I’m on your side fellow FReeper. Please, I wish no offense.
Just for background:
I do research in volcanics & rock formations. Been doing this for over 3 decades. Sierra Nevadas, Lassen, Tahoe, and volcanoes in the PNW. I have instrument systems that detect precursory anomalies to seismic events & have numerous patents.
But don’t take my word on the foundational structure of the spillway(s) - the same answers come from the highly respected & world renowned dam expert Scott Cahill - in excellent detail: “Expert: What You Need To Know About The Oroville Dam Crisis” https://www.youtube.com/watch?v=rl7Sm15unPQ
The Original Dam Geology Report, page 22-23 identifies the fractured rock “weathered rock” to depths of 20 to 30 feet. This rock is referred to as “rotten” as it is highly fractured. Scott Cahill uses the same words - “rotten rock”.
Keep in mind that the Original Dam Geology Report was very well done, but they didn’t have an exhaustive measure of fractured rock depth - just drilling references. Now that the Emergency spillway eroded so alarmingly, it exposed the depth of the “rotten” rock to at least 36 feet (from the hole/crevasse near the east end of the ES).
We now have direct evidence of the fractured rock depth in the spillway region as bottom of this 36ft thickness of fractured rock is at 797ft. The top surface of the main spillway slab is 814ft. Subtracting the 2ft thickness of the concrete slabs, the spillway comes to 15ft higher than the 797ft depth of the highly fractured rock (”rotten rock”).
Because of this subsurface erosion risk the spillway has integrated drainage capture. Thus the vents on the sidewalls of the spillway walls. Again, since the vents on the sidewalls are clearly seen jetting out volumes of water, there is a flow underneath the concrete slabs that threaten a weakening and cracking from the weight & any non laminar flow of the spillway flow water volume. This again is described by Scott Cahill (you don’t have to take my word for it). (Scott says that these systems should only “trickle” water - not the high “jetting” rate seen). He knows this is serious to the stability of the design.
Remember that the slabs of concrete have expansion joints. It is not a solid pour for the length of the spillway. Compromised expansion joints are fracture failures waiting to happen. This is exactly what happened in the mid section blowout of the main spillway. So there is a real risk of this same failure occurring from subsurface erosion in the upper section. You won’t have much warning with a high volume flow (such as 100,000cfs).
There is a reason why they have a person stationed along the upper spillway along with the array of high intensity illumination lights. They know this risk to the spillway. They would immediately shut off the flow if a failure suddenly developed.
(Thus the reason for an overabundance of caution in watching for spray anomalies - that would be the first sign.) I’m all for everything working out. God willing. There is too much at stake. I too have family downstream to this dam.
God Bless
I’m part of one of the groups/agencies mobilized to support DWR at Oroville. Plenty up to date - with boots on site info you can’t glean from people’s interpretations standing back and looking at photos. This isn’t my first rodeo.
At this point I am wondering if the rain even matters. The dam seems to be on the verge of a failure cascade if not already. The main spillway cannot be shut off long enough to make the repairs so as long as the water flows, the earth erodes back towards the dam.
Good info. Thanks.