That looks like a lot of bed rock has been eroded and taken away.
That looks like a lot of bed rock has been eroded and taken away.
That looks like from today. Water was still coming over the weir at dark yesterday.
The scour shown in 617 shows though there is rock, it is badly fractured and jointed allowing water to dislodge and move it away. The photo that is most telling for me is the one in #589 which shows how close the scour came to the north-end base of the emergency spillway - maybe only 25-30 feet based on the workers (shown in the picture for scale).
Lots more photos at this link:
http://www.latimes.com/visuals/photography/la-me-lake-oroville-spillway-pictures-photogallery.html
Hi GD,
This “hole” reveals a crucial problem. In the Original Oroville Dam Geology report, for the “spillway” design, it identifies that rock of “good quality” would be deeper down from the surface. The hole shows highly fractured (or rotten) rock that is structurally unsound in withstanding a “erosion flow” condition.
Why is this a crucial issue? The main spillway is at 814ft (surface elevation of the spillway concrete slabs at the spillway water exit). Using this hole erosion depth (36ft) of fractured/rotten rock and subtracting the original elevation before the hole (833ft), the fractured/rotten rock reveals a depth down to 797ft. This is 17 feet lower than the 814ft spillway concrete slab surface.
Note: This hole is only 303 ft from the west edge of the main spillway channel.
This means that an overtopping emergency spillway reservoir level (901ft) that initiates an accelerated erosion of this now proven fractured/rotten rock level of 797ft is a differential of 104ft.
IF the emergency spillway weir gave way, due to accelerated fractured/rotten rock “back erosion”, the 104ft water height differential & the power of scouring/turbulence would be a major problem (understatement).
This is likely why there is a focus on quickly plugging this location (i.e. rock bags + fast set concrete methods). The problem is that any overtopping would find another weak spot in the emergency spillway fractured/rotten rock.
-— clip from Original Dam Geology report
“An overpour spillway can be utilized at this site by affording moderate protection to the rock near down stream toe. A natural saddle at about elevation 865 and a ravine below it offers a suitable spillway location. Rock of good quality should be found about 30 feet below surface in this saddle.”
Original Dam Geology report link: See page 22 in book copy:
http://www.waterboards.ca.gov/waterrights/water_issues/programs/bay_delta/california_waterfix/exhibits/docs/RestoretheDelta/RTD_101.pdf