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  1. MortarBoarder

    MortarBoarder Member

    So perhaps 3-5 days after the storm the levels will be too low for the main spillway. Decreases after that will depend on the power house and associated plumbing (ignoring evaporation). We hear that the power company has plans to fix the power lines fairly quickly, and it looked yesterday as if dredging was about to start. I guess we soon will be watching the helicopters dancing with power towers.
  2. aczlan

    aczlan Member

  3. Shadowwalker

    Shadowwalker New Member

    New member here. First, thank you all for your posts on this subject. Although most are above my head, I still would like to add something I found about earthquakes on the Cleveland Hills fault about the draining of Lake Oroville in 1975 for repairs and the subsequent earthquakes that were created when the lake was refilled. Our drought lowered the lake and has refilled this winter perhaps causing the same conditions. Could this be the trigger for the lower portion of main spillway crumbling?


    From the report:


    The location of Lake Oroville and the areal distribution of historical earthquakes and known faults are shown an figure 1. Three other earthquakes of M 5.0 to 5.9 have occurred since 1900 an or near the Foothills fault system within 60 km of Oroville. The first two occurred in 1909, 60 km east of Oroville (Toppozada and others, 1978) and the third in 1940, 60 km north of Oroville (Bolt and Miller, 1975). Thus, the occurrence of the 1975 M 5.7 earthquake within 70 km of Oroville was not without precedent.

    Two factors suggest that Lake Oroville (maximum depth 220 m; storage capacity 413 billion m3) contributed to both the location and timing of the 1975 earthquake. The first factor is the proximity of the earthquake to the lake, and the extension of the causative fault to the lake as indicated by geologic, seismologic, and geodetic data (Department of Water Resources, 1979). This provides a possible avenue for water under pressure as high as 20 bars, resulting from a water depth of more than 200 meters into the fault zone (Lahr and others, 1976). The second factor is the occurrence of the earthquake following an unprecedented seasonal fluctuation in lake levels. This factor is illustrated in figure 2a, which shows lake levels (in meters above sea level) and number of earthquakes per month within 40 km of Oroville from 1964 to 1976. During the winter of 1974-1975, the lake was drawn down to its lowest level since filling to repair the intakes to the power plant. This unprecedented drawdown and subsequent refilling was followed by the earthquake sequence of 1975.

    I also found there have been 2 micro quakes and 1 quake above the 2.0 (attributed to quarry blasting) over the last week.

    Although the emergency spillway was originally built on “hard” rock could the earthquakes in the vicinity of Oroville since the dam’s creation be the cause of the fissures/faults in the “hard” rock?
  4. JCL

    JCL New Member

    The powerlines are non energized and PG&E expects them to be lost.
  5. CRB93

    CRB93 New Member

    Downstream flooding likely a consideration. Levees discussed earlier in the thread.
  6. Vicki W

    Vicki W New Member

    I have only seen cfs figures for releases... This again highlights the complications of communicating about science.
  7. Mick West

    Mick West Administrator Staff Member

    Did they say that? I'd suspect they would prefer not to change anything until they get the lake down to 850'
  8. yellowsubmarine

    yellowsubmarine New Member

    Croyle said a couple of times, "don't be surprised if you see it (reduced flows), don't worry if you see it". It seems they think the storms coming in won't raise the lake level and they can keep reducing it even at lower outflows, so that maybe they can get down to 850-860 even with the storms. I think he wanted to be very clear that if it happened people should not take it as a sign that the existing spillway is damaged. Maybe they will wait until they get to 850, but the fact he mentioned it here when it would be a couple of days to get to 850 even if no rain makes me think they'll try to reduce it sooner.
    But he also said this is based on the current models they have for the weather and inflows and "there are error bars"...
  9. alec

    alec Member

    One basic framing of this would be that (typically, not always!) erosion increases with CFS. It also increases with S=seconds=Time....the more volume and the longer duration the more erosion. Local , specific results will vary, it's not written in stone.
  10. SeanT

    SeanT Member

    Croyle said by design the max output changes with lake elevation, as levels drop they'll start lowering the output.
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  11. yellowsubmarine

    yellowsubmarine New Member

  12. SeanT

    SeanT Member

  13. MortarBoarder

    MortarBoarder Member

    Previous page of this thread says they're going to try to fly the towers out with helicopters, then bring them back to new footings. Going to move the lines higher up the hill, out of the way of all the mess and forthcoming construction.
  14. If the spillways are constructed on a sheeted dike complex (1), the large variances in rock type associated with this type of formation combined with faulting, folding, and low T metamorphism that occurred during obduction will result in areas of bedrock more prone to chemical weathering than others. The steeply dipping beds will enhance the downward migration of meteoric water, exasperating this differential weathering pattern. Being situated hundreds of feet above the river has left this bedrock exposed to the elements for a million years more or less and has led to chemically weathered bedrock extending far below the surface in places while inches away horizontally is fresh bedrock.

