I’ve been using “peripheral thingie” to talk about a California Delta water conveyance but Jeff Mount has done me one better: “punnel” (a peripheral tunnel).
jfleck at inkstain
A few thoughts from John Fleck, a writer of journalism and other things, living in New Mexico
Except the opposite of peripheral is paralel, and a paralel tunnel would also be a “punnel”. So you’d have to do, like, periphunnel and paralunnel. Or tunnipheral.