A retention basin enclosure includes a plurality of prefabricated concrete vertical wall segments, a baffle unit and at least one steel connecting dowel. Each wall segment includes a horizontal top end that defines a notch and two vertical edges that define at least one cylindrical bore. The baffle segment includes a vertical edge surface that defines a plurality of cylindrical bores and a horizontal planar member from which a plurality of protrusions extend upwardly therefrom so that the baffle segment reduces water flow velocity. Two baffle vertical wall members each have a planar vertical surface that defines a plurality of cylindrical bores disposed so that at least one of the cylindrical bores defined by concrete baffle segment can be placed in alignment with at least one of the cylindrical bores defined by the planar vertical surface.