Oroville Dam Drains in The Spillway Walls - How Do They Work?

[CRM114]

I have been involved in a few spillway designs, and havent run across a design where drains are gushing like this:

Source: https://twitter.com/theCaGuard/status/832014271022133248


I am guessing these drains are connected to the underdrains, so there is a lot of pressure under there that could lift panels.

 

[Ecomorphologist]

In post #43, samfriday asks

Any other comments on the footage?

Content from External Source

. I do have one, more of a question for someone who is knowledgeable about engineering and/or the construction plans. I notice in Alan Kuentz's video, when he zooms out a bit, the spillway is visible in the right foreground, and you can see flow pouring in through drain holes in the side wall. That flow has to be coming from some form of drain system, which is fully charged with water: either perforated pipes in saturated soil some distance to the sides in the hillside, or just local drains, in drain rock just outside the spillway sidewall. Maybe no pipes at all, just the drain rock and something to keep it from coming right through the drain holes. The latter seems implausible to me, if there's drain rock all the way down the very steep hillside, well, that's a really steep gradient, and drain rock percolates very quickly. It would take a LOT of water continually entering that side drain to back up so that every one of those drain holes is flowing. There would be surface flow outside the spillway, up at that level, not just below the breach. On the other hand, if those drain holes are the outfalls for a hillside drain system, it would mean there's a whole lot of water saturating however much of the hillside the drains are in. And continually flowing in. One wonders, is it really raining that hard already there? Whether it is or not, where's the water coming from that's pouring through those drain holes into the spillway?

 

[retired mech eng]

In post #43, samfriday asks

Any other comments on the footage?

Content from External Source

. I do have one, more of a question for someone who is knowledgeable about engineering and/or the construction plans. I notice in Alan Kuentz's video, when he zooms out a bit, the spillway is visible in the right foreground, and you can see flow pouring in through drain holes in the side wall. That flow has to be coming from some form of drain system, which is fully charged with water: either perforated pipes in saturated soil some distance to the sides in the hillside, or just local drains, in drain rock just outside the spillway sidewall. Maybe no pipes at all, just the drain rock and something to keep it from coming right through the drain holes. The latter seems implausible to me, if there's drain rock all the way down the very steep hillside, well, that's a really steep gradient, and drain rock percolates very quickly. It would take a LOT of water continually entering that side drain to back up so that every one of those drain holes is flowing. There would be surface flow outside the spillway, up at that level, not just below the breach. On the other hand, if those drain holes are the outfalls for a hillside drain system, it would mean there's a whole lot of water saturating however much of the hillside the drains are in. And continually flowing in. One wonders, is it really raining that hard already there? Whether it is or not, where's the water coming from that's pouring through those drain holes into the spillway?

I agree with you, and with the same point made by several other posters, that understanding the purpose and configuration of the wall drains is critical for armchair experts such as those on this blog. Can we not ask the authorities for this info? Does someone have a contact?

 

[CRM114]

Lowering flows to remove debris from diversion pool, scheduled to drop to 90,000 CFS at 11 AM

Source: https://twitter.com/CA_DWR/status/832297248423763968


http://cdec.water.ca.gov/cgi-progs/queryF?s=ORO

I agree with you, and with the same point made by several other posters, that understanding the purpose and configuration of the wall drains is critical for armchair experts such as those on this blog. Can we not ask the authorities for this info? Does someone have a contact?

I can tell you these drains are typical in concrete spillways and they drain the water that seeps under the slab or behind the walls. Modern designs try to minimize this, and you would expect only a trickle. The discharge ports are elevated above spillway flow so there is no backflow.

I would be concerned with the amount of flow, because that means significant head loss in the drain pipe. Wherever the water source originates, water pressure is significantly higher than the discharge point. Also, typical designs would have gravel around a perforated pipe as the intake. This much water could indicate that gravel is gone and/or there are voids under the slabs or walls.

 

[Ecomorphologist]

I can tell you these drains are typical in concrete spillways and they drain the water that seeps under the slab or behind the walls. Modern designs try to minimize this, and you would expect only a trickle. The discharge ports are elevated above spillway flow so there is no backflow.

I would be concerned with the amount of flow, because that means significant head loss in the drain pipe. Wherever the water source originates, water pressure is significantly higher than the discharge point. Also, typical designs would have gravel around a perforated pipe as the intake. This much water could indicate that gravel is gone and/or there are voids under the slabs or walls.

