Running SWMM Simulations: Options, Execution, and Results

After building your SWMM model with subcatchments, junctions, and conduits, it is time to run the simulation and analyze results. This tutorial covers simulation options, executing the model, and interpreting the output.

The option names and result-review workflow here follow the EPA SWMM 5.2 User’s Manual. Appropriate methods and time steps remain model-specific and must be checked for numerical stability and suitability for the decision being made.

Simulation Options Overview

Before running a simulation, you must configure the analysis options. Access these through Project > Analysis Options or by double-clicking the Options node in the Browser Panel.

The options dialog has several tabs controlling different aspects of the simulation.

General Options

The General tab sets fundamental simulation parameters.

Process Models

Select which processes to simulate:

For typical stormwater models, enable Rainfall/Runoff and Flow Routing.

Infiltration Model

Choose how SWMM calculates infiltration on pervious areas:

Flow Routing Method

The routing method determines how flows are calculated through the network:

Flow Units

Select your flow unit preference:

Date and Time Options

The Dates tab controls simulation timing.

Simulation Period

Antecedent Conditions

Higher dry days mean drier soil and higher initial infiltration capacity.

Time Step Options

Time steps control simulation resolution and accuracy.

Reporting Time Step

How often results are recorded to the output file.

• Fine enough to retain the peaks and timing needed for review
• Coarser intervals can hide short-duration instabilities or flooding

Wet Weather Time Step

Computational time step during rainfall periods.

• Fine enough to resolve runoff changes for the modeled subcatchments
• Check sensitivity instead of treating a generic interval as a default

Dry Weather Time Step

Computational time step during dry periods.

• May be coarser only where dry-weather processes change slowly enough
• Confirm that the choice does not distort continuous-simulation processes

Routing Time Step

Computational interval for routing flow and water-quality constituents through the conveyance system. It applies to every routing method; Dynamic Wave generally requires a much smaller value than Steady Flow or Kinematic Wave.

• Set an appropriate maximum for the shortest conduits and fastest hydraulic response
• Smaller values can improve stability but increase run time; confirm with result comparisons

Dynamic Wave Options

When using Dynamic Wave routing, additional options are available.

Variable Time Step

Enable adaptive time stepping:

Variable time steps can significantly reduce run times while maintaining stability.

Inertial Terms

Control how momentum equations are solved:

Select the inertial-term treatment deliberately and test its influence where rapidly varied flow or numerical instability affects study conclusions.

Force Main Equation

This setting applies only to conduits assigned the Circular Force Main cross-section. It selects the friction-loss equation used while those conduits flow under pressure; it is not a global choice for every full circular conduit.

Running the Simulation

Pre-Run Checks

Before clicking Run, verify:

1. All subcatchments have rain gages
2. All subcatchments have outlets
3. All junctions have valid inverts and depths
4. At least one outfall exists
5. Rain gage references valid time series

Executing the Model

1. Click the Run button in the toolbar, or
2. Go to Project > Run Simulation, or
3. Press F9

Monitoring Progress

During simulation:

• Progress bar shows percent complete
• Current simulation time displays
• Status messages appear for errors/warnings

Simulation Completion

When finished:

• Green checkmark: Simulation successful
• Yellow triangle: Warnings (review status report)
• Red X: Errors (simulation failed)

Reviewing Simulation Status

After running, review the status report: Report > Status.

Continuity Errors

Higher continuity errors indicate:

• Time steps too large
• Extremely steep or flat pipes
• Numerical instabilities

Flow Instabilities

The Status Report lists the links with the five highest Flow Instability Index values. The index counts local turns in each link’s reported flow series, normalizes that count against a random series, and ranges from 0 to 150. It does not list individual unstable time steps and is not, by itself, a pass/fail threshold.

• Inspect the high-index link hydrographs at a reporting interval fine enough to reveal oscillation
• Review non-converging nodes and routing-step statistics alongside the index
• Re-run with appropriate routing options or time-step changes and compare the affected results

Node Flooding Summary

Shows junctions that experienced flooding:

• Hours flooded
• Maximum flooding rate
• Total flood volume

Conduit Surcharge Summary

For conduits with a non-zero entry, the table reports hours full at both ends, at the upstream end, and at the downstream end; hours flowing above full normal flow; and hours capacity limited. Node surcharge is a separate table that reports hours surcharged and surcharge elevations.

Viewing Results

SWMM provides multiple ways to explore simulation results.

