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What is the purpose of the "Virtual Flow depth" calculation option?Product: SewerGEMSVersion: 08.11.XX.XXArea: Analysis/Computation, Problem ID#: 36733, GENERALINFORMATION
This calculation option is new to version 08.11.01.21 and can be used tocombat model result instability problems in cases of very low flows. Dryflow conditions can sometimes occur in storm and sewer models. A zero ornear zero flow can cause instability in the implicit numerical modelresults. In order to overcome this small flow instability, a virtualflow method is used. This method adds a small base flow to every conduitwhich has a depth that is less than the virtual depth value that youenter in the calculation options. After the calculation is finished fora time step, a filtering algorithm is used to filter out the virtualflow and depth so that the final results are more accurately presented.Values for the virtual flow depth field are typically between 0.0 and0.04ft. The default value of 0.04 ft has been tested and works well fora wide range of conditions. Generally speaking, a larger value willfurther increase stability while a smaller value may increase accuracyfor small flows for the same cases. So, if you experience unstable modelresults in cases of very low flows and depths, you may need to tryincreasing this value.