>>The concrete weir at the top of the emergency spillway was “just plopped on top of that ridge.<<
Just as I was about to post this research/conclusion independently this morning. Good to have an official come out and confirm the truth (the evidence is overwhelming that this was the case). Now they unleashed another can of worms. Waterflow under this structure is now a real part of the equation and very likely associated with the huge water bubbling in front of the this Weir. Another big issue is that this structure is not Solid Concrete. It is only a concrete shell that is filled with aggregate. Weepage, from leakage waterflow into the center filled aggregate can compromise the integrity of the strength of the Wier (capillary action). There are drain channels at the bottom of the Wier footing to keep the aggregate dry. A strong penetration of waterflow through the highly fractured rock may make these drains insufficient.
First image: Blueprints of Emergency Spillway & "Its Plopped on top of that Ridge"
Second image: It is sitting atop highly fractured rock, called "rotten rock" in structural terms. Erodes swiftly as proven by the recent 12,000cfs short time overtopping.
Here is another insanity: The original designers stated that this Emergency Spillway + the Main Spillway could handle 624,000cfs. Yet only a dinky 12,000cfs over the Emergency Spillway cut down to 797ft (36ft crevasse/hole) only 300ft from the main spillway. Yikes! Imagine if they tried what they said it could do?
==
----- https://archive.org/stream/zh9californiastatew2003calirich#page/92/mode/2up page 92-93: The emergency spillway, in conjunction with the flood control outlet, has the capacity to pass the maximum probable flood release of 624,000 cfs for the drainage area (peak inflow 720,000 cfs) while maintaining a freeboard of 5 feet on the embankment. The maximum probable flood has a probability recurrence interval in excess of 10,000 years.
More Design, Construction, Engineering Failure information:
The "best evidence" failure initiation of the Main Spillway had a substructure subsidence component. Turns out, the original test drilling was "sparse" and the location of where the "blow out" occurred & was missed (see green circle in main spillway - red circles are noted drill locations). Therefore, without "drill data" on the rock at that location, they may have been unaware of the lesser quality substructure conditions. Erosion/scouring under this "missed" location is believed to be why the concrete slab sunk. Then cavitation jackhammering of the concrete seam could occur from a turbulence void.
Another significant point: Engineers must have known of the geology at the deep crevasse/gash/hole erosion (result) as a test drill hole was done dead center to this location. Photos now reveal that the bottom of the gash is 36ft deep - putting the bottom (at the bottom water pooling) at 797ft.
So either the engineers were inexperienced with fractured rock (rotten rock) erosion, or the test drill result was not really inspected, or that someone didn't care, or ?. In any case, the result is a highly inaccurate assumption of the expected Emergency Spillway operation with the high rated curve tables of many hundreds of thousands of CFS performance - when this damage was done with only 12,000cfs in a short while.
The Dam design/construction archives actually state that vegetation (trees/shrubs) were ok in the ES spillway hillside area as they stated it would operate "infrequently". I would say they had no idea of what severe erosion would occur. Astounding.
P.S. The huge erosion and gashing cuts into the soil & fractured rock is the larger green circles. Most of this area was not test drilled. This again points to their belief in the "infrequent" scenario + the lack of knowledge of the fractured rock (or "rotten rock" - to hydraulic erosion/scouring & turbulence) swift decomposition from even small overtopping conditions.
I firmly agree with Scott Cahill's assessment (renowned dam expert) on the significant failure risk issues of the Emergency Spillway. He stated that he was not 100% sure of a non-failure to this dam structure (his words).
Good engineering is when all failure conditions are thoroughly researched, or well known from historical events, and thus designed with a safety factor.
With all of this, It is no surprise that heavy armoring of the downstream ES is underway. The revealed "fractured rock" is at levels below the main spillway - and it's close to the Emergency Spillway. Back erosion is the enemy.
