Methods and modules for use in a modular assembly are disclosed for retaining or detaining storm water beneath a ground surface. A module comprises a substantially horizontally disposed deck portion and at least one substantially vertically disposed side portion extending therefrom. The deck portion and side portion have respective end edges, and the side portion has bottom edges. The side portion and the deck portion define a longitudinal channel which is open at least at an end of the module. The side portion has at least one opening therein and defines a lateral channel in the module. The longitudinal and lateral channels are in fluid communication with one another. Each channel has about the same cross section and extends upwardly from the bottom edges to allow relatively unconstrained flow in the longitudinal and lateral directions.