Aerial View of Oroville Dam and its Damaged Spillway

[Humpy]

If the normal gates had enough capacity to meet all flood conditions, they would not have put in the emergency spillway, though. I can't find the flood of record at the dam site, but downstream a bit it is 360,000 cfs and the dam's main spillway is only 250,000 cfs.

It would depend on how early the gates were opened? Whatever can't be drained is stored until it can be drained later.

In the event of overwhelming rain, where the entire thing is fucked no matter what, they built in a point of failure. It's not something that I'd think was supposed to be used by choice.

 

[Zorba]

It would depend on how early the gates were opened? Whatever can't be drained is stored until it can be drained later.

In the event of overwhelming rain, where the entire thing is fucked no matter what, they built in a point of failure. It's not something that I'd think was supposed to be used by choice.

You have to be able to at least release the maximum possible inflow 1 to 1, because it is possible that you will hit you maximum possible inflow while your food pool is already full. This actually happened to a lot of lakes in Oklahoma a couple of years ago, the flood of record occurred when all flood pools were at 100% (the larger ones were actually given waivers to go up to 110+℅). There was actually a current going across the largest lake in the state as it outflowed over 400,000 cfs. Before the rain started, the state had been in a 4 year drought, and most lakes had less than 50% of their conservation pools full.

I agree that the emergency spillway wasn't installed with the idea that it would be used routinely or on a whim, but it is there to be used during a 500 year flood event with some margin. It was not installed simply to say they had it, but to cover certain plausible design cases.

 

[Humpy]

^^^
That all makes sense.

I had made an assumption that there was always plenty of storage capacity in reserve that, combined with what could be released, would handle every situation.

 

[0roo0roo]

Just remember, sanctuary cities are californias priority now.
Infrastructure so decrepit being released from a drought almost results in disaster.

http://www.mercurynews.com/2017/01/23/bay-area-rains-bring-a-bumper-crop-of-big-potholes/
http://www.mercurynews.com/2017/02/10/roadshow-finally-pothole-relief-coming-to-i-580/
http://losangeles.cbslocal.com/2017...es-on-l-a-streets-soars-due-to-frequent-rain/

 

[Carson Dyle]

If the normal gates had enough capacity to meet all flood conditions, they would not have put in the emergency spillway, though. I can't find the flood of record at the dam site, but downstream a bit it is 360,000 cfs and the dam's main spillway is only 250,000 cfs.

You can't predict every possible flood scenario. Or every emergency situation, such as a failure of the main spillway. With an earthen dam like that, it would be so horribly catastrophic if water overflowed the dam, that there had to be some kind of emergency outlet to prevent that from happening.

It appears to me that the emergency spillway did exactly what it was planned to do. Discussion elsewhere is about how the rock bed beyond this spillway was gradually weakened over the decades by being exposed to air and weather. Thus the erosion, and the danger from undercutting and the fear of a possible failure of the weir comprising the spillway.

 

[MikeMike]

Zorba...

Downstream flow means nothing. there's MULTIPLE tributaries that feed that river. the maximum inflow into Oroville reservoir was 190,435 CFS.

The spillway was rated at 240,000 CFS.

Obviously, it's not going to handle 240k at the current state, but they are running at 100k constantly and should be able to absorb the next storm with minor impact.
- The upcoming storm is colder, with less rain than the past. This has HUGE implications in the amount of water to expect.

 

[Zorba]

You can't predict every possible flood scenario. Or every emergency situation, such as a failure of the main spillway. With an earthen dam like that, it would be so horribly catastrophic if water overflowed the dam, that there had to be some kind of emergency outlet to prevent that from happening.

It appears to me that the emergency spillway did exactly what it was planned to do. Discussion elsewhere is about how the rock bed beyond this spillway was gradually weakened over the decades by being exposed to air and weather. Thus the erosion, and the danger from undercutting and the fear of a possible failure of the weir comprising the spillway.

There are hundreds of earthen dams around the country with no emergency, uncontrolled spillway. Typically controlled spillways have insane amounts of over capacity and are designed based on hydrology and codes. The installation of the emergency spillway would've been to account for one or more design conditions that could not be meet with the normal controlled spillway. It would not have just been thrown up there for looks, but knowing there were certain conditions that could press it into service. I am guessing they did a split spillway due to the cost of installing extra capacity in the main spillway, so they covered the very extreme, very low likelihood, design cases with the emergency spillway.

Note, there are massive earthen dams, such as Denison Dam, that use uncontrolled spillways as their main flood release mechanism. There are many reasons this is not typical, though.

