Oroville Dam Spillway Failure

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Don Turner

New Member
As I understand it, the spillways were not blasted, they were ripped with D9 Caterpillar bulldozers, then they cleaned out the loose rock:


Aaron Z
A reply to the Peterson Cat reference:
Be careful with using an industry publication as a factual source. They are justifiably proud that they got in on one of the biggest contracts the state had ever issued, and their audience is the other contractors that they might be able to do business with later, or concurrently for that matter. In this case, their numbers don't add up and are massaged for their benefit.
 

Pablo712

New Member
Minor turbulence but no erosion ... that is the profile of land there being exposed...


With erosion I think they refer to not let a rock hit the spillway structure or gates.
The swirl in that photo indicate that the water is passing fast and turbulent enough to "catch" a rock and send it to the spillway.
Slower discharge, less probability of a heavy rock get loose and in any case it will hit at slower speed.

As the spillway channel is "big" and is "cut" in the rock; I think that severe erosion is impossible in the upper channel with those flows levels


Those debris (mainly trees i think) also poses a danger
As we saw in a video pages ago, spillway doors opens from below to above, so reducing the flow also reduce the risk of any of those debris floating being "sucked" down and hit a spillway door; or worse, get stuck in a door opening and then catching an others small debris putting too much force in the door that could finally collapse/ejected and compromise the whole spillway.

EDIT: I saw liners in previous photos...They fail...maybe short ropes that weren't calculated for a 9xx feet deep lake so they ended sumerged and let those trees reach the spillway?



Sorry my bad English. I had been following from Argentina :)
 
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sushi

Member
that the water is actually coming from farther up the slope of the spillway, not from the surrounding land.

Evidence to support this assertion comes from that fact that the inflow to the reservoir from the surrounding drainage basin has declined due to several days without rain.
The drains on the spillway continue to significant flows. If this flow was drainage water from the adjacent hillsides it should have reduced due to lack of recent rains. Study of the images shows the drain flow has remained constant despite the lack of precipitation.
My own interpretation of spillway images is that the flow from the drains is greatest when spillway flows were highest i.e. around 100 k CFS.
 
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AleaEpiGeoRA

New Member
Regarding the discussion in previous posts about venturi effects contributing to the cause of the service spillway failure, a Google search using the words "spillway floor slab jacking Reclamation" returns this link:

https://www.usbr.gov/ssle/damsafety/risk/BestPractices/Chapters/VI-1-20150610.pdf

Reference 3 in this link: "Bureau of Reclamation, Uplift and Crack Flow Resulting from High Velocity Discharges over Open Offset Joints, Report DSO-07-07, Technical Service Center, Denver CO, December 2007" provides derivation of equations based on venturi principles, predictions using the equations, and results from large-scale laboratory experiments that verify the predictions. Modern construction has rubber or copper "water stops" incorporated in all joints between floor slabs, which should prevent this from happening, but the water stops can fail if the offset is too great.

Also, the material surrounding clay-tile pipes of the under-drain system can have improper gradation to stop erodible material from entering the joints between pipes. The pipe joints themselves can open by differential settlement so that, even if the filter material's gradations are proper, the filter material itself can enter the joints and wash away. Great care would have had to have taken place during construction on such a steep slope to ensure differential settlement would not occur.

The level of thoughtful detail and objectiveness of this website are very refreshing. Thank you all.
 
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Clare Krishan

New Member
make an educated guess as to how far below the surface the bedrock lies

I compiled three historic shots Oroville W-shoulder downstream elevations.png posted on twitter by Source: https://twitter.com/buttecohistory/status/831877037748953088
to illustrate my assertion re: modified saddle shoulder elevations and uncertainties re: where bedrock can be found and where infill may be lurking.

At left: in color taken early in construction before the spillway was laid -- natural hillside still predominantly old-stand tree-covered, light grey infill along banks of river (presumably from when diversion tunnel was built to redirect river to allow pouring of concrete seat at base of dam).

