OROVILLE: Highly porous rock

The possible failure of the emergency spillway from erosion caused the February evacuation of almost 200,000 people.

A cutoff wall will be constructed before Nov. 1 at the emergency spillway to contain any future erosion that might occur, said Erin Mellon, a spokeswoman with the Natural Resource Agency, which oversees DWR.

“If we did use the emergency spillway, then the erosion that caused the big problems we saw on Feb. 12 would stop at that cutoff wall,” she Mellon said.

The design team decided to construct a secant wall, which entails heavy concrete slabs anchored by concrete columns extending 20 to 60 feet below the ground, she said. The cutoff wall will be about 1,500 feet of concrete.

The consultants want to ensure holes for the columns are drilled deeper than existing weathered rock, according to the memo.

“Their erodibility and effect on secant wall stability should be addressed in the design,” the consultants wrote of the weathered rock zones.

The weathered rock, which initially formed below ground, has become a clay-like substance, which erodes quickly when exposed to high volumes of water, said Richard McJunkin, one of two geologists involved in the last geological study of the area in 1979.

The rock underneath the main spillway and emergency spillway is known as Smartville ophiolite, which after weathering has become highly porous, he said.

“It’s an old seafloor that got abandoned,” McJunkin said. “It’s been around a lot longer than the surrounding rock."

Mellon said the design team has addressed the recommendation since the April 25 meeting of the board of consultants.

DWR also has a goal of repairing the lower chute of the spillway by Nov. 1, the beginning of the next rainy season, the memo states.

The upper chute of the spillway will be used for one or two more rainy seasons until it can be demolished or replaced, according to the memo. The concrete slabs, which make up the entire spillway, will see an increase in thickness and have heavier reinforcements.

Inflows during the summer months from snowmelt will be managed by outflows from the Hyatt Powerplant and the Thermalito Afterbay, Mellon said.

“The lake levels are all managed with public safety as the main goal,” she said. “We’re very confident that we’ll be able to manage the summer snowmelt safety so we don’t have to use the spillway."