ERROR 107:
Cannot compute a valid time step.
Error 107 : A valid time step for runoff or flow routing calculations (i.e., a number greater than 0) could not be computed at some stage of the simulation.
First Check
In GeoSWMM, review the simulation options and confirm WET_STEP, DRY_STEP, ROUTING_STEP, and Dynamic Wave MINIMUM_STEP are greater than 0 before checking rainfall, controls, or hydraulic conditions.
1. Description
ERROR 107 means the GeoSWMM simulation was unable to select a runoff or flow-routing time step greater than zero. The first troubleshooting step should therefore be to validate the time-step settings in the final model input file.
Runoff and flow routing use separate time-step paths in the SWMM model: runoff checks begin with WET_STEP and DRY_STEP, while flow-routing checks begin with ROUTING_STEP and, for Dynamic Wave routing, the minimum-step setting. ERROR 107 occurs when the selected step becomes zero or negative. This can result from invalid time-step settings, values changed during import, conversion, or automation, or timing-alignment issues involving rainfall records, control rules, or routing events.
2. Technical Background
A. If ERROR 107 Occurs Immediately
Check the final GeoSWMM model settings used for the simulation.
Step | Action | Why It Helps |
|---|---|---|
1 . GeoSWMM Options Section |
| Confirms the values GeoSWMM uses during the run. |
2 . Runoff time steps |
| Ensures SWMM can select a positive runoff step. |
3 . Routing time step | Confirm ROUTING_STEP > 0. For Steady Flow or Kinematic Wave routing, this is the routing step used by the model. | Prevents an invalid routing step from stopping the simulation. |
4 . Dynamic Wave settings | When Dynamic Wave routing is used, confirm MINIMUM_STEP > 0 and MINIMUM_STEP does not exceed ROUTING_STEP. | Keeps the routing step within valid bounds. |
5. GeoSWMM imported or generated settings | Review GeoSWMM project settings, imported INP files, geodatabase edits, or any workflow that creates or modifies the model before the run. | Identifies values changed during GeoSWMM data preparation or model export. |
B. If the Simulation Advances Before the Error Occurs
Use this section when the GeoSWMM simulation starts successfully but stops later with ERROR 107. After confirming the time-step settings in Section A, check whether a timing issue is forcing the next calculation step to become invalid.
- Rainfall timing: check rain-gage and rainfall time-series records for incorrect dates, duplicate records, missing records, or inconsistent recording intervals.
- Control rules: confirm that control rules use valid timing and do not create conflicting or abrupt changes during the simulation.
- External or generated inflows: review lateral inflows and generated hydrographs near the time when the error occurs.
- GeoSWMM workflow changes: confirm that imported INP files, geodatabase edits, or other workflows did not change the final runtime options.
- Isolation test: temporarily disable suspect control rules, routing events, or inflows to see whether the error disappears.
C. If Sections A and B Are Valid
Use this section only after the direct time-step settings and timing inputs are confirmed. At this point, review model conditions that may create unstable routing or abrupt runoff response.
Conditions | Review |
|---|---|
Rapid or unstable routing conditions | Try a smaller positive ROUTING_STEP or review the Dynamic Wave variable time-step settings. If instability continues, check very short conduits, abrupt geometry changes, pumps, and regulators. |
Abrupt lateral inflows | Review connected subcatchment hydrographs and verify rainfall/runoff inputs that control inflow timing and magnitude. |
Note
GeoSWMM subcatchment attributes do not directly set the simulation time step, but they can contribute to abrupt runoff response. Review them only after the direct time-step and timing checks are valid.