Hacker News Reader

Comment by anotheryou | Hacker News Reader

Overflow means the actual spilling over the top (the emergency spillway) on to hill and street below? So we still have the broken main spillway in action or not?

View on HN

It's not just flowing over the top of the dam; the water is channeled through an area designed to handle overflow; but the channel is damaged.

See pic: http://s4.reutersmedia.net/resources/r/?m=02&d=20170213&t=2&...

I believe they're worried about the erosion tearing apart the hillside and thus destabilizing the entire hill; at which point the whole hill would just collapse.

LA Times has more great photos.

Google "Major sinkhole on spillway at Lake Oroville" to avoid the paywall.

http://www.latimes.com/visuals/photography/la-me-lake-orovil...

What I've been wondering is: What, if anything, did they do during all of these years of drought in order to ensure dams and all required mechanism remained sound and ready for when rains returned?

The lake near my house was down 120 feet. You could see land and structures never seen in decades. Prime opportunity to get in there to inspect and maintain said infrastructure. Not sure anything was done at all.

I am no expert in concrete but I would like to think the break in the Oroville spillway could have been detected and dealt with with cursory inspections over the last several years, when access was easy due to the drought.

I don't like it when folks are put in danger because other folks who are supposed to be working for us don't do their jobs. Not saying this is what's happening here, I don't know, but the question needs to be asked.

The spillway can be completely shut during normal operation of the dam. So the drought didn't provide any increased opportunity to do inspections.

It did in the sense that you could undertake projects that would be impossible had the lake been full. In other words, you had certainty of not needing the spillway for months/years.

It's pretty impossible to predict that the spillway won't be needed in a given wet season.

They share the data about the level of the reservoir:

http://cdec.water.ca.gov/histPlot/DataPlotter.jsp?staid=ORO&...

The wet season last year basically filled the reservoir up from its lowest levels. So a project started in the middle of 2014 would need to leave the system operable by December, and they have that same ~6 month period in pretty much every year.

The point is they had 6 to 10 years (however long the drought lasted) to get ready for this moment in time, even if a little bit at a time.

Nobody ever seems to do this. We always wait for disasters before fixing or maintaining anything. This, despite the fact that we pay taxes, plenty of taxes, to fully staff, train and employ departments with thousands of people who are supposed to actively be looking after these things.

It's the infrastructure equivalent of a sysadmin who never looks after security and then, when all hell breaks loose, proclaims we need to do something about security.

It's easy to say this after the fact, but infrastructure like that has a minimal budget for that sort of stuff. If we did this on every dam in the US, it would potentially amount to a billion dollars or more. At a time when the government is trying to claw back budgets, that sort of thing is never going to be approved.

Sure, but my point is that they have the same opportunity to do inspection whether there is a drought or not. So they've had the lifetime of the dam to do inspections and maintenance, not just a few years.

There's lots of possibilities. Maybe they didn't do inspections last year. Maybe they did inspections and noted damage that did not yet warrant repair. Maybe they did inspections and missed damage. Maybe they were unable to secure funds to repair the damage. Maybe the damage was noted too late to do repairs last fall. Etc.

The thing to do is wait and see if they come forward with a reasonable explanation of what went wrong and then feed that information into the maintenance plans at this dam and others.

Generally sink holes like what is affecting the spillway are caused by rain and flooding. So I'd say it is likely that the inspections all came up clear for the past X years, then the rain started, the sinkhole formed and the reservoir started filling at the same time.

What's interesting is that you know this. Which means that others should as well. Which means that, I don't know, perhaps modelling what might happen as rains returned, even conducting some tests with tankers full of water could be considered a prudent idea.

In other words, the idea that things could go bad once lots of water came back isn't a far-fetched concept. Yet we are reactionary. We don't seem to spend any time a-priory to try and get ready for such events. History is full of such fuck-ups.

Like I said in my other response; it comes down to money. If we took every "what-if" into account, we'd be wasting trillions of dollars each year on hypothetical scenarios.

This was the first time the spillway was EVER used in its existence; for all we knew at the time, it would have never been used.

