California Dept. of Water Resources investigators are trying to discover why a steel bulkhead failed on July 22 during routine hydraulic tests at Oroville Dam. Five workers were injured when a 6-ft-tall, 10-ft-wide nonstructural steel wall in a diversion tunnel at the dam’s powerplant collapsed during a routine annual hydraulic-valves test.

The incident shut the plant for about two hours until the bulkhead was repaired, according to Carl Torgerson, DWR chief of operations and maintenance.

The investigation will take about two months to conclude, says Pete Scheele, DWR field division chief. Officials from the state Occupational Safety and Health Administration are assisting DWR engineers with the probe.

Four workers suffered minor cuts and bruises, while the fifth was hospitalized with a broken leg and other injuries, Torgerson says.

The bulkhead houses one of two giant valves that regulate flow from Lake Oroville to the Feather River. “The partition gave way, and flying debris was the main source of injury,” Torgerson says.

Officials discovered a minor amount of hydraulic-system lubricant had leaked into a diversion pool below the dam. A cleanup crew removed the residue.

Oroville Dam is 70 miles north of Sacramento in Butte County and, at 770 ft, is the highest earthen dam in the U.S. The reservoir holds 3.5 million acre-ft of water. The hydro unit has three conventional and three reversible generators and a rated capacity of 645 MW.