What is the purpose of the "Virtual Flow depth" calculation option when using the Implicit solver?
This calculation option was introduced in version 08.11.01.21 and can be used to resolve model result instability problems in cases of very low flows. Dry flow conditions can sometimes occur in storm and sewer models. A zero or near zero flow can cause instability in the implicit numerical model results. In order to overcome this small flow instability, a virtual flow method is used. This method adds a small base flow to every conduit which has a depth that is less than the virtual depth value that you enter in the calculation options. After the calculation is finished for a time step, a filtering algorithm is used to filter out the virtual flow and depth so that the final results are more accurately presented. Values for the virtual flow depth field are typically between 0.0 and 0.04ft. The default value of 0.04 ft has been tested and works well for a wide range of conditions. Generally speaking, a larger value will further increase stability while a smaller value may increase accuracy for small flows for the same cases. So, if you experience unstable model results in cases of very low flows and depths, you may need to try increasing this value.
As seen in the below troubleshooting article, if you're having to adjust this calculation option because of many pipes with very low flows, you should consider omitting them from the model. This can be done by removing the dry pipes and adding the upstream load to the next-downstream manhole.
Troubleshooting unstable SewerGEMS and CivilStorm results using the implicit solver