Thank you CRM114! Exactly what I knew someone could contribute. It does seem like a lot of flow. Here's a screenshot from the DWR link provided above (http://pixel-ca-dwr.photoshelter.co...QU/DK-Oro-Spillway-damage-4203-02-15-2017-jpg). I think if you establish an identity you can download the jpegs themselves, I just did a screen grab, one of the photos from the opening Act of the drama.

 

[retired mech eng]

I can tell you these drains are typical in concrete spillways and they drain the water that seeps under the slab or behind the walls. Modern designs try to minimize this, and you would expect only a trickle. The discharge ports are elevated above spillway flow so there is no backflow.

I would be concerned with the amount of flow, because that means significant head loss in the drain pipe. Wherever the water source originates, water pressure is significantly higher than the discharge point. Also, typical designs would have gravel around a perforated pipe as the intake. This much water could indicate that gravel is gone and/or there are voids under the slabs or walls.

Thanks for your response. A bit more of my perspective follows (experienced with process safety issues in an oil refinery). Both the primary spill way and emergency spill way are essential "safeguards". They need to be robust, with the level robustness in proportion to the risk (probability x consequence). The safeguards must be "independent", so that the failure of one spill way does not impact the other spill way and does not impact the integrity of the dam. If the primary spill way, above the sink hole, is currently operating with large voids under the slab, and with a large amount of water under the slab, then "robust" is a term that doesn't come to mind. In my opinion (non-expert), the voids must be removed (pump with concrete?) or the complete spill way downstream of the gate needs to be replaced (once the first opportunity presents itself). Comments from others are welcome.

 

[Ecomorphologist]

Stuart Little, I noticed that too. Simplistically, it would seem to indicate that the main source of the water that's flowing out of the drains is high up on the slope, and is running down under the spillway floor or along the outside of the side walls, or both. When that water gets to the breach, it diverts, as water will always do, via the path of least flow resistance: the new ravine to kayaker's left. All other things being equal, if water were entering the drain system all along the whole spillway length, there would be discharge from the ports below the breach too. But one thing that might not be equal is that the flow resistance in the drains is higher up at the top of the spillway, and almost zero in the lower half, this would allow the lower half to drain in seconds or minutes as soon as the inflow at the gates was shut off, while it takes longer for the upper drains to empty (thus, still flowing in the photo). Regardless, one hypothesis about the original cause of concrete failure would seem to be piping of soil and gravel by those subsurface flows, creating poor support for the concrete above, or varying poor support and pressure, thus flexing, which concrete does not like very much.

Commenting on Mick West's photo just above in #58, it looks like the headcut on the left (kayaker's right) seems to have advanced a tiny bit upward relative to a large clump of shrubs there, but it's hard to be sure. That being soil and more-weathered rock than the hard gray stuff, it's easy to imagine that propagating suddenly under just the wrong conditions. But if the foundation under the upper spillway is hard rock close to the concrete, it would look terrifying but perhaps not actually jeopardize the spillway.

The thing about the additional new rainfall is its contribution to saturation and continually increasing pore pressure within the soil. When there is already a head cut scarp, really saturated soil can erode quickly and dramatically, not just progressively by rilling. The long term hillside there didn't have that head cut creating localized increased velocity, and lack of physical support for the soil that forms the scarp. As in a creek channel through a meadow, you can get whole chunks falling off. One never knows for sure, but it is certainly possible for erosion once there's a head cut scarp to happen differently than it did for the long term history of the slope.

 

[KenMH]

It strikes me as odd that the drains are so consistent in output if they are for controlling seepage in the area of the spillway adjacent to them. It appears they have a common source. Has this been worked out previously?

I had thought "maybe they are the drains for the main dams seepage?" but that then seems like doing it the hard way, not likely eh?

 

[stuart little]

"It appears they have a common source"... thats what i thought when i saw none of the bottom drains working after the sidewall blowout.. I figured there were pipes in the walls coming from above.. sorta how it looks

 

[stuart little]

nothing draining above this point in this photo is odd too IMO. I might see one drain above right near the crest that might be flowing ... pic seems odd/interesting.
note: another poster has brought up that if the flow was stopped the drains would cease to flow from top down... and that may be what this is indicating ... a recent shut down of flows and the draining of the system from the top down.. would be normal.

 

[CRM114]

Stuart, all. The drain flows seem diminished relative to when the spillway is flowing. Not sure if this is some residual drainage or a more constant flow. If constant, it would indicate to me it is groundwater fed by the reservoir, and to a lesser extent rain infiltration. Still a lot of flow. There is higher than desired permeability in the units these drain are serving.

