Incorrect Geometry for Custom Elliptical Pipes #144
Description
As noted recently on the Open SWMM Knowledge Base, SWMM produces inconsistent results for custom sized elliptical pipes. The problem stems from it using incorrect equations for the pipe's full flow area and hydraulic radius that came from SWMM 4.4. A corrected set of equations, derived from Table 4 of the Concrete Pipe Design Manual (American Concrete Pipe Association, 2011) are:
A = 0.8117 * (Height * Width)
Rh = 0.2448 * sqrt(Height * Width)
with all values in feet.
These equations should replace those used in lines 572-573 and 600-601 in file xsect.c, i.e.,
xsect->aFull = 0.8117 * xsect->yFull * xsect->wMax;
xsect->rFull = 0.2448 * sqrt(xsect->yFull * xsect->wMax);