What is the easiest way to open a new or different hydraulic model inside an existing MXD? (which may or may not already have an existing hydraulic model attached)
This is an area that we will be developing more seamlessly in the future. For now, you can use the following information. If you are starting with an MXD file that hasn't previously had a water or storm/sewer model associated with it, you can import the model file using the steps found in this link: Opening a Hydraulic Model in the ArcMap (ArcGIS) platform for the first time.
If the MXD file already has a hydraulic model associated it with it, the steps will require a little work ahead of time. If there is a model already associated with it, there is not an easy way to import the model file without some issues, such as multiple layers appearing in the ArcMap Table of Contents. To work around this, it is recommended that you create a template or "seed" MXD file. This will be an MXD that contains important information common across multiple usages, such as symbols and symbology. This can be useful for example when there is a need to create a copy of the model to do future work on.
Once the template or "seed" MXD is created, this can be the starting point for future modeling. Simply save the MXD file with a new name, then import the hydraulic model as you normally would.
This workflow will be improved in future releases of the programs.
How do I open a model that is already associated with an ArcGIS MXD file?
WaterGEMS for ArcGIS FAQ
SewerGEMS for ArcGIS FAQ