ERROR 111:
Invalid length for Conduit xxx.
Error 111 : Conduits cannot have zero or negative lengths.
Quick Fix — Start Here
Open the conduit named in ERROR 111 and check Use User Defined Length.
- If enabled, enter a positive Length value or disable it to use the GIS-derived length.
- If disabled, check the conduit’s GIS geometry and derived length.
Save the changes and re-validate the model.
1. Description
Every conduit in SWMM must have a positive length. The length represents the distance along the conduit’s flow path between its inlet (upstream) and outlet (downstream) nodes and is required for hydraulic routing.
ERROR 111 occurs during model validation when the conduit identified in the error message is passed to the SWMM engine with a length of zero or less. The error is detected before the simulation starts, so troubleshooting should focus on the conduit length being supplied to SWMM and how that length is being determined.
In GeoSWMM, conduits are represented as GIS line features. By default, GeoSWMM derives the conduit length automatically from the line geometry, so a valid conduit feature will normally have a positive length.
GeoSWMM also provides a Use User-Defined Length option when the modeled conduit length needs to differ from the GIS-derived length. This can be useful when a modeler needs to apply a specific design length or preserve established conduit lengths from an existing or legacy SWMM model within the GIS-based modeling environment.
When Use User Defined Length is enabled, GeoSWMM uses the specified value instead of the GIS-derived length. If a valid positive value is not assigned, the conduit can be passed to SWMM with an invalid length and trigger ERROR 111.
2. Technical Background
Conduit length is required by EPA SWMM for hydraulic routing and affects calculations such as conduit slope, flow routing, travel distance, and storage.
For example: Slope = elevation drop / Length — a zero length makes the slope undefined (division by zero). Conduit length also affects normal-flow calculations and Dynamic Wave routing, where the conduit geometry and length are used to represent hydraulic conditions along the link.
The fundamental SWMM engine requirement is Conduit Length > 0
Where GeoSWMM Gets a Conduit's Length
GeoSWMM can determine conduit length from one of two sources:
- Use User Defined Length unchecked (default) — GIS-driven length: GeoSWMM calculates the conduit Length automatically from its GIS line geometry. For a valid line feature with distinct endpoints, the computed value should normally be positive.
- Use User Defined Length checked — User-defined length: GeoSWMM uses the Length specified by the modeler instead of the GIS-driven value or geometric length. This is meant for cases where the GIS length doesn't represent the intended hydraulic length.
A key condition to check when ERROR 111 occurs is whether Use User Defined Length is enabled without a valid positive Length value. Figure 1 should show the default GIS-derived length state beside the user-defined length state so readers can quickly compare the two cases.
Figure 1. By default (left), GeoSWMM computes a conduit's Length from its GIS geometry, which is essentially always valid. When Use User Defined Length is checked but left blank (right), SWMM reads a zero length and reports ERROR 111.
3. Diagnostic Checklist
Use the following checks to determine which length source applies to the conduit identified in the error message before making changes.
Check | What It Tells You |
|---|---|
Is Use User Defined Length enabled? | If yes, GeoSWMM uses the user-defined value instead of the GIS-derived length. Confirm that the assigned Length is greater than zero. |
Is Use User Defined Length disabled? | GeoSWMM uses the length derived from the conduit’s GIS line geometry. Confirm that the feature has valid geometry and a positive derived length. |
Is the selected length appropriate? | If a user-defined value is used, confirm that it represents the intended design, field, plan, or legacy-model length and uses the correct project units |
4. Solutions
Start with Use User Defined Length to determine which conduit length GeoSWMM is using. If it is enabled, review the user-defined value first. If it is disabled, review the GIS-derived length and conduit geometry.
Table 1 : Troubleshooting steps
Step | Action | Why It Helps |
|---|---|---|
1 | In GeoSWMM, identify the flagged conduit and open its properties
| Isolates the conduit causing the validation failure before any changes are made. |
2 | If Use User Defined Length is enabled, assign a valid value — or disable the option
| Addresses an invalid user-defined length before investigating the GIS geometry. |
3 | If Use User Defined Length is disabled, inspect the conduit's GIS geometry
| Identifies GIS geometry problems that may prevent GeoSWMM from obtaining a valid conduit length. |
4 | If the feature is not intended to represent a pipe or channel, review the link type
| Helps ensure that the model element represents the actual hydraulic behavior rather than only satisfying the validation requirement. |
5 | Save in GeoSWMM and let the engine re-validate
| Confirms that a valid positive conduit length is being supplied to SWMM. |
Recommended Practice — Use GIS-Derived Conduit Length
In GeoSWMM, use the GIS-derived conduit length as the default approach. This keeps conduit length directly tied to the mapped line geometry, reduces manual input, and helps eliminate missing, zero, or inconsistent user-entered length values.