Map Display

Use the time slider to animate results:

1. Move the slider to select a time
2. Map colors show values at that instant
3. Click Play to animate through the simulation

Color variables you can display:

• Nodes: Depth, head, volume, flooding, quality
• Links: Flow, velocity, depth, and the Capacity view (for conduits, the fraction of full cross-sectional area occupied by flow)

Time Series Graphs

Create graphs of results over time:

1. Select an object (node, link, or subcatchment)
2. Go to Report > Graph > Time Series
3. Choose variables to plot
4. Click OK to generate graph

Useful Time Series Plots:

Profile Plots

View hydraulic grade lines through your system:

1. Go to Report > Graph > Profile
2. Select a path from upstream to downstream
3. SWMM shows water surface and conduit profiles

Profile plots help identify:

• Surcharged pipes (water above crown)
• Hydraulic bottlenecks
• Backwater effects

Scatter Plots

Compare two variables:

1. Go to Report > Graph > Scatter
2. Select X and Y variables
3. Useful for comparing modeled vs. observed data

Statistical Reports

Summarize results statistically:

1. Go to Report > Statistics
2. Choose object and variable
3. Select statistics (mean, max, frequency analysis)

Understanding Key Results

Hydraulic Performance Diagnosis

Start with conduit flow, depth, the hydraulic grade line, and the applicable project criteria. A reported maximum flow, Max/Full Flow ratio, or Capacity map value does not by itself prove that a conduit is undersized, especially under Dynamic Wave routing. Backwater, downstream boundaries, pressurization, controls, link offsets, and the timing of system inflows can all change the reported state.

Before proposing a size or slope change:

• Review node HGL and depth together with conduit flow and depth through the controlling period
• Check downstream tailwater, outfall boundaries, storage, controls, and possible backwater
• Verify node inverts, conduit offsets, dimensions, roughness, and the project’s elevation mode
• Confirm runoff and routing continuity and inspect instability or non-convergence warnings
• Compare the modeled state with the governing surcharge, freeboard, flooding, and velocity criteria

Flooding Analysis

Use the Node Flooding Summary to locate and quantify modeled overflow, then diagnose why it occurred. Check HGL profiles, downstream boundary conditions and backwater, storage and ponding, control actions, inflow assumptions, continuity, and the governing performance criteria. Flooding is an output condition, not a stand-alone pipe-sizing diagnosis.

System Response Time

How quickly does the system respond to rainfall:

• Time from peak rainfall to peak runoff
• Time from peak runoff to peak outfall flow
• Lag indicates storage effects or long travel times

Volume Balance

Verify water is accounted for:

• Total rainfall volume
• Total runoff volume
• Total outfall volume
• Storage volume remaining

Common Issues and Solutions

No Flow in System

Possible causes:

• Subcatchments not generating runoff (check imperviousness, rain gage)
• Subcatchment outlets not connected
• Adverse slopes in pipes

Solutions:

• Verify rain gage has data
• Check subcatchment outlet properties
• Review conduit slopes

Excessive Flooding

Possible causes:

• Conveyance restrictions or pressurized reaches
• Downstream tailwater, backwater, storage, or control settings
• Incorrect inverts, offsets, geometry, roughness, or inflows
• A performance criterion that the modeled system does not meet

Solutions:

• Trace the controlling HGL and flow path through the flooding period
• Verify survey data, link offsets, project elevation mode, boundaries, controls, and inflows
• Resolve material continuity, instability, or convergence problems before interpreting the result
• Evaluate design changes only after the system-level cause and governing criterion are established

Simulation Instability

Possible causes:

• Routing time step too large
• Extremely steep slopes
• Very small conduits

Solutions:

• Reduce routing time step
• Check for unusual geometries
• Enable variable time step

High Continuity Errors

Possible causes:

• Large time steps
• Flow exceeds numerical limits
• Incorrect units

Solutions:

• Decrease time steps
• Check for very high velocities
• Verify all units are consistent

Saving and Sharing Results

Project and Results Files

SWMM stores the project definition separately from simulation results:

Exporting Results

1. Status and summary reports: Use File > Export > Status/Summary Report
2. Time-series tables and summary tables: Select the required cells, then use Edit > Copy To
3. Graphs: Make the graph active and use Edit > Copy To for clipboard or file output
4. Binary results: Use a documented reader for the .out format when automated post-processing is required

Next Steps

You now have the skills to run SWMM simulations and analyze results. Continue learning:

1. Add complexity: Include storage units, pumps, and orifices
2. Explore LID: Model green infrastructure practices
3. Continuous simulation: Run multi-year simulations
4. Calibration: Match model to observed data

Summary

Running SWMM simulations involves:

1. Configure options: Select routing method, time steps, and duration
2. Verify the model: Check all connections and properties
3. Execute: Run and monitor for errors
4. Analyze: Review status report and examine results
5. Iterate: Refine model based on findings

SWMM output supports an engineering decision only after the model inputs, numerical behavior, boundary conditions, and governing performance criteria have been reviewed together.