Scott Cahill: Collapse Risk At The Oroville Dam Is Still Unacceptably High

Remember the crisis earlier this year at the Oroville Dam?

The overflow from California's winter of heavy rain threatened to overpower our country's tallest dam. A cascading failure of the dam's main gates, its primarily spillway AND its emergency spillway had the world watching hour by hour to see if a catastrophic breach was going to occur.

Fortunately, the rains stopped long enough for the situation to be brought under control. The dam remains in place and repair crews have been working all spring and summer.

But should we breathe easy at this point? Not at all, says dam safety expert Scott Cahill. Our readers will remember Scott from the excellent technical assessment he provided in the thick of the crisis earlier this year. In our earlier podcast with him, he explained how the real tragedy at Oroville was that for many years, small and affordable maintenance projects that easily could have prevented the crisis were diverted (in his estimation, the cost of making the needed repairs was quite small -- around $6 million. But for short-sighted reasons, the repairs were not funded; and now the bill to fix the resultant damage will likely be on the order of magnitude of over $200 million. Which does not factor in the environmental carnage caused by flooding downstream ecosystems with high-sediment water or the costs involved with evacuating the 200,000 residents living nearby the dam).

And the pattern appears to be continuing. In this week's podcast, Scott details a number of concerning structural risks visible at Oroville that are again being de-prioritized, or ignored all-together. And as before, straightforward and inexpensive projects that have high potential to prevent a catastrophic failure of the dam are not being pursued:

They've begun the repairs on the bottom half of the spillway, but the tragedy and loss from the bottom half of the spillway failing has already been realized. No one is worried about the bottom half of the spillway. On the other hand, they've done nothing yet with the upper half of the spillway -- which is what would cause a catastrophic failure of the dam. It's amazing how much money they've already spent, and yet their priorities are such that they haven't abated the liability at all.

So yes, we've made the bottom of the principal spillway, the concrete slues, more sound. But it's not the bottom of the dam that will fail, of course, it's the crest -- the top of it -- where the gates are. That's still highly suspect.

There are additional issues involving the unwanted moving of water through the dam -- the so-called "green spots". These are areas where water is migrating through the dam, probably through the indigenous soils adjacent to it. I've walked on these [at Oroville] and you can stick your foot down, and like your backyard after a torrential rain, water actually comes up into the footprint after you remove your foot. This is not a good situation. I believe there is a lot of movement of water through that dam, including at the structure itself that houses the gates that control the flow down the principal spillway.

There's nothing wrong with embankment dams in general, they're wonderful dams. But they rely on the mass of the earthen embankment itself to offset the forces that try to slide or rotate it into failure. When we see water migrating through a dam, it can potentially cause failure of the dam because it offsets the mass all that earth. Plus, there's a lot of river rock and sand in this embankment. River rock, as we all know, is round. Anyone can understand how a pile of round rocks, if the fines have been washed out from between them by water and the rocks then vibrated, for instance, by seismic activity, weakens the system. These concerns are very, very serious. I believe that this situation is occurring in multiple places across the Oroville dam -- and yet this is simply not being discussed.

Click the play button below to listen to Chris' interview with Scott Cahill (41m:06s).

This is a companion discussion topic for the original entry at

CA builds a metro high speed train instead of repairing the Oroville damn. US bails out criminal banks instead of fixing government spending and Wall Street.

I’ve been trying to keep up with the dam repairs at least a few times a week. My favorite reporter is Juan Browne and his blancolirio channel on Youtube. He puts out a video with information at least weekly (generally.) His latest video came out 8/18/17. Here it is:

