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  1. JCL

    JCL New Member

    Thank you. Also on page 133 in regards to the ESpill: "In part of the emergency spillway, an additional 10' of excavation was required to reach acceptable foundation rock, resulting in considerable time for excavation and placement of the backfill concrete to subgrade".
    This doesn't mention any type of anchoring of the Espill, but they worked hard to get down to prime bedrock, which is good. Did they think this massive structure wouldn't need any type of anchoring if they got to such nice bedrock?

    Also on page 133 when talking about building the main spillway I thought it was interesting that it sounds like they had a harder time with the outlet gate rock they were working with: "The slopes in the flood control outletgate section proved to be lower quality rock than anticipated. There were several large seams running parallel with the chute. The planned anchor bars were replaced with grouted rock bolts, pigtail anchors, and chain-link surface covering in that area".

    I am having a hard time figuring out why the rock at the outlet gate was so different than what they say they worked with to lay the Espill on? It also talks about having to air blast down below the outlet gate to get to foundation rock, but the outlet gate was so different.
    Thank you
     
  2. aczlan

    aczlan Member

    I wonder how endangered the emergency spillway actually was and how much was due to "appearances" of having the erosion near the spillway.
    If they had to rip down to rock like the Peterson Cat history page talks about to place the spillway and emergency spillway, how much danger was there of the spillway actually failing (vs a "possible risk" where people were evacuated because they didn't know what the rock was going to do)

    Aaron Z
     
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  3. CRM114

    CRM114 Member

    I can think of no dam that has had It's emergency spillway tested intentionally. That is not typical for many reasons. One, you don't control the water, and you have to receive a lot of water to test, meanwhile, your users go thirsty if you are intentionally holding water. Two, you'll never get the full design flow so you can fully test. Three, emergency spillways are allowed to have damage, so why damage just to test? Four, intentionally holding water back above flood storage just to test puts people d/s at risk, as Oroville just proved.

    Update: Probably the biggest reason is if you are flowing the emergency spillway, you are at maximum risk - why go intentionally to max risk level to test something if you can't isolate that something?
     
    Last edited: Feb 16, 2017
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  4. SFX

    SFX Member

    That issue has not come up. But now that you mention it, [ i think] that would be a serious problem. If the emergency spillway "wall" failed and a catastrophic amount of water happened, [i think] it would of course damage both the dam and anything else near the river, since the water would flow towards the dam as well as downstream.

    Dam failure might not happen, but the damage would have been a catastrophe. If the rock also eroded where the emergency spillway wall is, so that even more water could be released, it would have been very bad for all things considered.
     
    Last edited by a moderator: Feb 16, 2017
  5. Pozzolith

    Pozzolith Member

    I concur. Had not this failure occurred, we all wouldn't be here in this discussion. Concrete slabs-on-grade never perform well without support. Something changed to cause this void, most likely increased water infiltration. How did it get there? Cracks? Poor joint sealing?
     
  6. Brad P

    Brad P New Member

    [​IMG]

    [​IMG]

    I have never really thought about a sacrificial plug (apparently call a fuse plug) spillway before. Here are a couple of interesting examples. Note that with these structures there is a limit to how much the water level will go down.

    [​IMG]

    This is a better picture the first example.

    Edited to add third picture.
     
    Last edited: Feb 16, 2017
  7. Mick West

    Mick West Administrator Staff Member

    Last edited by a moderator: Feb 16, 2017
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  8. whoosh

    whoosh Member

    Noting that weather forecast QPF includes the moisture in snow (at approx 10:1). NAM 06Z showing that to be about a foot average over next three days, which I read to be the bulk of the well under 2" QPF avg. over the watershed thru noon Sunday.

    Some folks interested in sublimation and snowmelt may find interest in this NOAA site.

    And, while reviewing yesterdays CDWR photos, I call attention to the deferred erosion repair at the face of the weir apron near the right center of this photo.
     
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  9. KenMH

    KenMH Member

    Hi, new member, used to live in Chico and have biked and hiked the emergency spillway area.