At center: topped out dam with disturbances in vegetation pattern on on either side of spillway from heavy equipment transit tracks.

At right: practically naked saddle shoulder covered in light red-colored earth (IMHO indicating more use of in-fill since bedrock schist would be a darker grey-green), with no evidence of gulleys seen on old map topology (for example 1930s era map: http://www.historicaerials.com/ using GPS coordinates 39.538889,-121.485556 ) Where did they go, where does the run-off water they used to transport now go? Of particular interest to me was the NW fork of the creek at the boat ramp (now under the ogee weir of the emergency spillway).
 

Joe Ebeni

New Member
Ok now I'm confused. If 860 is the top of the intake cut, how would they reduce water level by 50' from 901?
Isn't the top of the spillway at 850? Post by Scott Gates shows "Flood Control Elevation 850" and it provides a capacity at that level. So it seems reasonable that the 50' target was the largest feasible drawdown since the power plant flow =0
 

Steve Funk

Senior Member.
Here is an interesting documentary on spillway damage and repair at Glen Canyon Dam. The part relevant to this thread starts about 5:20 in part one of three. It describes the cause of spillway erosion as cavitation. This occurs when imperfections in the surface cause the flow to raise above the surface, creating a short stretch of partial vacuum, followed by a water hammer effect. This was most pronounced where the slope of the spillway changed. That is consistent with what happened at Oroville, since the damage currently appears to start where the spillway gets steeper. It would suggest that the slightly lower slope upstream of the damage would be relatively somewhat more resistant to damage. Source: https://www.youtube.com/watch?v=dHpKvQ9XHV4
 

Mick West

Administrator
Staff member
Isn't the top of the spillway at 850? Post by Scott Gates shows "Flood Control Elevation 850" and it provides a capacity at that level. So it seems reasonable that the 50' target was the largest feasible drawdown since the power plant flow =0

850' is the level at which they are deemed to be in flood control. i.e. above 850 they are managing a flood, below 850 they are not. The bottom of the spillway is 813

850 is also approximately the top of the outflow gates. Below that level they reduce the outflow, but they do not stop it until they get below 820
20170216-224757-kbsha.jpg
 

deirdre

Senior Member.
It describes the cause of spillway erosion as cavitation.
that explains why they mentioned the patches were smooth, i was wondering why that was so important.

“It’s common for spillways to develop a void because of the drainage systems under them,” said Dossey.

While everything may have appeared normal back in 2013, 2014, and 2015; what happened Tuesday when the earth gave way, is anything but.

“There wasn’t any evidence that anything more needed to be done,” said Dossey, “repairs were smooth and looked like they were good and secure.” http://sacramento.cbslocal.com/2017...how-oroville-dam-spillway-previously-patched/
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Paveway IV

New Member
Interesting article from McClatchy on Wednesday, Feb. 15th:
Oroville puts focus on dam spillways – aging and some never tested

Mentions New Don Pedro emergency spillway use in 1997
..During that flood, more than 70,000 cubic feet of water per second poured out of the reservoir, including down the unlined emergency spillway. The torrent scoured out dirt and trees, destroyed a road and dumped debris in the river channel downstream. Afterward, the district had to remove “thousands and thousands and thousands of cubic yard of dirt and debris from the river,” said Curtain...
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Also discusses need for downstream channel sizing to match spillway. Curious about the claimed downstream capacity for Oroville - it seems the 100,000+ cfs release is already problematic.


In and around Modesto, hundreds of homes were flooded, along with three sewage treatment plants, which gushed raw sewage into the Tuolumne River. While use of the emergency spillway prevented water from destroying New Don Pedro Dam, the levees downstream were easily overwhelmed by the 70,000 cfs surging from the reservoir.

