While messing around with TNT, I've found you can cause sand to move horizontally by deleting the bottom of a sand pillar while a block of partially-submerged TNT goes off. When sand is falling, I've noticed it becomes an entity, 1) Because it can catch on fire, just like entities, and, 2) Because it does not collide with the player. Entities are known for being affected by TNT blasts, such as TNT/Arrow cannons, so I wondered how TNT would affect the falling sand. The results:
I can foresee certain devices/traps using this. My first thought when finding this was the wall to your base being covered in pillars of sand, and, when the enemy attacks, the sand is instantly catapulted over the wall and on top of them, suffocating them.