I should have used a different term, for clarity. By overflow I meant that the estimated amount of water in the reservoir exceeded its total storage capacity and they had to use spillways to drain the excess. We have a geo engineer on our team and he's saying that while primary spillways are made of concrete, the secondary (emergency) spillways are made of soil and materials and can deteriorate quickly.

Overflow over the top of a dam this size and type will almost certainly result in complete structural failure. The overflow we've seen so far is a secondary canal diverting excess water away from the main dam.

Is there an article discussing the dangers of continuing to use the main or emergency spillways? I'm not clear on why the downstream destruction is so dangerous. (And I apologize if it's completely obvious, maybe I'm just dense on this.)

The spillway is damaged and eroding as water flows through it. As it erodes it opens up such that more water can flow, accelerating the erosion to the point where the entire hillside erodes away and the dam essentially fails.

I can understand those concepts just fine, but when I look at the dam in Google Maps, it is not clear to me how this is specifically going to occur. The main spillway is on the hillside next to the dam, and the emergency spillway is on the same hill, yet further from the dam.

You have to remember that the hill _is_ the dam (of the type[1]). so when water flows down that spillway, it's going to pick up dirt with it - making the hill smaller and smaller, ie. making the dam thinner and thinner.

[1]https://en.wikipedia.org/wiki/Embankment_dam

The dam is the earth type of dam, but the portion the spillway is on is not part of the dam, at least as I am looking at it in Google Maps:

https://www.google.com/maps/@39.5400137,-121.4931958,3617m/d...

But if it eats up the spillway upwards it engangers the little part of tho damm where the vales are. If you do not have a concrete lip like on the auxillary spillway and it just flushes over the top of the hill it will quickly carve a lot deeper, so you have more water than just what was held back by the smaller wall.

Also: the top 70 meters or something is still a lot of water!

You're drawing distinction between the 'man made' part of the dam and the 'natural' dam, which exists on paper but not in the static force analysis. If that natural portion of the dam becomes unable to support the weight of the water, there will be a collapse.

The area around either spillway may continue to erode away. The main dam structure is currently not in danger. They don't want to run the main spillway at all but they also don't think they can rely on the emergency spillway.

It would still be a catastrophe, the outflow will go from inches per hour to feet per hour.

Okay, this makes more sense. The problem is that the spillways will fail in the sense that the downstream capacity cannot handle the volume of flow, so will flood those regions, as well as possibly flood areas not typically even near water as it carves its own paths.

California's damming of the Sierras and construction of the aquifers were primarily about flood control. Supporting the growing population came secondary (that's the easy part). An enormous effort was put into diverting melting snowpack into mountain valleys. This essentially created the northern and middle parts of the Central Valley, on top of creating sustainable year-round sources of fresh water for the developing coastal cities.

Downstream destruction is of concern because that entire region depends on flood control. The habitability and agricultural production of the corridor between Oroville and Sacramento relies upon, and was created by, the flood control of Plumas, Yolo, Butte, etc counties. There are a number of canals, diversions, and reservoirs both above and below the Oroville to further control flow the dam as it reaches our rivers. However, most of the re-routing is also man-made and thus untested for such an event.

The two spillways have different issues:

For the main service spillway, the main concern is that the flow of water will cause erosion damage upstream of the existing damaged site[1]. If the damage to the concrete continues up the spillway to the top, it could render the spillway inoperable.

For the emergency spillway, the main concern is that a continued flow of water would erode the soil off the hill to the point that the hill would no longer support the spillway (the concrete lip at the top of the hill). If this happened, the spillway would fail. The effect wouldn't be as severe as if the dam failed (because there would still be a large hill in between the water and where it wants to flow), but the erosion along the path of the water would get out of control pretty quickly.

[1] https://en.wikipedia.org/wiki/Headward_erosion

A picture is worth 1000 words: https://sandrp.files.wordpress.com/2017/02/screen-shot-2017-...

Dirt hillsides are not designed to have 100,000 cubic feet of water rolling down them. They tend to erode extremely quickly and can quickly turn into uncontrolable discharges if the lip erodes.

Reply on news.ycombinator.com