Problem

Is the Channel link element treated as prismatic or non-prismatic? If I have a cross section node at each end, will it keep a constant geometry for the channel or will it interpolate between the geometry of the cross sections?

With the Explicit (SWMM) solver, the following user notification appears when using channels:

"SWMM does not directly support channel elements. Non-prismatic channels are converted to SWMM style prismatic channels. The export sets the shape of the SWMM channel based on the start cross section if it exists, otherwise the stop cross section is used"

Solution

The answer depends on which product or which numerical solver you are using, which is selected in the Calculation Options. There are four solvers:

GVF-Rational - available in StormCAD, CivilStorm and SewerGEMS
GVF-Convex - available in SewerCAD and SewerGEMS
Implicit Dynamic - available in SewerGEMS and CivilStorm
Explicit Dynamic (SWMM) - available in SewerGEMS and CivilStorm

Currently, with the Implicit Dynamic numerical solver, channels are treated as non-prismatic; the geometry is interpolated over the length of the channel based on the upstream and downstream cross section elements' geometry. The exception is if there is only one cross section node connected to the channel, in which case it uses that.

With the other solvers, channels are treated as prismatic, with the geometry being constant over the length of the link, based on the Start Node's cross section. If the Start Node is not a cross section, then the geometry of the link is based on the cross section Stop Node. So for instance if you're using the StormCAD or SewerCAD product, channels will always be treated as prismatic. There is no need to convert channels and cross sections to conduits and transitions as these solvers will automatically handle the channels as prismatic. If you have adequately spaced cross sections and relatively small changes, there should not be any notable impact of the prismatic assumption.