Deep
If a tsunami is higher than your forehead you better move those legs and find a tree to climb, because that thing is deep!
Picture this tsunami rushing past the tree you managed to climb up. It's a deep one, way higher than your forehead, but you're safe here. Legs exhausted, sure, but safe and sound.
しん
ふか
None
You wait until the tsunami is only as deep as your shin (しん). Then you climb down the tree and wade home in the shin-deep water.
Imagine all the debris floating in the shin-deep water now, bumping up against your shins. Probably hurts, considering how sensitive shins are.