A modular assembly is provided for managing the flow of fluid beneath a ground surface. The assembly can feature a plurality of modules, each having a deck portion and opposing sidewalls extending downward therefrom. The opposing sidewalls can slope outward and away from one another as they extend downward from the deck portion. The modules further comprise a shoulder for supporting a link slab, and to support and separate modules that are stacked during transportation or storage. The sidewalls can define an interior fluid passageway having a flared configuration from top to bottom. The link slab and sidewalls of adjacent modules can define an exterior fluid passageway in fluid communication with a lateral fluid channel. A method is also provided for making a precast concrete module for use in the modular assembly.