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  1. deirdre

    deirdre Moderator Staff Member

    i still dont see how that applies to the parking lot area. But dont try explaining further just for my sake.
     
  2. Scott Gates

    Scott Gates Active Member

    Cool ... seems strange to run all the way across thru the dam .... but looks like it ... pretty sure they are the cut shown in this historic photo - main spillway right above it then emergency spillway runs up the inclide to right - parking lot is edge of trees

    [​IMG]
     
  3. Scott Gates

    Scott Gates Active Member


    A predesigned staged failure point in the dam ... a part of the structure designed to allow failure but to do so slowly so as to spread the release over time ...
     
  4. Scott Gates

    Scott Gates Active Member

    The parking lot area - which only comes into play at levels above the spillway - could be a designed weak point allowing staged failure - at far enough distance away from the dam itself to save the dam.
     
  5. deirdre

    deirdre Moderator Staff Member

    but how does 'weathered bedrock' play into that? i can see the road being a "sacrificial plug", but he lost me with the bedrock talk. takes a long *** time for even weathered bedrock to erode. :) so... he is saying the ROAD is the sacrifical plug?
     
  6. JamesV34

    JamesV34 New Member

    New poster, been following this for a few days.

    I've been to Oroville Dam a few times, most recently twice in 2014 (Feb & Oct) to photograph a variety of spots during the drought.

    A question was raised about the grating partially visible on the lake side of the dam near the spillway. I actually asked someone working at the lake about this when I was there and they are in fact intakes, but from what he told me they were unfinished.

    There are two you can see near the road that leads over the top of the dam on the way to the boat launch lot that's next to the emergency spillway, but apparently at some point a decision was made to build a couple more and at some other point progress was halted on them. Their placement would have been at a higher level than the original ones along the lakeside.

    I don't know if they did anything to finish them and put them into operation between late 2014 and now, but when I was there they didn't even have any of the trash grating the others do.

    If anyone would like to look at what I saw when I visited the two times I mentioned, you can go here, but I'm throwing a couple into the comment. There are some close-ups of those intakes in the Oct gallery. I also explored the Lake Shasta area in order to see a number of spots that were never demolished prior to the dam's completion.

    I've appreciated the information here from the standpoint of an observer, and it's been pretty eerie to see what's happened at Oroville so soon after I checked things out.

    [​IMG]

    [​IMG]
     
    Last edited: Feb 15, 2017
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  7. Scott Gates

    Scott Gates Active Member


    That debris is at appx 902.5 elevation ... top of spillway is 901 .... the road and base is build on top of that
     
  8. JCL

    JCL New Member

    On page 133 in regards to the drain system on the main spillway it mentions this "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"

    Having trouble figuring this out in regards to how this pertains to the emergency situation with the Espillway? Thanks!
     
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  9. MortarBoarder

    MortarBoarder Member

    Yup. "Intake Structure" also on page 121, figure 59, of "California State Water Project, Volume III"
    Again, doesn't show what connects to it. Also, has different intake grille than the stainless steel power house intakes.
     

    Attached Files:

  10. MortarBoarder

    MortarBoarder Member

    Cross section of the weir shows there's a drain underneath the weir, extending from about half-way out to the downstream face. The upstream face of the weir is covered with rubber. So the intent is to keep the concrete dry (preventing wet concrete, which would rot the steel rebar and destroy the weir). Rubber keeps out the dam's water, and drains carry away any water from underneath.

    Edit: Image from four pages back...
    [​IMG]
     
    Last edited: Feb 15, 2017
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  11. MortarBoarder

    MortarBoarder Member

    As rock whisperer pointed out, some of the rock is weak weathered rock and some is hard rock. Speculation, sorry, based on what he pointed out.

    That entire parking lot area (although no parking lot is shown in most construction diagrams) is fairly flat, gently sloping down to behind the emergency spillway weir. The low wall which is an extension of the weir can't take much pressure, but it doesn't have to.

