Problem

Is it possible to model a conduit that is larger on the downstream side compared to the upstream side? In other words, can a conduit change size as it moves downstream? (be non-prismatic)

Solution

Conduits are assumed to be prismatic - their section shape and size remains constant along its entire length. If you want to model a conduit where the size is changing, you would need to model the conduit as several conduits with varying size.

For instance, consider a case where a conduit that is 60 feet in length and is 12 inches in diameter on the upstream end and 24 inches in diameter on the downstream end. In order to model this change, you will need to model this as more than conduit separated by a transition or manhole element. You could model this as three conduits, each 20 feet in length. The first conduit would be 12 inches in diameter, the second conduit would be 18 inches in diameter, and the third conduit would be 24 inches. Between these would be a transition element at the appropriate invert elevations to account for the slope. If needed, you could use the Element Property Inferencing tool to help define the elevations. If you wanted a finer level of detail, you could use more conduits as well.

Another option would be to use the channel and cross section elements, if what you need to model conforms to one of the available types in the cross section element. For more information on this, see the article under "See Also" below.

Or, you could accept the prismatic assumption of conduits and select a conservative size for modeling purposes.

See Also

Are Channels prismatic or non-prismatic?