Excavating a City/Moving tons of sand (Brainteaser: Lots of Math/Physics needed)

[Destil]

That brings up an interesting question ... what is the pressure on the elemental plane of water? (You could ask the same of the elemental plane of air.)

Manual fo the planes lists both as 'normal' or somesuch.

 

[Neijin]

From FRCS ~p 58

"An unattended object cannot pass through a portal."

 

[Someone]

I think the prismatic wall idea is a winner. The only possible problem is that the prismatic wall can be only vertical, though maybe high enough so the sand flows through it naturally. If not, a prismatic sphere would do the trick. The duration is long enough to destroy huge quantities of sand without resorting to make it permanent.