    It seems to me that dam was built in the weakest point of a ridge, where water had previously found bands of rock that could be eroded, does this make the area around the dam at more risk for erosion than the areas typical geology? It does not surprise me to see a lot of rotted rock in the images (oranges and reds in images of exposed cuts) they are the reason the river cut through the ridge there in the first place, aren't they?

    Edited:grammar/spelling
     
    Last edited: Feb 16, 2017
  10. Geonerd

    Geonerd New Member

    Last edited: Feb 16, 2017
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  11. SeanT

    SeanT Member

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  12. Mick West

    Mick West Administrator Staff Member

    Well I just hope they include a better safety margin in their estimates this time.

    The outflow chart (as I read it) does not call for less than 100K cfs until around 852 ft, so they must be factoring in main spillway erosion risk as a factor.
    [​IMG]
     
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  13. Scott Gates

    Scott Gates Active Member

    I've found that curious, as well as a similar failure to address the issue at the juncture between the ogee weir and parking lot wall, and the sideslope there. Both areas were responsible for significant erosion. That said they hardened the areas that these locations contributed to the erosion of.

    I also suspect they have - in the interim - dug up construction and soils plans - as we have here - and realized the builders went to considerable effort and expense to insure the ogee weir was on solid fondational bedrock and that their excitement about the scour was probably unwarranted as it did not threaten the bedrock foundation of the weir ...
     
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  14. EricL

    EricL Member

    Not having looked back at the previous few pages yet, I'll just assume that JCL's comments are being mentioned here for the first time. The fact that as much as 10' of additional excavation was performed to reach "acceptable foundation rock" shows that they were indeed paying attention to rock quality, and it may well be that the weir is not as susceptible to being undermined by headward erosion as might be feared (naturally, reinforcing the erosion-prone areas immediately downstream of the weir, as they have been doing, still makes good sense). Second, his quoted document also shows that the weir itself was not placed directly on the existing bedrock, at least not at the locations of significant over-excavation (in the trade we call it "undercutting", not to be confused with the type erosion sharing that name which might occur later). The weir was constructed on top of a layer of "backfill concrete" which was placed to re-establish the planned subgrade. This is a process I had suggested in a much earlier post may have been used, and it's reassuring to see written evidence that it was done that way. Structures of this type, founded on good bedrock (either directly, or indirectly through undercut backfill as described here), do not need to be anchored.

    I spoke to an engineer about the air-blasting procedure, and learned that it's a very old, time-tested method for making it possible to create a watertight seal between the concrete base and the bedrock.

    As to the rock at the outlet gate being different, I can look for the earlier posts if need be, but several people have mentioned the highly variable quality of the bedrock at this site, and the spotty nature of severe erosion below the emergency spillway's weir illustrates that variability. The fact that they used a different anchoring technique at the gates of the main spillway where lower-quality rock was encountered may be somewhat worrisome, in that it implies that they may not have made a great effort to undercut (dig to greater than the planned depth so that the poor-quality rock is removed, then backfill with concrete, as described for parts of the emergency spillway's weir). That may have been the procedure down the length of the spillway as well, as suggested by the great amount of erosion into poor-quality bedrock that occurred so quickly at the location of the failure (I believe that this is likely rock that could have been removed and replaced with concrete at the time of construction, if only the future risk had been recognized). The fact that the erosion appears to have stopped progressing in spite of the strong flow over the last few days suggests that most of the exposed rock is durable, but that pocket of rock which now is gone was not.

    In the designers' defense, at the time of construction they may have been more concerned with simple support of a concrete structure (where the anchoring is typically done simply to prevent it from sliding on such a slope), and did not have a global experience base to tell them that they might need to consider the possible long-term loss of weathered rock beneath the structure as the years went by (it's not always reasonable to expect people to recognize the risk of an event of a type that no one has seen happen before). Since the failure occurred at a location of apparent poor-quality rock, I believe that the most likely cause of the initial failure was the loss of adequate support over time (an experienced soils engineer I've spoken with tells me that as well - but he hasn't written this officially anywhere so that's anecdotal too. Still, I'll try to find photos which illustrate how much different the rock at that location appeared from that of the surrounding, durable rock, before the resumed flow washed it away).
     
