Here's a breakdown of the etymology:
* "Desbris" originally meant "fragments" or "broken pieces" and was formed from the verb "desbrier" meaning "to break into pieces".
* Over time, "desbris" evolved into "débris" with the addition of an accent mark.
* The French word "débris" was adopted into English, retaining its original spelling, even though it has been pronounced with a slightly different emphasis than the French pronunciation.
The "s" at the end of "debris" might seem redundant, but it's a remnant of the older French form.
Therefore, the spelling of "debris" is a reflection of its French origin and evolution. It's a reminder that English language has borrowed words from many other languages, and sometimes their original spelling is retained for historical and cultural reasons.