Giovanni,

You have identified a metric bug in SWMM 4.4. The minimum orifice length was hard wired to a value of 200. You were getting a length of 200 meters and a roughness of 0.003454. If SWMM internally was using a length of 60.96 meters than the roughness will be 0.00641 as you can see
below:

PIPE LENGTH.......................... 200.00
MANNINGS ROUGHNESS................... 0.00354

PIPE LENGTH.......................... 60.96
MANNINGS ROUGHNESS................... 0.00641

While SWMM should have been using 60.96 meters for the orifice length the effect of the extra length was to decrease the roughness so that at least you should have the same flow and the same velocity in the shorter and longer equivalent pipe - even though the program was contradicting the blue book.

Regards,
Bob Dickinson