As Reservoir Rises, Rebuilt Oroville Dam Spillway to Be Used for First Time

Update, 8:55 a.m. Monday: The Department of Water Resources announced Monday that it will begin releasing water down the rebuilt Oroville Dam spillway on Tuesday. The release is designed to help manage the level of Lake Oroville, the reservoir behind the dam, as storms continue to roll across Northern California and this season's large snowpack begins to melt.

Details on the spillway reconstruction and 2017 disaster follow:

Original post, Tuesday, March 26: The agency that manages Oroville Dam says the facility's rebuilt spillway is likely to be pressed into service for the first time as soon as next week.

The Department of Water Resources announced Tuesday that Lake Oroville has risen close to the point where the agency will need to release water to maintain empty reservoir space for runoff from incoming storms and spring snowmelt.

DWR said it will give the public between 24 and 72 hours advance notice of a release, which can be expected to cause relatively rapid rises on the Feather River downstream of the dam.

In the meantime, the department said construction crews are extracting heavy equipment from the spillway area and removing a construction road that had been built at the base of the new concrete chute.

Spillway releases are a normal function of dam operations. They're designed to prevent reservoir water from rising to the point where it could spill over the top of a dam -- thus weakening the dam structure and threatening a catastrophic failure.

Even though the upcoming releases are likely to be relatively modest, the event will get widespread attention because of the 2017 crisis at the spillway, which prompted evacuation orders for 188,000 people living in communities along the Feather River.

Parts of the 3,300-foot concrete spillway chute began to disintegrate on Feb. 7, 2017, as dam managers released water to make way for heavy runoff unleashed by a series of warm winter storms.

Uncertainty about whether the spillway could be repaired led DWR to shut down the spillway temporarily to inspect damage, then to resume flows to test their effect on the structure, then halt them again.

Runoff cascaded into the 3.5 million acre-foot reservoir as forecast, triggering a rapid rise in the lake and raising the possibility that floodwaters would tumble over a quarter-mile-long weir adjacent to the top of the spillway and down a steep hillside that dam designers had designated as an emergency spillway. That had never happened in the dam's 49-year history.

Early Feb. 11, water topped the emergency weir and began sluicing down the unlined natural slope. DWR had insisted during earlier expert reviews of the dam that the rock beneath the hillside could withstand huge flows of water without disastrous erosion, but the relatively modest flow that began dumping a porridge of mud, rock and trees into the channel below showed otherwise.

Parts of the hillside gave way so rapidly that by the afternoon of Sunday, Feb. 12, some DWR employees voiced the fear that the emergency weir at the top of the slope would collapse. If that happened, an uncontrolled released from the lake would result in a wall of water smashing through Oroville and other communities along the Feather River.

That possibility prompted Butte County Sheriff Kory Honea to order an evacuation — which quickly led to similar warnings in neighboring Sutter and Yuba counties. The orders were lifted two days later.

Subsequent investigations, including an exhaustive inquiry conducted by a team of outside experts, concluded that a host of design, construction and maintenance problems led to the main spillway failure.

The independent forensic team also found that early geologic studies of the hillside below the emergency weir had documented the presence of highly erodible rock in the area -- a fact that didn't come to light because subsequent reviews of the Oroville facility simply failed to "diligently review" the available records.

The main spillway has been rebuilt to modern specifications, and an extensive area at the top of the emergency spillway has been reinforced with a variety of concrete structures -- all at a cost of $1.1 billion.

The Federal Emergency Management Agency has reimbursed the state for about $333 million of the recovery and rebuilding project so far. But FEMA recently balked at covering another $306 million in costs for the project, citing pre-existing conditions on both the main and emergency spillways.