I have only heard "debris is/was" and if there are many, use "pieces/piles of debris are/were".
From ncsu.edu (styling mine)
Debris doesn't have a grammatical plural in English. If you want to talk about a lot of debris—or if you want to pick it up—you have to gather it into plural piles.
You may be wondering how a grammarian can tell that debris is singular instead of plural. There are two clear signs:
First, debris always takes a singular verb: "Debris is knee deep in the backyard."
Second, we always refer to it by a singular pronoun: "The debris is knee deep. I don't know what to do with it."
Debris is not the only noun in English without a plural. Collective nouns that define a set of inanimate objects (luggage, dinnerware) are also regularly singular, as are nouns that refer to noncountable masses of substances (bacon, lox).
As Hurricane Fran has taught us, however, debris would be easier to pluralize than to carry to the curb.