There are two spillways, the main spillway and the emergency spillway.
The main spillway is the concrete spillway, and it developed a hole in late January. Due to very real concerns that it would deteriorate, the decision was made to use the emergency spillway until the concrete spillway would be fixed.
The emergency spillway is basically a concrete weir--when the lake level reaches a certain height, it spills out over concrete and runs down the hillside. This is considered an emergency spillway, and erosion is usually expected for such spillways (the regulatory definition of an emergency spillway is basically "it can be used once and then it needs a long downtime while it's repaired).
When the emergency spillway was in use, what happened was the very same erosion that was underpinning the main spillway was also occurring near the concrete weir. If left unchecked, the fear was that it would start eroding the concrete weir leading to potential collapse of the concrete weir. This prompted the decision to evacuate as much water as possible using the damaged main spillway which greatly exacerbated the damage (as the pictures show, a large section of the spillway now fails to exist, and it looks like most of the lower spillway would need to be torn up instead of being unused).
I'm not a geologist, and I certainly don't have detailed nature of the local geology. But my suspicion is that the prolonged drought followed by the intense wet season destabilized the slope (by weakening the organic stabilization of the slope and then loading it heavily with water), so spilling any dam water anywhere on it would lead to massive erosion (including undermining the concrete structures). The emergency spillway is supposed to be able to carry far more water loads. I don't think the severe-drought-then-extreme-rain scenario was envisioned during the dam construction (although it really should be after this event), so I don't know if the spillway would have performed as badly in "expected" conditions.
Also not a geologist, and not familiar with local geology. Packing so much water in a very small channel really helps water erode anything. If i remember correctly, it's thought Niagara falls moved a few miles in a day. That seems terrifying to me, it's a long drop down. Waterfalls slowly work their way upstream, obviously. Less obviously, a little weakness lets more water though, compounding the abrasive effects. Which allows more water, which cuts away the land faster.
So yeah, i don't think the dryness had anything to do with destabilizing anything. Water is just really good at cutting. The dam created a huge reserve of water, so lots is available to cut.
This is my understanding: the regular spillway suffered damage while it was being used, and had to be shut off. I don't think it's clear whether that was due to some kind of oversight or it "just happened". If the damage had not occurred, everything would've been fine, but it forced the use of the earthen emergency spillway, which eroded faster than anticipated. There had been calls to upgrade the emergency spillway to concrete, but these were not pursued for reasons of cost. So it was a combination of unexpected damage to the regular spillway and inadequate construction for the emergency one.
There is often the case of why was there so much water in the first place. I dont know much about this dam but here in Sweden the companies running water dams have been known to save up too much water in expectation of better prices. This leads to emergency dumping of water if it rains too much and the downstream areas gets flooded. Not this badly but a few houses gets water in the basement sort of flooding.
In California, sometimes they save up too much water, but it's because rain patterns are hugely variable. They don't know if the next big rainstorm will be next year or next month. So sometimes they get it wrong, but better to have too much water than too little.
There was previously observed damage to the primary spillway noted in 2005. A $100m repair was requested of state and federal (FERC) authorities, during the GOP-lead Bush and Schwarzenneger (national and state) administrations, but was rejected.
Present damages now exceed $100m, and repairs are likely to be several times the initial estimates.
So Brown and the CA Democrats are off the hook for ignoring a $100M repair, because Ahnold didn't fix it? IMO that makes them both the same. Actually, it makes the current administration worse because the increase in damage happened on their watch.
From my understanding, it wasn't so much the rainfall/flow as it was the damage to the spillway.
If the spillway doesn't experience structural failure, there's no problem. But it did, so flow through the spillway had to be reduced, which led to the emergency spillway coming into play and they had to continue using the damaged spillway, failure or not.
I guess it ends up depending on how failure is defined for a given context. It didn't fail in the sense that the area still holds back water. It did fail in the sense that they believed it to be unsafe to continue using it.
Considering they kept using the spillway even though it was damaged (because they hadn't repaired it as they should have due to lack of funds), and they had to use the emergency spillway (which wasn't reinforced because they didn't do it 12 years ago when they should have). I vote more than expected rainfall (216% of normal by some accounts - though they might have also been storing more water than normal due to the drought) and lack of proper funding to keep the engineering tolerances where they were supposed to be.
But I'm more inclined to suspect the problem is larger. For example that anthropogenic climate change is increasing the amplitude of the El Nino effect.
More than a decade ago, federal and state officials and some of California’s largest water agencies rejected concerns that the massive earthen spillway at Oroville Dam — at risk of collapse Sunday night and prompting the evacuation of 185,000 people — could erode during heavy winter rains and cause a catastrophe.
Three environmental groups — the Friends of the River, the Sierra Club and the South Yuba Citizens League — filed a motion with the federal government on Oct. 17, 2005, as part of Oroville Dam’s relicensing process, urging federal officials to require that the dam’s emergency spillway be armored with concrete, rather than remain as an earthen hillside.
The groups filed the motion with FERC, the Federal Energy Regulatory Commission. They said that the dam, built and owned by the state of California, and finished in 1968, did not meet modern safety standards because in the event of extreme rain and flooding, fast-rising water would overwhelm the main concrete spillway, then flow down the emergency spillway, and that could cause heavy erosion that would create flooding for communities downstream, but also could cause a failure, known as “loss of crest control.”
Agree that deferred maintenance is the primary culprit, based on what everyone is saying... The spillway should have withstood the flow of water from the extensive rainfall if operating within specifications.
While there was a lot of rain, and climate change is affecting long-term weather patterns, I think it would be misleading to associate the Oroville Dam problems to climate change.
Water management is difficult in California because you never know if this rainstorm will be the last of the season, or if the next one will overflow everything. It becomes a balancing act of when to let water out of the dams.
Was this way more rain than it was designed for? Or was it structural failure within design tolerances that caused all the damage?