Lund, the UC Davis engineer, said the 1997 flood demonstrates how dam spillways and the channels below them must be designed in unison. “In the case of New Don Pedro, the channel is way undersized,” said Lund. He called it “criminal” there isn’t more channel capacity near Modesto, a city of more than 200,000 people.
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The article goes on to mention an unlined emergency spillway being added to Isabella Lake Dam as shown below
Isabella_espill.png

It will use a labyrinth weir instead of the ogee crest weir on Oroville's emergency spillway:

tumblr_mavd8kX71K1qlic7co3_1280[1].jpg


The article also mentions the Folsom Dam Auxiliary Spillway project. This is a concrete dam an relied on its only spillway for flood control. The auxiliary spillway being added is pretty similar to Oroville's main spillway. Some project pictures here.
 
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Mick West

Administrator
Staff member
Also discusses need for downstream channel sizing to match spillway. Curious about the claimed downstream capacity for Oroville - it seems the 100,000+ cfs release is already problematic.

The flooded areas are low lying riverside parks and agricultural land. The region expects occasional flooding. There's always flooded fields round Sacramento.

Overall though the entire system is close to the limits due to rainfall.
 

Scott Gates

Active Member
Once again - construction images - showing 1964 ...




Here is the appx location of the main spillway, emergency spillway and parking lot ...



And here is the construction statement from page 133 - yet again:

"In part of the emergency spillway, an additional 10 feet of excavation was required to reach acceptable foundation rock, resulting in considerable additional time for excavation and placement of the backfill concrete to subgrade."

Page 133
 

sushi

Member
Repairs status as of 8AM Thurs 2/16 ... appears they have filled and armored most of the top bench down to the orig access road to parking lot ...

Source: https://youtu.be/wD7WzYVW4iU

If you look to the bottom right of the video posted by Scott, you can see the plume of water exiting one of the drains on the main flood control spillway. The area of land behind this drain is disrupted and tapers into the area scoured by the water which went over the auxiliary spillway. This area of land, and the contours, appear unlikely to be feeding water to the drains visible in the sidewalls of the main flood control spillway. This would confirm the fact of these sidewall drains as being fed by a drainage system lying either underneath the main flood control spillway, or beneath the back-fill behind the main flood control spillway sidewalls.

This lends support to the thesis that a fault in this drainage system was a factor in the failure of the main flood control spillway. In the images of the main flood control spillway taken during the inspection of the damage it appears as though the subsurface material washed out to the right hand side creating a cavity which undermined the cement floor of the spillway.
 

Clare Krishan

New Member
which carried all the way up to the top of ridge
Apologies for unclear post - will clarify here
the approach wall you are talking about in the photo.
re: erosion site in Figure 182 of gateway elevation cited at https://www.metabunk.org/oroville-dam-spillway-failure.t8381/reply?quote=200941 (to avoid contravening the no click rule, here's an excerpt
upload_2017-2-16_21-39-56.png

The emergency spillway (at left above) isn't "all the way to the top of the ridge" as Scott describes.

Labelled "groin" the bedrock is only 840' in elevation at the base of ogee weir (≠ TOP elevation at 901 ft).

The gap in the approach wall (at top center above) shown in CDRW photo I posted earlier (deleted for Not meeting Posting Guidelines sorry for the inconvenience) demonstrates turbulence but not erosion. Since the wall is only shown in plan (elevation not shown from rear) I can only guess how tall that section of the escarpment cut is - certainly beneath the spillway road at 870' but perhaps higher than the ogee weir at 840'

spillway wall eroded excerpt.png

The right-side of shute labelled "berm" and "cut escarpment" in red above present a risk to the permeable surfaces at 872' rising to 888' when passing water in excess of 901' (ie flood overspill levels across the spillway).

Only 100 ft separate the abuttment of the dam berm rising to 922' indicated behind dashed line labelled "Contract limit" in red (schematic above not photo above). Moisture penetration under pressure could infiltrate slowly via seepage into the dam itself.
 
