St. Elmo Drainage System Study and Upgrade (“Chattanooga Big Dig”)

The City of Chattanooga, Tennessee, selected Consor to provide structural and hydraulic evaluations, survey, design, and construction inspection services for the St. Elmo Drainage System Study and Upgrade project, which later became coined the “Chattanooga Big Dig.”

The Chattanooga “Big Dig” provides the storm drainage backbone to accommodate a 100-year storm event for a 1,100-acre drainage area encompassing the historic St. Elmo District, large parts of Alton Park, and South Chattanooga with one outfall into Chattanooga Creek, which is a tributary to the Tennessee River. Harnessing the power of the potential water volume from this large of an area created extreme challenges. This project was not only massive in size and volume but also equally massive in its required planning and coordination. The impact on vehicular and pedestrian traffic had to be managed and required coordination with property owners, local schools, and events for the timing of the closure of Broad Street (a primary roadway).

Extensive coordination with the railroad for two large-diameter tunnels was required to permit infrastructure crossings. The discovery of contaminated soils further complicated the massive amount of excavation required for the project, resulting in significant environmental impact considerations and permitting requirements. Coordination with endangered bat migration patterns was also needed before construction. Planning and engineering for this project reached across multiple disciplines with unique goals and requirements, but all had to come together to not only engineer unique solutions but merge all the solutions, requirements, time windows, and schedules to execute a successful project that protects the environment while attaining the functional project goals.

To prevent a catastrophic failure of their stormwater infrastructure and potential water contamination from a former landfill, the City of Chattanooga took action to redesign the storm conveyance in the historic St. Elmo area of the city. The new storm drainage is designed to accommodate a 100-year storm event for an 1,100-acre drainage area. The discovery of contaminated soils complicated the project and brought in additional stakeholders. The new infrastructure has been designed with expansion in mind to pave the way for future development of the area in conjunction with the planned extension of the Chattanooga Riverwalk.