How UE4 decide sort order of each polygons in same mesh (translucent material)??

I am making water shader based on ocean material of ryan.

so I made a mesh like (screen shot1)

big plane is water surface and small pieces are splash mesh

I made splash are moved. but there is a sorting problem.

(screen shot 2) - water surface opacity 80%
(screen shot 3) - water surface opacity 100%

always water surface is rendered front of splash.

I would like to know how UE4 decide which polygon is upper.

is there any solution to change sort order??

thank you.

ps. I am maya user

Have a look at the bottom of the picture