A culvert is a handy dandy thing, but no one knows where it got its name.
Possibly it was from a long-forgotten engineer who fancied being the first to build pipes for dealing with annoying and unwanted bits of water. Some engineers do things like that.
Whether the engineer's name was Cuthbert Vertulus, or Cloaca Venatia Tubula Fluminal, or something else, we will never know. Too late now. But this invention lives on.
And it's ever so handy.
As noted in the opening.
Because...well, because culverts drain water away, as in away from trails and into bogs, which, if you've been paying attention, you know are places best left, undisturbed, to themselves.
Which is why it's reasonable to have culverts. Because culverts keep us safe from those creeping bogs.
For example, take your cross drainage culvert. A cross drainage culvert is a conduitical conveyance (round or square — it doesn't matter) fashioned of native rock, or of wood, or of factory-hammered and galvanized metal, or of plastic or concrete, and it channels water across a trailway, from one side to the other.
But unlike so many of the tricks that trail builders like to spring on us, while the culvert does in fact shoot water crossways, it makes sure that that water goes under a trail (and not messily over the top of it, as peevish waters prefer when left to their own devices), moving said water from a ditch or catch-basin on the high side of a trail to somewhere or other (We don't really care where, do we?) on the low side.
And in fact culverts can route entire creeks under trails, and in that case they are called stream bed culverts, and are quite handy indeed, remaining, perhaps, internally slimy (but acting so discreetly that we normally never even notice) while still successfully repelling bogs.
And that's a plus for us, isn't it?