After working on a project in ArcMap integrated mode and saving an .mxd, why does the model not appear when reopening the .mxd?
After reopening the map, you must load the WaterGEMS/SewerGEMS/HAMMER project from the Hydraulic Model Manager. For example for WaterGEMS this would be under WaterGEMS > View > Hydraulic Model Manager.
To open an existing model, click the Open Hydraulic Model button. You can also close a hydraulic model or add a new model through the Hydraulic Model Manager.
WaterGEMS for ArcGIS FAQs