    When the emergency spillway weir is overtopped, the water on the parking lot is slowly moving around the weir, across the low concrete wall, then across yards more of rock, sort of. At low overflow levels, water tends to run back toward the weir. But there's not much room in the vicinity of the road. With several feet of water crossing there, only some of it can tumble back down toward the weir, which limits that route. By the time there's 10-20 feet of water (thanks, topographic map!), water is a wide river flowing across the parking lot, hardly bothered by the little bump, some coming back toward the weir, and some flowing down the next ravine over.

    At some point, the weak weathered rock at the edges gets cut away and starts wearing upstream, and this concentrates the flow at those points. Harder rock resists the wearing. This cutting is slowly eating upstream, back toward the parking lot. But it has to find weak points along the whole length of the parking lot (and the yards from the parking lot to the ravine). That is a huge sacrificial plug. The towns underneath have seen the flooding because all this is exceeding the 180,000 cfs bottleneck downstream. To drain the dam, that long channel has to keep chewing its way along that mess.

    Actually, what will probably be happening is the tiny spot between the end of the weir and the parking lot will be trying harder to cut through the edge of that flat table. Because nobody's been able to get the spillway working, and Noah's just bobbing around upstream, that will cut a ravine slowly through the edge of that big table. If the icecap is still melting and still feeding 400,000 cfs, the whole process continues. If the level has gone down, that narrow ravine will become the only outlet for the dam and after each rainfall will slowly chew through the whole mountain until it drains the whole reservoir.

    Edit: Paveway IV image from four pages back:
    [​IMG]
     
    Last edited: Feb 15, 2017

  12. Pictures and comments here show very narrow seams of fractured rock further up the spillway. But they are perpendicular to the water flow and again very narrow ... spillway concrete bridges these narrow bands - would seem they are not a threat?[/QUOTE]

    There is a photo floating around that I am unable to relocate possibly due to noise hogging key words in my search. It is an aerial photo of the main spillway under construction, the main gates completed, no concrete poured on the spillway downstream, but fully excavated. The bedrock has large patches with an orange hue, ~10 meters in size, inbetween gray bedrock. Whoever finds that photo will earn themselves free beer!
     
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  13. MortarBoarder

    MortarBoarder Member

    You've confirmed something that was unclear. The emergency spillway was placed directly on bedrock. No mention of laying wood or anything under the concrete. Rock was obviously cut for the key on its upstream side, but they didn't try to grind the whole surface flat. (The key tool seems to have not cut a "V", but did need a wedge shape on the upstream side, as fill is noted there.) It probably resembles the somewhat smooth surface visible behind the weir. But it's a somewhat bumpy surface, so the concrete fitted itself into whatever shape that was underneath it. By now, the wooden drain locations are probably somewhat larger drains due to rotting of wood.
     
  14. MortarBoarder

    MortarBoarder Member

    That sounds interesting. Keeping in mind that at some point the anchors were excavated into the rock, that would tend to spread rock fragments around. That doesn't explain such large patchwork, though, as the drilled out areas for #11 rebar anchors would be much smaller than 10 meters. If there's a checkerboard, then there might be a checkerboard of anchors. A theory needs a confirmation test. Look in a close-up for dots of grout around each anchor, so stipples inside a square of one color could be a pattern of anchors. If the picture has any anchors in it already.
     
  15. James L

    James L New Member

    Yeah, I'm sorry, but this theory makes no sense in the context of the geology of that ridge and the structures next to it. Why build an Emergency 'Auxiliary' Spillway on a rock ridge, and claim in documentation that it can pass multiple hundred thousand's of CFS, the evident point of which is to be lower than and a certain protection of, the main spillway and more importantly the dam itself, and then design a known erodible weak point smack next to it that would certainly fail during the very event the emergency spillway is supposed to allow for? Could it not be that the rock outcropping rises to that elevation and there is simply no need to extend the weir the rest of the length to the hillside? The topography of the rock in that spot is the natural extension of what the engineers put in place. A couple of feet of concrete is surely sufficient to perform the same function as the ogee section if laid on a semi-solid bed of rock. Moreover, the flow rate and energy of passing water will be lower through that section as there is no steep falloff right in front of it, further indicating that no elaborate construction is needed. The only seeming oversight here is that the potential for lateral flows in front of the adjoining ogee and parking lot sections could generate a weak point at the base. That is, if you assume the engineers did not closely consider how the ogee is tied to the underlying rock and its integrity.

