Updated 9:10 a.m. Tuesday

Federal regulators have asked the officials who operate Oroville Dam -- and who are in charge of the $500 million-plus effort to rebuild and reinforce the facility's compromised spillways -- to explain small cracks that have appeared in recently rebuilt sections of the dam's massive concrete flood-control chute.

In a previously undisclosed October letter, the Federal Energy Regulatory Commission told the state Department of Water Resources to document the extent of tiny cracks that have showed up in some of the spillway’s brand-new concrete slabs. FERC also asked DWR what, if any, steps might be required to address the issue.

In early November, DWR told the dam's federal overseers in a cover letter to a classified memorandum that steps taken to build a stronger spillway -- such as an added layer of steel reinforcement -- caused the hairline cracks.

The letter said the agency found "the hairline cracks are a result of some of the design elements included to restrain the slabs and produce a robust and durable structure."

The letter added that the cracking "was anticipated and is not expected to affect the integrity of the slabs."

FERC did not respond Monday to requests for further details on the extent of the cracks. But in a Nov. 21 letter, it said it had reviewed the Department of Water Resources report and agreed with the department's conclusion that the cracks "do not warrant repair at this time."

DWR spokesperson Erin Mellon said in an email Monday evening that hairline cracks are "something you expect to see" in concrete slabs as massive as the those in the rebuilt spillway, which measure 30 feet by 37.5 feet. "These cracks are not abnormal, nor do they cause a concern," she said.

Mellon said DWR will continue to monitor the concrete and that the agency, along with the spillway contractors and outside experts, is looking at a refinement to the concrete mixture to minimize cracking.

"However, considering these hairline cracks do not cause a concern, the mixture may remain the same going forward," she said. "We anticipate that hairline cracks would still form even with a refined mixture."

DWR's Nov. 7 letter to FERC mentions a technical memorandum that the department classified as critical energy/electric infrastructure information, or CEII. Under post-9/11 federal law, CEII documents can be viewed by members of the public or media only if they agree to sign nondisclosure agreements -- a provision that effectively places them beyond public view.

The upshot is that the evidence for and reasoning behind DWR's statements about the cause of the cracking is not available for independent assessment.

Robert Bea, a professor emeritus of civil engineering at UC Berkeley and a veteran analyst of structure failures, said that DWR's letter leaves "a lot of uncertainties regarding the implications of the reported micro-cracking."

Bea, who heads a Berkeley-affiliated group that has issued several reports this year highly critical of DWR's management of the Oroville facility, added that cracks in the concrete surface are potentially serious and require urgent attention.

"Cracking in high-strength reinforced concrete structures is never 'to be expected,' " Bea said in an email. Even small cracks could increase stresses in the concrete when it is under "service loading" -- for instance, when large volumes of water hurtle down the structure at speeds approaching 90 mph.

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

DWR inspection and repair records going back to the 1970s documented widespread cracking of the 3,000-foot-long spillway chute -- largely because of the thinness of the concrete covering the drainage system below the concrete slab. On at least two occasions, sections of the steel rebar inside the slab were found to be corroded and in need of replacement. After the spillway failed last February, some of the steel rods intended to anchor the slab to underlying rock were also found to be corroded.

The new spillway design aims to prevent those extensive concrete problems by introducing a series of improvements in the spillway design. In addition to more careful foundation preparation than that done for the original 1960s structure, the chute features much thicker concrete, increased steel reinforcement, stronger slab anchoring, interlocking slabs, waterstops to block the flow of water beneath the slabs, a redesigned sub-slab drainage system and electronic instrumentation to monitor the intrusion of water beneath the concrete chute.

The failure of the spillway and the subsequent severe erosion in an adjacent hillside used as an emergency spillway triggered the evacuation of about 180,000 people along the Feather River. Rebuilding the main spillway and reinforcing the adjacent emergency spillway will cost at least $500 million, DWR says.

The cost of the emergency response to the crisis -- including removal of 1.7 million cubic yards of debris, rock and mud that blocked river channel below the shattered spillway -- will top $100 million. State officials have said they will ask the Federal Emergency Management Agency to pick up 75 percent of the cost of the total costs of response and reconstruction.

• 6:25 p.m. Monday: This post was updated to include a Department of Water Resources response to federal regulators questions about the Oroville spillway concrete.
• 9:10 a.m. Tuesday: Post updated to include newly released Nov. 21 FERC response to DWR report on spillway cracks.