Steady Flow vs Kinematic Wave vs Dynamic Wave

EPA SWMM Routing Methods Explained

Understanding routing in EPA SWMM is essential for building reliable and defensible stormwater models. While runoff generation determines how much water enters your system, routing determines how that water actually moves through it.

In this article, we’ll break down what routing is, how it works in SWMM, and how to choose between the three available routing methods: steady flow, kinematic wave, and dynamic wave.

What Is Routing in EPA SWMM?

In EPA SWMM, routing refers to how flow is conveyed through a hydraulic network over time.

Once runoff is generated in subcatchments, routing determines how water moves through:

• pipes

• channels

• junctions

• storage elements

Routing directly affects:

• flow timing

• water depth in pipes and nodes

• hydraulic conditions like surcharge and backwater

Importantly, routing does not change how much runoff is generated—it controls how that runoff is distributed through the system over time and space.

What Is Hydraulic Routing?

Hydraulic routing describes the time-varying movement of water through a conveyance system.

At each computational time step, SWMM calculates:

• flow rate

• water depth

These calculations account for storage within the system, meaning water can be temporarily held in:

• conduits

• junctions

• storage units

Because of this, routing is a core component of SWMM’s hydraulic modeling framework and plays a major role in how realistic your results are.

Runoff vs Routing: A Critical Distinction

SWMM separates modeling into two connected processes:

1. Runoff Generation

Occurs in subcatchments based on:

• rainfall

• infiltration

• surface storage

• imperviousness

2. Hydraulic Routing

Takes that runoff and moves it through the system.

These processes use different equations and assumptions, and your routing choice determines which hydraulic behaviors your model can represent.

The 3 Routing Methods in EPA SWMM

SWMM provides three routing options, each with increasing complexity:

1. Steady Flow

2. Kinematic Wave

3. Dynamic Wave

Choosing the right one depends on what behaviors your analysis needs to capture.

1. Steady Flow Routing (Simplest)

Steady flow routing is the most basic option available.

Key Characteristics:

• No time-varying behavior

• No storage effects

• Flow is translated downstream without delay

Limitations:

• Cannot represent unsteady flow

• Cannot simulate real hydraulic behavior like backwater or surcharge

When to Use:

Only in situations where:

• timing and storage are not important

• simplified assumptions are acceptable

2. Kinematic Wave Routing (Moderate Complexity)

Kinematic wave routing introduces time-varying flow and allows runoff to move through the system as a wave.

Key Characteristics:

• Flow driven by gravity and slope

• Resistance represented by roughness

• Limited storage representation

Limitations:

• Cannot simulate all hydraulic conditions

• Restrictions on network topology:

  • limited loops

  • limited outlet configurations

Benefits:

• More realistic than steady flow

• Computationally efficient

When to Use:

• Simple systems

• When full hydraulic behavior is not required

3. Dynamic Wave Routing (Most Comprehensive)

Dynamic wave routing is the most advanced and flexible option in SWMM.

Capabilities:

• Backwater effects

• Pressurized (surcharged) flow

• Flow reversal

• Complex network layouts (non-dendritic systems)

Why It Matters:

This method can represent a much wider range of real-world hydraulic behavior, making it the preferred choice for detailed analysis.

When to Use:

Dynamic wave routing should be used when your model needs to capture:

• downstream controls

• surcharging

• complex system interactions

• realistic system performance

Choosing the Right Routing Method

The correct routing method depends entirely on your modeling objectives.

Ask yourself:

• What hydraulic behaviors need to be represented?

• Does the method support those behaviors?

If your model requires:

• backwater

• pressurization

• flow reversal

• complex connectivity

→ Dynamic wave is required

If not, a simpler method may be acceptable—but only if its assumptions align with your goals.

The Importance of Routing Time Step

The routing time step directly affects:

• numerical stability

• solution accuracy

Key Considerations:

• Dynamic wave typically requires smaller time steps

• Large time steps can cause:

  • instability

  • inaccurate results

SWMM includes variable time stepping, which adjusts the time step during simulation to improve stability.

Choosing a time step is always a balance between:

• accuracy

• stability

• computational efficiency

Common Routing Issues in SWMM

Routing problems often appear as:

• warning messages

• continuity errors

• unexpected hydraulic behavior

Common causes include:

• violating routing method limitations

• incorrect network topology

• poor time step selection

SWMM provides diagnostic outputs to help identify these issues. Reviewing them is critical to ensuring your results are reliable.

Best Practices for Reliable Results

To improve model defensibility:

• Select a routing method based on required behaviors

• Verify the method supports your system conditions

• Use appropriate time steps

• Review diagnostic outputs carefully

• Document your assumptions

Documentation should include:

• routing method

• time step

• key modeling decisions

This ensures transparency and allows others to understand and trust your results.

Final Thoughts

Routing controls how water moves through your SWMM model—and ultimately determines which hydraulic behaviors can be represented.

• Steady Flow → simplest, most limited

• Kinematic Wave → moderate complexity, some limitations

• Dynamic Wave → most comprehensive and realistic

Choosing the right method—and validating it with diagnostics—is essential for producing accurate, defensible models.

Want to Learn SWMM Faster?

We offer a free EPA SWMM training course with 20+ lessons covering everything from fundamentals to advanced modeling.

Check the link in the description to get started.

Conclusion

Stormwater infiltration is one of the most important processes in hydrology and stormwater management. By understanding infiltration factors, modeling methods, and practical applications, engineers can design systems that reduce flood risks, improve water quality, and support resilient urban development.

At Clear Creek Solutions, we provide resources to help you build this knowledge, including a free 24-video EPA SWMM course that walks you through modeling basics step by step.