ERROR 108:
Ambiguous outlet ID name for Subcatchment xxx.
Error 108 : The name of the element identified as the outlet of a subcatchment belongs to both a node and a subcatchment in the project's data base.
Quick Fix — Start Here
Locate the subcatchment identified in ERROR 108 and check its assigned outlet ID. Confirm whether the same ID exists as both a node and a subcatchment.
If it does, determine the intended receiving object, assign a unique ID to the conflicting object, and re-validate the model. For imported networks, use the GeoSWMM Network Analysis Tool to help identify duplicate or inconsistent node/outfall IDs.
1. Description
ERROR 108 occurs when a subcatchment outlet ID matches both a node and another subcatchment. Because either object can be a valid SWMM outlet, the engine cannot determine where the runoff should be routed.
In GeoSWMM, the Object Property Editor validates object names during editing, and the Watershed Delineation tool assigns unique names automatically. Therefore, this issue is most likely to be encountered in imported or edited model data, where conflicting IDs may already exist in the source dataset.
2. Technical Background
EPA SWMM allows a subcatchment to discharge either to a drainage-system node or to another subcatchment as run-on. When the outlet ID matches an object in both categories, the reference is ambiguous and ERROR 108 is reported. A subcatchment outlet ID must identify only one valid receiving object. Duplicate IDs within node classes are a separate data-quality issue; ERROR 108 specifically occurs when the same outlet ID exists as both a node and a subcatchment.
3. Resolution
Start with the subcatchment identified in ERROR 108. Confirm its outlet ID, identify the conflicting objects, and determine which object is intended to receive the runoff.
Table 1 : Troubleshooting steps
Step | Action | Why It Helps |
|---|---|---|
1 | Identify the flagged subcatchment and its outlet ID
| Identifies the ambiguous outlet reference. |
2 | Check for the naming conflict
| Confirms the conflicting objects that must be distinguished. |
3 | Confirm the intended receiving object.
| Prevents a naming correction from changing the intended runoff routing. |
4 | Assign a unique ID and update the outlet reference
| Removes the ambiguity while preserving model connectivity. |
5 | Save and re-validate the model
| Confirms both the naming correction and the intended drainage connection. |