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sushi

Member
This is the image referred to in the prior post. Apparently Mick has enlarged it to show greater details.

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.
 
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Pozzolith

Member
And as a corollary to that, those highway slabs were notorious for rocking under the continued repeated truck loading. Most have since been replaced with CRCP (continuously reinforced concrete pavement), or by edge doweling.
More on concrete slab control joints from the American Concrete Paving Association.

http://www.acpa.org/wp-content/uplo...oint-Design-and-Construction-FINAL-100214.pdf

A concrete slab WILL crack as it cures, no matter the reinforcing within. The trick is to control the cracking to locations that are planned, and design for load transfer between the now independent slabs with dowel bars.

https://www.fhwa.dot.gov/pavement/concrete/pubs/07021/07021.pdf

These joints are sealed to retard water infiltration, and is a continuous maintenance issue on concrete paved highways.

http://wikipave.org/index.php?title=Joint_Sealing

The purpose of joint sealant is to minimize infiltration of surface water and incompressible material into the joint system [1][2][3]. Sealants also reduce dowel bar corrosion potential by reducing entrance of de-icing chemicals. Pavement engineers have recognized the need for concrete pavement joint sealants for many years. Sealant use dates back to the early 1900's [4][5]. Today, nearly every agency building and maintaining concrete roadways or airports requires joint sealing.
 
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Clare Krishan

New Member
all the way up to the top of ridge.
an additional 10 feet of excavation was required
the approach wall you are talking about in the photo.

How sure are we that excess saddle shaved off to level the parking lot was only 10ft? Using the attendant's cubicle for scale (min 10ft for human occupation) that escarpment cut is closer to 25 to 50 ft.
parking lot saddle.png

We known from Figure 182 posted by Shadowwalker that the neck of the saddle was lower in the plans (I'm not going to try to defend that point a 3rd time: 3 deletions suffice) and attested here. As you can see visible in foreground the ogee crest weir shown in CDWR photo above (dated 2-15; published 2-16 at http://www.dailymail.co.uk/news/article-4226368/Cracks-offer-clues-California-dams-troubles.html) doesn't match the historic construction photos -- perhaps original back fill eroded away by force of overspill? Or the construction of the emergency spillway was not yet final in those old shots.
 
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Pozzolith

Member
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.

I would argue that it would take a significant flow to scour the sub-grade from beneath the slab. Minor infiltration through the cracks we've seen in the pictures likely would not wash out weathered rock. There had to have been some sort of failure to allow sufficient water beneath the slab and initiate scour.

It would be interesting to see some pictures of that spillway under construction.
 

Pozzolith

Member
Here's a pic with a different angle.



Looks to be quite a bit more soil (as opposed to weathered rock) on the side of the initial failure.
 

Scott Gates

Active Member
Apologies for unclear post - will clarify here

re: erosion site in Figure 182 of gateway elevation cited at https://www.metabunk.org/oroville-dam-spillway-failure.t8381/reply?quote=200941 (to avoid contravening the no click rule, here's an excerpt
upload_2017-2-16_21-39-56.png

The emergency spillway (at left above) isn't "all the way to the top of the ridge" as Scott describes.

Labelled "groin" the bedrock is only 840' in elevation at the base of ogee weir (≠ TOP elevation at 901 ft).

The gap in the approach wall (at top center above) shown in CDRW photo I posted earlier (deleted for Not meeting Posting Guidelines sorry for the inconvenience) demonstrates turbulence but not erosion. Since the wall is only shown in plan (elevation not shown from rear) I can only guess how tall that section of the escarpment cut is - certainly beneath the spillway road at 870' but perhaps higher than the ogee weir at 840'

spillway wall eroded excerpt.png

The right-side of shute labelled "berm" and "cut escarpment" in red above present a risk to the permeable surfaces at 872' rising to 888' when passing water in excess of 901' (ie flood overspill levels across the spillway).

