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That ‘if necessary’ probably proves to be true most of the time, but I think the use case they mention (levees built on compressible alluvial soils) can easily make it the better choice.

For levees, there’s also the even lighter https://en.wikipedia.org/wiki/Inflatable_rubber_dam (but those do have to be grounded)

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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.

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