I am finding that when I use a transverse weir with an assigned side slope of 3 there is an error and the model wont run. I didn't think that transverse weirs use a side slope so shouldn't this value be ignored?

In the input file, geometry of the weirs are included with the other conduit cross-sections. For rectangular open cross sections (which is used for transverse weirs as well as side and roadway weirs) a third parameter (Geom3) can be added to indicate the number of side walls removed in computing wetted perimeter for conduits. For this parameter possible values are 0,1, or 2. So there is a check to see whether , if so an error message is activated.

However, even if there is an acceptable value (between 0 and 2) for Geom3 of a weir, still SWMM does not use it and internally changed it to zero for transverse, side, or roadway weirs.