    Note that the height of the front face of that weir seems (I say seems because we can't see through the ground to where the actual footing is) to be decreasing in the direction of the parking lot for the very reason that the rock outcropping is rising there. In fact the entire spillway ridge is largely rock, and though it might end up being insufficient to prevent an undercut of the weir, it is not going to fail in total. So, there is absolutely no need for a sacrificial plug. At best you might slow the release of 40' of water, whereas in providing such a sacrificial plug, you almost ensure its release. Either the weir survives overflow (this seems silly to write), or it does not. That is very different from a dirt coffer dam which has no such geology in which to create a protecting structure. If there were a sacrificial plug, it should exist on the dam, not on the spillway area, which provides very little help.

    Disclaimer: I work in IT, not in geology or civil engineering.
     
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  16. CRM114

    CRM114 Member

    Regarding the drains, there is some pretty obvious weepage from the d/s face of the weir visible in this photo from yesterday:

    http://i.imgur.com/rx1HIA2.jpg

    Drains may not be functional in these areas or the internal rubble may have some layers that dont drain well.
     
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  17. Paco

    Paco New Member

    This second hand information, but as I was told: Those are abandoned power plant intakes for a smaller power generation plant that would be across from the current Hyatt plant. The rock was found to be "not suitable" for tunnels and they were abandoned. They go down approximately 30ft and stop. They aren't connected to anything.
    This is why they do not share the same mesh-style covering as the actual live intakes for the Hyatt powerplant.
     
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  18. We saw how quickly half the road was washed away with 15k cfs. This spillway is designed for at least 20 times that flow, the road won't last long and is inconsequential. As for the ability of the weathered bedrock not to resist flowing water was proven as evidenced by the extreme erosion of that hillside headcutting right to the base of the ogee weir and elsewhere. I'm not out there with my tape measure, but those holes are deep, as evidenced by photos on this blog. The ogee weir was threatened and tens of thousands evacuated because of 15k cfs. Geologists knew how weak this bedrock was, much was removed by bulldozers, and they knew how prone this rock was to flowing water, they hydraulically mined parts of the bedrock during dam construction.
     
  19. mortimers

    mortimers New Member

    So is it the general consensus that the purpose of using the emergency spillway was a way to keep the water level in the reservoir high while having a crippled main spillway?

    Meaning, they knew the main spillway was damaged so in order to not have to do what they are doing now (dramatically lower the level) they wanted to use the emergency spillway as an auxiliary and keep the main running at a reduced rate.

    I think the idea has been stated that the obvious purpose was to keep as much of the main spillway intact as possible and save repairs, but I don't know how much value those acre-ft of water is worth.
     
  20. deirdre

    deirdre Moderator Staff Member

    not anywhere near the parking lot. and the parkinglot is intact still. i think i am just completely misunderstanding the terms you are using to describe whatever it is you are trying to describe. And it doesnt matter soon they will turn the emergency spillway into an auxiliary spillway (with large concrete apron) which will speed up the flow in the future ergo annihilating any 'giving the people more time to evacuate' "sacrificial plug" benefit.
     
  21. Mick West

    Mick West Administrator Staff Member

    No. The water is worthless this month, in fact they would pretty much prefer it would vanish into thin air, as it's just either taking up valuable space or contributing to flooding downstream.

    They did not want to use the emergency spillway at all. But they also wanted to use the main spillway as little as possible, as more use could damage it. So they tried to minimize the use. Unfortunately things did not work out, they ended up using the e-spillway, and found it was pretty useless.

