It sure looks like a tailwater issue. Even during the 1-year event, the peak elevation in CB2 is higher than the grate in CB1 (downstream catch basins show flow through the grate).

Also - CB1 numbers are suspect as the grate is below the crown of the outlet pipe.

Thanks TerryScan for taking the time to look at this. In your experience, is there a way to fix situations like this? Any parameter I change doesn't seem to have an effect.

Thanks

If you look at your device routing for each CB, water can only reach the culvert outlet by first flowing though the grate, which is several feet higher. That would be OK if the CB only received water from above-grade through the grate, put it also needs to be able to pass the pipe flow from the upstream CB without forcing it to jump up through the grate.

Short answer: Delete the grate. (Device #3) The grate is not an outlet device on the CB, so it shouldn't be used to limit flow to the culvert.

This will allow water to exit freely through the culvert -or- overflow through your secondary outflow.

For details please see www.hydrocad.net/sewers.htm Note the section entitled "What about the grate?"

Peter Smart
HydroCAD Software
www.hydrocad.net