Only 100 ft separate the abuttment of the dam berm rising to 922' indicated behind dashed line labelled "Contract limit" in red (schematic above not photo above). Moisture penetration under pressure could infiltrate slowly via seepage into the dam itself.


Just one of many plans - shows the existing grade at 900' ...




Before construction of weir - above Ronnies head .... Soil level at appx 900' next to structure (922 at top)...





Finished ground level reservoir side of emergency weir same height as the top of the approach wall - appx 875 ...


 
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Pozzolith

Member
See discussion on sub-thread "Oroville Dam Main Spillway Waterfall Erosion Watch" at post #41 https://www.metabunk.org/oroville-dam-main-spillway-waterfall-erosion-watch.t8402/page-2#post-200755 re: chemical weathering (oxidation) of schist bedrock *accelerated* during heat of drought years.

Good information there. Also worth pursuing is the possibility of a broken drainage pipe. That condition might provide sufficient flow to initially erode soils outside the flume wall, then underneath the wall, and then finally beneath the spillway slab.
 

aczlan

Member
A reply to the Peterson Cat reference:
Be careful with using an industry publication as a factual source. They are justifiably proud that they got in on one of the biggest contracts the state had ever issued, and their audience is the other contractors that they might be able to do business with later, or concurrently for that matter. In this case, their numbers don't add up and are massaged for their benefit.
Which numbers are incorrect?

Re the drainage under the concrete spillway and the weir, I ran across the following (cross posted in the spillway erosion thread ):
Pages 133/134 of https://ia800302.us.archive.org/3/i...lirich/zh9californiastatew2003calirich_bw.pdf]

Drain System.
The foundation drains designed for the spillway included nearly vertical NX holes drilled 65 feet into the foundation rock of headworks
Monoliths 25 and 26 and extensive perforated pipe systems on the foundation surface under the headworks, chute, and higher portions of the emergency spillway weir.
Much of the drain system on the foundation surface was modified during construction.
The original 4-inch-diameter, horizontal, pipe drains under the chute were redesigned in accordance with a recommendation from the Oroville Dam Consulting Board.
The pipes were placed on a herringbone pattern to give them a downward slope and enlarged to a 6-inch diameter. The longitudinal collector system was enlarged proportionally and modified slightly. The effect of these modifications was to increase the system's capacity and its self-cleaning ability.
The pipes remained on the foundation enveloped in gravel which projected into the reinforced-concrete floor of the chute.
Similar drain pipes were impractical to place on irregular rock surfaces under the headworks and emergency spillway weir. The contractor was allowed to substitute wooden formed drains of equal cross- 133 sectional area. These forms were cut to fit the irregular rock surface and remained in place after the concrete was placed over them.
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Edited to replace the quote tags with ex tags

Aaron Z
 
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sushi

Member

If you view the bottom right of the first few frames of this video you can see one of the sidewall drains in operation. The water is under significant pressure and is seen to jet from the sidewall in volume.

That water appears to be collected from the upstream portion of the intact spillway. This suggests that even an intact spillway will permit significant amounts of water to percolate between adjacent concrete slabs.

If the sidewall drain failed such that the same volume of water was diverted through the underburden beneath the spillway slabs, rather than being removed and returned to the spillway main channel, then it appears likely it would remove significant amounts of material, undercut the spillway, and lead to slab collapse and failure.

The material in the area of the failure (The "waterfall") appears to be unconsolidated material subject to slumping. Such material would be easily washed out from underneath the spillway resulting in the creation of a major void leading to slab failure.

---
Edit
Was attempting to respond to pozzolith's prior post " Minor infiltration through the cracks we've seen in the pictures likely would not wash out weathered rock. There had to have been some sort of failure to allow sufficient water beneath the slab and initiate scour."
 

sushi

Member
Which numbers are incorrect?

Re the drainage under the concrete spillway and the weir, I ran across the following (cross posted in the spillway erosion thread ):


Aaron Z

Great find Aaron.