    Even worse would be if the spillways are built into the unit stratigraphically above the sheeted dikes, metavolcanics intruded by the dikes (1). Pillow lavas with their chill margins are fragile when fresh, and are even more susceptible to chemical weathering than the dikes.

    Diabase might be an incredibly strong rock when fresh, but not all of the main spillway is built directly on this competent bedrock as is evidenced by the original blowout and images of the spillways construction. There are many sections of the main spillway directly below the gates that were built on chemically weathered bedrock. With a 100k cfs flowing for the foreseeable future, with higher rates possible considering the long range forecasts, another blowout is a real possibility.

    The emergency spillway is not auxiliary, and i don't believe any competent geologist present during it's construction would have considered its use as anything but a last ditch effort. It was designed as a fail-safe to prevent the loss of the main dam during the event of up to half a million cfs inflow to the lake during a 200 to 1000 year event. The ogee weir extending from the main spillway gates was likely built to protect the main spillway gates. The bedrock was excavated deeper below the ogee weir to find more competent bedrock than the concrete wall extending northwards from the ogee weir, which was built to protect the ogee weir. The lack of concrete in the far NW corner of the parking lot where we saw considerable erosion and helicopters lowering bags of rocks into is not lack of foresight, but designed weakness built into a weak structure. The parking lot is built on highly weathered bedrock and is designed to function as a sacrificial plug located as far from the dam itself as possible, similar to the Auburn coffer dam failure of 1986. As headcutting progresses into the parking lot, water at elevation 900 and above is skimmed off. Once headcutting reaches the lake, then downcutting commences, "safely" lowering the lake till competent bedrock is found, maybe a hundred feet down, leaving the vast majority of the lake still in the lake and Oroville dam still standing, no matter the magnitude of the storm.

    The costs associated with building a sound "auxiliary" spillway due to the amount of bedrock that needed to be excavated to reach the competent stuff, probably discovered after work had begun, could have led to such a design. In the event that the emergency spillway saw huge flows (not a mere 12k cfs), the flows would hydraulically mine out the weak bedrock so that a future main spillway and gates could be constructed that actually would stand the test of time. This was before water quality standards for fish was a concern.

    Footage of helicopters ferrying in bags of rock is made for TV “look, we are doing everything we can” But the bedrock on either side of the ravine they were filling is just as weak as what had been washed out. If water were to return, downcutting would simply erode either side of the reinforcements or choose another channel to erode. The only way to make this emergency spillway an auxiliary with how things currently stand is to rock armor and concrete nearly the entire hilltop, which is what I expect to see done over the course of the next few weeks. But even then, without excavating out the weathered bedrock across the entire emergency spillway to a depth similar to the ogee weir closest the main spillway gates, a blowout similar to the original will be a real possibility.

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  15. Pat Dilling

    Pat Dilling New Member

    Croyle said that the reason they would reduce outlet flow from the spillway has to do with the formed trough on the lake side of the flood control structure (main spillway) and the effect of the water flowing through it as the water level lowers. Perhaps because of the potential for erosion? He stated that was in their operating manual/protocols.
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  16. Mick West

    Mick West Administrator Staff Member


    It's at 14:00 there.
    "If we...continue to remove water from the reservoir at this 100,000 cfs rate then that can affect the rock conditions up front of the flood control structure, and we don't want to compromise that at all. There's a design model when they have reservoir elevation at certain limits they put limits on how much you can discharge at a certain time, and again we'll be stepping that down based on the reservoir water surface elevation to make sure that we operate within the design criteria of that flood control structure."

    So basically as the lake level gets lower they need to reduce the flow - probably because it gets more turbulent in the channel behind the gates.

