Here is why I think it is deeper than the 20’ you speculated.
First of all, I could be wrong — ;-P
But I am basing that on the fact that all the original pictures had no reference for scale and I thought of the spillway much narrower that it is. Then I saw at the pictures with all the trucks out on it and I realized that this this has about eight gates that are 33’ each so it must be about 275’ wide or almost as wide as a football field is long.
Now when the spillway was shut down some weeks back at the start of all this we had slab out and a nice hole — looked almost manageable. Now with the last two weeks of running this spillway and what has gotten cut out below, when I look at the water cascading down in that bigger hole and running fully to the side, I bet it has cut a hole 22% of the width of the concrete I see — about a fifth as deep and the slab is wide. Well dang, that is 60’.
Maybe I am wrong but I bet it is closer than 20’
Man o’ Man this apple autocorrect is a pain in the rear.
>>Important Correction: Width of Main Spillway is 178.7ft <<
The 33’ x 17’-7” construction blueprints are height and width of each “gate” instead of width and height. (note: blueprints do not specify - ordering has to be extracted from image orientation).
The 8 gates provide 140.7 feet of open waterflow width into the main spillway. There are 7 concrete-piers that separate each bay with a “double width” concrete pier in the center. With each concrete pier stated as 5 ft thick, this comes to 40 ft. However, I believe the center “double width” concrete pier is slightly less than a full 2x thickness.
This width correction stemmed from looking at the string of maintenance vehicle sizes relative to the spillway. Couldn’t get enough truck lengths to get to the inverted length width numbers.