H2OMAP SWMM

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 0.0100000%
25 0.0040000%
50 0.0020000%
75 0.0013333%
100 0.0010000%
200 0.0005000%
300 0.0003333%
400 0.0002500%
500 0.0002000%
600 0.0001667%
700 0.0001429%
800 0.0001250%
900 0.0001111%
1000 0.0001000%
2000 0.0000500%
5000 0.0000200%
10000 0.0000100%

via Blogger http://ift.tt/NJKKp2

Leave a Reply