Troubleshooting Bypass Link Errors in Storm and Sanitary Analysis

How frustrating is it to step through each of the tedious tasks of configuring Storm and Sanitary Analysis to analyze your pipe network, just to get this daunting result:

Error 600: On grade Inlet does not have two outlet links. At least one stormwater sewer pipe and a bypass gutter link must be defined.

Your first impulse may be to search the manual that comes with Storm and Sanitary Analysis. The manual is 544 pages of very useful information. But in the error section, you will find two words describing the error: self explanatory. And after concluding that the manual will be of no help, Googling the error hits on a few results from the AutoCAD Civil 3D forum, all of which offer no resolutions or suggestions other than configuring the “on grade” inlets as sag inlets. The error will disappear, but all of those bypass links that were painfully configured to the network are omitted from the analysis, which is not the preferred result.There are a handful of other errors that may pop up and they are usually due to little errors or oversights that can go unnoticed.

To resolve this, you must troubleshoot in three areas:

1. The Civil 3D pipe network
2. The bypass links in SSA
3. The inlet nodes in SSA

Step 1: Evaluate the Pipe Network in Civil 3D

When the pipe network is laid out horizontally in plan, Civil 3D is configured to use parameters set up in the rule sets to position the pipes vertically. And while this does a fairly good job in keeping each pipe under the ground and maintaining a proper slope of a pipe from its upper end to its lower end, before taking the network into SSA, there are some things to check.

1. Often when we lay out storm sewer along a street, we’ll create the main pipe run on one side of the street starting from the upper end of the system running all the way to the outfall. Then we place structures on the other side of the street opposite to the structures in the main pipe run. When those structures are placed, we tie them into the main pipe run on the other side of the street with a pipe. In the image below, structure 4H was just placed along with a pipe tying it into structure 4A; structure 4A is part of the main pipe run.
   If you profile the pipe run for structure 4H to 4A, you will see that its invert is lower than the main pipe run, as shown below. When the pipe network is imported into SSA, this can cause the SSA analysis to error. Before exporting pipes out to SSA, inspect these crossings and correct any pipes that are not draining positively into the main system. Once you are assured that all is flowing through the system and reaching the outfall without issue, you are ready to continue.
2. Also check the surfaces assigned to your pipes and structures. If a pipe or structure is associated to a surface that it isn’t physically under, there will be some negative elevations in your exported pipe network. It’s also important to make sure the rim elevations assigned to your structures are correct, so that when you model your bypass links the upper and lower end elevations will make sense. For instance, is the physical position of the curb inlet triggering an elevation from the gutter flowline or the top of structure?
3. Do configure the structure properties for inlet locations as “on sag” if they are placed in low points. This will prevent them from generating bypass links when imported into SSA.

Proper preparation of your pipe network in Civil 3D prior to transferring them over to SSA will save a lot of time by saving you the arduous task of performing those manual edits in SSA.

Step 2: Configuration of the Bypass Links in SSA

Configuration of bypass links are a common cause for errors.

1. SSA assumes every inlet is “on grade” and attaches bypass links to everyone of them once imported, unless you configure them as “on sag” in Civil 3D. If you forget to do so, just delete the extra link from the sag inlets. Then delete the orphaned outfall node. Proceeding with the analysis at this point could cause errors because deleting the link and node will not reconfigure the inlet as “on sag”. You will have to open the inlet properties and change the inlet location property to continue.
   
2. The obvious task at hand is to reconfigure the default bypass links so that they connect to the proper inlets, as well as configuring each with the cross-sectional shape of its open channel shape. But once they are all physically linked to the bypassed inlet and the inlet receiving the bypass flow, the properties of each bypass link must be configured. Not doing so will cause errors especially when the elevation of the inlet invert is lower than the elevation of the outlet invert.
   

Step 3: Configuration of the Inlet Nodes in SSA

After the bypass links are configured, the properties of each “on grade” inlet node must be checked to be sure that the roadway/gutter bypass link value is set. Usually this is automatically set at import when the bypass links are created, but may reset to a blank after reconfiguring the bypass links. Not checking this value may result in errors.

The items posted above review three places you can check to track down errors around bypass links. As some of the posts in the forums would suggest, setting “on grade” inlets to be “on sag” inlets will produce an error-free analysis, but it does so by omitting bypass flow for “on grade” inlets. The consideration of bypass flow is critical to the design of our pipe network which will ultimately be transferred back to Civil 3D for the completion of construction plans.

– Cyndy Davenport