This image reveals the drain system design. Two longitudinal drain pipes are captured showing the downslope rise of the longitudinal pipes AND the outward spacing of the higher pipe. This outward spacing "jog" is important as it allows clearance for the vertical access cleanout pipes. Without this "jog" outward the overlapping downslope longitudinal pipe would block the vertical pipes that are needed for inspection and cleanout (maintenance - if needed) to each underslab drain pipe.
Each longitudinal drain pipe is connected to a given set of underslab drains (not revealed in this image). If a person was clever they could estimate the number of drains connected together based on this photo and then reverse engineer the slope of the spillway. These drains are angled downhill such that water "self flows" all of the way to the end point. Each longitudinal drain section "end" will be at a spillway sidewall opening to allow the water to trickle or flow back into the main spillway chute. Note that the pipes may not look angled properly, but the camera image has an optical effect demonstrated by the image's artificial curvature of the leveling concrete edge (see edge just above center workers' helmets).
It is likely that the first drain outlet will be connected to the original spillway section that will remain for 2017/2018. I would anticipate that the designers would want to keep these drains isolated from the new spillway drains as a problem could occur from the old spillway section "pre-charging" the new spillway drains from any potential "excessive flows". (excessive flows from the know design issues of the highly cracked slabs, clay seam underslab voiding channels [further erosion] facilitating "piping", and old style contraction joint "seams" without "waterstops" to block water penetration).
Another revelation to this design is that it evidences that they likely plan on placing back fill material behind the new sidewalls. This back fill material provides protection for the piping, insulation to cold temperatures, and would provide important structural support to the vertical sidewalls.
Underslab Upper Spillway drain pipe dumps into a longitudinal drain pipe. Longitudinal drain connects to a given number of nearby slab drains, then angles outward, downslope and then into upper spillway sidewall to dump back into the spillway chute.
