Torrential rainfall that inundated dozens of vehicles on the highway Tuesday evening was somewhere between a 200- and 500-year flood event, experts said, exceeding the 50-year standard of the Interstate 24/Interstate 75 interchange widening project.
The 50-year storm standard for the state project is part of drainage guidelines that transportation officials said were followed by the contractor on the $161 million, second phase of the interchange project.
Under state guidelines spelled out in plan documents, Wright Brothers Construction Co. is responsible for design and construction of the project’s entire storm water management system, including bridges, storm water transfer structures, inlets and collection systems. All runoff flowing into the project area, whether it originates from within the project or not, had to be accounted for in the design of the drainage system.
Tennessee Department of Transportation guidelines require the contractor to analyze and document existing drainage systems and not disturb existing flood walls along I-24 owned by the city of Chattanooga, documents said. The contractor was also asked to avoid overwhelming adjacent systems outside the project.