What stopping Minato doing this?

[xias]

Hey i love minato a lot but i dont think its that easy.

 

[ninthgate]

The pressure in the trench would probably crush the kunai too, making it useless. . . Probably why MInato hasn't done something like that.

High pressure water can crush a kunai made of steel?

Well, for starters, the pressure. Think about how pressure works.
If you open a floodgate (barrier) then what happens? Does the air go into the water's area, or does the water spill out?

I don't think the barrier+FTG works like a portal, for instance, th juubi's TBB suddenly appeared at the kunai location, it didn't flow out of anything. The barrier is just a temporary storage space for objects before they are teleported.

The only problem is that minato cant warp his barrier around an object. Barrier itself is a plain...

I thought the Kyuubi and Juubi's TBBs were objects? What criteria makes something not an object?