Multiple equipment failures, unheeded alarms and insufficient staffing at the city’s largest wastewater treatment facility are the likely causes of a “nearly catastrophic” flood that dumped millions of gallons of untreated sewage into Santa Monica Bay last summer, according to an official report obtained by The Times. Although a full understanding of what triggered the Hyperion Water Reclamation Plant crisis may never be completely determined, study authors said there was “little or no evidence” that the failure was caused by a deluge of concrete and wood that were illegally dumped into the city’s vast sewage system — a claim initially made by sanitation officials.