Kevin Clark did a post earlier this year on “When a Pipe Network Structure isn’t a Pipe Network Structure”.  Autodesk Storm and Sanitary Analysis (SSA) uses the STM file format as the mechanism for exporting a Civil 3d pipe networks out for consumption with SSA and the return trip back into Civil 3d – these settings within the command setting become critical.   Kevin’s focus was on Civil 3d 2010 and what it did during the export to Hydraflow Storm Sewer.   Let us take it one step farther – Autodesk Storm and Sanitary Analysis.

Autodesk Storm and Sanitary (SSA) reads and writes the STM file format.  SSA converts the Hydraflow structures into Autodesk Storm and Sanitary Structures.  On import, Autodesk Storm and Sanitary will import the Pipes (Lines in the STM file) as Conveyance Links of the same shape (Circular, Elliptical, or Box) and size.   The STM structures will import into SSA as either a Junction, Inlet or Outfall.   The STM Manhole and Headwall are created as Junctions.   The Outfall in the STM becomes – <don’t hold your breath> the Outfall in SSA.

Inlets get some special treatment.   Unlike the Hydraflow, SSA’s analysis is concerned more with the conservation of mass and momentum.   STM Inlets on grade can target downstream inlets or can be directed OFFSITE.   Unfortunately, SSA is unable to calculate a model when all possible exits are not accounted for, SSA will create a secondary OFFSITE Outlet that collects that unfortunately mis-targeted flow.   With that said, if you took the time to target your inlets in STM, SSA takes the time to preserve that connectivity for you.  Using the Hydraflow gutter statistics in the STM file, a roadway channel that routes your overland flow to the next target is automatically created.

That gutter channel is connected to the offsite outlet.  To redirect that gutter flow, simply select the conveyance link between the inlet and the OFFSITE outlet, right-mouse-click, and select Connect To/From.  Follow the dialog box to select the Upstream node and then the downstream node and you have succeeded in fixing that gutter flow link.  Repeat until all OFFSITE nodes are disconnected, and then delete those nodes as they are not needed.

Hopefully, you now understand where and why some of those items appear in SSA.  For more information, please refer to page 478 thru 480 of the Autodesk Storm and Sanitary Analysis Users Guide.