4.2.2: Erosion below the Auxiliary Spillway
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Image captured from a video taken by drone on February 12, 2017 at 9:30 am: (Video from the CA Department of Water Resources)
The lake behind the dam is on the upper right, and the spillway extends from the right side of the image to the break in slope near the top. Beyond the break in slope, water is flowing over a parking lot and curb, which it was not designed to do. Note the road in the upper left of the image. Also note that there are no apparent gullies near the base of the spillway, but there is one starting to form near the break in slope.
This image was taken from a different angle at about 3 pm: (Video from CA DWR)
The lake behind the dam is at the top of the image, and the break in slope near the top. Beyond the break in slope is toward the left. The road is under water running across the bottom of the image. Note the prominent gully extending upward toward the spillway near the center of the image. This gully formed between 9:30 am and 3 pm!
Here's what it looked like by 3:30 pm:
The rapid erosion of the head of this gully triggered emergency management people to decide to evacuate the population. If erosion continued to the spillway and the spillway failed, a 10 m high wall of water might have rushed down the Feather River.
Here is an overhead view after they lowered the water in the dam to below the emergency spillway: (I labeled the "Danger Area".)
Here's an image of the area from February 16, 2017, showing the progress of engineering work:
The dark grey area is where the "Danger Area" gully was. Note that it is huge compared to the construction trucks nearby. It has been lined with boulders and concrete. You can see the cement truck and boulders beyond the dark grey area if you expand the image.