Drainage - Pipe capacity and area calculated incorrectly

[Allan McManus]

In a drainage network I am currently working on, I have discovered a few pipes which do not have adequate capacity. This seemed strange to me since the catchment flows are quite low.
Investigating the issue further, it appears that 12d is calculating the full pipe area (Af) incorrectly, and thus the pipe capacity also appears wrong.
The vast majority of pipes in my model have full pipe area equal to the area of a circle with diameter=pipe size.
However a small number of pipes have an area smaller than this. Is there a reason that 12d would do this?
The only hint of a reason I can find is that these pipes are drowned. However the fact that a pipe is drowned has never caused this issue for me before.
Does anyone know what might be causing this problem?

[Owen Thornton]

Is the grate level at either end of the pipe, below the pipe obvert? Doing this turns your pipes into open channels, with a lower capacity.

(Not sure what you mean by "drowned". This is something that should only be happening if the tailwater level is set higher than a grate level. Happy to investigate your model, if you send data to support@12d.com.)