What does the "Number of Barrels" field mean for a conduit? What impact does this have?
The "Number of Barrels" enables you to specify the number of hydraulically identical conduit barrels that make up the conduit. For example a value of 2 means that the single conduit element represents two parallel pipes with the same shape/diameter, roughness, slope and inverts (that both connect to the same upstream and downstream element, at the same elevation).
This is useful for modeling parallel pipes instead of having to lay out two separate conduit elements.
Note that this will also impact other hydraulic results such as the Full Capacity. For example two pipes with the same size, roughness and slope will have different full capacity of the number of barrels is different. (for example two barrels with have double the capacity of one)
In the design constraints, the "Allow Multiple Barrels?" option enables the design algorithm to use more than one identical section in parallel, up to the specified Maximum Number of Barrels. This means that the constraint based design algorithm can adjust the number of barrels in addition to the section size to attempt to meet design constraints.