Subject: WARNING 04: minimum elevation drop used for Conduit - What Does this Message Mean?
This message means that the elevation drop across the link is less than the minimum allowable drop or (0.001 /3.048 meters)
Elevation1 = Link Offset Upstream + Upstream Node Invert
Elevation2 = Link Offset Downstream + Downstream Node Invert
Internally Elevation1 – Elevation2 should be greater than 0.001 /3.048 meters. If it is not then SWMM 5 or InfoSWMM will use the minimum drop or0.001 /3.048 meters
It simply is a rule that does not allow flat slopes as the flat slopes mean no normal flow calculations. You should not have to worry about this warningmessage.
Here is an example of a conduit in which the rule is applied. The rule is applied to link U-104 because it is flat and has no slope.
via Blogger http://www.swmm5.net/2013/07/warning-04-minimum-elevation-drop-used.html