Stormwater management is more than just keeping streets dry. It also means keeping water clean.

The Florida Department of Transportation (FDOT) is responsible for managing the water that runs off the state highway system. Regulations require an annual target amount of nitrogen and phosphorous to be removed from stormwater runoff. To do this, FDOT constructs best management practice (BMP) facilities in the roadside right-of-way to capture and treat the runoff.

However, water from places other than the road system also makes its way into the BMPs, even if they are designed just for roadway runoff. The two sources of runoff “co-mingle.”

Schematic showing co-mingling and bypass configurations of Best Management Practice (BMP) facilities. This research project tested the effectiveness of BMPs to treat co-mingles runoff water.   Source: UCF

FDOT has two options for dealing with co-mingled water. First, the agency is allowed to bypass the offsite water away from the BMPs, but that can be costly. The second option is to accept the extra water in existing BMPs that are designed for roadway runoff.

That raises the question: Does the extra volume of offsite runoff overwhelm the BMPs and lessen their ability to remove pollutants?

That is the question that FDOT and the University of Central Florida Stormwater Management Academy set out to answer.

Training BMP Trains

The core challenge for the project was with the model that practitioners use to assess the effectiveness of BMPs. The model, called BMP Trains, had no method to assess the effects of co-mingling. Because BMP Trains is the platform that water management regulating agencies and consultants use in the state of Florida, it was difficult for FDOT to know the best circumstances to co-mingle offsite water or to bypass it.

The purpose of the project was to modify BMP Trains so it could assess the effects and value of co-mingling.

“Bypassing offsite flow is costly and time consuming,” said Dr. Martin Wanielista, UCF Professor Emeritus and Co-Principle Investigator for the project. “In many cases, it may not be necessary to bypass. In fact, co-mingling may remove more pollutants at a very low cost. Quantifying the effects of co-mingling in our BMPs could garner significant cost savings for FDOT and other agencies.”

Hypothesis: A Runoff Race

The research team’s hypothesis was that, since roadways are made of impervious materials, roadway runoff reaches BMPs faster than water from surrounding areas. That runoff would then have a head start and more time to be treated before offsite water co-mingled. Because onsite and offsite water largely enters the BMPs at different times, there would be available capacity in the onsite BMP to treat offsite waters.

“Roadway runoff and offsite runoff are rarely going to be in the BMP at large volumes at the same time,” said Catherine Earp, FDOT Drainage Design Engineer. “Given that, we wanted to know if existing treatment facilities could handle both.”

To test this hypothesis, the team created a series of simulations to expand and test the Excel-based BMP Trains tool. They created 75 simulations for each of the five Florida meteorological zones that are used for stormwater treatment. The simulations re-created stormwater runoff scenarios using three main parameters:

  • Ratio of offsite to onsite runoff volume
  • Delay of offsite runoff to reach an onsite BMP
  • Treatment size of the onsite BMP

To make the simulations as realistic and nuanced as possible, the team also incorporated data that match real-world sites. This included complex land use such as a mixed residential and commercial area with a highway; directly-connected impervious areas like roof tops, driveways, and roads adjacent to each other; and varying soil conditions. A cost analysis routine was also added.

Map of the five meteorological regions used for stormwater management. This project tested multiple co-mingling scenarios for each region.    Source: UCF

Single or Co-mingle?

The simulations calculated the average annual capture volume of pollutants for each scenario. The results were impressive.

“In almost every case, it was more beneficial to route offsite flow through an existing BMP, even if the facility was designed just for roadways,” Earp said. “That is very valuable information when making decisions about how to deal with co-mingling situations.”

“The results were extremely positive,” Wanielista said. “Agencies can save money by co-mingling and remove more pollutants.”

Based on the results of the project, BMP Trains was modified to include calculations for pollutant removal effectiveness of co-mingled water; cost calculations; and an improved routine for estimating runoff from catchments with multiple soil and land uses.

Improving the State of the Art

For the updated model to be fully implemented state-wide, it had to be approved and adopted by regulatory officials and other practitioners.

“Once we verified the results of this research, we took it to the regulators,” Earp said. “They agreed that, in many cases, there was no need to bypass offsite flow and that there would be environmental benefit from co-mingling, without expanding the size of our BMPs.”

Wanielista said the real power of the updated model is that it lets practitioners and regulators make more informed decisions. For example, BMP Trains can also aid in determining when co-mingling should not be done.

“If it is not cost effective to run offsite water through an existing facility, agencies are still allowed to bypass it,” he said. “This project provided a method for assessing the opportunity to co-mingle and quantifying the benefits.”

This new methodology is now the standard practice for evaluating stormwater runoff, not just for FDOT, but also for many local agencies.

“Cities and counties that have the option of treating offsite water in an onsite BMP are using the results of this research,” Wanielista said. “We’re seeing more and more of them utilizing this tool and realizing real cost savings.”

Further Reading

BDV24-977-16 Removal Effectiveness of Co-mingling Off-site Flows with FDOT Right-of-way Stormwater
Final Report | Summary