The City of Shawnee Saves Dollars with Stormwater System Evaluation

Knowing when to replace aging corrugated metal pipe (CMP) can be an involved process that requires commitment and resources from cities managing infrastructure. There are obvious signs of CMP failure that are easier to identify once they occur, such as crushed, bowed, or deformed pipes or a rusted pipe bottom. But, staying on top of pipe system conditions across the city to identify pipe deterioration before it turns into a big failure can be difficult.

One way the City of Shawnee, Kansas monitors their storm sewer is by regularly video inspecting their system. This helps them stay in front of failures and is less costly than making emergency repairs once the pipes fail. When we began working with the city on improvements to Johnson Drive between I-435 and Woodland Drive, we knew they wanted to not only resurface the road, but also make any needed storm system repairs at the same time. They wanted to coordinate this work to minimize the construction impacts and maximize funding.

With this in mind, we began by evaluating the storm sewer system in the project area to determine the pipe condition and hydraulic capacity. Here are the steps we took:

1. Conducted a field survey to gather location and elevation information on the existing pipe system.
2. Reviewed the city’s pipe inspection video footage and noted issues.
3. Categorized issues by severity and gave each a condition rating.
4. Developed a pipe capacity model to evaluate pipe hydraulic capacity.
5. Examined the gutter spread to determine if additional inlets are necessary to capture design flows.
6. Performed manhole and inlet inspections to identify if the storm structure condition warranted the replacement of the structure.

From this, we discovered:

• Pipe that required replacement, pipe that could be lined to prolong the service life, and pipes that could remain in place.
• That some additional inlets needed to be extended up the roadway to address the existing gutter spread issue.
• Storm structure improvements, ranging from complete structure replacement to structure top replacement to reconstructing the inlet throat.

Using this data, we made a recommendation to the city on next steps with a cost comparison. Then, they determined how they’d like to move forward, using that information as a guide.

The full analysis of the system showed they could remove and replace some components and repair or leave others. The state of their infrastructure did not warrant a full rebuild. This allowed them to stretch city dollars and minimize construction disruptions. To improve the stormwater system efficiency and service life in the project area, we provided a range of solutions customized to meet the City’s needs that were aimed at keeping construction cost down.

Stormwater infrastructure should be evaluated regularly. The City of Shawnee, Kansas has implemented a long-term strategy that helps them protect their assets, and it often translates to dollars saved. Soon, it will help them qualify for the new Johnson County Stormwater Management Program (SMP) funding structure.