Construction of a new spillway at the Oroville Dam in northern California—the largest dam in the U.S.—is underway and is expected to be completed sometime in 2018, according to the California Department of Water Resources. The dam replaces the previous spillway, which was damaged by heavy flooding in February.
Problems at the Oroville Dam began, when the dam’s main sluice was damaged after a winter season of record rain and snowfall, following five years of drought. Torrential rainfall caused water levels to rise so quickly that large amounts needed to be released to prevent the dam from rupturing and sending a wall of water to the communities below.
The force of the cascading water was so strong that it created a large hole in the main sluice, requiring the use of an emergency spillway. This safety backup, however, also nearly failed because the dirt spillway, which had never been fortified by concrete, began to erode, increasing the risk of damage to the dam. In anticipation of a possible disaster, almost 200,000 residents living below the dam were temporarily evacuated.
The dam’s new construction has proved to be contentious at times, with residents expressing concern about small cracks that have appeared in the freshly laid concrete. Rainy season is just ahead and residents are anxious about the possibility of another flood.
State officials said cracking is normal, however, and federal regulators agreed that no immediate repairs are necessary, but not everyone is convinced.
“We heard that in 2009 when we saw DWR fixing cracks on the spillway, that it was completely normal, that it was no concern,” Oroville resident Genoa Widener told the Associated Press. “And then we were told to run for our lives. So you telling us that it’s normal is not enough.”
So far, about a third of the spillway has been fully rebuilt, while the rest has been fortified for the winter with plans to finish it next year. The project is expected to cost about $500 million.
In preparation for the upcoming winter, Lake Oroville was drained about 80 feet below its normal level, providing extra reservoir storage for incoming water from winter rain and spring snowmelt. On Wednesday, the lake was 200 feet (61 meters) below its maximum capacity, the AP said.
Residents are also upset because state officials have closed a scenic road spanning the top of the dam during the construction. They have deferred a decision about whether it will ever be re-opened due to safety concerns. Several residents said the road closure has cut off their access to recreational areas, the AP reported.