kanat

A covered canal

Said covering could be wooden, thatch, stone, dug completely underground.

The difference between kanat, culvert, or drain pipe is the size.

A kanat is sufficiently large that it can float a boat or raft, almost always with a path to one side.

Culverts are drains/pipes large enough to allow passage of most sebban, usually for maintenance or cleaning.

Drains do not allow for such passage.