Concerns over hairline cracks that have been observed in some of the Oroville Dam’s new flood control outlet (FCO) concrete chute slabs have been dismissed.
Repairs to the dam were undertaken after engineers spotted a hole in the main concrete spillway of the dam in February. California’s Oroville Dam is the tallest in the United States.
Investigations found its main spillway — an overflow for excess water — had suffered erosion damage as a result of the heavy rainfall which had spread through the state after several years of drought.
But after repairs started, which led to partial closure of the spillway, the dam’s 275m crest was breached and water started to flow down the auxiliary (emergency) spillway, which was just a channel cut into the hillside. Erosion damage soon appeared at the auxiliary spillway’s head, undermining the concrete weir. It led to water crashing down the main spillway at 2,831m³/s and evacuation orders being issued for 180,000 residents living in the danger zone, downstream.
A huge repair job is underway and California’s Department of Water Resources (DWR) has met its construction targets to get the dam ready for winter. However, in October removal of the concrete curing coverings over recently placed, erosion-resistant concrete slabs (placed for the new FCO spillway chute) had revealed a number of small cracks in the upper surface of the concrete slabs.
DWR then took some remedial actions including adjusting the concrete mix design and contacting concrete experts. The Federal Energy Regulatory Commission asked the DWR to undertake several areas of action including mapping the cracks and evaluating possible causes.
Following the assessments, in a letter to the DWR, Federal Energy Regulatory Commission regional engineer Frank L Blackett said: “We concur with your assessment that the current condition of the hairline cracking does not warrant repair at this time.”