    Last edited: Feb 16, 2017
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  15. Mick West

    Mick West Administrator Staff Member

    Interesting current standard practices:
    https://policy.nrcs.usda.gov/OpenNonWebContent.aspx?content=32418.wba


    20170216-104502-f3p6q.

    Dental concreting seems to be what they are doing now.
     
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  16. wrorke

    wrorke New Member

    I interpret this chart as not calling for any particular discharge rate. It does not specify the prescribed discharge rates, but rather it represents the maximum allowable discharge rates. The discharge rate from the main spillway should never exceed 100K cfs if the water is below 852 ft, but discharging at any lesser rate (or even not at all) is allowable.

    You are doing a most excellent job with these threads. Kudos to you.
     
  17. Don Turner

    Don Turner New Member

    I also noted in the original after- construction report the change from a simple rock reinforcement (what might be better described as a rock dowel) to a somewhat more robust system of rock bolts, anchors and chainlink. By today's standards of reinforcement it was an upgrade, but at the time it was typical that rock reinforcement used techniques derived from open pit mining operations. Unfortunately, corrosion of steel in the ground occurs over time, and needs to be replaced. Open pit mines are viewed as a temporary feature and the down slope consequences are not as severe as a dam with a lot of people below it. Today we would use fully grout encased anchors to acount for the predicted design life of the structure.
     
  18. SeanT

    SeanT Member

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  19. JustCurious

    JustCurious New Member

    DWR looking to cut outflow even more by late weekend while NWS is beginning to raise rainfall estimates

    "Incident plans from the DWR, Cal Fire and Butte County Sheriff's Department call for water releases to begin to taper on Saturday, settling Sunday to around 60,000 cubic feet per second."

    Source: http://www.latimes.com/local/lanow/la-me-ln-oroville-weather-forecast-20170216-story.html

    We may get to see what effects this may have on the main spillway by reducing to outflow.
     
  20. KenMH

    KenMH Member

    Is this pipe,
    [​IMG]

    Part of what is indicated as 'drains' in the lower right of this image?
    [​IMG]

    If so, it looks like it has been broken a long time, rusted out as water flowed around the outside. If that were the case I would suspect the flat(ish) discoloured area below might be from water leaking over time from a broken underground drain. If the dam weir drains are leaking into the layered crumbling bedrock over time, well that's not good is it? It's all speculative, thought check please? Sorry if that is to alarmist. It just looks like a section of drain pipe to a junction slightly, and it looks like it's close to where it says 'drains' on that diagram, sooo.... It could be old 1800's mining equipment used as fill for all I know.
     
  21. Mick West

    Mick West Administrator Staff Member

    • Inflow now 25Kcfs, expect inflow to fluctuate between 25K cfs and 45K cfs.
    • 3 Priority areas, presumably in front of the weir.
      • 1 - is 100% complete
      • 2 - 25%
      • 3 - 69% complete
    • Helicopters limited by weather
    • Ramping down output at 5K every 2 hours to 80K, then will stop.
    • Need to reduce debris to bring water level down at the Hyatt power plant as it might be flooded
    • Important to make the plant working again, so they can use it to get water out the lake. 14,000 cfs
    • Also need Hyatt plant to maintain river flow later (when they shut off the main spillway)
     
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  22. Mick West

    Mick West Administrator Staff Member

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  23. KenMH

    KenMH Member

    I have some fears about this that might be typical to many Californians, hopefully they can examined by some experts.

    As a Californian that has hiked that slope, I imagine it just cutting through all the way to the reservoir if water ran over it for even a few weeks.

    From what I know of California geology as an amatuer geology fan and rock climber, the mesozoic volcanics there are highly metamorphosed from faulting, when thinking of bedrock think of rocks that have been shattered and lay together from proximity. The tectonics have taken place after subduction turned to transform faulting so the rock was starting to be uncovered and weight on it was less and less as it was sheared and buckled into ridgelines this river found a way through the weakest points of. So this 'competent' bedrock might be stuff that in other areas would be considered unappropriate.

