The evacuation order came as the authorities reported that the dam's emergency spillway – which was used for the first time in the 50–year history of the dam, could fail at any time, causing severe flooding to the communities around the lake.
Oroville has an average annual rainfall of about 31 inches, but this year, since October, Feather River that begins at Lake Oroville had already seen 25 inches of rain, according to the California Department of Water Resources (DWR). What is more, water from the Sierra Nevada mountain range, which is experiencing one of its wettest seasons, is also ending up into the lake. On Friday, February 10, water levels at the Oroville Dam were a little less than 2 meters from the dam's top.
To prevent overflow, managing authorities had started releasing water from the lake, when they noticed that large pieces of concrete were missing from the dam's primary spillway and a hole almost the size of a football field and at least 40-feet deep had been formed in the lower part of the channel, due to erosion. The emergency spillway, which is an embankment covered with trees, was put into use on Saturday, February 11, when the lake topped 901 feet, thus its capacity. On Sunday, though, authorities found out that the emergency spillway was also eroding and the warning for immediate evacuation was issued.
Managers inspecting the primary spillway over the weekend decided it could handle larger water flows and by Sunday night they were sending 100,000 cubic feet per second down the main spillway, while the normal downstream flow is 50,000 cfs. The flow of water on the emergency spillway eventually stopped. As of 8 a.m. Monday (11 a.m. ET), Lake Oroville had dropped to 897.2 feet, according to the DWR and it continues to drop at a rate of about 4 inches an hour. In case this did not happen, the DWR had a contingency plan to use helicopters to drop bags of rocks into the damaged part of the spillway, in order to temporarily plug the hole.
The erosion that had caused the initial panic appears to be slowing down and current water release is within the capacity of downstream channels, stated the DWR. The threat of severe flooding has subsided, however, the dam condition is still considered as highly unstable. People are urged to remain away from the region, as the area is expecting more rain on Wednesday and managing authorities are trying to fortify the emergency spillway before water levels at the reservoir begin rising again.
Image gallery credits: CNN