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SAN FRANCISCO, Feb. 13 (Xinhua) -- The fear of possible failure of the emergency spillway at Oroville Dam in Northern California, the United States, forced Sunday about 180,000 people to evacuate along the Feather River.
The California Department of Water Resources (DWR) said that the order to evacuate was based on information it had received.
The information raised concern that erosion at the head of the spillway threatens to undermine the concrete weir and allow large, uncontrolled releases of water from Lake Oroville, the second largest man-made lake in the state of California, capable of storing more than 3.5 million acre-feet, or 4.4 cubic kilometers, of water.
While reaffirming that Oroville Dam, the tallest in the United States with an earthfill embankment 770 feet, or 230 meters, high on the Feather River east of the city of Oroville, is sound, the DWR clarified that the dam is a separate structure from the emergency spillway.
The DWR said Sunday afternoon a hole developed in the emergency spillway structure, a concrete lip on the north side of the dam, as water cascaded down the dirt ravine. Flow over the spillway weir began Saturday morning and the erosion appeared to be spreading upward toward the structure.
Officials said if the structure were undercut by the enlarging chasm, it could fail, and the water behind that barrier would come down the hill uncontrollably into the diversion pool and down the Feather River, setting the stage for the possibility of massive flows exceeding the capacity of downstream channels and flooding into Oroville and communities south of the city.
To avert more erosion at the top of the emergency spillway, the DWR had doubled the flow down the dam's main spillway, so as to lower the water level in the lake more rapidly and stop water flowing over the emergency spillway.
As highways leading out of the area were clogged, Butte County Sheriff Kory Honea was quoted as saying by local newspaper Oroville Mercury Register that the decision to evacuate tens of thousands of people was an "extraordinarily difficult" one to make.
There was no word from the DWR and local authorities on when people would be allowed back into the area, and Chris Orrock, a spokesman for the state agency, said people would not be let back until officials are confident that the area is safe.
Oroville is about 113 kilometers, north of California's state capital of Sacramento, or 240 kilometers northeast of San Francisco.
Oroville Dam was built by the DWR from 1961 to 1968 as one of the key features of the California State Water Project (SWP) to serve mainly for water supply, hydroelectricity generation and flood control.
