Pipe with low flow exhibiting zero velocity

When analyzing stormwater or sewer networks in Autodesk Civil 3D using Devotech iDAS, you may encounter a situation where pipes with a very low flow exhibit zero velocity.

This phenomenon occurs because the SWMM engine, which Devotech iDAS utilizes for its analyses, reports a zero velocity if the pipe's flow depth falls below 0.003 meters (0.01 feet) as per SWMM code guidelines.

To address this issue, one effective strategy is to increase the pipe roughness coefficient. For example, adjusting Manning's roughness coefficient from 0.012 to 0.015 has successfully resolved the zero velocity reporting problem in certain scenarios.

For further insights into this issue, refer to the discussion on OpenSWMM:

https://www.openswmm.org/Topic/31660/low-flow-zero-velocity