Buoyant forces apply to things in the ground. HDPE pipes without adequate backfill or anchoring will float out of the ground, like zombies, in flood conditions where the pipe itself contains air. Your local street department has probably seen it. Also, for expedient flood protection, those tubular dams are typically filled with water.

My reaction is that "if necessary" isn't really an "if". It is necessary, or your levee may float off precisely at the time you need it to work.