If the officers are worried the criminals are going to have time to flush the evidence down the drain if they knock... the solution is simple - turn off the water (at the curb or cut off service to the block) at the same time you knock on the door.

If what I've heard about some of the US drugs laws are true - having them try to flush the evidence would only make things worse. What I've been told is in the US it is the weight (or volume) of the controlled substance that determines the charge - so 1 hit of acid on a piece of paper is a (relatively) minor charge, but put that hit in a drink and get caught and you are considered to have 1 drink's worth (weight or volume) of acid and are charged accordingly. So take a few grams/8-balls of coke and dissolve it in water (that stays in the trap because the water is off) and suddenly the dealer is looking at charges for 1-2 kg worth of eau-du-toilette coke.