OROVILLE, California // Residents below the tallest dam in the United States near Oroville in Northern California were ordered to leave their homes after authorities said an auxiliary spillway was in danger of collapse.
“Immediate evacuation from the low levels of Oroville and areas downstream is ordered,” the Butte County sheriff posted on social media. “This in NOT A Drill. This in NOT A Drill. This in NOT A Drill.”
State authorities and engineers on Thursday began carefully releasing water from the Lake Oroville Dam about 105 kilometres north of Sacramento after noticing that large chunks of concrete were missing from a spillway.
The earthfill dam is upstream and to the east of Oroville, a city of more than 16,260 people.
At 230 metres high, the structure, built between 1962 and 1968, is the tallest dam in the United States, 12 metres higher than the famed Hoover Dam.
Water started flowing over an emergency spillway at the dam on Lake Oroville, for the first time on Saturday after erosion damaged the Northern California dam’s main spillway.
It was the first time the emergency spillway had been used in the reservoir’s nearly 50-year history.
“This is a very unusual event for us here in Oroville,” said Eric See, spokesman for California’s department of water resources.
Officials hoped to avoid using Oroville Dam’s emergency spillway, fearing it could cause trees to fall and leave debris cascading into water that rushes through the Feather River, into the Sacramento River and on to the San Francisco Bay. Crews prepared for several days, clearing trees and brush.
Water was expected to continuing flowing over the emergency spillway for 38 to 56 hours, Mr See said on Saturday. In addition to the emergency spillway, water is also flowing through the main spillway that was significantly damaged from erosion, he said.
Unexpected erosion chewed through the main spillway earlier this week, sending chunks of concrete flying and creating a 200-foot-long, 30-foot-deep hole that continues growing. Engineers do not know what caused the cave-in that is expected to keep getting bigger until it reaches bedrock.
* Reuters and Associated Press
