I have four corridors for ditches. Three are created with one assembly (10' flat bottom with 4:1 slopes to existing ground, 7' deep), and one is with another assembly (204 flat bottom with 4:1 slopes to existing ground, 9' deep). One of the small ditches runs into the larger (actually a detention pond,) and a second smaller ditch exits the detention pond. I am trying to merge these three corridors but have not had any success. They are all on the same baseline. I am pretty new to corridors and have never merged corridors before. The ACAD Help File examples discuss merging regions that are separated, but right now my smaller ditches cross the boundary of the larger corridor. I have changed the stationing of one of the smaller ditches to bring it outside of the boundary of the larger one, but still am unable to merge them. When I try to "Select a region to merge into" the command line says, "Nothing selected, pick within a region boundary." Do I need to first create corridor surfaces in order to perform this operation? If not, what could be the problem?
added: I am using C3D 2011 on Win 7