    So now they are forced to use the main spillway. They are repairing the e-spillway not so they can use it again but just incase the main spillway fails.
     
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  22. Mick West

    Mick West Administrator Staff Member

    They cleaned the bedrock, but where did they "mine" it?
     
  23. alec

    alec Member

    Re; #724 general consensus:
    Count me out on that one.
    It's unlikely in this situation, that loss of valuable water is driving decision making.My guess is that the top priority is to keep the dam functioning. These dams serve multiple purposes, two of which are flood control and irrigation. I have heard, personally read somewhere recently(no source, anecdotal,trust me sorry,research it..???) someone claiming that leaving space below peak capacity was a plan to bankrupt farmers. Running an enormous reservoir, in early February , filled to the brim , in one of the wettest years in history, in my judgment, by choice would be outright insane, likely a violation of any operations manual, and the least desired state of affairs for anyone with any responsibility. I'd expect dam operators, by screening, by law, by temperment are extremely level headed and act cautiously . My opinion, flat out no.
    To serve the flood control requirement, I'd guess it's required, quite possibly legally mandated to maintain a margin appropriate for substantial inflows. The spillway is a necessary part of the dam. The season is young and wet . The water system being operated here is complex with multiple downstream effects. Lag times in flows are enormous. You have to have large margins of safety to adjust in time for the unforeseen. Preservation of the spillway, essentially the regulator valve, and preservation of the dam for a significant chunk of California's water distribution system are paramount and far outweigh considerations of water and spillway repair costs.
    Water does have a value and I'm certain you can easily find that value. The last number I recall was, and my memory could be completely off on this , if I remember in the order of $225 -450 per acre-foot and part of the cost was based on the quality of water . My recollection, again, my memory may be slightly off on this, is that LA DWP considered the E. Sierra Water a particularly high quality of water that was mixed with other waters . Please chime in and correct me if on off on those last two statements .
    It also must be remembered that in the initial stages, the damage increased drastically, and could arguably be described as rapidly self destructing . It would have been absurd to not be alarmed by the rapid uphill degradation of the spillway. Would you radically increase the flow in this critical piece of infrastructure under these conditions? At this time of year? With an already flooded Central Valley? With a city below the dam? Where likely many of your friends and family live?
    Emphatically, Hell No!
    count me out of that nonsense!
     
  24. Mick West

    Mick West Administrator Staff Member

    Fixed that for you:
    20170215-222243-l34n5.

    The water isn't dropping off a weir into a bottomless pit, or a hillside. The other side (as you illustrate) is at about the same level. It's also flowing pretty slow.

    The weir route will carry more water than the road route.
     
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  25. Mick West

    Mick West Administrator Staff Member

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  26. Paveway IV

    Paveway IV New Member

    The discussion about the integrity of the emergency spillway is kind of going in circles here, folks. While there is a brief mention of the loss or impairment of the main spillway in some of the literature here, I really don't think that was part of the emergency spillway design, hence the designation as emergency (not auxiliary) spillway.

    Consider this:

    If there was serious flooding, the flood control (i.e. 'main') spillway was designed to handle 295,000 cfs at full flood pool height of 917'. At that height, the emergency spillway would also be flowing about 295,000 cfs. The downstream system can barely take 150,000 cfs - it's flooding a lot of areas at 100,000 cfs now. At a flood pool height of 917' and nearly 600,000 cfs, Oroville would be under several dozens of feet of water. That would already be catastrophic - no need for hand-wringing about 20' deep erosion scars toppling the emergency weirs. What's another 100,000 cfs on top of 600,000 cfs? They are not worried about saving anything downstream at that point - they only want to preserve what's left of the dam.

