I'm trying to understand context around the design of the dam spillway vs. historical rainfall and usual flow...
Was this way more rain than it was designed for? Or was it structural failure within design tolerances that caused all the damage?
Was this way more rain than it was designed for? Or was it structural failure within design tolerances that caused all the damage?
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.
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.
https://en.m.wikipedia.org/wiki/Institute_for_Creation_Resea...
Dont underestimate water and human greed.
Present damages now exceed $100m, and repairs are likely to be several times the initial estimates.
In reality, it failed well before the stated tolerance and developed holes. Once that happened all bets were off.
Wikipedia has more info; it gives the capacity of the gates above the spillway as 150,000 cu ft/s, and says the spillway developed cracks in 2015.
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.
The unusually wet year and failure of the emergency spillway then forced them to run the main spillway at a high rate despite the damage.
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.
http://www.mercurynews.com/2017/02/12/oroville-dam-feds-and-...
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.”
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.