. During this video, he flew over the dam in his mighty Luscombe. The aerial views of the dam start at 4:10 and end at 5:55. The focus is more or less on the repairs to the spillway. At 4:45 is a good shot of the project looking south along the dam axis. No green spot is evident to me in any of the shots. If the green spot were from dam leakage, it should be a nearly continuous feature (as long as the reservoir level is above the leak point.) After the flyover, Juan is showing still shots and providing comments. At 6:15, The head gates to the spillway are shown. Juan mentions that a new access road has been made for gate repairs. Are they actually repairing the gates or did they just make a good show of it? I really don't know... At 6:35, he's showing the "upper portion of the spillway which has been rock bolted to bedrock." (I've seen rock bolting activity off and on going in the upper section since major repairs began earlier this summer.) Juan continues showing images progressing down the spillway and discussing the ongoing work. At the bottom, he works his way back up the spillway with images taken from the other side. By 10:22, he's reached the top again and then shows California DWR drone footage released 8/17/17. That ends at 12:22. I know from past experience that regulators in charge of public projects are loathe to accept any culpability for a failed project. They normally don't have any monetary incentive to limit their actions, but they will catch hell if anything fails (again.) As a result, they generally go overboard on requirements only to paper their butts in case something fails. Scott was saying that the dam regulators are essentially the dam operators. That changes the dynamic. Now, the regulators have a financial incentive to be reasonable. We'll see if Scott's worries become issues. Since the construction season is short and this project is complicated and large, choices had to be made to split the work into at least 2 dry seasons. They chose to leave the upper spillway and work on the destroyed lower spillway. Reinforcement of the upper spillway has been accomplished. (I can't imagine that an inexpensive technology like ground penetrating radar wasn't used to locate and identify voids under the upper spillway.) I haven't seen any footage where work was being done on the spillway gates. Until Scott mentioned it, I didn't think it was an issue. As far as the emergency spillway is concerned, Scott is right that 3% of the design flow almost caused the dam to fail. This isn't hyperbole! The head cut erosion near the north end of the emergency spillway was getting close to the weir foundation. (That is what prompted the emergency evacuation orders in February.) To stop that erosion, the dam operators opened the spillway gates wide open and caused all the damage to the spillway. Part of the repairs has been to install a cutoff wall down slope from the weir (to keep head cutting from impacting the dam) and also to cover the erodible rotten rock with concrete. The cutoff wall is complete except for a section under power lines. Until the power has been rerouted, the drills can't safely operate that close to the high voltage. There's still lots of time for this particular phase to be completed before the rainy season begins. There's lots of work to be done before the rainy season begins. We've got about 10 weeks to go. Grover

More on the “green spot” Scott discusses in the podcast:

Oroville Dam's 'green spot' raises new worries that the nation's tallest dam could be facing a breach danger from a slow motion leak five months after it overflowed (DailyMail) A 15 member team at UC-Berkeley has spent months researching a 'green spot' on the Oroville Dam The spot is located near the left abutment on the backside of the nation's tallest dam in northern California The team published a 124-page report that says the structure could be facing a breach danger from a slow motion leakage failure California's Department of Water Resources claim the spot is due to rain, vegetation, or is a natural spring Report author Robert Bea disagrees with the DWR and says if the spot was caused by rain, it shouldn't just be concentrated in one specific area
And here are photos from the same article: Click here to read the full DailyMail article

The news report comes from England! TPTB must want it to collapse to create another diversion for the masses. Trump should declare an emergency and have the FEDs take control. But he probably already is bought out; we will see tonight.

The article you linked makes it seem like the green spot is a new feature. It isn’t. You can check on Google Earth and see that it was evident in 1998. That is as far back in history that Google Earth has past photos available for this location.
The greenery is much more evident in spring. The green spot dries up and turns brown by June/July. If water were flowing through the dam, the vegetation wouldn’t dry up and would be much more evident in the dry season. Earthen dams can be quite variable in composition. There was likely a change in material composition near that point that gives it a higher field moisture capacity. Perhaps that area had more fine aggregate or plasticity (but still within construction specifications) than the surrounding areas. It could be that innocuous. I’m not saying it is good for this to happen, but it is not automatically bad either.
Here is a stylized cross section schematic of a typical earthen dam from: The green spot is approximately where the downstream “Riprap Protection” arrow points. As an uncle used to tell me when I skinned my knee, “it’s a long way from the heart.”

Grover -
I’m not sure that Scott would say age is a plus in this case.
As I’m not an engineer and so am very leery of speaking for him, I believe he said in the interview that the ‘multiple’ signs of foundation pressure (migrating water through the dam) he observed while at Oroville had likely been there for some time. His concern seemed to be that – given enough time – these could eventually undermine the embankment’s integrity.
Now, you may be right that any seepage is too small/far away to risk a breach. But the fact that a highly-experienced dam safety engineer like Scott is so concerned, as well as is a 15-member team from Berkeley that published a 124-page report on this very risk, is enough for me to be comfortable that it’s a topic worthy of discussion.
As always, any data/evidence/sound theories to the contrary are welcome. I think your video from Juan Browne is a good addition to the discussion – although there’s not much in it to refute or support the foundation pressure situation (his camera shot of the embankment was so distant that even though a green spot isn’t apparent, it can’t be ruled out, either, IMO).
Having no real engineering expertise or on-the-ground experience at Oroville, though, I’m not qualified to weigh in authoritatively on the green spot issue.
To that end, I’ve invited Scott to chime in here in the Comments. I can’t guarantee he will, but he did so with the earlier podcast we recorded with him, so I’m hopeful.

