[Jim W N]

they were "redesigned" or modified during actual construction to be angled upslope

Why would a drain pipe point uphill?

[abb]

I think he meant to say downslope. From that original drawing that showed the drain to be horizontal, the drains were angled slightly to take advantage of the spillway slope.

By doing that, the terminal end of the drain at the spillway center would be higher in elevation than the other end that drains into the collection pipe.