Feb 13, 2017

Nearly 200,000 remain under evacuation near California Dam

Oroville, CA
USA
by
Olga R. Rodriguez and Dan Thompson
,
Associated Press
Photo: California Department of Water Resources
Photo: California Department of Water Resources

Archivist Preface

A warmer atmosphere drives more extreme precipitation across all storm types, and warming temperatures convert snowfall to rainfall, which in turn can melt snow pack. Both factors increase the risk of flooding.[1][2]

Climate conditions in California are polarizing further, amplifying the flood/drought pattern that has repeatedly visited the state over the last century.[3][4] Overall basin dryness has increased, due to warming temperatures which dry out soils and amplifies drought conditions. This in turn decreases flood risk.[5] However at the same time, as rainfall is concentrated into extreme precipitation events and as increasingly less precipitation falls as snow, flood risk increases.[6] These trends combine to drive weather toward extremes at either end of the drought/flood spectrum.

Article Excerpt

Nearly 200,000 people remained under evacuation orders Monday as California authorities try to fix erosion of the emergency spillway at the nation's tallest dam that could unleash uncontrolled flood waters if it fails.

About 150 miles northeast of San Francisco, Lake Oroville — one of California's largest man-made lakes — had water levels so high that an emergency spillway was used Saturday for the first time in almost 50 years.

The evacuation was ordered Sunday afternoon after engineers spotted a hole on the concrete lip of the secondary spillway for the 770-foot-tall Oroville Dam and told authorities that it could fail within the hour.

...

Late Sunday, officials said the evacuation orders remained in place despite the fact water was no longer spilling over the eroded area.

"There is still a lot of unknowns," Butte County Sheriff Kory Honea said at a news conference. "We need to continue to lower the lake levels and we need to give the Department of Water Resources time to fully evaluate the situation so we can make the decision to whether or not it is safe to repopulate the area."

About 188,000 residents of Yuba, Sutter and Butte counties were ordered to evacuate.

...

Unexpected erosion chewed through the main spillway during heavy rain earlier this week, sending chunks of concrete flying and creating a 200-foot-long, 30-foot-deep hole that continues growing. Engineers don't know what caused the cave-in, but Chris Orrock, a Department of Water Resources spokesman, said it appears the dam's main spillway has stopped crumbling even though it's being used for water releases.