, 2017 - 09:30pm PT
|
hey there say, august west, and tuolumne_tradster..
thanks for the updates... oh my...
|
|
tuolumne_tradster
Trad climber
Leading Edge of North American Plate
|
|
|
Feb 28, 2017 - 09:37pm PT
|
There are ultra-mafic (peridotite) and serpentine (hydrated olivine and pyroxene) outcrops east of the Oroville reservoir that are associated with the regional northwest trending Melones fault zone. Serpentine is commonly associated with ophiolites. They represent the basal layer of the Ophiolite Sequence, the upper mantle, that has been hydrated as it ascends up through the crust to the surface along regional, deep seated faults.
One of the largest outcrops of Serpentine in California is the Cedars north of Cazadero associated with the Coastal belt Franciscan Complex.
http://www.supertopo.com/climbing/thread.php?topic_id=1410539&msg=1411905#msg1411905
|
|
tuolumne_tradster
Trad climber
Leading Edge of North American Plate
|
|
|
Feb 28, 2017 - 10:03pm PT
|
If there is that much active erosion going on as the flow stops, why didn't it erode all the way to the gates when the flow was 100K cfs?
The headward erosion that occurred at the emergency spillway is exactly why they called for an evacuation. The ~ 30 ft wall below the emergency spillway was in danger of failure as the high flow rates immediately began eroding that soil and vegetated "emergency spillway" slope that should never have been subjected to this intense erosive force. Every tree that spillway that was toppled on the spillway, created a hole that immediately became a focus for deep erosion and gully formation. At this point, the only option they had was to go back to using the main spillway. In retrospect, we were fortunate that there was enough time between the atmospheric river events that there wasn't a more serious disaster.
Fortunately it appears that the upper part of the main spillway, where it is lower angle before the main drop is underlain by reasonably hard bedrock that remained coupled to the concrete foundation of the spillway. However, where the failure occurred, it appears that the spillway was underlain by "soft" weathered &/or fractured bedrock as mentioned ^^^ that became decoupled from the concrete spillway since it was last seriously tested in 1997. The area of the spillway where it steepens is also where you would expect cavitation forces to be significant. Once this area of the main spillway failed, it diverted the flow off to the right side of the spillway where it eroded a deep gully down to the river removing all the "soft" or fractured bedrock in its way.
http://www.mercurynews.com/2017/02/17/oroville-dam-what-made-the-spillway-collapse/
|
|
healyje
Trad climber
Portland, Oregon
|
|
However, where the failure occurred, it appears that the spillway was underlain by "soft" weathered &/or fractured bedrock...
I guess I'd beg to differ - a lot of the spillway was just poured straight on the dirt that made up the hillside and only part of the original failure area was actually poured with any kind of rock immediately under it. Wouldn't be surprised if the highest underlying rock didn't determine the slope of the whole rig in order to keep costs down.
If it had been in NH no one would have thought twice about blasting the sh#t of the whole raceway and building it on a decent and stable base.
Also, the upper portion of the spillway that's intact for the moment is no doubt as paper-thin and shoddily built as the rest of it was and should all be ripped out for a fresh start - hopefully with a bit more concrete and rebar.
|
|
tuolumne_tradster
Trad climber
Leading Edge of North American Plate
|
|
I guess I'd beg to differ - a lot of the spillway was just poured straight on the dirt that made up the hillside and only part of the original failure area was actually poured with any kind of rock immediately under it. Yes, I agree..."soft" bedrock => weathered bedrock => soil (aka "dirt")
What is left is the extent of competent bedrock...the criminal evidence has left the crime scene and washed away down the Feather River.
|
|
Reilly
Mountain climber
The Other Monrovia- CA
|
|
a lot of the spillway was just poured straight on the dirt that made up the hillside and only part of the original failure area was actually poured with any kind of rock immediately under it.
Bridges have nothing under them, but most are adequately engineered.
|
|
WBraun
climber
|
|
That bridge is bomber.
I wouldn't hesitate to drive over it.
I know all about engineering, I use the best engineering, I know many of the best engineering words.
What's to worry .....
|
|
healyje
Trad climber
Portland, Oregon
|
|
"soft" bedrock => weathered bedrock => soil (aka "dirt")
Thanks, that would definitely escape most layfolk...
|
|
neebee
Social climber
calif/texas
|
|
hey there say, ... wow, guys... thanks for all the updates...
and others share, as to all this...
|
|
neebee
Social climber
calif/texas
|
|
hey there say, a friend of mine, is curious...
is there more updates on the oroville dam?
thanks... :)
|
|
tuolumne_tradster
Trad climber
Leading Edge of North American Plate
|
|
neebee: the WDR website is probably the best place for updated information on the Oroville Dam Spillway Incident. Click on the Oroville Spillway Incident menu item. Each photo includes an explanation on the right side bar.
https://pixel-ca-dwr.photoshelter.com/index
|
|
tuolumne_tradster
Trad climber
Leading Edge of North American Plate
|
|
This is one of the best photos so far that shows the geology beneath where the main spillway failure occurred...
My speculation is that the iron oxide stained rock, immediately beneath spillway failure, contained "soft" bedrock that was chemically altered by hydrothermal fluids possibly while these rocks were still part of a late Jurassic volcanic arc. These altered rocks were more vulnerable to chemical weathering during the last several 100K years in the Sierra foothills and easily eroded once the overlying concrete spillway became decoupled. There also appears to be a structural discontinuity (yellow dashed line), possibly a low angle fault, that could serve as a preferential pathway for modern groundwater flow. The blue-green bedrock beneath the low angle structural discontinuity, appears to be competent, meta-volcanic bedrock.
|
|
Reilly
Mountain climber
The Other Monrovia- CA
|
|
Why was that concrete only about 4" thick?
Did they think it was to be a residential driveway?
|
|
tuolumne_tradster
Trad climber
Leading Edge of North American Plate
|
|
Looks thicker than 4 inches in this photo...
|
|
|
SuperTopo on the Web
|
Let us know!