Had this discussion before (where you agreed post 2355) in where post 2,354 reveals the "optical illusion" of the true slope of the broken drain (see link - it shows a downslope at/near -2.5 degrees from horizontal).
** Took your original image and modified it: The green pipe "fix" is connected as a "erosion bypass repair" to a 12" drain that had been interconnected with the broken coupling elbow in the sidewall. The interconnection piping to the sidewall elbow is missing from the erosion and spillway chute fracturing.
The newly revealed "other" 12" sidewall longitudinal drain revealed itself after the recent spillway run as it facilitated erosion of debris that covered it prior. Perhaps there is a misunderstanding in thinking this "footing" drain was stated as connecting to the elbow above. This would be incorrect as the slope angle to this "footing" drain would have been positive (i.e. non-gravitational "flow" oriented). What the picture reveals is the natural "overlap" of sidewall drain piping. At some point, this longitudinal footing "collection" drain has a coupling that angles piping towards its next sidewall elbow drain outlet. The destroyed section further down the spillway is where this transition would occur (if not at the junction of the outline of the herringbone drain @ the broken edge of the main spillway).
For the Curious: an Optical illusion - Gravity Drain Pipe design flows downward..
Abb: I think you have it about right. It clarifies that manual iso layout sketch I posted upthread.
Abb's original sketch re: under slab drainage design..
Updated/modified sketch to illustrate what is in the image (prior discussion)
