High water releases have eroded the base of Lake Oroville’s spillway

Faced with rising reservoir levels, state engineers on Wednesday were trying to figure out how much water they could send down Lake Oroville’s damaged spillway.

The state planned to make test releases, knowing the flows likely will enlarge the 30-foot-deep hole in the concrete spillway of California’s second-largest reservoir.

Workers discovered the erosion Tuesday and stopped the high releases the state was making to maintain required space for incoming runoff.

To partially counter the spillway shutdown, they increased releases through Oroville’s power plant on Wednesday. But there is a limit to that.

That leaves the state with a difficult choice: keep sending water down the spillway, which will worsen the damage, or let the reservoir fill, which would send flows gushing down a separate emergency spillway that is not paved or gated.

Department of Water Resources officials want to avoid doing that if possible, but as a precaution workers are clearing trees and debris from downstream areas. Engineers also are looking for ways to bolster the spillway.

“The dam is sound, and no imminent threat to the public exists,” the department emphasized in a news release.

It is unclear why flows gouged a hole 200 feet long and 75 feet wide in the concrete chute that functions as the spillway for the earthen dam.

“Something failed under the spillway to cause it to collapse,” water resources spokesman Doug Carlson said.

Oroville is the key reservoir in the State Water Project, which sends Northern California supplies to the urban Southland.

The recent parade of storms that have pounded Northern California filled reservoirs to above-average levels for this time of year, forcing managers to ramp up releases to make room for more inflow.

After the state halted spillway releases Tuesday, so much runoff continued to flow into the reservoir that Oroville went from 80% full to 84% overnight.

“At this point, they have to be prepared to use the broken spillway,” said Ron Stork, who has studied Oroville operations as a policy staffer for Friends of the River, an environmental group. “If they don’t, the reservoir is going to rise and there is no place to put a big inflow.”

“I don’t think this is an impending disaster,” he added. “But this is a fairly serious set of dilemmas that DWR is going to have to solve.”

Typically, reservoir releases are made through Oroville’s powerhouse and the spillway is used only for high releases. The last time managers opened the spillway valves was in 2011.

bettina.boxall@latimes.com

Twitter: @boxall

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UPDATES:

5:40 p.m.: This article was updated with coments from environmentalists and new information from the Department of Water Resources.

12:30 p.m.: This article was updated with new information from the Department of Water Resources.

This article was originally published at 10:05 a.m.