A pitfall full of vipers

[Shin Okada]

I am planning to put a pit fall which is not that deep, but contains a lot of vipers in it. I am planning to use Viper Swarm which is in the new Fiend Folio.

But how can I estimate EL, or XP for such a trap? Shall I just consider that encounter includes that pit (CR 2 or 3?) and a Viper Swarm (CR 3)? But if PCs does not trigger the trap, they will never meet those vipers. Are there any guidance for handling such kind of trap ( a pitfall with a creature in it)?

 

[Henrix]

According to S&S a pit trap with monsters at the bottom should be treated as two separate traps, that is as a trap encounter followed by a monster encounter.
Just as if the monsters were behind a door.

In your case an El 2-3 and another EL 3 encounter.

I would prefer to just increase the traps CR slightly, as if it had been set with spikes.