Thank you Christ G for your suggestion on how to simulate weirs with changing height and length. I have a similar question on culverts. The number of circular barrels and invert elevation of culverts may change over time. One way I can think of is to use overflow gates with closed top to mimic this kind of culverts. Just wonder if anybody has better idea?
Using drop gates to model a culvert that changes invert elevation would be the simple approach, if that gives good enough answers.
Alternately, you could put it a bunch of different culverts at different heights (each one would have to be a separate culvert group). You could then use the Advanced Rules (see User’s Manual) to control which culverts have flow. The Rules allow you to set a given culvert group to have a zero flow.