    I believe this is the graph they will use:

    I've marked 100K cfs, which is the recommended rate at 852 feet. Then down to 50K cfs at 838 feet, and all the way down to 5K at 820 feet

    http://www.water.ca.gov/orovillerel...baCity_Levee Dst1_Appendices_Pages_10-177.pdf
    (Page 107, "Oroville Spillway and Flood Control Outlet Rating Curves" 1963)
    Last edited: Feb 15, 2017
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  17. alec

    alec Member

    as a guess, and only a guess, and not a situation I'd care to make decisions based on guesses.
    If the spillway is not eroding anymore (and I do mean anymore! like ZERO, requires measurements, scale? inch, mm, ) it's conceivable that spillway flow could be increased, perhaps substantially. The upper part of the spillway is currently intact, hopefully sound, and i think i recall, max concrete spillway flow is upward of 250,000cfs. (check this number!)
    The crux of the matter would seem to be why has erosion stopped ? That's distinct from slowed down. Current concrete spillway stability is a godsend in this multi-variate situation. Different flows might produce different results. If the 100,000cfs currently is sufficient to achieve near term desired reservoir capacity, it seems prudent to continue with a stable regime. I believe the stated goal was a 50 foot ( =851' elevation) drop by Wednesday.
    Testing other flows might be a possibility , were erosion to increase as a result (rate dependent... measure this!!! continuously real time!! even now) and stability decrease that would be a high consequence . It's also conceivable that given high inflow conditions, or other developing situations, it might be desirable, or essential, to increase outflow.....knowing that the spigot could be upped a bit , or a lot, might have some value.
    Briefly, back to erosion, it might be worthwhile to go beyond merely measuring the location of the current erosion, but also measure (and I do mean measure!!!, tape , yardstick, photo from fixed safe, repeatable location, laser, GPS....all of the above?? ...real numbers required!!!) downslope , undercut or other soil and rock structures that are currently receding in order to give an early warning that the erosion is proceeding. Likely, if slippage above cutbank is observed even without failure, given flows and precip, it's fair to judiciously project that micro slice of terrain will heed gravity's urging. While keeping an open mind that one's projection may not occur , it might be a useful tool for erring in the direction of prudence.

  18. Mike Smith

    Mike Smith New Member

    Just to reiterate; the Bernoulli effect is not how aerodynamic lift is generated, and it is not how hydrodynamic lift for planing craft is generated either.

    Both generate lift as a reaction to displacing a medium (air, water) downwards.

    [edit] I am hesitant to link directly, but Wikipedia has an excellent treatment of the subject on their Lift (force) page.
    Last edited: Feb 15, 2017
  19. alec

    alec Member

    It sounds to me like they are describing the effect of reduced reservoir water mass on the underlaying or formally underlaying rocks i.e., "isostatic rebound"
    Check me on this please water engineers. (after
  20. MortarBoarder

    MortarBoarder Member

    Look up that term and you'll find it's relevant when there's been a mile of ice on Duluth.
    So it's not applicable until May. :)
  21. Mick West

    Mick West Administrator Staff Member

    Obviosuly they can't release ANY water through the spillway when the lake level get below the sill of the main spillway (Flood Control Outlet), which is 813.6 feet. The lake level also limits how much water will flow through the gate, regardless of how open the gates are. They simply can't do more than a certain amount when the lake is low - not to avoid damage, there just isn't the head of water.
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  22. Mick West

    Mick West Administrator Staff Member

    Yeah, really don't think isostatic rebound is a factor here.
  23. MortarBoarder

    MortarBoarder Member

    I request that further invocation of a Bernoulli concept provide a link to something which mentions its relevance to watercourses. Spillway design guidelines seem to not discuss it.
  24. Ethan O'Connor

    Ethan O'Connor New Member

    I don't have the cite handy, but the isostatic depression after filling the reservoir from empty was about 5cm.
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  25. MortarBoarder

    MortarBoarder Member

    No. There are several studies about Oroville Dam quakes, and any effects were minor. The dam may have settled (and become stronger) a little. One fault jiggled. I've found no mention of an effect on surface rocks. The effects on instruments in the dam were reported.

    Notably, I have seen no mention of movement around the dam. For any project like this, there are a number of survey marks and I'm sure that they checked that nothing had moved. At present, there are three NGS GPS and height marks in the immediate vicinity of the dam.
  26. alec

    alec Member

    thanks on this.
    Is it a correct reading that, worst case , top 100' of reservoir flows out, dam preserved, below 801' elevation, remaining water retained, if all functions "safely"?
  27. Andreas McZane

    Andreas McZane New Member

    Is it only me who thinks that positioning of the two power line towers is faulty as being so close to the main spillway?
    In case of emergency spillway use, there's still a chance that water flows to their direction and washes them out. In case they fall towards the main spillway, I'm pretty sure that creates some damage to the spillway concrete, just enough to initiate a crack.
  28. deirdre

    deirdre Moderator Staff Member

    and you 'know' this how?
  29. [The crux of the matter would seem to be why has erosion stopped ?[/QUOTE]

