Hmm
That's challenging (but not because it's difficult...in fact it's very easy).
I'll post here a workaround (nurbs based NOT mesh) this weekend
In the mean time:
setting up things/test case in Microstation (and get extremely frustrated by a variety of tools - but I confess that I hate meshes , he he)
testing some ideas in the benchmark (that pure humble Rhino)
using the patch command
and testing the nurbs(the lake . that is)
more this weekend
best, Peter