Regarding no drainage below the break: there could be a lateral pipe aligned with the axis of the spillway connecting each drain, so they each act as relief of a higher local drainage. If that is the case, then obviously the source of water is above the break.

The pic with the red circle definitely indicates these drains are likely tied together - so water move along the lateral to the lower drains. Also, the difference in flows between the two "spillway off" pics likely indicates the first was influenced by higher groundwater storage. So that first pic is either influenced by local rainfall or groundwater is depleting because the spillway was just shut off.

My bottom-line conclusion from all this is the gushers in the pics I posted at the top of Page 2 are mainly sourced from spillway flows, since there had been little local rain in the days prior to those pics. So there is a lot of water getting under the spillway/walls.

 

[retired mech eng]

Stuart, all. The drain flows seem diminished relative to when the spillway is flowing. Not sure if this is some residual drainage or a more constant flow. If constant, it would indicate to me it is groundwater fed by the reservoir, and to a lesser extent rain infiltration. Still a lot of flow. There is higher than desired permeability in the units these drain are serving.

Regarding no drainage below the break: there could be a lateral pipe aligned with the axis of the spillway connecting each drain, so they each act as relief of a higher local drainage. If that is the case, then obviously the source of water is above the break.

The pic with the red circle definitely indicates these drains are likely tied together - so water move along the lateral to the lower drains. Also, the difference in flows between the two "spillway off" pics likely indicates the first was influenced by higher groundwater storage. So that first pic is either influenced by local rainfall or groundwater is depleting because the spillway was just shut off.

My bottom-line conclusion from all this is the gushers in the pics I posted at the top of Page 2 are mainly sourced from spillway flows, since there had been little local rain in the days prior to those pics. So there is a lot of water getting under the spillway/walls.

Regarding the mysterious drains and the ongoing discussion, I found a photo (summer? year?) where there is a small spill way flow, and yet the drains are clearly dry. To me this is probably saying that the drains were not always used to remove water under the deck, but may have been installed in anticipation of such. I will try and track down the year/month of the photo.

 

[KenMH]

and yet the drains are clearly dry

I see them as clearly draining water down the face of the walls of the spillway, what are we looking at? What am I missing? Aren't the dark vertical streaks evenly spaced along the spillway walls the water from the drains?

 

[Scott Gates]

There is no flow from the drain on the right wall adjacent to where the hole opened. That would imply the pipe to the wall outlet was either blocked or disconnected. A simple blockage would merely increase flow at the other outlets. A disconnected pipe would release water in the area around the break. Such a condition could undermine the slab in that vacinity. It would be of interest to know how long ago (months or years) that outlet stopped flowing when the others were flowing.

I noticed this right off ... not only is there no drain from the right wall but there is also no drain from the left wall over and up one drain:



In the after image there is weak flow with the left drain. The no flow right drain was at the point of damage - and the erosion outside the wall has apparently wiped out the downstream drains as well.

These drains start after the crest of the spillway above. They are placed high in the sidewalls (I presume so they are above the flow when in use. That also however means they are not draining water from outside the walls where drains are located - too close to surface and in the gravel next to the wall.

The logical extension is each one drains water from higher up in the spillway thru gravity.

That the break occurred at exactly the spot of a diagonal line drawn from the poorly operating left drain and the not operating right drain seems too big for coincidence ...

 

[CRM114]

I see them as clearly draining water down the face of the walls of the spillway, what are we looking at? What am I missing? Aren't the dark vertical streaks evenly spaced along the spillway walls the water from the drains?

I have to agree they are draining, albeit more of a trickle.

In response to retired mech engineer: the drains arent just holes in concrete I am about 99% sure. They are connected to a pipe network that goes under the slab that is likely perforated to collect seepage.

 

[KenMH]

I am not sure if this image appeared in the first thread but it belongs here.



here is a closeup (attached file) I made in photoshop, zoomed but not adjusted which I might try later. I circled some areas where it looks like the thicker base is a conduit for water and it's coming out regularly on the outside.

 

[Scott Gates]

Page 133-134: "Spillway drains"

"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-sectional area. These forms were cut to fit the irregular rock surface and remained in place after the concrete was placed over them."

Content from External Source

This fits pretty exactly with the seeming pre-damage failure of the drain system in almost the exact area of the damage.

https://ia800302.us.archive.org/3/i...lirich/zh9californiastatew2003calirich_bw.pdf

 

[CRM114]

I am not sure if this image appeared in the first thread but it belongs here.

here is a closeup (attached file) I made in photoshop, zoomed but not adjusted which I might try later. I circled some areas where it looks like the thicker base is a conduit for water and it's coming out regularly on the outside.

