The Mill Creek/Peaks Branch/State-Thomas Drainage Relief Tunnel (Mill Creek Tunnel) is one of the largest projects Halff is doing, with a $320 million total cost. It is a project that many of us will never actually see.
The Mill Creek Tunnel is the largest tunnel currently under construction in the United States. It will provide needed stormwater drainage relief to help save lives and protect residents, schools, medical facilities, businesses and streets. The tunnel will stretch from the State-Thomas District in Uptown Dallas through the Mill Creek and Peaks Branch watersheds, and will include five large inlet structures. Once complete, the 5-mile-long tunnel will sit 70 to 150 feet below ground to provide 100-year flood protection for nearly 2,200 commercial and residential properties in the east Dallas area, including Baylor Medical Center and Fair Park. The tunnel’s capacity will be 160 million gallons of water.
Halff was selected to design a drainage relief system for Mill Creek and two drainage-relief projects for Peaks Branch. Halff developed and analyzed 10 alternatives for implementing these projects. The adopted plan is a combined system that provides drainage relief to Mill Creek and Peaks Branch. The tunnel component of the project was revised during the design phase to provide flood relief for the State-Thomas area.
Once the project began, Halff’s Water Resources team developed XPSWMM hydrodynamic models for the Mill Creek, Peaks Branch and East Peaks Branch watersheds, as well as conceptual drainage master plans for Mill Creek, Peaks Branch and East Peaks Branch.
Halff’s Public Works team developed construction plans for the 5-mile tunnel, which consists of a 2-mile-long, 35-foot diameter segment and a 3-mile long, 30-foot diameter segment. Halff also led the design effort for the 30,000 gpm dewatering station that will empty the tunnel for maintenance and inspection.
The new tunnel, excavated using a tunnel-boring machine (pictured above), will play a major role in preventing future flooding and property damage. Starting at White Rock Creek, the tunnel will connect to the existing storm-sewer systems and offload systems downstream of the tunnel. The downstream systems will have near 100-year capacity once the tunnel is online.
