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In pics: Authorities warn US’ tallest dam may collapse, thousands evacuated

Residents below the tallest dam in the United States near Oroville in Northern California were ordered to evacuate immediately on Sunday after authorities said an auxiliary spillway was in danger of imminent collapse.

world Updated: Feb 13, 2017 20:08 IST
Water flows down Oroville Dam's main spillway, near Oroville.
Water flows down Oroville Dam's main spillway, near Oroville.(AP Photo)

Almost 200,000 people were under evacuation orders in northern California on Monday after damage to the overflow channel of the tallest dam in the United States raised fears the spillway could collapse.

The reservoir of the Oroville Dam, located 120km north of the state capital Sacramento, had been completely full after several weeks of heavy rain.

Read: California’s crumbling Oroville dam spillway prompts urgent evacuations

The Oroville Dam discharges water at a rate of 100,000 cubic feet per second over a spillway. (AFP Photo)

The dam itself was not in danger of collapse, the California department of water resources said, but there were major fears over the emergency spillway due to erosion damage on its concrete top after water began pouring over it on Saturday.

Authorities were releasing 100,000 cubic feet (2,830 cubic meters) of water per second from the main spillway, bringing down the level of reservoir Sunday so that there was no more flow into the emergency channel.

California department of water resources personnel monitor water flowing through a damaged spillway on the Oroville Dam in California, US. (Reuters Photo)

Although the immediate threat had passed, the evacuation order remained in place as authorities evaluated the state of not just the auxillary spillway but also the concrete-lined main spillway, which was damaged as well with a large gash caused by recent heavy volume.

The damaged spillway with eroded hillside over the Oroville Dam. (Reuters Photo)

California governor Jerry Brown asked the federal emergency management agency on Friday to declare a major disaster due to flooding and mudslides brought on by the storms.

“I’ve been in close contact with emergency personnel managing the situation in Oroville throughout the weekend. It’s clear the circumstances are complex and rapidly changing,” he said in a statement.

The area of erosion on the lower half of the Oroville Dam spillway. (Reuters Photo)

The earth fill dam is just upstream and east of Oroville, a city of more than 16,000 people.

Helicopter footage from news station KCRA showed water pouring down the auxiliary spillway, which is parallel to the main spillway.

The Oroville reservoir level close to the top of Oroville Dam, after a series of storms led to an increased water level. (Reuters File Photo)

At 770 feet high, the structure, built between 1962 and 1968, is the tallest dam in the United States, besting the famed Hoover Dam by more than 40 feet.

In a Facebook post ordering the “immediate evacuation” of low-lying areas of the city of Oroville as well as downstream communities, the Butte County Sheriff’s department warned that a “hazardous situation is developing with the Oroville Dam auxiliary spillway.”

Water flows down Oroville Dam's main spillway. (AP Photo)

The water flowed through a gorge to a secondary reservoir, which was also very full, before draining into the Feather River that runs through Oroville, a city of about 20,000 people.

A resident waits in traffic to evacuate Marysville, California on Sunday. (AP Photo)

About 188,000 people in downstream communities had been ordered to flee on Sunday afternoon as water was still gushing over the top of the wide auxiliary spillway.

An official of department of water resources takes a picture of water going over the emergency spillway at Oroville Dam. (AP Photo)

This is the first time in the dam’s history that water has flowed above the overflow channel. The structure is expected to face further strain in the coming months as northern California faces its wettest ever winter.