    Headward migration has encountered competent bedrock again. There are more zones of weathered bedrock underlying the spillway between the original blowout and the gates. The spillway may fail in another spot without needing to headcut its way up. I'm sure DWR is dreading the prospect of a tell tale rooster tail appearing closer to the main gates. They will have eyes on it 24/7, ready to close the gates at a moments notice.
  30. alec

    alec Member

    Not to belabor a minor point, but both the 5cm depression and the earlier cite on the 5.7 quakes , and their probable cause are consistent with reservoir content having an effect (measurable in the first case) on underlaying rocks. Using conventional English , it appears the dam operations manual recognizes this also.
    appreciate any clarity on clearing up my misread. Thanks.
  31. deirdre

    deirdre Moderator Staff Member

    Moderator Note - deirdre
    Please remember Metabunk is an accurate, sourceable information forum. Please remember the 'outside readers' who are reading the thread for verifiable information. Be mindful of not making statements as if they are fact* unless you provide clear sources to back it up or these sources were posted previously.

    *use modifiers such as "i imagine", "i think", "my guess would be.." etc unless you have documentation proving statements.

  32. Legion V

    Legion V New Member

    This is the best theory I have come across explaining the original Main Spillway failure. "Sailing Mark", could you please rewrite the portion of your post that deals with the spillway failure in less technical, more accessible way? Thank You. P.S. If you don't mind sharing, what is your profession?
  33. Ln X

    Ln X New Member

    I have a question about the hillside just below the newly constructed road by the emergency spillway: won't it erode quite a bit under heavy rains?

    Since there is so much exposed topsoil and already gulleys formed, won't the rainfall create streams which make the gulleys a bit deeper and thus take away a bit more of the ground support which helps to prop up the weir section of the dam (immediately adjacent to the emergency spillway?)

    Put it this way, I live in England and even a small storm which lays down an inch of rain in a day will cause even the smallest streams and gulleys to roar with water in the hills and mountains. Throw in all the exposed top soil and there is a danger of small mudslides and possible scouring close to the temporary road. It won't come close to the discharge that happened last Sunday and Monday, but still... The engineers need every bit of the emergency spillway they can and extra erosion will not help their task.

    Heavy rainfall will not help erosion around the newly formed cliffs -- the ones by the main spillway -- either...

    Given this why aren't there dozens of concrete mixers and hundreds of dumper trucks patching up the entire section of eroded emergency spillway? Why hasn't it been done before these new sets of storms arrive?
    Last edited: Feb 15, 2017
  34. stuart little

    stuart little New Member

    i'd imagine rain erosion is last on every bodies list right now. totally irrelevant ... its the foot of the weir and immediately downstream thats the priority and not for rain... but for some odd # of 1000's of cuft going over the wall.
  35. Yup. There's solid bedrock somewhere down there.
  36. Mick West

    Mick West Administrator Staff Member

    Stuff going on near the power station at the base of the dam.
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  37. deirdre

    deirdre Moderator Staff Member

    the part i quoted. is there construction documentation for dams that explains this technique of sacrificial areas?
  38. Scott Gates

    Scott Gates Active Member

    Nope ... there IS a small drain for the basin between weir and new haul road. But it is primarily for rainwater I suspect.

    Culverts cause concentration which causes erosion.

    The whole point of a weir wall like this is maintaining laminar flat flow over the area ... all flowing in one uniform depth and one direction - a flat, uniform sheet - so there are no concentrated flow points to begin erosion. The best way IMO would be to make the top bench below weir flat all the way across ... same with slope down to the road. Harden the bench, the slope to the road, the road base and the slope down from the road .... then let water take its course down the hillside - which we've seen can easily handle the flow ... being largely blue/green bedrock
  39. alec

    alec Member

    Hmmn, guess it's time for a calculation on top 100' of reservoir capacity, just to file back in the databanks. That takes some of the uncertainty out of things.
    Very much appreciate your ultra clear geologic descriptions.
    Thanks metabunk,
    Lastly, anybody find any ultra-clear late afternoon/ early eve imagery? Somehow the idea of high resolution infrared, perhaps even low resolution infrared imaging keeps kicking around in my head as a potentially useful tool to get an early jump on fugitive water flows....probably work well at night too.
  40. Scott Gates

    Scott Gates Active Member

    The use of the emergency spillway was not an auxiliary use. It was put into use because of an emergency prompted by damage to the main spillway.

    I think your find is a good catch - but in this case use as an auxiliary to the main is still emergency use
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