I've been working with TINs for years and have only ever been satisfied when using 3D polylines for breaklines.
I am currently transitioning to using terrains and after reading the documentation in ArcGIS, it seemed that I ought to be able to apply a polygon as a breakline.
Intrigued by the possibility, I decided to try it with a LiDAR data set I received from a project partner along with a 2D polygon feature class of wetted stream channel planform.
I specified the wetted channel polygon as a hard breakline, built the terrain, and ended up with two very deep (0 elevation) troughs along the extent of each bank.
Based on my reading the software should create nodes where the polygon perimeter intersects triangle edges and interpolate z-values along those edges, but that does not seem to be the case. Is this actually possible?
I am using ArcGIS 10.0 SP4 with an ArcInfo license and 3D Analyst and Spatial Analyst extensions.