Problem

When importing a polyline shapefile in Modelbuilder, the resulting pipes are not connecting at the expected locations. Or, multiple pipes are seen connected to the same node.

When performing a second update (build model) in ModelBuilder, pipe connectivity changes or you notice that the ModelBuilder summary indicates a change, when nothing has changed in the source or destination.

Solution

If you are seeing multiple pipes connected to the wrong node, this could be due to duplicate labels in the selected "key" field. Check the data in the field you select for the key field (or your GIS-ID field, if using the GIS-ID method) and ensure that there are no duplicates. See more information on how to use ModelBuilder here: Using ModelBuilder to Import External Data

Unexpected connectivity can also be due to the way that the polylines are connected in your source file, the tolerance you specified and/or the order of establishing connectivity by creating new nodes if none found at endpoint, or connecting to existing nodes within the specified tolerance.

For example, a pipe may be connecting to a newly created end node from another polyline within the spatial tolerance specified, instead of connecting at a point closer to the end of the pipe. A second ModelBuilder run ("build model" button) might end up changing the end node connectivity of that pipe if a closer end node was created during the first ModelBuilder process, after that pipe was already connected.

To help avoid this problem:

1) Use the snapping feature in your GIS application (assuming the source is a shapefile or geodatabase) and ensure polyline endpoints connect at other polyline endpoints where possible.
2) Where possible, import end nodes layers (point features) along with the polyline/pipe layers, so the program doesn't need to auto-create junctions.
3) Use a smaller tolerance for cases where multiple junctions are close together.

Note: It would also be worth checking the scale of the source data. If the source data has a scale that is very small, it may be close to or smaller than the tolerance itself. If that occurs, pipes may connect to unexpected nodes. If this occurs, you may need to adjust the scale in the source program, like ArcMap. 

Example:

Example of how the shapefile source could have been set up (separate colors are separate polylines). A smaller tolerance in ModelBuilder would also help if the junction points here are too close together.

See Also

Creating a shapefile/feature class to ensure proper pipe and junction connectivity using ModelBuilder

Preparing GIS data for use in the hydraulics and hydrology products

Building and updating a model using ModelBuilder