Analyzing EPA SWMM Results

Running a SWMM model is only half the work. Results must be interpreted with the inputs, boundary conditions, numerical checks, and governing criteria before they support a design or compliance finding. This tutorial covers SWMM output files, graphing tools, statistical summaries, and techniques for extracting the information you need.

The file descriptions and result views below follow Chapters 9 and 11 of the EPA SWMM 5.2 User’s Manual.

Understanding SWMM Output Files

After a simulation, SWMM generates several output files:

The binary .out file contains detailed time series that SWMM uses for graphs and tables. The .rpt file provides human-readable summaries.

Status Report Structure

The status report (.rpt) contains these sections:

1. Analysis Options - Simulation settings used
2. Runoff Quantity Continuity - Water balance for hydrology
3. Flow Routing Continuity - Water balance for hydraulics
4. Subcatchment Runoff Summary - Totals by subcatchment
5. Node Depth Summary - Node depths and maximum hydraulic grade
6. Node Flooding Summary - Flooding duration, rate, and volume where flooding occurred
7. Link Flow Summary - Maximum link flow and conduit hydraulic results
8. Conduit Surcharge Summary - Duration of full, above-normal-full-flow, and capacity-limited conditions

Checking Continuity and Mass Balance

Runoff Continuity

The runoff continuity table shows:

Interpreting Continuity Error:

Flow Routing Continuity

The routing continuity shows:

Using the Summary Tables

Subcatchment Runoff Summary

Key columns in the subcatchment summary:

The runoff coefficient allows quick comparison with Rational Method assumptions:

Node Depth Summary and Node Flooding Summary

Use the Node Depth Summary to review maximum depth, maximum hydraulic grade, and their timing. Compare elevations with the node rim, ground surface, connected conduit crowns, boundary data, and the governing freeboard or surcharge criteria.

Use the separate Node Flooding Summary to review where SWMM reported flooding, its duration, maximum flooding rate, and total flood volume. Interpret those values with the ponding option and ponded-area inputs: without effective surface ponding, overflow leaves the modeled conveyance system.

Link Flow Summary and Conduit Surcharge Summary

The Link Flow Summary reports maximum absolute flow and, for conduits, results such as maximum velocity and maximum/full-flow and maximum/full-depth ratios. These are reported hydraulic states, not universal pass/fail criteria; compare them with node HGL, downstream boundaries, controls, and the criteria governing the project.

The separate Conduit Surcharge Summary reports hours full at both ends or at either end, hours above full normal flow, and hours capacity limited. Do not apply a universal objective to minimize every non-zero duration. Determine whether the condition is acceptable for the specified event and design criterion, and diagnose backwater, pressurization, controls, boundaries, and continuity before changing conduit size or slope.

Storage Summary

For storage units (detention ponds):

Calculate detention efficiency:

Creating Time Series Graphs

SWMM’s graphing capabilities help visualize system behavior.

Hydrograph Plots

To create a flow hydrograph:

1. Select Report > Graph > Time Series
2. Choose object type (Subcatchment, Node, or Link)
3. Select specific object
4. Choose variable (Runoff, Depth, Flow, etc.)
5. Click Add to plot

Common hydrograph comparisons:

• Inflow vs. Outflow: Shows detention effect
• Multiple Subcatchments: Compare contributing areas
• Pre- vs. Post-Development: Assess impact

Profile Plots

Longitudinal profiles show system behavior along a path:

1. Select Report > Graph > Profile
2. SWMM uses the defined conduit linkages
3. Select starting and ending nodes
4. Choose a specific time or animate through simulation

Profiles reveal:

• Hydraulic grade line position
• Surcharged reaches
• Backwater effects
• Grade breaks and flow restrictions

Scatter Plots

Compare two variables:

• Rainfall vs. Runoff
• Inflow vs. Outflow
• Upstream vs. Downstream flow

Scatter plots help identify:

• Rating curve behavior
• Threshold effects
• System response characteristics

Statistical Analysis

Peak Values Summary

Extract peak values for design verification:

Volume Calculations

Total volumes are critical for:

• Detention sizing verification
• Water quality volume capture
• Flood volume estimates

From the summaries:

Use consistent units when multiplying runoff depth by area; convert the result to the required volume unit. Do not multiply an instantaneous runoff flow rate by area.

