State water resources officials and federal regulators caused the failure of the Oroville Dam spillway in February by ignoring long-established guidelines and neglecting their duty to manage risks and detect flaws, a scathing report by a Berkeley engineering expert concluded Thursday.

Robert Bea, a professor emeritus of engineering at UC Berkeley, said in his analysis of the causes of the spillway failure at the nation’s tallest dam that the “progressive deterioration” of the chute could have been prevented if proper procedures had been followed.

“The gated spillway was managed to failure” by the California Department of Water Resources and the Division of Safety of Dams and “regulated to failure” by the Federal Energy Regulatory Commission, said Bea’s report, issued by the Center for Catastrophic Risk Management.

Bea and Berkeley colleague Tony Johnson blamed “long-term continued use of these inappropriate standards, guidelines, procedures and processes” over several decades.

Bea’s report comes as contractors work around the clock fixing the main spillway, a large portion of which disintegrated Feb. 7 after heavy rains filled the reservoir to the brim. That failure forced operators to deploy the dam’s emergency spillway, sending water over a bare hillside, which caused near-catastrophic erosion and forced the evacuation of more than 180,000 people from downstream communities.

Subsequent reports concluded that the main chute didn’t have water stops to seal joints and prevent leaks from weakening it. Also, there was too little steel reinforcement in the structure, and the pipes that drain water beneath it were made of clay instead of superior PVC.

Bea revealed potentially catastrophic problems in both the primary and emergency spillways caused by flaws that were flagged to state and federal officials going back more than a decade. He said the cratering of the main spillway occurred in a spot where cracks and other defects had been found repeatedly since 2009.

His latest report outlines design flaws in the base slabs of the spillway, poor foundation work, a faulty drainage system and ineffective maintenance of the 770-foot earthen dam that was completed in 1968. Bea said many of the defects were uncovered in inspections dating back to 2008, but were not adequately repaired or resolved by the state Department of Water Resources, which runs the dam.

He said the faulty work resulted in the fissure that opened on the 1,730-foot-long spillway.

State officials have said they did everything they could to keep the dam safe. A team of engineering experts, not including Bea, is investigating the design, maintenance and management of the dam’s two spillways.

In a statement, the Department of Water Resources said it welcomes “additional analyses and studies and will ensure this report is shared with the forensic team.”

The state’s main contractor, Kiewit Corp. of Omaha, Neb., hopes to have the compromised main spillway rebuilt by November.

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