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"When the grey rock has all of those fractures and veins that run through it and underneath it, I don’t see how the grey rock is very much more resistant to erosion than the brown rock."

Here is evidence to the question:

Core Sample Analysis - A hard Amphibolite rock seam (blue grey color) observable in the middle of the longer core sample being inspected

That core sample had fractures that would force any structural design to require a consolidation of the material via high pressure grout injection. But that still would be a poor option as this sample amphibolite is fractured there is a mixture of weathered rock present. This core sample is very unusual in that the amphibolite is "sandwiched" as a laminar layer fold or seam. It is not surprising there are fractures in this narrow "seam". In addition, that core sample is not a representative of large emplacements of blue grey bedrock (amphibolite) found in large sections of the original main spillway (image below).

The image demonstrates the resiliency integrity of large sections of good fresh blue grey Amphibolite - compared to "highly fractured weathered rock". This section took quite a beating from imbedded debris within a powerful hydraulic pounding when the spillway was operating at 100,000 cfs (note: part of the flow was still coming down this section with debris, even though a greater flow was forming the "canyon"). There were chunks still coming down in this section. Yet the #11 anchor rebar is observed intact within the Amphibolite and much of the spillway is still anchored into the Amphibolite foundation. The damage to the concrete reveals the jackhammering pounding debris environment during this flow (spalling & rebar entangling). If the Amphibolite was highly fractured, you would see a significant elevation difference from erosion compared to the leftover concrete slab grade.

Bottom line: The structural conditions of the main spillway and the emergency spillway hillside are complex. Before any proposed designs are suggested, care must be exercised in thoroughly sampling the geologic substructure to determine what stripping to do, what grout consolidation is required, and what areas are too risky to construct upon.

I encourage your insightful queries, discussion, and skilled technical abilities. In these FReeper discussions, and in decades of complex failure analysis, I have found the best input from great souls such as these discussions have yielded. No need for disclaimers here (degree/expertise/etc) & no questions/comments should be inhibited by such. btw- often times, it is a technician or some other person who notices something unusual, yet crucial to the failure investigation, that experts (some not all) have discounted. That is why I will interview the tech’s after getting the initial failure crisis challenge explained by the experts.

You know in the end the fix will be to just cover everything in a thick layer of concrete and re-bar.

3D Model simulations of a "flip bucket" & how a spillway flow could "jump a distance".. Concerns in the force vector stresses in anchoring such a design

Continuing prior discussion w/ abb: (see post links below)

For those interested - the original Main Spillway was model tested with a "flip bucket". This is simply a curved structure designed to "flip" the waterflow into a "jump". (see images below). This design was abandoned as the powerful jump caused significant hydraulic erosion forces on the far side bank of the Feather River. Even when a very large curved "redirection wall" was tested (on the far river bank), the designers changed to using four large "flip bucket type dissipation blocks" that dissipated the flow energy into an aerated upward trajectory that essentially transformed the flow into a billowing turbulent mass. This design minimized the far side bank erosion of the Feather River.

Color images below are from 3D flow software analysis of a spillway "flip bucket" jump design. Marked up on the image is the force vectors that the spillway chute concrete (& substructure) would experience from transferring the water's downflow inertia into an upward trajectory. The most important question is meeting the requirement of the f3 force vector. This force requires substantial anchoring in the "weathered rock" region of the damaged end of the upper main spillway. If this design is not adequate, or the substructure is not fully characterized and stabilized, there could be a risk of a possible "downslope tumble failure". (not a good scenario <- understatement). Thus the discussion of whether there is any good competent bedrock at the anchoring point and how deep would the excavation be to get to it.

Proposed "Flip Bucket" option to Jump the Gap over the plunge pool

Poor Bedrock concerns for anchoring new "Flip Bucket" proposal (compared to good anchoring to Amphibolite of modified "flip bucket type dissipation block design" MS)

Force vector "downslope" stress (f3) on weak "weathered rock" section of damaged main spillway. Substructure design challenge to anchoring instability and a possible "downslope tumble failure".

Original spillway model testing of a "flip bucket". Flows were discovered to cause extreme erosion damage to the far side of the Feather River if implemented. Design changed to incorporate "dissipation blocks" (i.e. to dissipate the energy & aerate the trajectory flow into a billowing turbulence mass).