CASE STUDY: Merritt Road (Dallas TX)
A Thin Green Line Between Love and Hate
The City of Rowlett made a bold choice in redeveloping Merritt Road, a two-mile 14.2 million Capital Improvement Project that serves as a connector for the President George Bush Tollway. Funded by the Regional Tollway Revenue Fund, Dallas County and The City of Rowlett, the innovative design for Merritt Road is intended to support this area as a new technology corridor and to attract related business. There is a municipal movement in transportation to construct roadways that don’t just serve as a corridor for vehicles, but also for pedestrians and bicyclists. Rowlett had seized the opportunity to pursue a “complete street” concept that expanded the usability and potential for growth beyond the traditional curb-to-curb model. The center median swale was less than 10’ wide and a little over an acre, which made it a challenge for maintenance crews to access it. Traditional bioretention was not feasible with the limited area for treatment and long-term maintenance, leaving the engineers with a decision to use FocalPoint High Flow Biofiltration.
The primary function of the swale was to convey water from the roadway into the biofiltration system. The FocalPoint treats the first 1.5” of runoff and the remaining storm event is managed through overflow inlets. The FocalPoint system provides water quality treatment while minimizing the filtration bed footprint to less than 1,000 ft2 for the entire roadway. Traditional bioretention would have taken up 10 times that area. Designers chose to place sod throughout the swale and using native plantings for the biofiltration system to minimize maintenance. During construction, the threat of silt and clay clogging the system was mitigated by using a “cap & seal” method that encases the entire system in a protective geotextile cover. Even though the construction phase didn’t have any significant problems, it didn’t take long to run into an issue. After a couple of rainfall events it became apparent the water velocity in the swale was too fast, washing some of the sod on top of the filtration bed. Gabion walls and check dams were placed in the swale to help dissipate the flow. The high velocities from the runoff were not anticipated during the design phase but were easily solved with a simple retrofit.
Merritt Road took advantage of next generation bioretention, focusing on preventive maintenance design and reducing grey infrastructure. This simple approach allowed the City of Rowlett to have a low-cost maintenance budget and minimized risk of clogging during construction. Maintenance includes mowing, picking up trash and replacing the mulch layer on the FocalPoint once a year.