Adam wrote:
I'm not sure that Scott would say age is a plus in this case.
Adam, Lots of news articles lead the reader to panic because they lead the reader to think it is a new problem that just manifested itself. I get angry at those alarmist pieces. This problem has been around at least 19 years and likely longer. Is there degradation in that time? Will this problem destroy the dam? I don't see it as a huge issue. By the same token, I don't expect the dam to last forever. In fact, I bet that it will fail within a billion years. It likely will fail much sooner. Of course, so will every other man made structure. Then again, so will every natural structure. We (and nature) don't build things to last forever. We can't afford to do so. Risk is just a fact of life. Earthen dams all have common features as noted by the stylized schematic I posted earlier. The core is a clayey material that limits water movement. The shells armor the core to keep it from eroding. Filters are a gradation between the fine grained core and the coarse armoring. A good dam design can actually heal breaches by settling. (Not saying that there is a breach, just that it may be self limiting.) All earthen dams leak. How much and how muddy is the key. Also, the filter water will become muddy before the dam is in jeopardy of failing. Are the operators reporting problems? I haven't heard of any, but that doesn't mean there aren't any. For a core breach to be evident on the surface of the dam about a third of the way down slope, lots of really bad things have to happen. First, there has to be a core breach. Then, the fines from the core breach have to plug up the filter with fines. Then, the fines have to plug up the armoring riprap so that water would find it easier to manifest itself on the surface rather than percolate through the dam structure unnoticed. Why would this water only manifest itself in the springtime? That simply doesn't make sense to me. That the green spot has existed for some time is rather comforting. It was likely a result of construction material variability rather than a manifestation of a new problem. I'm not someone who believes everything that government says. I question motives behind events and reporting of events. In this case, I haven't seen any evidence that would cause me concern. I haven't looked at the head gate problem that Scott mentioned, and frankly, I see that as a greater potential issue than the green spot. I also agree with him that the emergency spillway issue could have destroyed the dam. That problem is being addressed. Problems were identified and have been prioritized and the highest priority problems have been and are being addressed - at great cost. As I mentioned in a prior post, there is too much work to be done in one short construction season. Some work has, by necessity, been delayed until next year. Everyone is betting that this next winter weather will be less severe than last winter's weather. Contingencies are being engineered into the temporary construction products in case the winter is more severe. I haven't seen (nor looked intensely) where federal monies have been spent on the repair efforts. The repair costs should be borne completely by the dam operators and their stockholders. Had they maintained the dam properly, none of this would have happened. Since they chose to delay maintenance, they should be held accountable for that decision. Bailing them out just reinforces that they can get away with shoddy decisions. That means that the public can expect shoddier decisions in the future. Their financial based decisions impact the lives of all the people downstream of the dam. Downstream isn't just confined to the Sacramento River channel. It is the whole broad valley that is potentially impacted. All of those people have a stake and should be concerned that sound decisions are being made. Adam, I would enjoy conversing with Scott on this forum. I have a background in geotechnical engineering, but my work experience wasn't with dams. (Nevertheless, the fundamentals still apply.) Scott has insights, based on his experience, that I simply don't possess. I will question him politely but firmly. Absent his involvement in this forum, I will post my concerns with his stated positions. What else can I do? Grover

I am aware of the rock anchors that were placed in the upper spillway back in the Spring. I am also aware that some repairs, apparently minor, have been discussed for the gate structure. The details have not been disclosed because they are conveniently Secret. FERC has questioned the condition and age of the gate mechanisms themselves in a number of pieces of correspondence that span several years. I doubt very much that any significant repair will be done to them this year. The concrete structure that houses the gates has major structural issues that I also don’t think will be addressed this year, although some minor concrete repairs might be made. There is undoubtedly a large void under the gate structure and upper spillway that critically needs to be properly filled in order to help stabilize the structure. The binding and misalignments of the gates as cited in the inspection reports year after year, and the large crack and other cracks as well, are a result of loss of support under the structure, alliowing it to shift in normal operation. There also needs to be a cut off wall UPstream of the gate structure to prevent future migration of water under the structure. The gate structure is the greatest point of vulnerability for this dam in the upcoming season.
As for Mr. Brown, his photos and videos, although selectively chosen, are appreciated. In our view, his credibility slipped a bit when he changed his tune. Initially he was quite critical of DWR, as was warranted. Then he magically became their cheerleader overnight. In recent months, it seems he has moderated back a bit to be less than 100% favorable to them, but still inclined to see positives, ignoring the continuing conditions that put people downstream at risk and the various credible reports, such as Dr. Bea’s, that highlight and illuminate them.

The green spot, an evolution of porting water through the surface of the tallest dam in America is serious, but there are many failure modes going unaddressed on this dam. Juan Brown is a great way to hear what the DWR wishes you to hear. They have hired a public relations team to “spin” the issues. They have public meetings where they employ Delphi technique to quiet an misdirect the conversation. They do everything but maintain and repair.