    Even at a flood pool height of 901', the dam was designed to be able to spill 250,000 cfs - that is still almost double what the downstream system can handle. That volume of release would still be catastrophic, with widespread flooding and damage downstream. Erosion of the emergency spillway with a few more feet of water and a larger release - say, 400,000 cfs total - isn't going to be much more catastrophic than 250,000 cfs. Does it really matter if the roof of your house is under 15 ft of water vs. 25 ft? Would a torrent of water from the failure of the emergency spillway damage much else if bridges were already torn out and downstream towns inundated?

    From an engineering standpoint, I would guess they were thinking along the lines of "Things are so borked by 250,000 cfs/901' that the integrity of the emergency spillway is inconsequential."

    While they may have factored in a partially-usable main spillway into reserve calculations, they obviously (from everything we've seen so far INCLUDING the design) did not anticipate 'auxiliary' usage of the emergency spillway. That despite the careful use of weasel-word qualifiers like 'rare' and 'occasional'. They were pretty plain about an acceptable level of damage resulting from its use at all. And if it were to be used at all, they give calculations (including release rate graphs posted here) as a combination of the full main spillway flow and whatever additional is going over the emergency spillway.

    I have no expertise in dam construction, but has anyone really seen many dams constructed with 'auxiliary' spillways? I mean, if there is any impairment in the flood control spillway, then that's already an emergency. It seems perfectly reasonable to design the flood control spillway to handle 1000-year floods. Yes, a fully-armored auxiliary spillway would have been nice (and rather convenient for 200,000 people in the current case) but how common is that anywhere else? Should every dam have an auxiliary spillway in case the main one goes out? Don't most dams just overtop instead of having a dedicated emergency spillway? (I'm asking here - my armchair reckoning is that most have no secondary/emergency spillway).

    It seems we're arguing about an enhancement to a feature the original engineers put in as an 'extra' to save the dam, not to control what would already be catastrophic flooding from the maximum main spillway release. Since it's there, sure - it makes sense to throw a few tens of millions 'upgrading' it now. I really can't see the original design as a deficiency thought considering that few other dams' designs accommodate an impaired flood control (main) spillway during a serious flood.
     
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  27. Mick West

    Mick West Administrator Staff Member

    The problem with that is that if you overtop a gravity embankment dam (which is basically a big pile of dirt and rocks), then it fails from erosion in hours. i.e totally fails, 700 feet fails, millions of acres flooded. It would look like the Teton Dam failure.

    20170215-233412-hhf28.

    Dams like Shasta or Hoover don't have an emergency spillway because they are made of concrete. The emergency spillway on Shasta is essentially the big cylindrical gates on the top of the dam
    20170215-233220-rvuxd.

    With an embankment dam you want the spillway as FAR away from the pile of dirt that is the dam as possible. Consider the New Melones Dam, normally water is let out through tunnels, but it has an (unused) emergency spillway, as it must.

    20170215-232725-b94my.


    Or Don Pedro (which looks like it used the unlined e-spillway quite safely)

    20170215-233756-vas3y.

    That's the way you do it!
     
    Last edited: Feb 15, 2017
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  28. deirdre

    deirdre Moderator Staff Member

    From the last pic there.. looks like the right side of the spillway the bedrock is more "solid"/continuous, then the other side that 'appears' to have non-bedrock gaps.
     
  29. Any one see steel reinforcing or bolting of the concrete to the bedrock that is being done.
    It probably does extend under the road. Your zoom in appears to show a concrete curb though. The photo does show a lot of fresh bedrock exposed. Looking at google maps and page 94 of https://archive.org/stream/zh9californiastatew2003calirich#page/94/mode/2up
    shows that up to 100 vertical feet of bedrock was removed to make the parking lot. Perhaps my sacrificial "soft plug" at the entrance to the parking lot theory might not be so soft. I will now rescind that theory.
     
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  30. Mick West

    Mick West Administrator Staff Member

    The problem here was that you nearly had the emergency spillway failing at 55K cfs going down the main spillway with an overtopping of the weir of perhaps 10K cfs. Yes the emergency spillway works as an overflow, but it's also supposed to not fail at the first trickle of water.
     
