Matt,

If you call for an orifice, EXTRAN actually substitutes an equivalen "pipe" with a Manning's n determined by a formula to convert from the orifice loss equation to the Manning.s loss equation. I recommend you use a conduit which allows you to set the "minor" loss coefficients. You will want to set the pipe length to take care of the short pipe problem (see p 13-14 of the EPA SWMM Manual).

Ron Kilmartin
ronkilmartin@attbi.com