A 6" pipe will move a significant volume of water. With a herringbone pattern I would presume the pipes commence near the center-line of the spillway with one arm going to the left sidewall and another to the right sidewall.

A failure of one of those 6" drain pipes would flow water through the gravel back-fill and, in those areas not immediately supported by competent bedrock, would permit the erosion of the under-burden beneath the slabs.

The images of the main spillway chute in the area of the "waterfall" show unconsolidated material subject to slumping. The images of the damaged chute show an area of competent blue grey rock downhill from a void devoid of bedrock likely due to the fact this area of the chute was built over weathered rock which was friable and removed by the water stream.
 
How sure are we that excess saddle shaved off to level the parking lot was only 10ft? Using the attendant's cubicle for scale (min 10ft for human occupation) that escarpment cut is closer to 25 to 50 ft.

It says "additional 10 feet". The then-existing saddle was trimmed down to the planned bottom of the emergency spillway weir. But then they found the rock there was bad in places, so they had to go down another 10 feet in places to reach good rock, then filled that gap back up with concrete.
 
Is this pipe,


Part of what is indicated as 'drains' in the lower right of this image?

No, it is not a drain. The drains are still trapped under the weir. It also has been mentioned in this discussion that some of the drains [ were might have been ] fashioned out of wood due to uneven rock shapes.
 
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EricL

Member
Which numbers are incorrect?

Re the drainage under the concrete spillway and the weir, I ran across the following (cross posted in the spillway erosion thread ):
The pipes were placed on a herringbone pattern to give them a downward slope and enlarged to a 6-inch diameter. The longitudinal collector system was enlarged proportionally and modified slightly. The effect of these modifications was to increase the system's capacity and its self-cleaning ability.
The pipes remained on the foundation enveloped in gravel which projected into the reinforced-concrete floor of the chute.
Aaron Z
I found this very interesting, especially since last night I started thinking that a series of localized drains would be far safer than a single inter-connected one (because resistance to flow within the single system could allow high pressures to build up farther down the slope, not just within the drain system but beneath the floor). Anyway, I feel better about the idea since now, since originally, one poster even said the idea was "silly".
A failure of one of those 6" drain pipes would flow water through the gravel back-fill and, in those areas not immediately supported by competent bedrock, would permit the erosion of the under-burden beneath the slabs.
It's true that a break in a pipe could do as you say, but I will point out that that the majority of undermining incidents of more typical structures, in cases where the undermining is associated with drain systems, are due to localized erosion where the flow is entering intact collection pipes, so the water can have great powers of erosion even before it is collected by the system (no link, but a soils engineer I know has dealt with repair of such problems a number of times).
Minor infiltration through the cracks we've seen in the pictures likely would not wash out weathered rock. There had to have been some sort of failure to allow sufficient water beneath the slab and initiate scour.
This is not necessarily correct, since when the spillway is running rather full, the volume of flow associated with every drain is significant, showing that all locations under the floor have water which is being replenished (from floor leaks) just as fast as each individual drain can be seen to remove it. The flow is clearly significant at times.
The right hand sidewall appears to lack a water jet in the expected position of a drain. All the other drains appear to be discharging water. This would suggest a problem with that specific drain.
It could also show that the pocket of weathered rock which later photos showed was present at that location was simply providing an easier route for water to exit that area, leading either out of the spillway area completely, or downhill to the location of the next drain, or some combination of both. Either of these alternatives seem at least as likely, given the large pocket of weathered rock later shown to be at that location, and would give the same result - a non-flowing drain outlet.
Also, the material surrounding clay-tile pipes of the under-drain system can have improper gradation to stop erodible material from entering the joints between pipes. The pipe joints themselves can open by differential settlement so that, even if the filter material's gradations are proper, the filter material itself can enter the joints and wash away. Great care would have had to have taken place during construction on such a steep slope to ensure differential settlement would not occur.
I love seeing that someone already knows this. This problem continues to this day with perforated plastic pipes, at least regarding the aspect of placement of filter material (since differential settlement is no problem with continuous plastic pipe). Proper placement of filter fill is critical, and little things that make such work more inconvenient (as suggested here regarding tile joints) can lead to inconsistencies in work quality.
Reference 3 in this link: "Bureau of Reclamation, Uplift and Crack Flow Resulting from High Velocity Discharges over Open Offset Joints, Report DSO-07-07, Technical Service Center, Denver CO, December 2007" provides derivation of equations based on venturi principles, predictions using the equations, and results from large-scale laboratory experiments that verify the predictions. Modern construction has rubber or copper "water stops" incorporated in all joints between floor slabs, which should prevent this from happening, but the water stops can fail if the offset is too great.
Good information. I hadn't thought of localized flow beneath the slab being driven by a pressure gradient that would drive it up though the cracks.
 