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  31. Mick West

    Mick West Administrator Staff Member

    Here's a view from the other side. Gives you more of a sense of the scale. Also the hillside looks vaguely like it was dug into, then filled in.
    20170215-235642-6qvgm.
     
  32. Paveway IV

    Paveway IV New Member

    My point is more like this, Mick: At the time the Oroville emergency spillway was used, the main spillway was only at 55,000 cfs or so. So what's the design spec that should be applied to ALL dams here?

    That kind of sounds like a design spec. but therer's a lot of grey area there. Let's say:

    "The emergency spillway must be able to handle flood control for the maximum probable flood for an entire season [or some other duration] if the main spillway is impaired to 20% [or some other figure] of its design capacity."


    If that's the way it should be done (which certainly sounds reasonable), then so be it. An engineer can understand clear specs - maybe this needs to be an explicit requirement today rather than a hazy one subject to debate by corporate lawyers. If this wasn't clearly part of the specs back then, then how much should an engineer or architect have pushed for that design choice fifty years ago (assuming they had mortgages and would have wanted to continue feeding their families)? Greedy management, corporate lawyers and enthusiastic accountants usually trump 'ethical engineers' (well, at least in my world).

    Then there's the matter of all the other dams that might be in the same situation as Oroville. If you say "This is the way it should be done," then the people in xville will argue that they are just as important as the people in Oroville. If Oroville Dam needs a robust emergency spillway, then all the other earth-fill dams should have the same requirement. Nobody should have to argue in court anywhere about it being a legitimate part of the re-licensing process. It's not just a problem unique to Oroville - they were just the unlucky ones to be first in line.

    Sounds like some shovel-ready infrastructure jobs for all that low-interest money waiting to be borrowed.
     
    Last edited: Feb 16, 2017
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  33. Milton

    Milton New Member

    I may have missed this discussion, but when they feared the imminent failure of the emergency spillway, and a possible 30' wall of water being thereby unleashed, was there any chance that water could have made it to the downstream face of the dam and eroded it from that side?
     
  34. Michael G

    Michael G New Member

    Not exactly. A concrete dam can be designed for water to flow over it, but it doesn't have to be and if it's not an overtopping would cause damage if not outright destruction.

    Thin-arch concrete dams like Flaming Gorge could be destroyed by overtopping.

    On Shasta (a gravity-arch hybrid), the spillway reduces its slope gradually to ensure that the water doesn't hit the base of the dam and cause damage and there's nothing in the path of the water once it leaves the spillway. (It's directed towards a rock cliff that deflects it somewhat, slowing the flow.) The upper cylindrical gates are only the final series of places to dump water from. In normal use, they will max out the powerplant first, then use lower release points that let water out through the face of the spillway. This video is Shasta right now. They're using only the top row of spillway valves which along with the powerplant have them releasing about 75,000 cfs. There are two lower rows of valves that could come into play before they had to open the upper gates. That could happen soon as they're about 5' from the top.


    Source: https://www.youtube.com/watch?v=kaBjrqkvPnE


    If all the pieces of the spillway combined could not handle the inflow, you'd have a problem. What might happen if significant amounts of water came over away from the spillway? I don't think anybody really knows. At the very least, powerhouse damage/destruction is likely.

    Hoover Dam (also gravity-arch) would probably be damaged by a major overtopping. At the very least, the powerhouse would be destroyed. As with Shasta though, there are multiple levels of redundancy allowing for water to be released, including the powerhouses, additional release gates tied to the powerhouse tunnels, and finally two emergency release tunnels that would handle additional overflow.

    Hoover Dam is a relatively easy case though. Flows are low enough that it took years to fill the reservoir, so the possibility of a sudden emergency is almost nonexistent. On the downside, the likelihood of those reservoirs filling up again in anything less than a decade is also nonexistent.
     