A quarter of a Billion or so has been sent to CA to plug the holes in the Oroville Dam by the Federal Government. The Billions of wasted money are the result of irresponsible stewardship of a major element of infrastructure and a culture that is a failure.
Already hundreds of thousands of people’s lives have been put in jeopardy. When confronted with their failure they have misled and lied and redacted documents. When men like Dr. Bea offer help they ignore them, then marginalize their words.
Even with this horrible bungling of responsibility, I, like Dr. Bea see nothing improving. My words speak for themselves and time will show who was right and who was wrong.

Thanks for participating in this forum. I’m angered that FEMA is pumping money into the dam restoration! The idiots who decided to postpone maintenance should be held accountable​ fired! The firing should start at the top and work its way down to any level that had material decision making power. Until the other dam operators know that their cushy, high paying job is in jeopardy for not making appropriate decisions, they will never change their modus operandi.
Before I comment any further, I’m going to read Dr. Bea’s report. For anyone else interested, I found the 78 page report embedded in a .pdf reader in this article:

I understand the concern but what does this have to do with “peak prosperity”?

I read through Dr. Bea’s report that was included in the link I posted in #11. I’m not sure that is the report you were referencing. Almost all of the report concerned the deplorable condition of the existing spillway. There was a brief mention of the head gates, and nothing about the green spot. Would you mind posting a link to Dr. Bea’s in-depth report?
I liked the title of your post - “Stop Fearmongering.” Thanks for posting that Juan Browne video. I would have posted it if no one else had. Although Juan isn’t an engineer, he’s got a good eye for noticing critical features. He’s taken the approach of showing the news about the construction features that he considers significant. In my opinion, he does a much better job than any other “news” source out there.
It looks like the new spillway is heavily reinforced with rebar! (Steel reinforcing is a double edged sword. See Our Future Is (Literally) Crumbling Before Our Eyes for examples of the other edge.) The concrete is much thicker than the original spillway and Kiewitt has gone to great pains to remove eroded bedrock before placing the concrete. That will form a stronger bond between the concrete and the competent bedrock. In the short to medium term, these are good measures.
Juan was part of the news conference where one of the questions (not his) concerned the green spot. The answer was that the green spot is being investigated and a report is due out in a couple of weeks. Juan also noted that 900 rock bolts have been installed in the existing upper spillway and that twice as many more are scheduled to be placed before Nov. 1st. That should provide some risk mitigation for spill releases this winter season. Of course, all of the upper spillway will be rebuilt next construction season.
Juan also noted that the reservoir is currently at about 763’ and is planned to be at 700’ by November 1st. That gives them at least 160’ of freeboard to work with. The dam operators will do everything in their power to keep from using the spillway this winter/spring. After all, why tempt fate? Unless Northern California receives another record breaking wet season, they should be able to meet that goal. Human nature is to bar the barn door after the horses escape. In essence, lots of horses escaped in February.
If anyone knows of DWR’s plans for repairing the spillway head gates or work that has been performed in the head gate area so far, please post a link. Thanks,

In an earlier life, in 1969, I was given a guided tour of the entire California Water Project, from Oroville to Los Angeles. At the time, I was doing the design and cost estimating for a similar project in Texas.

An incidental additive note to a post above: The purpose of the rock and random fill outside the clay core is to add mass to resist the horizontal force of the water against the embankment. Gravity. You have the weight of the water times the depth as a horizontal force acting against it. FWIW, the rock fill was a “re-cycling” of waste material from the gold-mining days.

While any failure of the dam would be a horrible local problem, the loss of irrigation water and of electrical generating capability would be a statewide long-term problem. Not just at the Oroville project but down at San Luis as well. And lesser, at the downhill recovery after the Tehachapi tunnel.

Apologies for the digression. Cross-fingers. We surely don’t need another Fort Peck disaster.

We’ve just been contacted by California’s Department of Water Resources (DWR), which invited us to direct our audience to their website where they are providing updates on the situation at Oroville to the general public:

Regularly updated information on the Lake Oroville Spillways Emergency Recovery Project is available at and should be a valuable resource for future guests and your podcast audience.
We've invited the DWR to appear on the podcast to provide whatever clarifications/updates they may have. No word back yet on their level of interest in doing so. Per usual, we'll keep you updated as we have more.

Juan Browne just posted his latest video of a boots on the ground tour with the engineers. This includes a discussion of what led up to the failure and a walking tour, detailing the rebuild construction.
no fear mongering, no conspiracy theories… just facts and data folks.
enjoy part one of likely three videos to cover all the material:

Juan Browne just posted his latest video of a boots on the ground tour with the engineers.
No fear mongering, no conspiracy theories… just facts and data folks.