Minimum Drop for a Link in SWMM 5
I wanted to discuss a slightly hidden feature of SWMM 5 that has a definite impact on flat or links with zero slope. SWMM5 uses an internal unit system based on feet and cfs even though externally you can use SI units. There is a minimum conduit drop between the upstream and downstream ends of a SWMM 5 link which is 0.001 feet. This is done so that every link has a non zero slope. However, depending on the length of the link the drop of 0.001 feet results in drastically different slopes. Here is a table of the length of link (feet) and the internal slope used in SWMM 5 based on the minimum drop of 0.001 feet. As you can see a very long link can have a very small slope which has time step issues - you need a smaller time step. Length Slope 0.1 1.0000000% 0.5 0.2000000% 1 0.1000000% 2 0.0500000% 3 0.0333333% 4 0.0250000% 5 0.0200000% 6 0.0166667% 7 0.0142857% 8 0.0125000% 9 0.0111111% 10 0.010...