If you look closely, that wall is actually hanging in the air! The water is snaking below the wall. THat picture shows one of the drains, however. I have circled in blue what appears to be a steel 45 degree elbow. There would have been another elbow, then a pipe that headed upstream to a perforated network of pipes, I'm guessing.

 

[CRM114]

Page 133-134: "Spillway drains"

"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-sectional area. These forms were cut to fit the irregular rock surface and remained in place after the concrete was placed over them."

Content from External Source

This fits pretty exactly with the seeming pre-damage failure of the drain system in almost the exact area of the damage.

https://ia800302.us.archive.org/3/i...lirich/zh9californiastatew2003calirich_bw.pdf

Wow, nice find! Good thing they increased the pipe diameter, because these things appear to be at or near capacity when the spillway is flowing. In modern designs these pipes might be sized to carry 10 times what you estimate they will need to carry as a safety factor. There is substantial water moving under this spillway, and substantial uplift pressures. Imagine the head loss as water goes through pores of gravel (if there is any left), into perforations, elbows, etc. then shoots out these pipes like in the first post of this thread. I'm pretty sure this is not what the original designers intended.

Update: e.g. this photo:

 

[KenMH]

If you look closely, that wall is actually hanging in the air! The water is snaking below the wall. THat picture shows one of the drains, however. I have circled in blue what appears to be a steel 45 degree elbow. There would have been another elbow, then a pipe that headed upstream to a perforated network of pipes, I'm guessing.

DUDE!!! Totally hanging in the air!!! Wow! darn low res images...

I see the elbow, seems like pretty clear proof the drains at least terminate nearly horizontally from the uphill section.

 

[Mview]

Fascinating discussion. Good general information on spillway drainage here starting on page 3-120:

Appurtenant Structures for Dams (Spillways and Outlet Works) Design Standard


Chapter 3: General Spillway Design Considerations

Content from External Source

https://www.usbr.gov/tsc/techreferences/designstandards-datacollectionguides/finalds-pdfs/DS14-3.pdf

 

[KenMH]

Fascinating discussion. Good general information on spillway drainage here starting on page 3-120:

Appurtenant Structures for Dams (Spillways and Outlet Works) Design Standard


Chapter 3: General Spillway Design Considerations

Content from External Source

https://www.usbr.gov/tsc/techreferences/designstandards-datacollectionguides/finalds-pdfs/DS14-3.pdf

Nice post, great find. I think Stewart Mountain dam with its dual gated spillways is my favorite damn now.

 

[sushi]

[Broken External Image]:https://mng-chico.smugmug.com/Oroville-Week-of-1-30-2017/i-sZJBSXt/A

The above image was originally posted by Mick. If you compare the right and left sidewalls you will see that all the drains are located opposite each other in the spillway walls. 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.

According to the red circle added by Mick, the missing drain flow is located adjacent to the initial area of damage to the flood control spillway.

 

[sushi]

A photo of the spillway Jan 27, 2017, failure was on Feb 8, just 13 days later. There might be some clues here
https://mng-chico.smugmug.com/Oroville-Week-of-1-30-2017/i-sZJBSXt/A

This spot is the top line of the original hole.

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

 

[aczlan]

Speaking of the drainage system, page 133 of: https://ia800302.us.archive.org/3/i...lirich/zh9californiastatew2003calirich_bw.pdf says:

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.

Content from External Source

Aaron Z

 

[retired mech eng]

Speaking of the drainage system, page 133 of: https://ia800302.us.archive.org/3/i...lirich/zh9californiastatew2003calirich_bw.pdf says:

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.

Content from External Source

Aaron Z

Perhaps this has been addressed elsewhere in the forum, but if the drain pipes are carbon steel, local corrosion can cause failure (as can erosion, etc). I would say those pipes have a limited life, especially where exposed to changing conditions (air, water, minerals, abrasion, dynamic forces, stresses from unsupported spans, etc).

 

[Mick West]

Regarding the history of the drains (i.e if there's been a broken drain that contributed to erosion). This is from May 19 2012:

http://mapio.net/pic/p-56412686/
http://www.panoramio.com/photo/78798367




And this is back in Nov 9 2007

 

[edi malinaric]

[Broken External Image]:https://mng-chico.smugmug.com/Oroville-Week-of-1-30-2017/i-sZJBSXt/A

The above image was originally posted by Mick. If you compare the right and left sidewalls you will see that all the drains are located opposite each other in the spillway walls. 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.

