I'd like to know if a jet blaster can work under water in a fully submerged pipe?

I have a 150mm VC stormwater main with a short section root blocked. The DS MH cannot be located, so it needs to be cleared from the US MH.
The pipe is fully backed up from recent rain. The blockage is about 15m along the pipe (from the US MH).

Assuming DS MH means downstream manhole, and UP MH means upstream manhole.

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I'd like to know if a jet blaster can work under water in a fully submerged pipe?

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No.

A jet blaster wouldn't clear roots even if the pipe were not filled with water. You must use what in the US is called a snake. I believe in UK it is called roding and you must use a cutting head. A cutting head will cut the roots but they will quickly grow back. You may slow their regrowth by periodically using a root killer, which is just copper sulphate . But they will come back, after all they have all they need to grow, water and food.