California shores up damaged Oroville dam after deluge swamps area

PUBLISHED : Tuesday, 28 February, 2017, 4:44am
UPDATED : Tuesday, 28 February, 2017, 7:25am

The effort to protect Northern California’s Oroville Dam entered a critical phase Monday as engineers began to shut off water flowing out of the damaged main spillway, giving officials their first unobstructed view of the eroded concrete spillway since a crisis prompted mass evacuations earlier this month.

The move starts a race against Mother Nature, with officials hoping to have crucial repairs finished before more rains arrive, something that could cause the reservoir to rise to critical levels again.

California Department of Water Resources acting director Bill Croyle said the spillway can only remain shut off as long as water levels remain at a level they consider safe.

The water level at the reservoir was at 838 feet Monday morning, data show. Crews will begin releasing water again when they finish cleaning up debris gathered in a pool at the bottom of the spillway or when the water level pushes back up to 860 feet because of melting snow and storm runoff, whichever comes first, Croyle said.

He estimated they have five to seven days to work before they have to turn the spigot back on.

Monday’s shutoff has been more than a week in the making and gives a host of different geologists, engineers and work crews a chance to assess the geology of the earth beneath the spillway and how it’s eroding under different conditions. At the same time, two cranes that have been floating on barges in a pool at the bottom of the spillway will have their first chance to begin dredging up hundreds of thousands of square yards of sediment, rock and vegetation that have kept the dam’s hydroelectric plant from operating.

Engineers and inspectors have had to wait until the water stopped flowing so the pool’s water level at the bottom was low enough to work.

Rain lets up in flood-ravaged California

The main concrete spillway was damaged earlier this month after a week of powerful storms and after an earthen, emergency spillway that was used when the reservoir reached capacity also rapidly eroded into the pool.

Officials need to keep the dam water levels in check because of damage to the emergency spillway that carries water when the reservoir goes above capacity. Damage to that spillway earlier this month prompted the evacuation of 100,000 people. Officials were able to use the damaged main spillway to reduce water levels, easing the crisis.

Record rain and floods swamp California

Interviews and records suggest that the near-catastrophe grew out of fundamental problems with the original design of the emergency spillway that were never corrected despite questions about its adequacy.

The “solid” bedrock that officials thought would stand up to the force of the spill was soft and easily eroded. The long concrete lip of the spillway was not anchored into the rock. Critical power lines were strung across the spillway, which consists of nothing more than an earthen hillside covered with trees and brush.