According to the red circle added by Mick, the missing drain flow is located adjacent to the initial area of damage to the flood control spillway.

Hi All

Mmm - if this was the place of a previous "make-good" and during the fix the drain pipe was sheared!!!!!

When was the last spillway repair carried out?

cheers edi

 

[Pozzolith]

Perhaps this has been addressed elsewhere in the forum, but if the drain pipes are carbon steel, local corrosion can cause failure (as can erosion, etc). I would say those pipes have a limited life, especially where exposed to changing conditions (air, water, minerals, abrasion, dynamic forces, stresses from unsupported spans, etc).

I'm not completely ready to give up on my root theory. Perhaps roots could have infiltrated at a joint in a drain line and stopped up flow, or displaced a connection. Roots WILL find moisture.

 

[Pozzolith]

here's the page with the video of this picture

http://www.nbcbayarea.com/news/loca...rops-After-188K-Evacuated-413594893.html?t=28

 

[KenMH]

great image find, clearly shows that is flowing water, strong evidence for a pipe (probably proof of a pipe for many).

 

[retired mech eng]

If you look closely, that wall is actually hanging in the air! The water is snaking below the wall. THat picture shows one of the drains, however. I have circled in blue what appears to be a steel 45 degree elbow. There would have been another elbow, then a pipe that headed upstream to a perforated network of pipes, I'm guessing.

here's the page with the video of this picture

http://www.nbcbayarea.com/news/loca...rops-After-188K-Evacuated-413594893.html?t=28

Thanks for posting this. This photo brings to mind another possible mode (future, and hypothetical only) of failure, that being the failure of the spill way walls, independent from, or prior to failure of the deck. To me (non-expert), this means that if the walls fail at a given location, it is likely that the slab will eventually fail at the same location, which will feed more failures further upstream. In the event that portions of the slab are well anchored to bedrock, the water will try to circumvent these anchored areas, constantly looking for the next upstream weak patch of deck.

 

[sushi]

Thanks for posting this. This photo brings to mind another possible mode of failure, that being the failure of the spill way walls, independent from, or prior to failure of the deck. To me (non-expert), this means that if the walls fail at a given location, it is likely that the slab will eventually fail at the same location, which will feed more failures further upstream. In the event that portions of the slab are well anchored to bedrock, the water will try to circumvent these anchored areas, constantly looking for the next upstream weak patch of deck.

The above image was posted to the main thread. The wall anchors are visible extending down as black lines beneath the wall footer. This suggests the wall segment is/was intact. The void shows an area which has been scoured of either soil or weathered rock. Competent bedrock is shown abutting the near spillway wall.

Other images of the same area of failure suggest an initial under-slab scour proceeding from image left to image right in a diagonal down-slope path beneath the slab. The water cut an opening beneath the still intact spillway wall and the removed detritus was carried out from under the spillway and down along the outside of the north side (image right) spillway wall.

All the evidence seems to suggest a failure of the under-slab drainage system leading to failure of the spillway deck.

Spillway images dating from 2007 show negligible flow from the drain outlets in the vicinity of the spillway deck failure. It is possible the under-slab deterioration proceeded undetected over a period of years.

EDIT
Found the 2007 spillway image up thread and have added it in. The red arrow indicates a drain location which shows no output. This suggests there may have been a problem at this location that went undetected for 10 years.

 

[retired mech eng]

The above image was posted to the main thread. The wall anchors are visible extending down as black lines beneath the wall footer. This suggests the wall segment is/was intact. The void shows an area which has been scoured of either soil or weathered rock. Competent bedrock is shown abutting the near spillway wall.

Other images of the same area of failure suggest an initial under-slab scour proceeding from image left to image right in a diagonal down-slope path beneath the slab. The water cut an opening beneath the still intact spillway wall and the removed detritus was carried out from under the spillway and down along the outside of the north side (image right) spillway wall.

All the evidence seems to suggest a failure of the under-slab drainage system leading to failure of the spillway deck.

Spillway images dating from 2007 show negligible flow from the drain outlets in the vicinity of the spillway deck failure. It is possible the under-slab deterioration proceeded undetected over a period of years.

EDIT
Found the 2007 spillway image up thread and have added it in. The red arrow indicates a drain location which shows no output. This suggests there may have been a problem at this location that went undetected for 10 years.

