Wet Spots on Spillway: Were Not Expected

March 2018 aerial photo of the Oroville Dam main spillway shows wet streaks on the roller compacted part of the chute, which the Department of Water Resources said were expected. Photo by Gonzalo (Peewee) Curiel

AGENDA 21 RADIO

Oroville >> First there was the green spot, now there’s the wet spot.

There was quite a buzz around Oroville last week about aerial photos that showed a wet streak down the face of the repaired main Oroville Dam spillway.

But it was something that had been expected, and the Department of Water Resources said as early as January that it might happen.

The water is flowing on the roller compacted concrete portion of the spillway. RCC is more permeable than the structural concrete on the rest of the spillway. So when rainfall builds up moisture in the soil outside the walls of that part of the chute, some can trickle through, according to DWR spokeswoman Erin Mellon.

It appears the water is following a line from the walls along the layers of RCC, which are piled up like a massive layer cake, and then trickling down.

There is no drainage system in the part of the chute, so any water that gets in there has to come out someplace.

It’s a short-term problem. The second phase of repairs on the main spillway are scheduled to begin May 1, and that will include placing a 2 1/2 foot-thick layer of structural concrete over the RCC. A drainage system will also be added beneath that concrete.

The walls on that part of the chute will also be replaced with structural concrete.

April 23, 2017 Oroville Dam Left Groin Area Above Hyatt Power Plant. Photo Paul Preston, Copyright 2017 AENN

“Green Spot” at Oroville Dam by Paul Preston Copyright 2017 AENN

An aerial photo of the Oroville Dam main spillway shows wet streaks on the roller compacted part of the chute, which the Department of Water Resources said were expected. Photo by Gonzalo (Peewee) Curiel

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