In building a model, a user occasionally runs into situations where there are gaps in the modeling data for which it is difficult to fill in the information. For example elevation data may be missing form a manhole or some property may be missing from a pipe. In such cases, it is desirable to let the model draw "inferences" about those values from property values from nearby elements. For example, elevations from upstream and downstream manholes can be used to interpolate values for manhole elevations between those with known properties or the diameter or material from the downstream conduit can be used to fill in the diameter and material for conduits for which those properties are missing.

Bentley SewerGEMS fills in elevation data such that the slopes of pipes being inferred are constant along a reach and uses the downstream pipes as the basis for inferring missing values for pipe properties. In the drawing below, the elevations are interpolated between the known elevations and the physical properties of the downstream pipe are used to fill in the properties of the missing pipes. However, if the user has manually entered some of the physical properties manually, these values will not be overridden. Loading information (sanitary load/infiltration rate) and head loss type (AASHTO, HEC-22) are not inferred but are taken from the manhole prototype.

Where:

• Gu = upstream ground
• Gd = downstream ground
• Zu = upstream invert
• Zd = downstream invert
• N = number of additional manholes
• D = diameter (rise) of downstream pipe

In Bentley SewerGEMS , element property inferencing is only available for gravity elements.

There are two general cases for this tool

1. Layout new manholes - In this case, the user knows the elevations of the manholes at the beginning and end of a run and the physical properties of the downstream conduit but has not yet placed the intervening manholes and pipes. The user indicates the number of manholes that will be places in the gap and they will automatically be inserted. This is useful in laying out new systems.
2. Follow existing path - In this case, the user has laid out the manholes and the conduits between them. The user employs this tool to assign properties to the elements between the upstream and downstream nodes. This is more useful in filling in details of the system when the path is already determined whether for a new or existing run of conduits.