Tons of earth washed down the hillside below the Oroville Dam emergency spillway in February. Three projects have been approved for the site. But none of the projects include the construction of a concrete chute to carry water to the channel below.
The Department of Water Resources has identified a list of factors that may have contributed to damage to the area below the emergency spillway. They include the lack of concrete to disperse water flowing over the spillway, "erodible rock and soil" and the "absence of erosion protection downstream."
Erin Mellon with DWR says crews will not address the area directly below the spillway this year, but instead are building a cutoff wall 750 feet down the hill.
"The top of the wall is going to be flush with the ground" says Mellon. "So the wall is going to be dug underground 35-to-65-feet deep."
The cutoff wall is designed to prevent erosion at the bottom of the hill from creeping uphill and threatening the structure at the top.
Drilling began on the first of nearly 500 holes two weeks ago through the top layer of dirt and rock and down into the bedrock. The holes will be filled with concrete to make the wall.
Nothing will prevent erosion above the wall if the emergency spillway is used within the next twelve months.
DWR says next year, crews will work on projects to protect against erosion above the cutoff wall. They include a buttress to strengthen the (1,450-foot-long) emergency spillway and ( an equally wide) "splash pad" that will connect the emergency spillway with the cutoff wall 750 feet below.
As for an actual concrete chute to the channel below? Mellon says it's one of several possibilities.
"That could be an additional flood gated control spillway," Mellon says. "It could be armoring of the emergency spillway so that you have concrete all the way down to the water. It could be adding new outlet systems to help add different ways of releasing water from the reservoir. Everything is on the table and will be looked at by DWR."
Nearly two million cubic yards of debris from the emergency and main spillways have been removed from the channel below Lake Oroville.