This is just a note on how SWMM5 uses zero Subcatchment Slope in its Runoff Calculations:
1. If the slope is zero then the engine will NOT calculate Runoff as the WCON parameter is zero (Bullet 4 in Figure 1)
2. If you do want Runoff from all of your Subcatchments then you need to check the slope value in your Hydrology by doing a data query (Bullet 2 in Figure 2)