    Water demands drive public policy, which drives politics, which drive bad decision making on crucial infrastructure... :p

    Having lived both in Tahoe and Chico the rock quality from the granitic plutons to the volcanic metamorphics is just night and day for rock climbing, the metamorphosed stuff won't take a humans weight in most cases before blocks separate.

    That is all amautuer opinion, I have been there but not dug down to bedrock. It's something I have been wanting to ask more educated people like Rock Whisperer about since discovering this thread when this all started as soon as the evacuation order went out. So, any clarification of my 'Californians don't trust rocks to stay in place, or government to know what its doing' fears?

    If anyone outside California wants a quick visual primer on the geology here is a University of California public lecture that covers a lot of history and possible future scenarios for California plate tectonics.
     
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  24. Tom W

    Tom W New Member

    There are a couple of reasons that I can think of off-hand for them wanting to reduce the flow.

    One is to taper downstream flow so that the dams and levees don't get overburdened by the output of Oroville plus local runoff of the coming rainfall. Running a river system is a balancing act of sorts. We have a pretty elaborate one here in
    Tennessee run by TVA. In times of heavy rain, flood control becomes the obvious concern. Otherwise, navigation and power generation are priorities as well.
    The second might be that they want to reduce turbulence at the bottom so that they can clear out/dredge the pathway towards the dam in preparation for re-starting some generation (which will help lower the water level too).
    Third, of course, they probably want to minimize wear and tear on the damaged main spillway.
     
  25. Mick West

    Mick West Administrator Staff Member

    Last edited: Feb 16, 2017
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  26. Jeff Semenak

    Jeff Semenak New Member

    Last edited: Feb 16, 2017
  27. Don Turner

    Don Turner New Member

    Than
    Thank you for the picture. Where does it come from and is there a view more straight on that might show clearly the rock joints and separation of the joints? I would also be looking at the slope for any remnant presplit hole casts indicating the blast charge size and degree of shattering of the back slope from the blast. An overshot slope would indicate too much charge used and resulting openess in the joints. A cursory look at this slope indicates it was adequately constructed for its intended use. I am still interested in the results of the spillway rock excavation and how the final grade looked before concrete placement.
     
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  28. Jeff Semenak

    Jeff Semenak New Member

    Those drains seem to be releasing a fair amount of water in your, before shot.
     
  29. alec

    alec Member

    Again, excellent clear photo Mick. This photo clearly shows the differential erosional qualities of the "bedrock". The piles of orange talus fallen from above tell the whole story and the deep concern about how that would handle large flows for extended periods. The orientation of the fractures is also important.
    It's easy for me to imagine a high flow over "stable bedrock" suddenly quarrying off a large block , which then hammers it's way to the bottom of the gorge.
    An area of concern I've mentioned before is also visible in this image. Follow the rock cut downhill to the clearing that levels out before reaching the pylons. I'm very interested in seeing this area from different perspectives. My concern is the terrain between the concrete spillway and the cut channel in the unpaved emergency spillway, beginning at the earlier roadcut of the road that crosses the spillway. This would come into play only if there were substantial sustained flow over the weir. Likely there are areas of concern (parking lot) that would develop earlier.
    I don't know how to define "substantial flow". I will again mention multiple weeks on the Colorado at less than 10,000cfs and camping above Crystal Falls with the ground vibrating from that flow all night. At 100,000 cfs, that would be 10 low flow(10,000cfs) Colorados over the spillway unto a surface full of soil and not scoured clean. I've seen at Oroville already a brief flow that peaked at 12,000-cfs and it's damage. It'd be nice if that goes down the concrete spillway instead.
    Anybody up in Oroville care to tell us what 100,000cfs pumping out of the spillway feels like up close and personal?
    Last thing on this differential erosional composition of the area: The spillway apparently has reached a meta-stable point at 100,000cfs. I wonder if a few contributing factors to that might have to do with the spillway remnant being on "good" bedrock now (with the distinct random possibility that "bad" bedrock is very nearby) , that there's not a destructive harmonic vibrating at current flow, and that as someone prior suggested that the 100,000 cfs flow is launching the water airborne so far that the water amount in a destructive turbulent pattern and saturating immediately below the earth currently supporting the spillway has been reduced substantially and is partially contributing to the reduction in spillway erosion. (too many words, not enough grammar, apologies)
     
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  30. Mick West

    Mick West Administrator Staff Member

    It's from here.
    https://mng-chico.smugmug.com/Oroville-Week-of-5-9-2016/i-FJMxJTc/A
    I think there's some more side-on shots posted here too.
     
