Math Help - Isosceles Triangle

An isoceles triangle 10 feet across the top & 20 feet deep is partially submerged in water. Express the area of the submerged portion as a function of the deapth h.

I presume the base of the triangle is the TOP in this case and the apex is stuck in the mud?

You should notice similar triangles:

1) The whole thing.
2) The submerged portion.

Corresponding parts of similar triangles are similar?