Discussion Topic |
|
This thread has been locked |
tuolumne_tradster
Trad climber
Leading Edge of North American Plate
|
|
|
Feb 17, 2017 - 09:30am PT
|
Holding steady @ ~ 80K cfs Outflow...
|
|
cleo
Social climber
wherever you go, there you are
|
|
|
Feb 17, 2017 - 09:55am PT
|
my point is that this forecast is not used in make flood control decisions.
I'm not so sure that's always true - I notice a ramping up of releases from Folsom Reservoir 3-4 days ahead of the January pineapple express storm (of course that's run by USBR, not DWR).
I've also heard discussion of designing flood control operations taking into account forecasting... which would have the effect of making the operations less conservative, not more conservative. (e.g. more water saved for summer). The old flood operations aren't taking into account forecasting, but they are designed to account for maximum credible storm inflows to protect downstream communities.
Just my 2-cents heresay - if i had more time I'd look into this further.
|
|
Ed Hartouni
Trad climber
Livermore, CA
|
|
|
Feb 17, 2017 - 10:24am PT
|
the current issue with the Oroville dam flood control diagram is nicely illustrated by TT's plot above, and the 1997 weather behavior which was much more like the historic record, that is, after the "unusual" rain in early January, that year settled into the usual snow fall in the Sierra, which released later in the spring and summer. The plot shows that it might be difficult to reach prescribed flood control volume in time to mitigate the next winter storm. This causes problems for the dam operators (having to make use of the "emergency spillway" and to pass the inflow out as quickly as possible, thus flooding downstream areas).
When it rains all winter, the basic assumptions of the history based flood control diagrams is not met. So the designed "conservation pool" has to be managed differently. By filling the reservoir in mid January there was little "leeway" for additional wet winter storms, there have been two, and a third on the way. That coupled with the need to manage the spillway outflow, and to prevent downstream flooding (that is what flood control is about) certainly raise questions (and these are not new questions) regarding the management of the system.
|
|
rick sumner
Trad climber
reno, nevada/ wasilla alaska
|
|
|
Feb 17, 2017 - 10:30am PT
|
Started snowing here at 5500'. At least the precipitation is starting as snow at comparatively low altitude. Keep your fingers crossed.
|
|
cleo
Social climber
wherever you go, there you are
|
|
|
Feb 17, 2017 - 10:39am PT
|
This is what I'm talking about - Folsom operators are bouncing the levels around that TOC line - actively releasing ahead of forecasts. Oroville was not doing that.
There has been a lot of criticism in the past 4-5 years of dam operators releasing water and not saving it during drought. I've heard that most operations guides were developed in the 80s and 90s, before 10-day forecasting became reliable. Many were saying that operations *with forecasting* could run the water levels higher, because they'd have a week or so to dump desperately if a big storm were forecasted.
|
|
WBraun
climber
|
|
|
Feb 17, 2017 - 10:45am PT
|
Makes sense cleo
You and Ed should be the dam operators .....
|
|
cleo
Social climber
wherever you go, there you are
|
|
|
Feb 17, 2017 - 10:46am PT
|
Well there is a DWR job opening...
|
|
tuolumne_tradster
Trad climber
Leading Edge of North American Plate
|
|
|
Feb 17, 2017 - 11:45am PT
|
Check out this post from the metabunk.org blog site RE variability in the quality of the bedrock beneath the spillways...
Rock Whisperer, and others, have reported that the bedrock in the area of both spillways is of variable quality. Some portion of it is competent rock and this appears in the images in a blue gray shade. Other portions appear to be subject to weathering or oxidation such that the rock becomes friable. This weathered rock takes on a reddish / orange hue. One poster with experience rock climbing in the the area around Chico and Oroville suggests the weathered rock will not hold the weight of a human being.
I have some experience dealing with corrosion events which are similar processes to rock weathering. Both are oxidation events. Oxidation may be accelerated by both water and heat. Since California has experienced a number of years of drought it is clear there was an extended period during which little use was made of the flood control spillway. Local rain would still have fallen and this may have passed through cracks in the spillway floor. The cement floor of the spillway would have heated during the day and this may have resulted in heat transfer to any voids beneath the spillway floor. The result may have been accelerated weathering of once competent rock.
If we now move forward in time to the present day with its high levels of rainfall then it is possible some amount of this rainfall percolated into the control joints between abutting slabs. This rainfall may have filled any existing voids and initiated a new drainage path through the weathered rock bypassing the installed drainage system and inhibiting water from exiting the "missing" drain in the image annotated by Mick presented in post 844.
Once the flood control spillway was put back in service the additional water, and the increased head, would have acted to initiate scour along any new drainage path. Over a period of time this would ultimately remove a significant amount of under-burden and enlarge the void to the point that the spillway floor suffered a collapse. Given the volume of water, and the high pressures, any incipient point of failure would quickly be enlarged, the material support for the spillway floor would be removed and the floor slabs would fail, likely collapsing into the void and then being hammered and broken by the water flow. The outcome would be something akin to what may be viewed on the images of the workers inspecting the flood control spillway after the the large void developed.