    Last edited: Feb 16, 2017
  31. edi malinaric

    edi malinaric New Member

    Hi Tom - it's not so much that they want to reduce the flow - it's that as the water level drops, the maximum flow that can pass through the spillway automatically reduces.

    The line labelled "Flood Control Outlet Rating" gives you the maximum flow rate through the spillway vs dam water level. Note that this curve is for the condition where the spillway valves are fully open all the time.

    The line labelled "Spillway rating" refers to the auxiliary spillway and gives you the flow rate over the AS as the water level increases beyond 901 feet

    At that point (901 feet) conditions change as the auxiliary spillway joins in and the total discharge follows the line labelled "combined".

    IIRC the water level topped out at 904 feet.

    At that water level the spillway could have passed 270 000 cusecs - but because of concerns about the spillway chute they closed the valves to limit the spillway flow to 100 000 cusecs (~ 37% of maximum possible flow)

    Also at that water level the auxiliary spillway can only discharge 30 000 cusecs.

    Now the water level is going down.

    All the time that the water level has been dropping they have been quietly opening the spillway valves to maintain that 100 000 cusecs discharge.

    When the water level reaches 852 feet the valves will be fully open and from that moment the flow rate will steadily reduce from 100 000 cusecs until it reaches zero at a level of 813.6 feet which is the floor level of the spillway.

    This Mech Eng salutes my Civil Eng colleagues - good going.

    cheers edi
     
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  32. 6850

    6850 New Member

    Historic USGS topographic map, showing the topography of the ridge from the main dam over to the parking lot. You can see that the parking lot area was originally high ground, over 950' in the center.

    With 3 supplemental images
    - Modern topo map
    - Dam features traced onto historic topo
    - Modern topo overlayed onto historic topo


    20170215_NewLabelsOldMap. 20170215_50overlay. 20170215_OldMap. 20170215_moderntopo.



    https://ngmdb.usgs.gov/img4/ht_icons/Browse/CA/CA_Bidwell Bar_288362_1950_24000.jpg
    https://ngmdb.usgs.gov/img4/ht_icons/Browse/CA/CA_Oroville Dam_293795_1970_24000.jpg
    https://ngmdb.usgs.gov/maps/TopoView/viewer/#11/39.5525/-121.3189
     
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  33. KenMH

    KenMH Member

    Could this indicate uncontrolled seepage from deep in the hill, possibly along that orange rotted bedrock seem? They never used the north power plant inlets according to an account here related from a park employee, I wonder if they dug into some really bad rock and that made the north intakes unusable.

    I dont want to speculate further. I can be a bit alarmed at dramatic things, hope I didn't go too far there.
     
  34. EricL

    EricL Member

    I believe you are half right on that. Water most likely contributed to degradation of highly weathered bedrock, and also contributed to the migration of loosened particles, leading to a slight drop in elevation (possibly a very small amount, initially). The original size of the area of reduced floor-slab support might have been significantly smaller than the area which subsequently eroded out, or perhaps it was nearly that size. There's no way for us to know.

    As to keeping water from getting under the slab, that would not be possible in the first place. It's not possible to avoid having cracks develop in a large slab (that's the reason control cuts are made in slabs which need to have a nice appearance, so the inevitiable cracks will form at those sawcuts rather than appear at random. It's likely that this slab too had sawcuts for control of cracking (again, not to eliminate cracking)). If the slab is properly reinforced, the cracks won't present any structural problem, but they are there, and with a thick layer of water flowing down the chute, the head pressure available to force water through those cracks would be significant.

