Incredible damage to the primary spillway at Oroville Dam in California has been revealed, showing the scale of work engineers face this week before a vital power plant can be reopened.
Officials stopped water flowing down the primary, or main, spillway on Monday 27 February so that engineers could clear debris from the bottom of the spillway, lowering water levels in a channel leading to the Hyatt Power Plant.
The plant needs to come online as soon as possible so that more water can be released from the reservoir, which has breached capacity in recent weeks following heavy storms.
It was the first time engineers were properly able to assess the damage after it was noticed on Monday 7 February. Photographs show the extent of the damage to the spillway, with large sections of the spillway and surrounding hills lost to erosion. The debris pile at the bottom of the spillway is roughly 764,555m³.
Repair works on erosion surrounding the second, emergency, spillway continue around the clock, with rock, aggregate and cement slurry being used to fill the affected areas.
California has suffered heavy rain in the last few weeks, leading to an increase in water levels at Oroville Dam. On Tuesday 7 February, erosion was discovered mid-way down the concrete chute. It was closed before gradually being reopened for testing.
Water levels crept up in the reservoir and on Saturday 11 February water ran down the emergency spillway for the first time in the dam’s 48-year history. By Sunday 12 February erosion was showing at the head of the emergency spillway, leading to fears of severe flooding. California governor Jerry Brown issued evacuation orders to more than 180,000 residents.
At the time a prominent dam expert, Andy Hughes, told New Civil Engineer he believed engineers made the wrong decision in blocking off the primary spillway when damage was initially discovered, as this risked the auxiliary spillway and could have led to dangerous flooding.
Water flows down the main spillway were ramped up despite the damage, to stop water running down the emergency spillway so engineers could apply temporary repairs. Water levels in the reservoir dropped well below the 275m limit, leading engineers to believe the reservoir could cope with more storms without the emergency spillway being used.
An Independent Board of Consultants was created by the California Department of Water Resources, which appointed five engineering experts to offer advice on repairs to the damage to both spillways.
Repair works to the emergency spillway continue around the clock. Construction crews have been installing foundations for structures which would slow water flows in case the emergency spillway is required again.