Skip to content


A Deep Dive into SWMM Modeling: 5 Common Mistakes

A Deep Dive into SWMM Modeling: 5 Common Mistakes

The Stormwater Management Model (SWMM) is a prediction tool used to measure stormwater runoff quantity and quality from drainage networks. For nearly a decade, it has also been used to analyze, plan and design both gray and green infrastructure. SWMM software and technology are continuously advancing, making it an increasingly crucial part of utility infrastructure planning. However, to fully gain the benefits of the model, it is essential to avoid the mistakes below.

Mistake 1: Listening to your gut instead of the data

When evaluating drainage system issues, it is easy to fall prey to bias. As humans, we are more likely to assume an issue is caused by something that we can see or something that has been an issue in the past. With most of our storm and sanitary infrastructure below ground, it is easy to fall prey to this bias and spend capital improvement plan (CIP) dollars on plants, outfalls, and public complaints.

These biases are reinforced and tweaked by our “gut feelings,” but they remain unsupported by data. To make an informed decision, it is best to look at a full picture of the system and its performance, which can be achieved through SWMM. The model can help identify what and where the issue is in your system, above or below ground, ensuring that CIP dollars are spent appropriately.  Upon identifying the problem, the model is also a valuable tool to analyze a variety of solutions to develop the optimal solution for each unique circumstance.

Mistake 2: Limiting flexibility in your CIP plan 

The CIP Master Plan provides an outline and roadmap of prioritization for how a community can improve and sustain its infrastructure. Incorporating SWMM into this planning process allows the municipalities to consider a variety of conveyance alternatives and innovative technology to address infrastructure needs and performance requirements. 

The CIP Master Plan also serves as an economic development tool that enables a community to plan for future growth. SWMM experts can put strategically placed meters in areas designated for future development. This ensures that proper infrastructure is in place so that community growth won’t be hindered. Having a holistic picture of the system allows the plan to progress smoothly and considers the most economically viable options for years to come.  Like a sewer system, the model is a living and growing asset.  As years pass and the system evolves, so does the model.  This provides flexibility and allows the CIP Master Plan to evolve with the system.

Additionally, SWMM empowers a community to allocate financing ahead of time to direct the sequence of projects. It allows communities to seek out the most cost-effective opportunities, such as a new technologies, special funding, or project sequencing to solve a problem. 

Mistake 3: Targeting quick fixes instead of root cause

Just like a doctor seeks to treat the whole patient, SWMM attempts to treat the entire system. A utility may be experiencing "symptoms" such as surcharging, flooding or overflows, but the source of these system issues may stem from another location. Instead of putting a bandage on these issues with a quick fix, SWMM helps to find the root cause. Once the root cause is discovered, it can be planned for and addressed in the CIP Master Plan. The solution can also be simulated in the model to identify any impacts on other areas of the system.  Using this strategy helps prevent a more catastrophic event in another area of the system, which saves money and protects the community.

Mistake 4: Not investing early in SWMM 

Municipalities may delay implementing SWMM because it's perceived as an added expense to increasingly tight budgets. However, not investing early in SWMM could prove more costly down the line. At a fraction of the total CIP program cost, an investment in SWMM provides the data necessary to verify, prioritize, and fund problems and solutions to reduce risk for a community’s infrastructure program. 

For example, let’s say that a municipality slowly began experiencing surcharging and basement backups. The initial assumption might be that the trunk line was not large enough. The municipality hires a consultant to design a parallel trunk line to increase conveyance capacity, which could cost millions. After some discussion, the municipality decides to complete a model of the system. The model identifies an area with abnormally low conveyance, suggesting a clog, collapsed pipe or constriction of some kind. The municipality completes field investigations (CCTV) and finds a collapsed pipe, which is replaced at a fraction of the cost of a new trunk line. In addition, the model remains an asset for future investigations.

Mistake 5: Not using SWMM experts to analyze the data 

SWMM is only the first step to making informed CIP decisions. To make the model useful, an expert will need to interpret the data. SWMM experts can help by identifying the optimal locations for meters to get accurate data and analyze it. They are aware of common problems from experience working with other utilities, and they use this experience to identify and solve root cause issues within a system. 

A model may return “no errors” when running, but a lack of errors doesn’t ensure correct information. SWMM experts ensure that the model output is valid, and they also drive model calibration to ensure that the data remains valuable. 

Keep your utility above water with SWMM

SWMM benefits the community. It saves money by finding the best long-term solution and fully identifies infrastructure problems by providing a complete picture of the system. The model supports data-driven decisions, giving confidence that the final solution is the best solution.  

Our experts at B&N have the experience you need to successfully implement SWMM into your utility and help maintain an up-to-date and relevant model configuration. We have a robust team of in-house design staff that specialize in survey, sewer, rivers and dams to analyze a modeling component.

To learn more about integrating SWMM into your community, please contact Brenton Hasenour or Danny Ketzer.

Brenton Hasenour, PE, Project Manager

Brenton Hasenour, PE 
Project Manager

Danny Ketzer, PE, Watershed Modeling Leader

Danny Ketzer, PE 
Watershed Modeling Leader