Nice graphic. A few of more questions:
1) Do you know when prior to Feb 2017 the spillway outflows were significantly greater than 50,000 cfs? Did they ever previously run the outflows at full capacity of 150,000 cfs?
2) Any educated guesses as to why there was damage at 50,000 cfs now and not after outflows were significantly greater? (Maybe the one-two punch of first weakening and then later, final breaking - and/or - the constant battering of hot/cold expansion/contraction pressure on the concrete over about 50 years?)
3) I've heard some experts call this one of the greatest engineering failures in history. Maybe that's too much, I don't know, but how do you rate the failure of the spillway after it ran for 24 hrs at about one-third capacity its planned capacity?
low bid?
As ER333 has pointed out, the undermining due to the flawed drainage system design produced (a) thinning leading to diagonal cracking that keeps getting worse, (b) voids under the slab due to wash-out which now hold more water for more washout, etc etc [except where voids are filled in recent repairs] and (c) the spillway walls were not ripped out previously and now the outflow has potential damage every time it is ramped up. (d) I think we are also seeing a further compromised underslab drainage system itself, water release may occur without the full release of pressure.
According to this, they operated at 160K cfs in 1997.
http://www.chicoer.com/article/NA/20170218/LOCAL1/170219704
“The weather this year, as bad as it seems, was much worse in 1997. There was a one-week period where everything was flooded.
People were being evacuated in Tehama, Hamilton City, Durham, Yuba City and Marysville. Butte Creek cut a new channel in the canyon. Highway 70 above Lake Oroville was badly damaged. No valley communities were spared. Roads, bridges and a section of railroad were washed away. It was relentless.
The inflow to Lake Oroville on New Year’s Day was 328,000 cubic-feet per second. By comparison, the peak inflow to Lake Oroville this month was 190,000 cfs.
The outflow in 1997 was 160,000 cfs. The peak last week was in the neighborhood of 110,000 cfs.
The difference, of course, is that in 1997 the spillway wasn’t broken.”
IMO, it wasn’t an engineering failure. It was a lack of proper preventive maintenance that caused the problem. Most concrete highway paving would last much longer than it does, if joint maintenance were done adequately.
But guess what gets cut when money’s tight? Maintenance. Governments never, ever cut payroll, they cut projects or material.
There’s never money to maintain, but endless funds to fix stuff once a catastrophe occurs.
“when prior to Feb 2017 the spillway outflows were significantly greater than 50,000 cfs?”
It was a very long time ago.
I checked releases since the 2013 repair to the spillway and a couple were 13,000 cfs, but all the rest were much, much lower. This was the first ‘test’ of the repair.
In all this heavy talk of the spillway’s ‘faulty’ drainage system it’s important to remember that it wasn’t designed to handle the water leakage from an unmaintained, cracked surface.
There is a bigger engineering picture here that frames the question on the "rating of failure". In this "picture" is the sequence of engineering judgements and events that continued in the face of stark evidence where other dam spillway designs have failed. I haven't put together the full sequence in a single post - only pieces of the whole story & history. Remember this dam was intended to sustain 1861/1862 flood scenarios.
Just a few notable points... the flood capacity of the combined spillway design was intended to handle "standard project flood" conditions of input rates of 440,000 cfs, by designing & rating the total combined spillway capacity of the dam at 646,000 cfs.
+Emergency Spillway stated design rating: 350,000 cfs. Failed. It couldn't sustain a meager 12,000+ cfs.
+Main Spillway stated design rating: 296,000 cfs. Failed. Blowout at 55,000 cfs.
In Engineering, designs have safety factor ratings (Factor of Safety FoF or Safety Factor SF) that are higher than the stated specifications (i.e greater than 1). IN the case of the spillways, this would translate into structural integrity greater than the stresses placed from the design rating - including knowing the erosion performance of the geology in the case of the emergency spillway.
One way of comparing the magnitude of failure: Right now the only operating spillway is damaged & limited to 50kcfs. This is a far sight short of a 646,000 cfs designed rating. In such high impact structures such as dams, there are no excuses. You must know its performance and be darn sure any unknowns are proven out. btw- They had later knowledge of failure modes after dams failed & they had time to evaluate/fix these issues in their spillway. This too should be factored into the failure consideration of "engineering judgement".