  35. DCMonkey

    DCMonkey New Member

    Looking at https://archive.org/details/zh9californiastatew2003calirich and judging from the general plan (Pg 94):
    General Plan E-Weir Area.JPG

    and this passage (Pg 92):



    ... it looks like the parking lot area both in front of and behind the weir was excavated from the existing terrain.

    Also, here's the parking lot weir section:

    Broad Crest Weir Section D-D.JPG
     
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  36. Michael G

    Michael G New Member

    The thing that seems most unique about Oroville is that they decided they needed an emergency spillway, but because its use wasn't part of normal operations, they decided not to ever test the thing. Even Hoover Dam -- where it took months to fill the reservoir to the point where the spillway came into use -- did that kind of test. Had a test been done under controlled conditions 50 years ago, they would have discovered the problem, fixed it, and we wouldn't be having this discussion. Flooding might still be an issue with high flows but the integrity of the structure would not.

    I'd say the major lesson to take away from this is: If you think you may need it, you have to test it. I see no evidence that they didn't design the thing to a reasonable standard, which is what you're asking for. But design is useless if you never test whether it works. If that means you need to shut down power generation and the main spillway and everything else for a year just to get water flowing over at a significant rate than that's what you have to do, just as they did on Hoover Dam.

    Unfortunately you won't see anybody talking about that on TV because it's hard to direct accusatory and sensationalist questions at people who are mostly dead by now. Like you, I suspect it wasn't the engineers who made the call.
     
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  37. Marc Kupper

    Marc Kupper New Member

    I'm pretty sure the unused intakes are in the far upper-left of that diagram. While the map is not to scale it shows the plugs somewhere between 500 to 600 feet in from the intakes rather than 30 feet that someone was told. Something that bugs me though is that there would have been very little tunneling needed for that intake. Most of it would have been constructed on the ground as a concrete tunnel and buried as the dam was constructed.

    Go to http://www.historicaerials.com/ and search for Oroville dam. Look at the 1973 topo or newer to orient yourself and then drop to 1966 and earlier. You can see the original path of the river. The diagram references this with "river" in the upper-left corner. The earlier years have topo mapping of the parking lot area that could be used to estimate the cutting. The 1969 aerial shows a fresh cut hillside to the north of the parking lot. The hill immediately north of the parking lot is 1014 feet. The main weir area looks like it was already at 900 feet. Both the main spillway and emergency spillway were placed in natural saddles.
     
  38. Brad P

    Brad P New Member

    [​IMG]

    Hoover Dam spillway at work (in the lower left corner of the picture.)
     
  39. Brad P

    Brad P New Member

    upload_2017-2-16_6-5-52.

    A couple of interesting things.

    Note that three lines above the highlighted "Maximum release" it says "Emergency Spillway". It also says "(in saddle on right abutment)" So there was an existing saddle where they built the emergency spillway.

    I have a problem with the idea that the "emergency" spillway was designed as an sacrificial plug though. Even if the Main Spillway was inoperable, the emergency spillway is designed to pass a 350,000 cfs flood. Why then would it start failing with only 12,000 cfs flowing over it? Would you really want it to wash out at that low a volume? Or even if its designed to wash out at 100,000 cfs, wouldn't you then say that the maximum release is 100k ? Also, note that the highest recorded flood was 250k cfs. If the Main spillway wasn't working at all, would you want the Emergency Spillway to fail below, say, 300k?

    If you want to make sure the dam doesn't overtop there are other, more accurate ways to do it. Along the Mississippi River some of the levies have galleries built into them that can be filled with explosives to remove the levies when they need to release the overall pressure on the levy system. This was done a couple of years ago just south of St. Louis.

    Edits made to correct spelling error and to make it more readable.
     
    Last edited: Feb 16, 2017
  40. Brad P

    Brad P New Member

    [​IMG]

    A simple reason for the cause of the initial failure would be that the 15" bottom slab wasn't designed to span 100 feet. Assuming some of the base material washed out before the failure, the bottom slab could have cracked and buckled exposing edges to the force of the water. For scale, the chute is 178 feet wide.
     
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