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JCL

New Member
Serious question: In the event of an ESpill failure could it be enough force to carry these bags of rocks downstream enough to contribute to the damage? Could we have a nightmare scenario where we find these bags of rocks strewn about the valley with a complete dam failure?
 

EricL

Member
Here's a pic with a different angle.



Looks to be quite a bit more soil (as opposed to weathered rock) on the side of the initial failure.
It looks to me like there's a bit of weathered rock on the embankment face toward the downhill side of that eroded hole. This, along with the fact that what's on the face of the main portion of the hole looks less like rock and more like soil, suggests that there had effectively been a "trough" of weathered bedrock at this location, sloping downhill away from the viewer. I find myself believing that, given the high flow rates of the drains, there was likely enough moving water present beneath the slab (simply flowing toward the collection points), to cause erosion here, even without any structural failure of the drains themselves. I also wonder what the subgrade at this location looked like back when the structure was built, and what their criteria were for judging acceptability. Perhaps this remark by Sushi explains or partially explains it.
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.

However, regarding this part:
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.
I believe the explanation is probably much more simple. I believe the rock just uphill of this location always was durable, just as the rock is obviously very durable at many other exposed locations down the hill from here (the rock immediately up-slope certainly has a very different appearance from the weathered rock that had been at the location of this pit before additional erosion removed it), and thus not susceptible to significant degradation. And again, the fact that the drain outlet at this location was not discharging immediately before slab failure doesn't suggest to me that drain failure was the initial cause, only that the lack of flow was the end result of easier flow routes already being present within the area of damaged rock. Still, it seems possible that damage to the drain might have occurred (especially with clay tile, if that's what it was) once the rock began to erode.
 

EricL

Member
Serious question: In the event of an ESpill failure could it be enough force to carry these bags of rocks downstream enough to contribute to the damage? Could we have a nightmare scenario where we find these bags of rocks strewn about the valley with a complete dam failure?
I tend to doubt that very much. The gradient of the river valley itself is much flatter than the slope of the emergency spillway. Once the water reaches the base of the river valley, the volume of flow will of course be the same, but the depth of the water will be greater and the speed of the water will be less. Because of the reduced speed, the ability of the water to carry large, dense objects would be much less as well. There might be something I'm overlooking regarding the working cross section of the river channel downstream which would negate the effect I mention here, but there's a big difference between water flowing down a mountain side and water flowing along a much flatter river bed, so for now, that's my perception of things.
 

CRM114

Member
A couple of links:

Speculations as to the cause of failure in the media:
http://www.chicoer.com/general-news...possible-culprit-in-oroville-spillway-failure
- Cavitation
- Drought drying material under slab, creating voids

Geological speculations:
https://www.google.com/amp/www.forb...oroville-dam/amp/?client=ms-android-sprint-us
He call the more orange formation "altered" and that would make it weaker.

There does seem to be something about this spot of the spillway, but correlation is not necessarily causation.
 
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