Report Identifies 28 Potential Causes of Spillway Damage at Oroville Dam

May 10 was an important day for the Oroville Dam.

But the last scheduled spill of the season wasn't the only release people were talking about.

Wednesday, the Department of Water Resources released preliminary findings of a forensic investigation into what factors could have contributed to damaging the spillways that led to a mass evacuation in February.

"Preliminary report would be called suspicions confirmed," said Republican state Sen. Jim Nielsen.

Nielsen was not surprised by the findings of 28 possible factors.

To name a few for the service spillway:

2. Large variations in slab thickness
3. Limited slab reinforcement
5. Corrosion and failure of reinforcing bars across tracks
11. Plugging or collapse of drains
12. Flow into foundation that exceeded capacity of drain pipes

And for the emergency spillway:

2. Hillside topography that concentrated flows and increased erosive forces
4. Absence of erosion protection

"It could be anything, it could be all of them at the same time," said Professor Saad Merayyan.

Merayyan is a professor of water resources engineering at Sacramento State.

He said the forensic findings don't point in one specific direction but do present the possibility that the spillways had damage that went long unnoticed.

"It's possible. Could be at small scale and then when a large amount of water is released from the reservoir, that caused that to basically magnify," Merayyan said.

That's something thousands of evacuees feared to be true all along. It's why many of them distrust the state agencies maintaining the dam they live just downstream from.

"Over the decades, it's very clear that the inspections of the dam and spillway have not been thorough, particularly in terms of going down deep into the soil profile," Nielsen said.

The DWR says none of its previous inspection reports showed any signs of spillway damage.

"So we didn't expect it, but we're certainly going to fix it and make sure nothing like it happens again," said Erin Mellon with the Natural Resources Agency.