    I am certain that one indication that water under the slab is inevitable and that the designers planned for it, is indicated by those drain outlets in the wall. After I talked with a soils and foundation engineer about the drain system, I (or we, since I agree with the engineer's idea) believe that it's a system designed to transfer water from under the slab back into the spillway. Since any volume of water tends to settle to having a level surface, fully-flooded conditions beneath the slab would result in that water, upon flooding the backfill behind the walls, being at the height of the drain outlets in the wall at some distance down the hill. The result is that each of those drains is removing water from beneath the slab which is at that same level, but higher up the slope. The conduction vessel for that water would be the backfill of the retaining walls on each side. The result of having this series of drains along the length of each wall would be a sloping, or perhaps slightly stair-step pattern of the water elevation within the retaining-wall backfill along the length of each wall. The bottom line is that it seems that a system was put in place to keep accumulated water from developing the enormous head pressure which otherwise could develop beneath the slab at greater and greater distances down the slope, since the head pressure at any given location would be only that of the height of the nearest wall drain. This is a logical interpretation of those drain locations which makes perfect sense based on what the under-slab water might do in their absence (at worst, too much pressure would be applied to the bottom of the slab, and at best, the water would flood out the top of the wall backfill and form a continuous stream along each side of the spillway), but I'd love to find a document describing such a method which I could reference. I believe those drains are an important part of the design.

    I mentioned in an earlier post that flowing water is more likely to erode material below slabs or foundations if the drainage occurs without the protection of filtering backfill material, but in this case, with the erosion likely occurring within a large pocket of native, weathered bedrock at some depth below any structural fill that might have been used (and there may have been no fill at all, just concrete on native rock), the presence or absence of filtering fill becomes a non-issue.
     
    Last edited: Feb 16, 2017
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  35. EricL

    EricL Member

    I believe it makes no difference whether that's one of the drains or some random buried object. The purpose of the drain system beneath the weir is to remove whatever bit of water that inevitably leaks from the upstream side, and finds its way beneath the concrete. Once that water is beneath the structure, it needs to have an easy means of egress to the downstream side so that it doesn't accumulate and become pressurized by full hydraulic contact with the impounded water on the upstream side, and thus be forced to move with enough energy to possibly cause erosion. If the water can simply escape along low-resistance pathways, no damage to the foundation soils/rock will be done.

    Once that drained water has moved away from the weir, normal practice would be to let it go where it wants (downhill). Providing a piped pathway to take that water to some distant location for the express purpose of protecting pockets of weathered rock within the path of the emergency spillway doesn't seem logical in light of the fact that the designers had planned (erroneously, we know now) for that same rock to tolerate the massive flow of a flooding event.
     
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  36. Don Turner

    Don Turner New Member

    Mick- is there supposed to be a link attached here? If I am misunderstanding, forgive me.
     
  37. Mick West

    Mick West Administrator Staff Member

  38. EricL

    EricL Member

    I'm new to the site and I'm not sure how to insert one of my previous posts here which is relevant to your question. It's post #775. To briefly repeat the main premise there, an engineer I talked to is very certain that those drains actually remove water from beneath the slab. Water beneath the slab is contiguous with that in the backfill of the sidewalls, so water at any given location beneath the slab can be drained back into the spillway by the first downhill wall drain that is at that same elevation or lower.

    In the photo you commented on (again, I wish I knew how to re-insert it at this point like site protocol says should be done), there's no flow at all in the spillway, so water that has leaked under the slab is not being replenished, so in turn, water that has been finding its way to the drains is probably already gone from the upper parts of the slope. If the spillway gates stay closed, all of the drains will eventually stop running, but it will happen in a steady progression down the slope.
     
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  39. Mick West

    Mick West Administrator Staff Member

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  40. Junkie

    Junkie New Member

    Right, obviously they *can* release less water than that, the question is why they're doing so.
    Another option is that they think they might run out of water in the lake before they get the power plant back in operation.

    All of the soil that's been washed down will settle somewhere. I suspect if you abruptly cut the flow, it'll settle sooner... and they really don't want it to settle near the dam. By reducing the flow, they can keep the flow active longer and hopefully get the power plant back in action to clear out that section of the river.

    This is just a guess, but it doesn't seem it'll take all *that* long to drain the lake down to spillway level if they keep the flow at 100k as long as they can.
     
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