This also serves to explain why the point of failure has not "marched uphill" toward the sluice control structure. The drainage system above the damaged area is functioning as it should, the water is being drained from beneath the slab and there is no opportunity for the creation of alternate, destructive, drainage path.
|
|
cleo
Social climber
wherever you go, there you are
|
|
|
Feb 17, 2017 - 12:09pm PT
|
As a geotechnical engineer, I think the above analysis is likely spot on...
Minor erosion under the spillway during 4-5 years of drought due to ??? soil collapse or chemical weathering OR due to hillslope saturation/heavy rain -->
Minor concrete cracking and infiltration of water -->
Accelerated Erosion underneath -->
Loss of support under Concrete Spillway + Heavy Water Load (has the spillway been loaded in 5 years?) -->
Collapse.
|
|
Ed Hartouni
Trad climber
Livermore, CA
|
|
|
Feb 17, 2017 - 05:09pm PT
|
...I'm figuring our how to retire and am failing miserably
|
|
aspendougy
Trad climber
Los Angeles, CA
|
|
|
Feb 17, 2017 - 06:31pm PT
|
In 1975, the Banqiao Dam in China failed. There were an estimated 171,000 dead and 11 million people displaced. Don't buy a house below a dam, unless you want to die quickly. The worst one in U.S. history was the Jonestown disaster back in the 1800's. It was in Pennsylvania, about 2,200 died.
|
|
labrat
Trad climber
Erik O. Auburn, CA
|
|
|
Feb 17, 2017 - 06:39pm PT
|
^^Holy crap. I had no idea. 1 in 2000 year flood! From Wikipedia ^^
Typhoon Nina
The heaviest rainfall was recorded along the Banqiao Dam where 1,631 mm (64.2 in) of rain fell, 830 mm (33 in) of which fell in a six-hour span.[10] These rains led to the collapse of the Banqiao Dam, which received 1-in-2000-year flood conditions. In all, 62 dams failed during the disaster
|
|
WBraun
climber
|
|
|
Feb 17, 2017 - 06:50pm PT
|
When I was in Borneo on top of a 1500 foot tower in a typhoon the rain was so powerful and hard it crushed our emergency tent.
The river below rose 10 feet during the night .....
|
|
john hansen
climber
|
|
|
Feb 17, 2017 - 07:04pm PT
|
Finally found some photos of the progress of the repairs below the emergency
spillway. Around 9:30 this morning.
I hope they can get the rip rap all the way along the base to protect the first 100 feet from the spillway at least, before it overflows again.
Click on the photos to enlarge. The one at bottom right shows what is really getting done,, those big areas of concrete could divert the water in unknown ways. Those concrete pumps can do pump an 11 yard truck in less then 15 minutes.. some construction and concrete companies are making some money right now.
https://twitter.com/CA_DWR/status/832644089359192064
|
|
Bargainhunter
climber
|
|
|
Feb 17, 2017 - 07:04pm PT
|
In my personal experience, increased head would definitely have acted to initiate scour along any new drainage path
|
|
c wilmot
climber
|
|
|
Feb 17, 2017 - 07:10pm PT
|
Rip rap or any dry stone structure needs a solidly anchored base to be built upon. I hope I am wrong but I am doubtful those rock piles will hold if the overflow is put to use.
|
|
john hansen
climber
|
|
|
Feb 17, 2017 - 07:43pm PT
|
Wilmot, I feel somewhat the same. I don't think the emergency spillway was built down to bedrock. I also think I read some where it was built on compacted fill , although I cant find where I read it.
To me the problem is when a ravine, or canyon, whatever you want to call it starts eroding it will keep cutting back at the top end, just like in the photos taken before the repairs. They have filled these with boulders , bags and slurry, but this would only funnel water into the area under the center
of the spillway. This could erode back to the base of the structure and undermine the spillway.
I will try to find the source about it being built on compacted earth.
Having done a lot of earth and concrete work, but never on this scale, I would think the spillway would have a pretty big footing maybe 40 feet wide and 10 feet or more deep.
That footing would be flat along the bottom built up with "compacted fill to grade". Probably the same stuff as the main dam.
B.O.F = Bottom of footing on any set of engineering plan.
Some where there is a grading plan for the dam.
I wonder about a topo map pre 1964
|
|
rottingjohnny
Sport climber
Sands Motel , Las Vegas
|
|
|
Feb 17, 2017 - 08:18pm PT
|
Hoping they didn't outsource the compaction to the chinese...
|
|
thebravecowboy
climber
The Good Places
|
|
|
Feb 17, 2017 - 08:27pm PT
|
that is one dam precocious gully
|
|
tuolumne_tradster
Trad climber
Leading Edge of North American Plate
|
|
|
Feb 17, 2017 - 08:33pm PT
|
fyi...
emergency spillway damage...not clear to me what was the original engineering design for the emergency spillway. Whatever it was, it failed miserably in record time.
main spillway damage...
|
|
|
SuperTopo on the Web
|
Let us know!