Here is one example I know of as an example of the over designed capacity of a typical dam, the Keystone Dam which is upstream of Tulsa experienced a 500 year flood in 1986, when the flood control pool was already over 100% capacity. The peak outflow was 350,000 cfs. The gated spillway is capable of 939,000 cfs, at which point nearly all of Tulsa would be underwater. Even using only 1/3 of the gate capacity over a few days, and much lower otherwise, they dumped 4 times of the full capacity of the lake over a 3 week period, or about 2 times the normal pool capacity of Oroville lake.

 

[MikeMike]

and the emergency spillway was rated at 435,000 CFS or something like that when it was designed.

The issue is most likely due to natural weathering of the exposed bedrock causing it to weaken and thus erode unexpectedly with the water level.

It may also be an issue of not understanding exactly how the emergency spillway was designed and what the expected erosion was around the weir.

 

[Carson Dyle]

It may also be an issue of not understanding exactly how the emergency spillway was designed and what the expected erosion was around the weir.

Or an issue of not having a whole hell of a lot of choice and trying to balance between two potentially catastrophic scenarios. Dump more water down the damaged spillway and it could destroy what remains of it, starting with taking out that power transmission tower teetering on the edge of the ravine.

 

[MikeMike]

Sorry, I can't quote, I throw errors when I do.

Those power lines don't supply power to the dam.

The dam is supplied by an underground feed and also has a built in generator in case of power failure to allow for operation of the spillway.

It was also a "well we may as well try and see what happens" as you said. If it worked as an emergency, there's no reason they wouldn't have just let it run its course that way and stopped utilizing the broken spillway.

 

[Zorba]

BTW: My whole point is that it was installed knowing it could be used. Knowing it was designed so that it could be used under extreme circumstances, it is pretty crazy how much damage occurred with very low flow rates or a short period of time. I am sure the engineers on site believed it would perform much better than it did, even if they did expect some erosion. I'm arguing against the idea that it was never designed to actually be used and that they people on site were just clueless.

 

[Leyawiin]

No, it's really not. In entertainment there's no such thing as bad publicity, in dams the equation is entirely different. Focusing this much attention on a possibly defective dam is not a boon to the economy and it most assuredly does not help property values anywhere downstream.

Name a single instance of any community that was helped long term by a short term influx of money spent by people there to clean up a disaster. Just one. It is idiocy of the highest order to suggest that this is not going to impact property values in the area downstream of that dam just because the town gets a one-time spending boost.

You can't compare a contaminated superfund site in rust belt upstate New York with a dam spillway system in California that can be fixed. Our population will keep growing and our real estate is worth more. If there's no flooding this will fade from memory especially once improvements and repairs are completed. We've had serious flooding in the past in other California communities (El NIno 1997) and they've all bounced back and their real estate prices have continued to climb. Not to mention scores of homes consumed by wildfires or damaged by earthquakes over the decades. Again, they're replaced or repaired and things get back to normal within a few years tops.

 

[Humpy]

The issue is most likely due to natural weathering of the exposed bedrock causing it to weaken and thus erode unexpectedly with the water level.

How much natural weathering can occur in 60 years?

 

[Humpy]

Or an issue of not having a whole hell of a lot of choice and trying to balance between two potentially catastrophic scenarios. Dump more water down the damaged spillway and it could destroy what remains of it, starting with taking out that power transmission tower teetering on the edge of the ravine.

The choice was between further damaging something that was already damaged, but overall functional and controllable versus doing something that had never been done before.

With a bunch of people living below.

If one is being cautious with a concern for safety, it's not the best time to be trying something new.

 

[Humpy]

BTW: My whole point is that it was installed knowing it could be used. Knowing it was designed so that it could be used under extreme circumstances, it is pretty crazy how much damage occurred with very low flow rates or a short period of time. I am sure the engineers on site believed it would perform much better than it did, even if they did expect some erosion. I'm arguing against the idea that it was never designed to actually be used and that they people on site were just clueless.

The part of the emergency spillway that was designed worked flawlessly. The overall system did not, because mostly it was just a hillside. I don't know how a design can fail so soon and still be considered to be a design.

Regardless, I disagree with the idea that it was designed and installed to be used in the way that it was a few days ago. The design appears to be for one-time use during catastrophic failure to slow down and spread damage away from the main dam and spillway.

 

[Jaskalas]

All that water flowing out of the emergency spillway and the water level still rose above that to flood a nearby parking lot and then uncontrollably spill out over bare land. Needless to say that design needs to be improved upon. The emergency spillway should have a secure base, and should have been placed at a lower elevation.

The evacuation and economic toll is due to the design flaw.