Project Details

Birnamwood Drive is located in northern Harris County, east of Spring, Texas. The existing roadway runs north/south between Fern Hill Drive and intersects Cypresswood Drive. The new section of Birnamwood is approximately 0.68 miles in length and extends from Fern Hill Drive north to Spring Creek Drive. This project is the first roadway project in the Houston area to implement Low Impact Development (LID) techniques and is viewed by Harris County as a stepping stone to implementing LID as its default roadway design approach throughout the County.

This roadway was required to manage a large (27-acre) drainage area, which was considered 95% impervious for safety reasons despite surrounding parklands. Since this was an initial LID implementation, the design aimed to stay close to standard designs while prioritizing safety. Key safety considerations included full curbs, limitations on ponding depth in the swale, and protection of sight lines related to vegetation. Stormwater enters the depressed center median via false-backed curb inlets.

During the design and consultation phase, potential construction and maintenance challenges were assessed. The primary conveyance feature utilized was a dry swale, prompting the design team to evaluate different biofiltration media for pollutant filtration.

What Convergent Water Technologies Provided

Convergent Water Technologies provided an advanced biofiltration solution tailored to Birnamwood Drive’s requirements. Two types of filtration media were considered:

• Option 1: Typical Low Flow Media – Engineered biofiltration media with a flow rate of less than 5 inches per hour. This option required nearly 900 square feet of bioretention surface area per acre of impervious drainage, leading to significant real estate needs, high excavation costs, and complex maintenance demands.
• Option 2: High Flow Media – High-performance media with a flow rate of 100 inches per hour. This solution required only 70 square feet of infiltration bed surface area per acre treated. The smaller footprint enabled the design team to use two small biofiltration systems, one in each of the two swales in the center median. This approach minimized construction and maintenance costs while improving overall constructability.

The FocalPoint High-Performance Modular Biofiltration System (HPMBS) was selected as the optimal solution. FocalPoint HPMBS integrates a high-performance open-cell underdrain, clog-proof bridging mesh, bridging stone, and a high-performance biofiltration media that exceeds 100 inches per hour in flow rate. This system provided superior water quality and drainage capabilities, accomplishing in 2,000 square feet what traditional bioswale designs would have required a full acre of biofiltration to achieve.

Additionally, various LID practices were implemented in the depressed center median, including native landscaping, check dams, and the FocalPoint HPMBS adjacent to extreme event outfalls. Plantings were chosen to maximize debris filtration, adapt to site conditions, and reduce maintenance requirements. Earthen check dams were designed to slow water velocity and extend the time of concentration. The FocalPoint HPMBS filtered contaminants before discharge into Spring Creek, while extreme rainfall events were managed via bypass pipes directly to the outfall. The detention capacity of the center swale eliminated the need for offsite detention, representing a significant cost-saving measure.

Why Birnamwood Drive (Harris County) Chose LID and FocalPoint

Birnamwood Drive selected Convergent Water Technologies due to its ability to provide a cost-effective, high-performance stormwater management solution. The FocalPoint HPMBS offered several key benefits:

• Space Efficiency – Required only a fraction of the area compared to traditional bioswale designs, reducing land use and excavation needs.
• Cost Savings – Lower construction and maintenance costs compared to conventional bioretention solutions.
• Enhanced Performance – Provided exceptional pollutant removal and high infiltration rates to handle stormwater efficiently.
• Sustainability – Integrated seamlessly with LID practices, helping to set a precedent for future roadway designs in Harris County.

By choosing Convergent Water Technologies, Birnamwood Drive successfully implemented an innovative, efficient, and sustainable stormwater management system, positioning itself as a model for future LID-based roadway projects in the region.