Cracks have appeared in some recently rebuilt portions of the Oroville Dam's $500 million concrete flood-control chute, and federal regulators want to know why.

The Federal Energy Regulatory Commission (FERC) sent a letter in October to the state Department of Water Resources (DWR), who operates the California dam, mentioning the small cracks that have surfaced in the new concrete section, and asking what could be done to address the problem, KQED News reported.

According to a response letter sent by the DWR in November, the cracks were caused by efforts taken to strengthen the spillway, including adding a layer of steel reinforcement. The DWR also stated that the cracking in the concrete "was anticipated and it not expected to affect the integrity of the slabs."

Later in the month, FERC issued another letter, and agreed with the DWR that the cracks do not need to be repaired, KQED News reported.

In an email to KQED News, Erin Mellon, a spokesperson for DWR, said the cracks are expected in concrete slabs as large as the ones used in the spillway, which measure 30 ft. by 37.5 ft. According to Mellon, the agency will keep an eye on the concrete, and plans to work alongside contractors and outside experts to find a refined concrete mixture to cut down on cracks.

"Considering these hairline cracks do not cause a concern, the mixture may remain the same going forward," Melon said. "We anticipate that hairline cracks would still for even with a refined mixture."

But Robert Bea, professor emeritus of civil engineering at UC Berkley, and a structure failures analyst, doesn't agree, KQED News reported.

"Cracking in high-strength reinforced concrete is never 'to be expected'" said Bea, as the cracking "develops paths for water to reach the steel elements embedded in the concrete and accelerate corrosion. Such corrosion was responsible for the degradation and ultimate failure of the steel reinforcing in parts of the original spillway."

The updated spillway is meant to prevent the original design's extensive concrete cracking issues, KQED News reported.

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