Where's the water? Apparently there were years of pre-warning signs [2] of an anomaly at the area where the main spillway failed. It is missing water. The main spillway has drain pipes under the concrete slabs that collect any penetrating sub-slab water and route it up into drain outlets in the concrete sidewalls.
btw- Scott Cahill, a dam expert, notes that a well designed/sealed/maintained spillway would have very little drain water during spillway operation [3] (should not be "jets of water" - these "jets" reveal how much volume and pressure the water may "scour" underneath the spillway slabs and substructure rock).
You can see water from these drains from 2013 and in 2017 (just prior to the "blowout failure"). Critically, the "blowout area" has missing water from the drain outlets. This is a critical indicator as a non-presence of waterflow indicates that water is not being collected (possible drain pipe failure) OR the water is not entering the drain either by an improper gravel layer emplacement OR the water has found a deeper flow path (or combinations of all three). Water has been observed to well up from under the main spillway slabs at the area of the blowout failure. The 2013 image reveals this. The 2017 image also shows a "defect" chip in a seam in the main spillway slab (at/near the blowout failure area). This 2017 picture was taken in the last week of Jan 2017, just before the Feb flooding & main spillway failure. Having this defect present would have likely initiated very damaging cavitation erosion from spillway flow (jackhammer effect on the concrete). Likely a precursor to the "missing water" substructure penetration = (i.e. potential sub-structure erosion deep under the slabs or undermining the "backfill concrete" chunks [1]).
= = reference notes:
1. Stagnation Pressure' as described by the Bureau of Reclamation: "Stagnation pressure refers to two conditions that can result in damage and/or failure of the spillway: (1) High velocity, high pressure flows enter cracks or open joints in the spillway flow surface (such as a chute), which results in uplift pressure that lifts (displaces) portions of the spillway conveyance feature; and (2) High velocity, high pressure flows enter the foundation through cracks or open joints in the spillway flow surface, which results in internal erosion of the foundation and loss of support of portions of the spillway conveyance feature [26]."
2. Alarms Raised Years Ago About Risks of Oroville Dam's Spillways:
..excerpt: "The cratering of the main spillway — which spiraled into the current crisis in Butte County — occurred in a spot where cracks and other defects had been found repeatedly since 2009, said Robert Bea, a professor emeritus and engineering expert at UC Berkeley." “My God, we had evidence that there was trouble going back to 2008, 2009,” said Bea, who at The Chronicle’s request reviewed 14 dam inspections from 2008 to 2016 conducted by the Division of Safety of Dams, which is part of the Department of Water Resources.
“Yes, they had detected the defects (in the main spillway) and yes, they had put into gear remedial measures,” Bea said. “Were those repairs sufficient? No. The result was a breach.”
.. end excerpt
http://www.govtech.com/em/disaster/Alarms-raised-years-ago-about-risks-of-Oroville-Dams-spillways.html
3. One highly respect dam design expert, Scott Cahill, had some very enlightening information regarding Oroville's main spillway & issues linked to his extensive knowledge on dam failures. Three of the key issues he noted in detail was (1) thorough & expert level maintenance (2) the Oroville main spillway had too high of an underflow of water beneath the concrete [as he noted by the amount, volume, and velocity of the "jetting" underdrains - this is a significant issue to address to prevent erosion scouring and sub-spillway pressurized leakage] - [i.e. this was an "early warning" that should have been taken action upon] (3) the original construction MUST be thorough on its bedrock/concrete and anchoring stability.
Workers in 2013 at pre-failure blowout area - notice the missing drain flow (red circle) and the notable upwelling of water in the main spillway.
Same missing waterflow in drain near pre-failure blowout area. notice the defect in the concrete slab seam at/near the pre-failure blowout area.
