SewerGEMS Modelling an outlet controlled basin

Question

Hi, 
 I have a rising main pumping into a gravity main. The rising main and gravity main is connected with a manhole. 
 Even though my pipes are sloped to the minimum and diameters seemingly adequate, I get spikes in the Froude number. I'm assuming that this happens because of the wave action and backflow occurring inside the pipe as a result of a sudden influx in fluid from the rising main. 
 Is there a way to simulate a type of outflow controlled attenuation structure that would assist in smoothing out the volume influxes from the rising main into the gravity main? 
 Kind regards, 
 Liezl

Jesse Dringoli · Answer

Hi Liezl, 
 It may be best to take a look at your model so we can check if the Froute number problem is "real" or related to model instability or some other factor. 
 
 Are you using the latest version of SewerGEMS? (10.03.04.53) Check under File > Help > About. See more here: Downloading OpenFlows Software 
 Are you seeing the Froude number message just at the beginning of the simulation, or throughout the duration? Typically some initial instability will occur as the dry pipes start to fill, and you may need to focus on later timesteps. 
 Are you seeing a high number for the "Continuity Error" in the Calculation Summary? You'll want to get that below ~5% before you can trust the results. See the following articles (depending on the solver you're using) for guidance: 
 
 Troubleshooting unstable SewerGEMS and CivilStorm results using the implicit solver 
 Troubleshooting unstable SewerGEMS and CivilStorm model results using the Explicit SWMM Solver

If this does not help, please provide a copy of the model for review by going to File > Save to Package, send us the resulting ZIP file (see two options here ), and reply back with any additional steps to reproduce.