A possible working sequence to avoid some of the later working problems with grass (hair) on a terrain.

Duplicate your terrain. And then,(Note the order) before anything else, click off the visibility. Then drag the hair into that selected (invisible) terrain and work on as you need in the vertex room!

You can then use your original terrain for whatever you had in mind, creating a beach line, as the target terrain for your water intersect. creating surface replications etc.

bwtr