A pair of crippled spillways at Oroville Dam can be repaired in part by November, but a good deal of the work will probably have to be done after the next rainy season, according to reports by an independent panel of experts.

The five-member panel, hired by the California Department of Water Resources, found during three reviews in March that reconstruction of the spillways at the nation’s tallest dam — the badly mangled main chute and the eroded emergency spillway — will likely take more time than is available this year.

But there is at least enough time, the panel said, to make fixes that ensure Lake Oroville doesn’t spill over and flood the region.

The release of the reports on Monday came a week after the state awarded a $275.4 million contract to Kiewit Infrastructure West Co., a division of international firm Kiewit Corp., to repair the damaged structures. The winning bid was nearly $44 million more than a $231.7 million estimate by department engineers.

“Complete repair of the emergency spillway is not anticipated by November 1, 2017,” said the panel, named at the behest of the Federal Emergency Regulatory Commission, in its March 31 report. “However, work will continue on the emergency spillway to ensure it can safely pass the original design flow criteria by November 1.”

The damage to the main spillway was first detected Feb. 7 as operators released water from Lake Oroville amid this year’s historic rainstorms. As a pothole opened up and then grew into a full collapse of the concrete chute, the water in the reservoir rose and began pouring over an emergency spillway that had never been used.

The emergency spillway sent water spilling over a concrete apron and down a bare hillside, and the hillside eroded quickly, prompting the evacuation of 180,000 people downstream amid fear of a catastrophic flood.

The panel of experts was hired to review design plans and technical details related to repairs of the spillways. Another forensics team is studying why the spillways failed, but officials said a final determination is not expected until sometime in the Fall. The larger goal is to prevent such a crisis from happening again.

Kiewit is expected to partially rebuild and stabilize both spillways before November. Any remaining work will be done in the summer of 2018, according to the preliminary plans. The state, citing security concerns, has declined to release the full scope of the work or full details about what went wrong, but the reports give an idea of the complexity of the project.

“Restoring the original (main) spillway will require that the entire lower section of the chute” be rebuilt, the March 31 report said. It outlines a scheme to buttress the end of the chute so water would flow into a plunge pool, which would direct flows down a carved-out canyon without causing further erosion.

“The design has already started, but the construction will not start until the middle or end of May,” said Dave Gutierrez, the former chief of the California Division of Safety of Dams. “The current plan is to make repairs all the way up to the gated structure. Any slabs that are downstream of that, those will get repaired” or replaced.

A March 10 analysis by the panel outlined some problems with the original spillway design, including too little concrete, compacted clay being used to fill depressions in the underlying rock, and leakage.

“The amount of drain water flowing from the pipe discharge openings along the spillway training walls seems extraordinarily large,” the report said. “It appears also that the drains are collecting leakage through cracks in the chute slab and/or defects in the construction joints between slabs.”

UC Berkeley’s Center for Catastrophic Risk Management released a report earlier this month saying the spillway wasn’t designed to meet today’s standards.

The author, Bob Bea, a professor emeritus and engineering expert at UC Berkeley, said the concrete on the main spillway at the 50-year-old reservoir was too thin and not adequately anchored. The finding is similar to what several state consultants said before officials started making their reviews confidential.

Bea also said the state failed to do adequate maintenance at the site, including repairing cracks in the spillway that allowed water to seep in and weaken the structure.

The big agencies that buy water from Lake Oroville are expected to pay for a large portion of the repairs, along with the federal government, state water resources officials said.

The state has been forced to continue releasing water down the main spillway, further testing the carved-out hillside around the broken part of the chute. It has so far handled the pummeling from the releases, which are necessary as snowmelt flows into Lake Oroville.

Crews have reinforced the main spillway with concrete and bolted it to the ground, and every crack on the chute has been sealed, officials said. Meanwhile, tons of concrete and boulders are shoring up the hillside below the emergency spillway, though the state hopes not to use it again.

Gutierrez, an expert on dam construction, said the new spillway will be designed before a final determination has been made about the cause of the collapse, but designers will use the “potential causes” laid out by the forensics team to avoid problems.

He said the forensics team will also review the actions taken by dam operators between Feb. 7 and Feb. 12 to determine if their actions helped the situation or worsened the crisis.

Peter Fimrite is a San Francisco Chronicle staff writer. Email: pfimrite@sfchronicle.com Twitter: @pfimrite