Giorgio,

This is an interesting question with many ramifications in SWMM 4 and SWMM 5:

First, if you used a value of 0.0 for AMEN in SWMM 4 the program will change the area to 12.566 square feet or 1.22 square meters. If you use a value of 0.01 for AMEN in SWMM 4 then the minimum area will be 0.01. If you were using 0.0 then SWMM 4 and SWMM 5 have the same behavoir in your model.

The minimum area in SWMM 5 is hardwired at present to:

#define MINSURFAREA 12.566 // min. nodal surface area (~4 ft diam.)

since the internal units are American units in SWMM 5. We will have to test how important having a minimum area of 12.566 is versus 0.01, for example, in SWMM 5. It probably does not matter a lot if you are using medium sized trapezoidal channels. Again if you were using 0.0 for AMEN in your SWMM 4 data set then SWMM 5 will use the same default area.

Regards,
Bob Dickinson
DickinsonRE@CDM.COM