Here d_runoff is the total runoff depth and A_subcatch is the subcatchment area.

Time to Peak

Identify time to peak from hydrograph data:

• Create a time-series table with Report > Table, then use Edit > Copy To for a clipboard or file copy
• Find maximum value and corresponding time
• Calculate lag time from rainfall centroid

Exporting Results

To Spreadsheet

Copy time-series results for detailed analysis:

1. Create a time-series table with Report > Table
2. Select the required cells, or use Edit > Select All for the whole table
3. Choose Edit > Copy To and select the clipboard or a file

SWMM’s File > Export menu exports maps, hot-start results, or the combined Status/Summary Report; it is not a general time-series export command.

To Reports

For report preparation:

1. Copy summary tables from .rpt file
2. Make a graph active and use Edit > Copy To for a clipboard or file copy
3. Create custom post-processing scripts

Binary Output File

Advanced users can read the .out file directly using:

• SWMM Output File Reader utilities
• Python with swmm-toolkit library
• Custom parsing code

Interpreting Common Results Patterns

Hydrograph shapes are diagnostic clues, not unique diagnoses. Compare each pattern with rainfall, inflows, control actions, boundary time series, HGL profiles, storage state, reporting interval, and continuity before assigning a cause.

Pattern 1: Rapid Rise, Slow Recession

Hypotheses to check include fast runoff response from small or steep subcatchments, the rainfall interval or temporal pattern, short overland-flow timing, and delayed release from surface, conduit, or storage volume. Groundwater or a downstream boundary can also prolong the recession.

Pattern 2: Double Peak

Hypotheses to check include multiple rainfall bursts, differently timed tributaries, a storage or control release, and time-varying downstream conditions. Compare component hydrographs before attributing the second peak to a single source.

Pattern 3: Flat-Topped Hydrograph

Hypotheses to check include an outlet or control setting, a downstream boundary or backwater, pressurization, storage behavior, flooding loss, and coarse reporting intervals. A flat top does not by itself prove that the conduit is at capacity.

Pattern 4: Long Recession Tail

Hypotheses to check include detention or network storage draining, groundwater inflow, tailwater, control operation, and the model’s initial state. Confirm the responsible volume and flow path with component time series and continuity results.

Troubleshooting Poor Results

High Continuity Errors

Investigate reporting and routing time steps, non-converging nodes, high-instability links, geometry, initial conditions, and the processes included in the mass balance. Re-run a controlled sensitivity case only after identifying a plausible cause; a smaller time step does not automatically cure an input or conceptual error.

Unrealistic Velocities

Check for:

• Incorrect slope values
• Wrong Manning’s n
• Missing invert offsets
• Boundary, backwater, control, or pressurization effects

Zero or Low Flow

Verify:

• Subcatchments connected to correct nodes
• Rain gages assigned properly
• Pervious/impervious fractions correct
• Outlets not blocked

Design Report Extraction

For typical design reports, extract:

1. Peak Flow Comparison

   • Pre-development vs. post-development peaks
   • Peak reduction from detention

2. Volume Summary

   • Total runoff volume
   • Detention volume utilized
   • Water quality volume captured

3. System Performance

   • Maximum depths at key nodes
   • Surcharging duration
   • Flooding locations (if any)

4. Outlet Performance

   • Peak outflow rate
   • Drain time for detention
   • Emergency spillway activation

Summary

Effective results analysis in SWMM requires:

• Checking continuity errors before detailed analysis
• Using summary tables for system-wide performance
• Creating graphs to visualize temporal patterns
• Comparing results to design criteria
• Documenting key metrics for reports

Take time to understand what the model is telling you. Unexpected results often reveal important design considerations or model issues that need attention.

References

1. Rossman, L. A. (2022). Storm Water Management Model User’s Manual Version 5.2. U.S. Environmental Protection Agency, EPA/600/R-22/030. Official PDF.

2. Rossman, L. A. (2022). Storm Water Management Model Reference Manual Volume I - Hydrology (Revised). U.S. Environmental Protection Agency, EPA/600/R-15/162A.

3. James, W., Rossman, L. A., & James, W. R. C. (2010). User’s Guide to SWMM 5. CHI Press.

4. Gironas, J., Roesner, L. A., & Davis, J. (2009). Storm Water Management Model Applications Manual. U.S. Environmental Protection Agency, EPA/600/R-09/077.