What should I use to model a vertical bend (change in slope) or change in size of my pipe? A Transition, Manhole, or Cross Section node?
The transition element is used to model a change in slope or size between two conduits whereas the Cross-Section element is used to model a change in slope or size between two Channels.
The difference is in where the cross sectional area of your pipe/channel is defined. If the cross sectional area of your pipe or channel remains constant over its length, you should use a Conduit. If there is a gradual change in cross sectional area over its length, or if you want to define the cross section at the end node, you should use a Channel.
For example if you have a 100 foot long circular pipe that is 12" in diameter for the entire length and you want to model a bend/slope change that occurs in the middle, you would use the transition node between two conduits. The Manhole node could be used in place of the Transition, but it includes a vertical vault/structure, whereas the Transition is enclosed, with a bottom and top elevation field. So, for a slope or size change where there is no structure between at the transition point, the Transition node may be better suited.
If you want to model a length of natural channel that widens at some point, you would use the Channel element and define the cross section/shape of key points in the channel at Cross Section end nodes.
The following picture shows an example of the use of a transition element between prismatic channel conduits:
Transition vs Manhole (Structure Type: Transition Node)