Thanks for your response. I did not mean to imply that wall failure was the root cause of what we have seen thus far, only that if wall failure were to occur (for various hypothetical reasons) prior to slab failure, that could cause upstream deterioration. I have modified my post #32 above in an attempt to clarify this. A possible future example would be the unsupported wall shown in the photo of post #30.

 

[wrorke]

What is this?

http://pixel-ca-dwr.photoshelter.co...fg/G00003YCcmDTx48Y/I0000p1.rqHqeHzk/BSB-6369




And here ...


http://pixel-ca-dwr.photoshelter.co...fg/G00003YCcmDTx48Y/I0000f2T6yyciD4Y/BSB-6369

One of these for every side drain?



http://pixel-ca-dwr.photoshelter.co...0tw_SKnTXG_k/KG-oro-spillway-damage-10164-jpg

 

[Mick West]

 

[aczlan]

Looks like a cleanout,inspection, or possibly a vent port for the drain system to me. It does not appear to be designed to handle much pressure (based on the thin top which might be perforated and only 3 attachment points).
If I see the elevation properly, those are above the level of the side drains.

Aaron Z

 

[CRM114]

Good find. These do seem to be associated with the drains. Possibly a collector manhole for the pipe network. E.g. - Based on the behavior I have seen, I'm guessing a pipe connects all these manholes upspillway, then a collector pipe diverts off from each manhole under the spillway to collect seepage.

 

[sushi]

Looks like a cleanout,inspection, or possibly a vent port for the drain system to me. It does not appear to be designed to handle much pressure (based on the thin top which might be perforated and only 3 attachment points).
If I see the elevation properly, those are above the level of the side drains.

Aaron Z

A prior post gave evidence that the drain system follows a herringbone layout with the drain lines commencing at the centre line of the FCS and moving diagonally down-slope to the the adjacent right and left FCS side walls.

My hunch is that the diagonal drains pass underneath the sidewall just beneath the fittings shown at the position of the yellow arrows. The drain line then runs down-slope to the first discharge port situated down-slope from the yellow arrow. As Aaron Z suggests these fittings likely serve as clean out, or inspection ports.

If one examines the annotated FCS image above, it is evident that the damage on the left side of the spillway cuts in a diagonal across the spillway surface. I suspect that this diagonal failure of the surface slab follows the path of one of the diagonal drain lines. If a herringbone pattern was overlaid on the image, with the left diagonal starting at the fitting under the arrow to the immediate left of the left toe of the damaged slab, it would reach the center-line of the spillway at a position level with the next up-slope inspection fitting.

Support for the thesis that these inspection ports are connected to each individual drainage line is provided by the damage on the right hand side of the spillway. We have the evident damage to the surface slab of the spillway. This damage halts along a horizontal line extending across the spillway just above the damaged section.

If you inspect the right spillway sidewall it is evident that the sidewall exhibits damage above the transverse horizontal line associated with the damage on the slab. The earth is clearly eroded above the main area of damage at a point on the sidewall where another inspection fitting would be expected to be located. I suspect damage to the drainage system resulted in the obstruction of the discharge port on the right side resulting in drainage water backing up and exiting through the top of the inspection fitting. This helped erode the earth along the right sidewall. As Aaron Z notes, the inspection ports lack any form of sealing mechanism so any back flow due to an obstruction in the vicinity of the discharge port would result in water backing up and flowing out of the inspection hole.

 

[sushi]

Good find. These do seem to be associated with the drains. Possibly a collector manhole for the pipe network. E.g. - Based on the behavior I have seen, I'm guessing a pipe connects all these manholes upspillway, then a collector pipe diverts off from each manhole under the spillway to collect seepage.

If you inspect the image annotated with yellow arrows starting at the left side image bottom, you see a sequence of discharge port flows as follows:

Strong (starting at the very bottom of the image)
Weak
Strong
Very Strong (adjacent to the area of damage
Weak
Strong (second discharge above the damaged area)

If there was a discharge pipe travelling down-slope along the sidewall, I would expect there to be a constant or increasing head the further down-slope the water traveled. Since there is significant variation in the strength of the discharge streams, this suggests there is no lateral connector running down-slope providing a common discharge rail.

The evidence in the image tends to support a series of individual drain lines running diagonally down from the centre of the slab, making a 45 degree down-slope turn beneath the inspection port fitting, and then running down to another 45 degree turn into the discharge port. One of the other pictures of sidewall damage showed a clear image of a 45 degree elbow attached to the exterior of the the sidewall (cannot find that image).

EDIT

Found the image. I believe it is from the right sidewall adjacent to the area of damage. Is

most easily seen when image magnified.