Oroville >> Small cracks have appeared in the new concrete in the main spillway at Oroville Dam, but the state says they were expected and nothing to worry about.

An explanation of the “hairline cracks” was called for in a letter the Federal Energy Regulatory Commission wrote to the Department of Water Resources in October. KQED radio in San Francisco first reported the previously undisclosed letter Tuesday.

The Oct. 2 letter from FERC said it understood DWR was concerned when the cracks were discovered after concrete curing covers were removed from some of the slabs poured this year.

It called for a report within 30 days that mapped the cracks, identified potential causes and spelled out remedies, “if necessary.”

The state has been rebuilding the spillway after most of it eroded away in February.

KQED also reported Tuesday on the cover letter to the DWR report submitted Nov. 7 in response to the FERC letter. The actual report was not available.

In the cover letter, DWR said it had evaluated the cracks and determined they were caused by efforts to create “a robust and durable structure.”

The features included anchoring the slabs to the foundation, placing the slabs on a layer of leveling concrete, and interlocking the slabs with keyways and a continuous layer of reinforcing steel.

“With the inclusion of these design elements, the presence of hairline cracks was anticipated and is not expected to affect the integrity of the slabs,” DWR said.

UC Berkeley civil engineering professor Robert Bea, a veteran analyst of structure failures, said cracking in high-strength reinforced concrete structures is never expected.

The cracking “develops paths for water to reach the steel elements embedded in the concrete and accelerate corrosion,” Bea wrote in an email. “Such corrosion was responsible for the degradation and ultimate failure of the steel reinforcing in parts of the original gated spillway.”

The state spent $500 million or more this year repairing the 3,000-foot-long main spillway and the emergency spillway, but an additional year of work will be required.

The state also spent $140 million responding to the initial crisis that occurred when the main spillway began to break up on Feb. 7. Subsequent heavy storms swamped DWR’s ability to handle the inflow with the damaged main spillway, and water began flowing Feb. 11 over the emergency spillway, which was just a concrete weir at the top of a bare hillside.

The following day, rapid erosion up the emergency spillway slope posed the threat of collapse of the structure, and evacuations of 180,000 people in the Feather River corridor were ordered.

Crews from Kiewit completed what DWR said are sufficient first-year repairs on the main spillway Nov. 1. DWR said it is expected to handle releases of 100,000 cubic feet per second if necessary.

Work is continuing to armor the emergency spillway, and the first phase of that work is expected to be done by the end of January.