ERROR 115:
Adverse slope for Conduit xxx.
Error 115 : Under Steady or Kinematic Wave routing, all conduits must have positive slopes. This can usually be corrected by reversing the inlet and outlet nodes of the conduit (i.e., right click on the conduit and select Reverse from the popup menu that appears). Adverse slopes are permitted under Dynamic Wave routing.
1. Description
Under Steady or Kinematic Wave routing, all conduits must have positive slopes, meaning the conduit invert must drop in the modeled flow direction from the inlet (upstream) node to the outlet (downstream) node. A negative computed slope of a conduit is flagged as an adverse slope.
ERROR 115 occurs during model validation when the effective upstream elevation (node invert + inlet offset) is lower than the effective downstream elevation (node invert + outlet offset). Steady and Kinematic Wave routing require a positive conduit slope; Dynamic Wave routing permits adverse slopes.
2. Technical Background
SWMM calculates conduit slope from the effective elevations at each conduit end and the conduit length:
- Upstream End Elevation = Upstream Node Invert + Inlet Offset.
- Downstream End Elevation = Downstream Node Invert + Outlet Offset.
- Slope = (Upstream End Elevation - Downstream End Elevation) / Conduit Length.
For Steady Flow and Kinematic Wave routing: Slope > 0
The Steady Flow and Kinematic Wave routing methods rely on the conduit bed slope as the primary driving term for normal-flow calculations, so a zero or negative conduit slope is incompatible with those routing assumptions and triggers ERROR 115 during validation. The Dynamic Wave method solves the one-dimensional St. Venant equations and can represent flat or adverse bed slopes because flow can be governed by the hydraulic grade line rather than bed slope alone.
In practice, ERROR 115 commonly falls into two categories: the conduit was digitized opposite the intended drainage direction, causing inlet and outlet nodes to be swapped, or the effective conduit elevations are genuinely adverse because of offsets, source-data errors, or a real physical configuration such as a siphon or backwater-influenced pipe.
In GeoSWMM, models built with the Watershed Delineation tool derive conduit direction from the terrain, so a reversed-conduit issue is less likely to originate from that workflow. For imported GIS or model data, or after manual GIS edits, verify that conduit direction follows the intended drainage path.
3. Resolution
Start with the conduit identified in ERROR 115. Confirm its direction and effective end elevations before changing model data.
Table 1 : Troubleshooting steps
Step | Action | Why It Helps |
|---|---|---|
1 | Identify the flagged conduit and check its slope
| Confirms that the conduit has an adverse slope and identifies the values involved. |
2 | Check the conduit direction
| Determines whether the adverse slope results from a reversed conduit rather than incorrect elevations. |
3 | Reverse the conduit if the direction is incorrect
| Corrects conduit orientation without changing valid elevation data. |
4 | If the direction is correct, verify elevations, offsets, units, and vertical datum
| Identifies source-data or elevation inconsistencies that can create an artificial adverse slope. |
5 | If the adverse slope is intentional, review the routing method
| Preserves the physical geometry while using a routing method that permits adverse slopes. |
6 | Save and re-validate the model.
| Confirms that the issue has been addressed without changing the intended drainage configuration. |
Note: Do Not Change Valid Elevations to Remove ERROR 115
First confirm the conduit direction and source elevation data. If the adverse slope represents the actual system, use Dynamic Wave routing rather than changing valid invert elevations or offsets simply to clear the error.